Anatomy and Physiology – Cardiovascular System and Blood

DIAGNOSTIC RADIOGRAPHY · NTA LEVEL 4 · SEMESTER ONE

Anatomy and Physiology – Cardiovascular System and Blood

CRT04101 · Anatomy, Physiology and Pathology

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Anatomy and Physiology – Cardiovascular System and Blood

Cardiovascular System

The cardiovascular system consists of the heart, blood vessels, and the
approximately 5 liters of blood that the blood vessels transport. Responsible
for transporting oxygen, nutrients, hormones, and cellular waste products
throughout the body, the cardiovascular system is powered by the body‘s
hardest-working organ — the heart, which is only about the size of a closed
fist. Even at rest, the average heart easily pumps over 5 liters of blood

throughout the body every minute.

The Heart

The heart is a muscular pumping organ located medial to the lungs along
the body‘s midline in the thoracic region. The bottom tip of the heart, known
as its apex, is turned to the left, so that about 2/3 of the heart is located on
the body‘s left side with the other 1/3 on right. The top of the heart, known
as the heart‘s base, connects to the great blood vessels of the body:

the aorta, vena cava, pulmonary trunk, and pulmonary veins.

The Heart

The heart is a hollow muscular organ which beats over 100,000 times a day
to pump blood around the body's 60,000 miles of blood vessels. The right
side of the heart receives blood and sends it to the lungs to be oxygenated,
while the left side receives oxygenated blood from the lungs and sends it out
to the tissues of the body.
The Heart has three layers; the ENDOCARDIUM (inner layer), the
EPICARDIUM (middle layer), and MYOCARDIUM (outer layer).
The heart is protected by the PERICARDIUM which the protective
membrane is surrounding it.
The heart has FOUR CHAMBERS, in the lower heart the right and left
Ventricles, and in the upper heart the right and left Atria. In a normal heart
beat the atria contract while the ventricles relax, then the ventricles contract
while the atria relax. There are VALVES through which blood passes
between ventricle and atrium, these close in such a way that blood does not
backwash during the pauses between ventricular contractions. The right and
left ventricles are divided by a thick wall (the VENTRICULAR SEPTUM),
babies born with "hole in the heart" have a small gap here, which is a
problem since oxygenated and deoxygenated can blood mix. The walls of
the left ventricle are thicker as it has to pump blood to all the tissues,
compared to the right ventricle which only pumps blood as far as the lungs.
The spleen
This is a large flat oval organ located below the diaphragm, it's main
function is to STORE BLOOD. The size of the spleen can vary, for example
it may enlarge when the body is fighting infection also it's size tends to
decrease with age. It is a non-vital organ and it is possible to survive after
removal of the spleen.
Perinicious anaemia is a Vitamin B12 deficiency resulting in a reduction in
number of erythrocytes.
Aplastic anemia is a failure of the bone marrow to produce the enough red
blood cells.

Septicaemia – bacterial toxins in blood.

Circulatory Loops

There are 2 primary circulatory loops in the human body: the pulmonary
circulation loop and the systemic circulation loop.
Pulmonary circulation transports deoxygenated blood from the right side of
the heart to the lungs, where the blood picks up oxygen and returns to the
left side of the heart. The pumping chambers of the heart that support the
pulmonary circulation loop are the right atrium and right ventricle.
Systemic circulation carries highly oxygenated blood from the left side of
the heart to all of the tissues of the body (with the exception of the heart and
lungs). Systemic circulation removes wastes from body tissues and returns
deoxygenated blood to the right side of the heart. The left atrium and left
ventricle of the heart are the pumping chambers for the systemic circulation

loop.

Blood Vessels

Blood vessels are the body‘s highways that allow blood to flow quickly and
efficiently from the heart to every region of the body and back again. The
size of blood vessels corresponds with the amount of blood that passes
through the vessel. All blood vessels contain a hollow area called the lumen
through which blood is able to flow. Around the lumen is the wall of the
vessel, which may be thin in the case of capillaries or very thick in the case
of arteries.
ARTERIES carry oxygenated blood away from the heart. They are thick
hollow tubes which are highly ELASTIC which allows them to DILATE
(widen) and constrict (narrow) as blood is forced down them by the heart.
Arteries branch and re-branch, becoming smaller until they become small
ARTERIOLES which are even more elastic. Arterioles feed oxygenated
blood to the capillaries. The AORTA is the largest artery in the body, taking
blood from the heart, branching into other arteries that send oxygenated
blood to the rest of the body.
CAPILLARIES distribute the nutrients and oxygen to the body's tissues
and remove deoxygenated blood and waste. They are extremely thin, the
walls are only one cell thick and connect the arterioles with the venules
(very small veins).
VENULES (very small veins) merge into VEINS which carry blood back to
the heart. The vein walls are similar to arteries but thinner and less elastic.
Veins carry deoxygenated blood towards the lungs where oxygen is received
via the pulmonary capillaries. The PULMONARY Veins then carries this
oxygenated blood back to the heart.
All blood vessels are lined with a thin layer of simple squamous epithelium
known as the endothelium that keeps blood cells inside of the blood vessels
and prevents clots from forming. The endothelium lines the entire
circulatory system, all the way to the interior of the heart, where it is called
the endocardium.
There are three major types of blood vessels: arteries, capillaries and veins.
Blood vessels are often named after either the region of the body through
which they carry blood or for nearby structures. For example,
the brachiocephalic artery carries blood into the brachial (arm) and
cephalic (head) regions. One of its branches, the subclavian artery, runs
under the clavicle; hence the name subclavian. The subclavian artery runs
into the axillary region where it becomes known as the axillary artery.
Arteries and Arterioles: Arteries are blood vessels that carry blood away
from the heart. Blood carried by arteries is usually highly oxygenated,
having just left the lungs on its way to the body‘s tissues.
The pulmonary trunk and arteries of the pulmonary circulation loop provide
an exception to this rule – these arteries carry deoxygenated blood from the
heart to the lungs to be oxygenated.Arteries face high levels of blood
pressure as they carry blood being pushed from the heart under great force.
To withstand this pressure, the walls of the arteries are thicker, more elastic,
and more muscular than those of other vessels. The largest arteries of the
body contain a high percentage of elastic tissue that allows them to stretch
and accommodate the pressure of the heart.Smaller arteries are more
muscular in the structure of their walls.
The smooth muscles of the arterial walls of these smaller arteries contract or
expand to regulate the flow of blood through their lumen. In this way, the
body controls how much blood flows to different parts of the body under
varying circumstances.
The regulation of blood flow also affects blood pressure, as smaller arteries
give blood less area to flow through and therefore increases the pressure of
the blood on arterial walls.Arterioles are narrower arteries that branch off
from the ends of arteries and carry blood to capillaries.
They face much lower blood pressures than arteries due to their greater
number, decreased blood volume, and distance from the direct pressure of
the heart. Thus arteriole walls are much thinner than those of arteries.
Arterioles, like arteries, are able to use smooth muscle to control their
aperture and regulate blood flow and blood pressure.
Capillaries: Capillaries are the smallest and thinnest of the blood vessels in
the body and also the most common. They can be found running throughout
almost every tissue of the body and border the edges of the body‘s a vascular
tissues. Capillaries connect to arterioles on one end and venules on the
other.Capillaries carry blood very close to the cells of the tissues of the body
in order to exchange gases, nutrients, and waste products. The walls of
capillaries consist of only a thin layer of endothelium so that there is the
minimum amount of structure possible between the blood and the tissues.
The endothelium acts as a filter to keep blood cells inside of the vessels
while allowing liquids, dissolved gases, and other chemicals to diffuse along
their concentration gradients into or out of tissues. Precapillary sphincters
are bands of smooth muscle found at the arteriole ends of capillaries. These
sphincters regulate blood flow into the capillaries. Since there is a limited
supply of blood, and not all tissues have the same energy and oxygen
requirements, the precapillary sphincters reduce blood flow to inactive
tissues and allow free flow into active tissues.
Capillaries contain small holes in their structure that allow oxygen and other
nutrients to pass through into organs and tissues. Specific types of capillaries
are determined by their functions, which affects their number and placement
in the body. Capillaries are the smallest types of blood vessels in the body
and operate as a network of many blood vessels woven together. They are
most numerous in areas of the body that require a higher amount of oxygen
and nutrient exchange
Veins and Venules: Veins are the large return vessels of the body and act as
the blood return counterparts of arteries. Because the arteries, arterioles, and
capillaries absorb most of the force of the heart‘s contractions, veins and
venules are subjected to very low blood pressures. This lack of pressure
allows the walls of veins to be much thinner, less elastic, and less muscular
than the walls of arteries.
Veins rely on gravity, inertia, and the force of skeletal muscle contractions
to help push blood back to the heart. To facilitate the movement of blood,
some veins contain many one-way valves that prevent blood from flowing
away from the heart. As skeletal muscles in the body contract, they squeeze
nearby veins and push blood through valves closer to the heart.
When the muscle relaxes, the valve traps the blood until another contraction
pushes the blood closer to the heart. Venules are similar to arterioles as they
are small vessels that connect capillaries, but unlike arterioles, venules
connect to veins instead of arteries. Venules pick up blood from many

capillaries and deposit it into larger veins for transport back to the heart.

Coronary Circulation

The heart has its own set of blood vessels that provide the myocardium with
the oxygen and nutrients necessary to pump blood throughout the body. The
left and right coronary arteries branch off from the aorta and provide blood
to the left and right sides of the heart. The coronary sinus is a vein on the
posterior side of the heart that returns deoxygenated blood from the

myocardium to the vena cava.

Hepatic Portal Circulation

The veins of the stomach and intestines perform a unique function: instead
of carrying blood directly back to the heart, they carry blood to the
liver through the hepatic portal vein. Blood leaving the digestive organs is
rich in nutrients and other chemicals absorbed from food. The liver removes
toxins, stores sugars, and processes the products of digestion before they
reach the other body tissues. Blood from the liver then returns to the heart
through the inferior vena cava.
Blood
The average human body contains about 4 to 5 liters of blood. As a liquid
connective tissue, it transports many substances through the body and helps
to maintain homeostasis of nutrients, wastes, and gases. Blood is made up of
red blood cells, white blood cells, platelets, and liquid plasma.
Red Blood Cells: Red blood cells, also known as erythrocytes, are
most common type of blood cell and make up about 45% of blood volume.
Erythrocytes are produced inside of red bone marrow from stem cells at the
astonishing rate of about 2 million cells every second. The shape of
erythrocytes is biconcave—disks with a concave curve on both sides of the
disk so that the center of an erythrocyte is its thinnest part. The unique shape
of erythrocytes gives these cells a high surface area to volume ratio and
allows them to fold to fit into thin capillaries. Immature erythrocytes have a
nucleus that is ejected from the cell when it reaches maturity to provide it
with its unique shape and flexibility. The lack of a nucleus means that red
blood cells contain no DNA and are not able to repair themselves once
damaged.
Erythrocytes transport oxygen in the blood through the red pigment
hemoglobin. Hemoglobin contains iron and proteins joined to greatly
increase the oxygen carrying capacity of erythrocytes. The high surface area
to volume ratio of erythrocytes allows oxygen to be easily transferred into
the cell in the lungs and out of the cell in the capillaries of the systemic
tissues.
White Blood Cells: White blood cells, also known as leukocytes, make up a
very small percentage of the total number of cells in the bloodstream, but
have important functions in the body‘s immune system. There are two
major classes of white blood cells: granular leukocytes and granular
leukocytes.
General types of blood cells: (each has many different sub-types)

 ERYTHROCYTES

(red cells) are small red disk shaped cells. They contain
HAEMOGLOBIN, which combines with oxygen in the lungs and is
then transported to the body's cells. The haemoglobin then returns
carbon dioxide waste to the lungs. Erythrocytes are formed in the

bone marrow in the knobby ends of bones.

 LEUKOCYTES

(white cells) help the body fight bacteria and infection. When a tissue
is damaged or has an infection the number of leukocytes increases.
Leukocytes are formed in the small ends of bones. Leukocytes can be
classed as granular or non granular. There are three types of granular
leukocytes (eosinophils, neutrophils, and basophils), and three types
of non-granular (monocytes, T-cell lymphocytes, and B-cell

lymphocytes). See also the

THROMBOCYTES

(platelets) aid the formation of blood CLOTS
protein substances. When the body is injured thrombocytes
disintegrate and cause a chemical reaction with the proteins found in
plasma, which eventually create a thread like substance called
FIBRIN. The fibrin then "catches" other blood cells which form the
clot, preventing further loss of blood and forms the basis of healing.
Platelets: Also known as thrombocytes, platelets are small cell fragments
responsible for the clotting of blood and the formation of scabs. Platelets
form in the red bone marrow from large megakaryocyte cells that
periodically rupture and release thousands of pieces of membrane that
become the platelets. Platelets do not contain a nucleus and only survive in
the body for up to a week before macrophages capture and digest them.
Plasma: Plasma is the non-cellular or liquid portion of the blood that makes
up about 55% of the blood‘s volume. Plasma is a mixture of water, proteins,
and dissolved substances. Around 90% of plasma is made of water,
although the exact percentage varies depending upon the hydration levels of
the individual. The proteins within plasma include antibodies and albumins.
Antibodies are part of the immune system and bind to antigens on the
surface of pathogens that infect the body. Albumins help maintain the
body‘s osmotic balance by providing an isotonic solution for the cells of the
body. Many different substances can be found dissolved in the plasma,
including glucose, oxygen, carbon dioxide, electrolytes, nutrients, and
cellular waste products. The plasma functions as transportation medium for
these substances as they move throughout the body.
Functions of the Cardiovascular System
Functions of the cardiovascular system. Blood circulates through a
network of vessels throughout the body to provide individual cells with
oxygen and nutrients and helps dispose of metabolic wastes. The
heart pumps the blood around the blood vessels.
The cardiovascular system is composed of the heart, blood and blood
vessels. It connects all parts of the body through arteries and veins, arterioles
and venules, and capillaries. Through this network, the blood delivers and
expels nutrients, gases, waste products and chemical messengers throughout
the body. Red blood cells transport oxygen, while white blood cells detect
infections and kill foreign microbes and toxins. Clotting mechanisms further
protect the body from blood loss when there‘s a wound. The cardiovascular
system works alongside the respiratory system to deliver oxygen to the
body‘s tissues and to remove carbon dioxide. It also helps maintain constant
body temperature through the process thermoregulation. The sweat glands,
smooth muscle around arterioles, skeletal muscle and endocrine glands all
work together to keep a normal body temperature. Hormones, which are
essential chemical signals used by the body to communicate with itself, are
transported by the cardiovascular system to other parts of the body to deliver
their message. Lastly, the cardiovascular system helps maintain fluid
balance, which is important to ensure efficient movement of nutrients, gases
and electrolytes in the cells.
The cardiovascular system has three major functions: transportation of
materials, protection from pathogens, and regulation of the body‘s
homeostasis.
Transportation: The cardiovascular system transports blood to almost all of
the body‘s tissues. The blood delivers essential nutrients and oxygen and
removes wastes and carbon dioxide to be processed or removed from the
body. Hormones are transported throughout the body via the blood‘s liquid
plasma.
Protection: The cardiovascular system protects the body through its white
blood cells. White blood cells clean up cellular debris and fight pathogens
that have entered the body. Platelets and red blood cells form scabs to seal
wounds and prevent pathogens from entering the body and liquids from
leaking out. Blood also carries antibodies that provide specific immunity to
pathogens that the body has previously been exposed to or has been
vaccinated against.
Regulation: The cardiovascular system is instrumental in the body‘s ability
to maintain homeostatic control of several internal conditions. Blood vessels
help maintain a stable body temperature by controlling the blood flow to the
surface of the skin. Blood vessels near the skin‘s surface open during times
of overheating to allow hot blood to dump its heat into the body‘s
surroundings. In the case of hypothermia, these blood vessels constrict to
keep blood flowing only to vital organs in the body‘s core. Blood also helps
balance the body‘s pH due to the presence of bicarbonate ions, which act as
a buffer solution. Finally, the albumins in blood plasma help to balance the
osmotic concentration of the body‘s cells by maintaining an isotonic
environment.
Knowing the functions of the cardiovascular system and the parts of the
body that are part of it is critical in understanding the physiology of the
human body. With its complex pathways of veins, arteries, and capillaries,
the cardiovascular system keeps life pumping through you. The heart, blood
vessels, and blood help to transport vital nutrients throughout the body as
well as remove metabolic waste. They also help to protect the body and
regulate body temperature.
The cardiovascular system consists of the heart, blood vessels, and blood.
This system has three main functions:
Transport of nutrients, oxygen, and hormones to cells throughout the body
and removal of metabolic wastes (carbon dioxide, nitrogenous wastes).
Protection of the body by white blood cells, antibodies, and complement
proteins that circulate in the blood and defend the body against foreign
microbes and toxins. Clotting mechanisms are also present that protect the
body from blood loss after injuries.
Regulation of body temperature, fluid pH, and water content of cells.
The cardiovascular system, also known as the circulatory system, is
composed of blood, blood vessels and the heart. The heart functions as a
pump to move blood through the blood vessels of the body. A circulatory
system is essential for large, multi-cellular organisms, such as humans and
animals, and provide at least five major functions that are necessary for life.
Transporting Oxygen and Removing Carbon Dioxide
One of the most important functions of the circulatory system is to supply
oxygen to all the cells in the body. Every cell in the body requires a constant
supply of oxygen to stay alive. Because most of the cells are not in contact
with air, the circulatory system must supply them with oxygen.
When a person inhales, air enters the lungs, and oxygen is then absorbed
across the membrane of the lungs into the bloodstream. This oxygen-rich
blood is pumped through the heart to smaller and smaller blood vessels
throughout the body. In the tiniest blood vessels, called capillaries, oxygen
diffuses out of blood and into cells. At the same time, carbon dioxide
produced by the cells is absorbed back into blood, which then returns to the
lungs, releases carbon dioxide and picks up more oxygen.
Transporting Nutrients and Removing Wastes
A second critical function of the circulatory system is to supply all the cells
in the body with nutrients and energy. After food is digested in the stomach,
it migrates through the intestines, where nutrients from food are absorbed
into the bloodstream. The blood also absorbs glucose, an energy source,
from the liver, which is the body's glucose distribution center. These
nutrients and energy are then transported to all the cells of the body, in a
manner similar to the transport of oxygen. Blood also absorbs the waste
products made by cells, and transports them to the excretory organs for

removal from the body.

Fighting Disease

In addition to nutrients and oxygen, the blood also carries around important
disease-fighting cells. The organs of the immune system, such as the spleen,
create many types of specialized cells that can kill foreign cells trying to
invade the body. The circulatory system is responsible for transporting these

cells from the immune system to all other parts of the body.

Transporting Hormones

Hormones are crucial chemical signals that the body uses to communicate
with itself. Hormones control many things such as growth, the reproductive
cycle and glucose metabolism. Hormones are created in one part of the
body, such as the brain or the liver, and then must be transported to another
part of the body by the cardiovascular system in order to deliver their

message.

Regulating Body Temperature

The cardiovascular system also plays a role in regulating body temperature.
If body temperature rises too high, blood vessels close to the skin dilate,
increasing in size. The larger surface area of blood vessels close to the skin
means more heat is conducted across the skin into the air. Conversely, if
body temperature drops, the blood vessels constrict, decreasing in size. The
smaller surface area of blood vessels next to the skin causes less heat to be
lost across the skin and retains more heat in the body.
To transport nutrients, gases and waste products around the body
To protect the body from infection and blood loss
To help the body maintain a constant body temperature
(‗thermoregulation‘)
To help maintain fluid balance within the body
Transportation of nutrients, gases and waste products
The cardiovascular system acts as an internal road network, linking all parts
of the body via a system of highways (arteries and veins), main roads
(arterioles and venules) and streets, avenues and lanes (capillaries).
This network allows a non-stop courier system (the blood) to deliver and
expel nutrients, gases, waste products and messages throughout the body.
Nutrients such as glucose from digested carbohydrate are delivered from the
digestive tract to the muscles and organs that require them for energy.
Hormones (chemical messengers) from endocrine glands are transported by
the cardiovascular system to their target organs, and waste products are
transported to the lungs or urinary system to be expelled from the body.
The cardiovascular system works in conjunction with the respiratory system
to deliver oxygen to the tissues of the body and remove carbon dioxide. In
order to do this effectively the cardiovascular system is divided into two
circuits, known as the pulmonary circuit and the systemic circuit.
The pulmonary circuit is made up of the heart, lungs, pulmonary veins and
pulmonary arteries. This circuit pumps deoxygenated (blue) blood from the
heart to the lungs where it becomes oxygenated (red) and returns to the
heart.
The systemic circuit is made up of the heart and all the remaining arteries,
arterioles, capillaries, venules, and veins in the body.
This circuit pumps oxygenated (red) blood from the heart to all the tissues,
muscles and organs in the body, to provide them with the nutrients and gases
they need in order to function.
After the oxygen has been delivered the systemic circuit picks up the carbon
dioxide and returns this in the now deoxygenated (blue) blood, to the lungs,
where it enters the pulmonary circuit to become oxygenated again.
Protection from infection and blood loss
Blood contains three types of cells as listed below and shown in the adjacent
image.
Red blood cells
White blood cells
Platelets
Red blood cells are responsible for transporting oxygen around the body to
the tissues and organs that need it.
As oxygen enters the blood stream through the alveoli of the lungs it binds
to a special protein in the red blood cells called haemoglobin. This can be
seen in the adjacent image.
The job of white blood cells is to detect foreign bodies or infections and
envelop and kill them, as seen in the below image.
When they detect and kill an infection they create antibodies for that
particular infection which enables the immune system to act more quickly
against foreign bodies or infections it has come into contact with
previously.
Platelets are cells which are responsible for clotting the blood, they stick to
foreign particles or objects such as the edges of a cut.
Platelets connect to fibrinogen (a protein which is released in the site of the
cut) producing a clump that blocks the hole in the broken blood vessel. On
an external wound this would become a scab.
If the body has a low level of platelets then clotting may not occur and
bleeding can continue.
Excessive blood loss can be fatal – this is why people with a condition
known as haemophilia (low levels or absence of platelets) need medication
otherwise even minor cuts can become fatal as bleeding continues without a
scab being formed.
Alternatively, if platelet levels are excessively high then clotting within
blood vessels can occur, leading to a stroke and or heart attack. This is why
people with a history of cardiac problems are often prescribed medication to
keep their blood thin to minimise the risk of clotting within their blood
vessels.
Maintenance of constant body temperature (thermoregulation)
The core temperature range for a healthy adult is considered to be between
36.1°C and 37.8°C, with 37°C regarded as the average ‗normal‘
temperature.
If the core temperature drops below this range it is known as hypothermia
and if it rises above this range it is known as hyperthermia.
As temperatures move further into hypo or hyperthermia they become life
threatening. Because of this the body works continuously to maintain its
core temperature within the healthy range.
This process of temperature regulation in known as thermoregulation and the
cardiovascular system plays an integral part.
Temperature changes within the body are detected
called thermo receptors, which in turn relay information about these changes
to the hypothalamus in the brain.
When a deviation in temperature is recorded the hypothalamus reacts by
initiating certain mechanisms in order to regain a safe temperature
range. There are four sites where these adjustments in temperature can
occur, they are:
Sweat glands: These glands are instructed to secrete sweat onto the
surface of the skin when either the blood or skin temperature is detected to
be above a normal safe temperature. This allows heat to be lost through
evaporation and cools the skin so blood that has been sent to the skin can in
turn be cooled.
Smooth muscle around arterioles: Increases in temperature result in the
smooth muscle in the walls of arterioles being stimulated to relax causing
vasodilatation (increase in diameter of the vessel).
This in turn increases the volume of blood flow to the skin, allowing cooling
to occur. We see this is in the adjacent diagram where blood that is
normally concentrated around the core organs is shunted to the skin to cool
when the body is under heat stress.
If however the thermo receptors detect a cooling of the blood or skin then
the hypothalamus reacts by sending a message to the smooth muscle of the
arteriole walls causing the arterioles to vasoconstrict (reduce their diameter),
thus reducing the blood flow to the skin and therefore helping to maintain
core body temperature.
Skeletal muscle: When a drop in blood temperature is recorded the
hypothalamus can also react by causing skeletal muscles to start
shivering. Shivering is actually lots of very fast, small muscular
contractions which produce heat to help warm the blood
Endocrine glands: The hypothalamus may trigger the release of
hormones such as thyroxin, adrenalin and noradrenalin in response to drops
in blood temperature. These hormones all contribute to increasing the
bodies metabolic rate (rate at which the body burns fuel) and therefore
increasing the production of heat.
Maintaining fluid balance within the body
The cardiovascular system works in conjunction with other body systems
(nervous and endocrine) to balance the body‘s fluid levels. Fluid balance is
essential in order to ensure sufficient and efficient movement of electrolytes,
nutrients and gases through the body‘s cells.
When the fluid levels in the body do not balance a state of dehydration or
hyperhydration can occur, both of which impede normal body function and
if left unchecked can become dangerous or even fatal.
Dehydration is the excessive loss of body fluid, usually accompanied by an
excessive loss of electrolytes.
The symptoms of dehydration include; headaches, cramps, dizziness,
fainting and raised blood pressure (blood becomes thicker as its volume
decreases requiring more force to pump it around the body).
Hyper hydration on the other hand results from an excessive intake of water
which pushes the normal balance of electrolytes outside of their safe
limits. This can occur through long bouts of intensive exercise where
electrolytes are not replenished and excessive amounts of water are
consumed.
This can result in the recently consumed fluid rushing into the body‘s cells,
causing tissues to swell. If this swelling occurs in the brain it can put
excessive pressure on the brain stem that may result in seizures, brain
damage, coma or even death.
Dehydration or a loss of body fluid (through sweat, urination, bleeding etc)
results in an increase in ‗blood tonicity‘ (the concentration of substances
within the blood) and a decrease in blood volume. Where as hyperhydration
or a gain in body fluid (intake of water) usually results in a reduction of
blood tonicity and an increase in blood volume.
Any change in blood tonicity and volume is detected by the kidneys and
osmo receptors in the hypothalamus.
Osmo receptors are specialist receptors that detect changes in the dilution of
the blood. Essentially they detect if we are hydrated (diluted blood) or
dehydrated (less diluted blood).
In response hormones are released and transported
system (through the blood) to act on target tissues such as the kidneys to
increase or decrease urine production. Another way the cardiovascular
system maintains fluid balance is by either dilating (widening) or
constricting (tightening) blood vessels to increase or decrease the amount of

fluid that can be lost through sweat.

The Circulatory Pump

The heart is a four-chambered ―double pump,‖ where each side (left and
right) operates as a separate pump. The left and right sides of the heart are
separated by a muscular wall of tissue known as the septum of the heart. The
right side of the heart receives deoxygenated blood from the systemic veins
and pumps it to the lungs for oxygenation. The left side of the heart receives
oxygenated blood from the lungs and pumps it through the systemic arteries
to the tissues of the body. Each heartbeat results in the simultaneous
pumping of both sides of the heart, making the heart a very efficient pump.
Blood pressure
You should be able to interpret pressure changes in arteries, capillaries and
veins. The chart shows how the pressure changes in the circulatory system –
starting at the aorta [aorta: The major artery that leaves the left side of the
heart, carrying oxygenated blood to the body tissues.] leading from the left
ventricle to the rest of the body, and back to the vena cava [vena cava: The
major vein that carries deoxygenated blood to the right side of the heart
from the body tissues.] leading to the right atrium.
Pressure changes in the circulatory system
Regulation of Blood Pressure
Several functions of the cardiovascular system can control blood pressure.
Certain hormones along with autonomic nerve signals from the brain affect
the rate and strength of heart contractions. Greater contractile force and heart
rate lead to an increase in blood pressure. Blood vessels can also affect
blood pressure. Vasoconstriction decreases the diameter of an artery by
contracting the smooth muscle in the arterial wall. The sympathetic (fight or
flight) division of the autonomic nervous system causes vasoconstriction,
which leads to increases in blood pressure and decreases in blood flow in the
constricted region. Vasodilatation is the expansion of an artery as the smooth
muscle in the arterial wall relaxes after the fight-or-flight response wears off
or under the effect of certain hormones or chemicals in the blood. The
volume of blood in the body also affects blood pressure. A higher volume of
blood in the body raises blood pressure by increasing the amount of blood
pumped by each heartbeat. Thicker, more viscous blood from clotting
disorders can also raise blood pressure.
Homeostasis
Homeostasis, or the clotting of blood and formation of scabs, is managed by
the platelets of the blood. Platelets normally remain inactive in the blood
until they reach damaged tissue or leak out of the blood vessels through a
wound. Once active, platelets change into a spiny ball shape and become
very sticky in order to latch on to damaged tissues. Platelets next release
chemical clotting factors and begin to produce the protein fibrin to act as
structure for the blood clot. Platelets also begin sticking together to form a
platelet plug. The platelet plug will serve as a temporary seal to keep blood
in the vessel and foreign material out of the vessel until the cells of the blood
vessel can repair the damage to the vessel wall.
Blood groups and blood transfusion
Blood transfusion is generally the process of receiving blood products into
one's circulation intravenously. Transfusions are used for various medical
conditions to replace lost components of the blood. Early transfusions
used whole blood, but modern medical practice commonly uses only
components of the blood, such as red blood cells, white blood
cells, plasma, clotting factors, and platelets.
Historically, red blood cell transfusion was considered when
the hemoglobin level fell below 10 g/dL or hematocrit falls below 30% (the
"10/30 rule"). Because each unit of blood given carries risks, a trigger level
lower than that at 7–8 g/dL is now usually used as it has been shown to have
better patient outcomes. The administration of a single unit of blood is the
standard for hospitalized people who are not bleeding, with this treatment
then followed with re-assessment and consideration of symptoms and
hemoglobin concentration. Patients with poor oxygen saturation may need
more blood. The advisory caution to use blood transfusion only with more
severe anemia is in part due to evidence that outcomes are worsened if larger
amounts are given. One may consider transfusion for people with symptoms
of cardiovascular disease such as chest pain or shortness of breath.[2] In cases
where patients have low levels of hemoglobin but are cardiovascular
stable, parenteral iron is a preferred option based on both efficacy and
safety. Other blood products are given where appropriate, such as clotting
deficiencies.
BLOOD
As blood moves throughout the body, it carries oxygen and nutrients to all
the places they're needed. Blood also collects waste products, like carbon
dioxide, and takes them to the organs responsible for making sure wastes
leave the body.
Blood is a mixture of cells and liquid. Each has a specific job:
Red blood cells carry oxygen to the body's tissues and remove carbon
dioxide. Red blood cells make up about 40%-45% of a person's blood and
live for 120 days.
White blood cells are part of the immune system, and its main defense
against infection. White blood cells make up less than 1% of a person's
blood.
Platelets are cell fragments that help blood clot, which helps to prevent
and control bleeding. A person's blood has about 1 platelet for every 20
red blood cells.
Plasma is a pale yellow liquid mixture of water, proteins, electrolytes,
carbohydrates, cholesterol, hormones, and vitamins. About 55% of our
blood is plasma.
Blood cells are made in the bone marrow (a spongy material inside many of
the bones in the body). A full-grown adult has about 10 pints of blood
(almost 5 liters) in his or her body.
What Is a Blood Transfusion?
A transfusion is a simple medical procedure that doctors use to make up for
a loss of blood — or for any part of the blood, such as red blood cells or
platelets.
A person usually gets a blood transfusion through an intravenous line, a
tiny tube that is inserted into a vein with a small needle. The whole process
takes about 1 to 4 hours, depending on how much blood is needed.
Blood from a donor needs to match the blood type of the person receiving it.
There are eight main blood types:
O positive
O negative
A positive
A negative
B positive
B negative
AB positive
AB negative
In emergencies, there are exceptions to the rule that the donor's blood type
must match the recipient's exactly. Blood type O negative is the only type of
blood that people of all other blood types can receive. Medical teams use it
in situations when patients need a transfusion but their blood type is
unknown. Because of this, O negative donors are called "universal donors."
People who have type AB blood are called "universal recipients" because
they can safely receive any type of blood.
A blood transfusion usually isn't whole blood — it could be any one of the
blood's components. For example, chemotherapy can affect how bone
marrow makes new blood cells. So some people getting treatment for cancer
might need a transfusion of red blood cells or platelets.
Other people might need plasma or only certain parts of plasma. People who
have hemophilia, a disease that affects the blood's ability to clot, need
plasma or the clotting factors contained in plasma to help their blood clot
and prevent internal bleeding.
Types of Blood Transfusions
Red Blood Cell Transfusions A patient suffering from an iron deficiency or
anemia, a condition where the body does not have enough red blood cells,
may receive a Red Blood Cell Transfusion. This type of transfusion
increases a patient‘s hemoglobin and iron levels, while improving the
amount of oxygen in the body.
Platelet Transfusions Platelets are a component of blood that stops the
body from bleeding. Often patients suffering from leukemia, or other types
of cancer, have lower platelet counts as a side effect of their chemotherapy
treatments. Patients who have illnesses that prevent the body from making
enough platelets have to get regular transfusions to stay healthy.
Plasma Transfusions Plasma is the liquid part of the body‘s blood. It
contains important proteins and other substances crucial to one‘s overall
health. Plasma transfusions are used for patients with liver failure, severe
infections, and serious burns.
Clotting of blood
Coagulation (also known as clotting) is the process
changes from a liquid to a gel, forming a clot. It potentially results in
homeostasis, the cessation of blood loss from a damaged vessel, followed by
repair.
Coagulation, in physiology, the process by which a blood clot is formed.
The formation of a clot is often referred to as secondary homeostasis,
because it forms the second stage in the process of arresting the loss of blood
from a ruptured vessel.
Platelets may stick to areas where the blood vessels are damaged and form
blood clots. Vacuities also are a major cause of damage to the blood vessel
walls. Diabetes. Diabetes external link icon increases the risk of plaque
buildup in the arteries, which can cause dangerous blood clots.
How do you get a blood clot?
DVT occurs when blood clots form in the deep veins of the legs or pelvis,
and is often caused by: prolonged sitting or bed rest. surgery or trauma
(especially hip surgery, gynecological surgery, heart surgery) medications
such as estrogen, and birth control pills with higher levels of estrogen.
Initiating the Clotting Process
A blood clot is a normal function of blood
cells that is used to repair damaged blood
vessel walls. Blood clots become a
problem when the blood "clots" are in an
artery or vein inappropriately.
Risk factors for developing blood clots in
arteries include high blood
pressure, cholesterol, diabetes, smoking,
and family history.
Risk factors for developing blood clots in
veins include prolonged immobility, including immobility after surgery,
hormone therapy (including birth control pills), smoking, pregnancy, and
genetic factors.
Causes of arterial blood clots include rupture of atherosclerosis
plaques, embolus from another location, and artery injury.
Causes of venous blood blots include stasis and chemical factors that
cause the blood to clot abnormally.
Symptoms of blood clots depend upon their location and whether they
occur in an artery or a vein. A blood clot in an artery that supplies blood
to the heart or brain may result in
o heart attack,
o stroke, or
o TIA (transient ischemic attack or mini-stroke)
When blood clots occur in a vein, symptoms may include
o pain,
o swelling,
o warmth, and
o redness.
If a forms in a vein in a leg or arm breaks; breaks off and travels to the
lung a pulmonary embolus can occur. Symptoms of pulmonary
embolism are chest pain and shortness of breath.
Blood clots are diagnosed initially by history and physical examination.
Other tests may be ordered depending upon the location of the blood clot.
The treatment for blood clots depends upon the location, but most
situations require the use of anticoagulant medications that thin the blood.
Medications used for blood clot treatment thin or anticoagulate the blood
include:
o apixaban (Eliquis)
o rivaroxaban (Xarelto)
o dabigatran (Pradaxa)
o warfarin (Coumadin)
o enoxaparin (Lovenox)
Complications of blood clots often depend upon their location.
Blood clots can be prevented by remaining active especially after
surgery, quitting smoking if you take birth control pills, controlling
andhigh blood pressure, high cholesterol, and diabetes.
The prognosis for a person with a blood clot depends upon the health of
the person, the location of the blood clot, and how quickly medical care is
accessed.
Picture of Blood Clots
Picture of blood clotting
Blood flows through the body in a continuous loop. Blood is pumped
through the body by the heart, but that same blood returns back to the heart
both by gravity and by muscles in the arms and legs contracting and
squeezing, or milking, the blood back to the heart. If blood becomes
stagnant, it may clot and cause potential life-threatening conditions.
The medical term for a blood clot is a thrombus (plural: thrombi). An
embolus refers to the situation in which the clot breaks away from its
original location and travels through the bloodstream to another location.
There are four potential outcomes regarding a blood clot. It will either
grow,
dissolve,
embolize, or
recannulate (a situation in which capillary blood vessels proliferate

within the clot to form new channels so that blood may resume flow)

Cardiac Cycle

The cardiac cycle refers to a complete heartbeat from its generation to the
beginning of the next beat, and so includes the diastole, the systole, and the
intervening pause. The frequency of the cardiac cycle is described by
the heart rate, which is typically expressed as beats per minute
The cardiac cycle is the sequence of events that occurs when the heart beats.
There are two phases of the cardiac cycle. In the diastole phase, the
heart ventricles are relaxed and the heart fills with blood. In the systole
phase, the ventricles contract and pump blood to the arteries. One cardiac
cycle is completed when the heart fills with blood and the blood is pumped
out of the heart.
The events of the cardiac cycle described below trace the path of the blood
as it enters the heart, is pumped to the lungs, travels back to the heart and is
pumped out to the rest of the body. It is important to note that the events that
occur in the first and second diastole phases actually happen at the same
time. The same is also true for the events of the first and second systole
phases.
The cardiac cycle refers to a complete heartbeat from its generation to the
beginning of the next beat, and so includes the diastole, the systole, and the
intervening pause. The frequency of the cardiac cycle is described by the
heart rate, which is typically expressed as beats per minute.
Cardiac cycle a complete cardiac movement, or heart beat, including systole
, diastole, and the intervening pause.
Cardiac Cycle: 1st Diastole Phase
During the diastole phase, the atria and ventricles are relaxed and the atrio
ventricular valves are open. De-oxygenated blood from the superior and
inferior vena cavae flows into the right atrium. The open atrioventricular
valves allow blood to pass through to the ventricles. The SA nodecontracts
triggering the atria to contract. The right atrium empties its contents into the
right ventricle. The tricuspid valve prevents the blood from flowing back
into the right atrium.
Cardiac Cycle: 1st Systole Phase
During the systole phase, the right ventricle receives impulses from

the Purkinje fibers and contracts.

The atrioventricular valves close and the semilunar valves open. The deoxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle.

The pulmonary artery carries the blood to the lungs. There the blood picks
up oxygen and is returned to the left atrium of the heart
veins.
Cardiac Cycle: 2nd Diastole Phase
In the next diastole period, the semi lunar valves close and the
atrioventricular valves open. Blood from the pulmonary veins fills the
left atrium. (Blood from the vena cava is also filling the right atrium.) The
SA node contracts again triggering the atria to contract. The left atrium
empties its contents into the left ventricle. The mitral valve prevents the
oxygenated blood from flowing back into the left atrium.
Cardiac Cycle: 2nd Systole Phase
During the following systole phase, the atrio ventricular valves close and the
semi lunar valves open. The left ventricle receives impulses from the
Purkinje fibers and contracts. Oxygenated blood is pumped into the aorta.
The aortic valve prevents the oxygenated blood from flowing back into the
left ventricle.
The aorta branches out to provide oxygenated blood to all parts of the body.

The oxygen depleted blood is returned to the heart via the vena cavae.

Cardiovascular System

The cardiac cycle is vital to proper cardiovascular system function.
Comprised of the heart and the circulatory system, the cardiovascular system
transports nutrients to and removes gaseous waste from the cells of the body.
The heart and its cardiac cycle provide the "muscle" needed to pump blood
throughout the body, while blood vessels act as pathways to transport blood
to its destination. The driving force behind the cardiac cycle is cardiac
conduction. Cardiac conduction is the electrical system that powers the
cardiac cycle and the cardiovascular system. It is what causes the
heart muscle to contract, sending nerve impulses that travel throughout the
heart wall.
Lymph and Lymphatic circulation.
The lymphatic system is a network of tissues and organs that help rid the

body of toxins, waste and other unwanted materials. The primary function of the lymphatic system is to transport lymph, a fluid containing infectionfighting white blood cells, throughout the body.

The lymphatic system primarily consists of lymphatic vessels, which are
similar to the circulatory system's veins and capillaries. The vessels are
connected to lymph nodes, where the lymph is filtered. The tonsils,
adenoids, spleen and thymus are all part of the lymphatic system.
There are hundreds of lymph nodes in the human body. They are located
deep inside the body, such as around the lungs and heart, or closer to the
surface, such as under the arm or groin.
The spleen, which is located on the left side of the body just above the
kidney, is the largest lymphatic organ, according to the U.S. National
Library of Medicine (NLM). It controls the amount of red blood cells and
blood storage in the body, and helps to fight infection. If the spleen detects
potentially dangerous bacteria, viruses, or other microorganisms in the
blood, it — along with the lymph nodes — creates white blood cells called
lymphocytes, which act as defenders against invaders. The lymphocytes
produce antibodies to kill the foreign microorganisms and stop infections
from spreading. Humans can live without a spleen, although people who
have lost their spleen to disease or injury are more prone to infections.
The thymus is located in the chest just above the heart. This small organ
stores immature lymphocytes (specialized white blood cells) and prepares
them to become active T cells, which help destroy infected or cancerous
cells.
Tonsils are large clusters of lymphatic cells found in the pharynx. According
to the American Academy of Otolaryngology, they are the body's "first line
of defense as part of the immune system. They sample bacteria and viruses
that enter the body through the mouth or nose." They sometimes become
infected, and although tonsillectomies occur much less frequently today then
they did in the 1950s, it is still among the most common operations
performed and typically follows frequent throat infections.
Lymph is a clear and colorless fluid; the word "lymph" comes from the Latin
word lympha, which means "connected to water,"
The lymphatic system is responsible for the following:
Cleansing the cellular environment
Returning proteins and tissue fluids to the blood
Providing a pathway for the absorption of fats into the bloodstream
Defending the body against disease
The lymphatic system is composed of lymph (or interstitial fluid), lymph
vessels, lymph nodes, lymph organs (e.g. tonsils, adenoids, appendix,
spleen,, thymus gland, and patches of tissue in the intestines called Peyer
patches), and lymphoid tissue. Monocytes and lymphocytes pass from the
bloodstream through the blood capillary walls into the spaces between the
cells in the body. When they pass into this lymph or interstitial fluid that
surrounds cells, they perform their protective functions. Monocytes change
into macrophages, destroy pathogens, and collect debris from damaged
cells. Lymphocytes are much more complicated and are essential to the
immune response, so they are discussed in the next section. Once monocytes
and lymphocytes pass into the lymphatic capillaries, the fluid is
termed lymph or lymphatic fluid.
Lymph moves in one directo to prevent pathogens from flowing through the
entire body. The system filters out the microorganisms as the lymph passes
through its various capillaries, vessels, and nodes. Lymph travels in the
following sequence:
From the interstitial spaces between the cells, then
Toward the heart through lymphatic capillaries.
To lymphatic vessels that carry lymph using a valvular system.
To the lymphatic nodes, which are also called lymph glands, that
filter the debris that has been collected through the use of
macrophages. These nodes can become enlarged when pathogens are
present. Note the major lymph nodes in the figure, including the
cervical, axillary, inguinal , and mediastinal nodes.
Then to either the right lymphatic duct or the thoracic duct, both of
which empty into the large subclavian veins in the neck.
Once in the venous blood, the lymph is then recycled through the
body through the circulatory system.
The organs in the lymphatic system are the spleen, the thymus gland, the
tonsils, the appendix, and Peyer's patches. the spleen is located in the upper
left quadrant and serves to filter, store, and produce blood cells; remove
RBCs; and activate B lymphocytes. The thymus gland is located is located in
the mediastinum and is instrumental in the development of T lymphocytes
(T cells). the tonsils are lymphatic tissue (lingual, pharyngeal, and palatine)
that helps protect the entrance to the respiratory and digestive systems. The

vermiform appendix and Peyer patches are lymphoid tissue in the intestines.

THE LYMPHATIC

SYSTEM IS

COOPERATIVE

 The lymphatic system
aids the immune system
in removing and
destroying waste, debris,
dead blood cells,
pathogens, toxins, and
cancer cells.
 The lymphatic system absorbs fats and fat-soluble vitamins from the
digestive system and delivers these nutrients to the cells of the body where
they are used by the cells.
 The lymphatic system also removes excess fluid, and waste products from

the interstitial spaces between the cells.

THE TRANSFORMATION

Arterial blood carries oxygen, nutrients, and hormones for the cells. To
reach these cells it leaves the small arteries and flows into the tissues. This
fluid is now known as interstitial fluid and it delivers its nourshing products
to the cells. Then it leaves the cell and removes waste products.
After this task is complete, 90% of this fluid returns to the circulatory

system as venous blood.

WHAT IS LYMPH?

The remaining 10% of the fluid that stays behind in the tissues as a clear to
yellowish fluid known as lymph.
Major lymphatic ducts. (Courtesy of NIH/NCI)
 Unlike blood, which flows throughout the body in a continue loop, lymph
flows in only one direction within its own system. This flow is only
upward toward the neck. Here, it flows into the venous blood stream
through the subclavien veins which are located on either sides of the neck
near the collarbones.
 After plasma has delivered its nutrients and removed debris, it leaves the
cells. 90% of this fluid returns to the venous circulation through the
venules and continues as venous blood.
 The remaining 10% of this fluid becomes lymph which is a watery fluid
that contains waste products. This waste is protein-rich due to the

undigested proteins that were removed from the cells.

LYMPHATIC CIRCULATION

The lymph is moved through the body in its own vessels making a one-way
journey from the interstitial spaces to the subclavian veins at the base of the
neck.
 Since the lymphatic system does not have a heart to pump it, its upward
movement depends on the motions of the muscle and joint pumps.
 As it moves upward toward the neck the lymph passes through lymph
nodes which filter it to remove debris and pathogens.
 The cleansed lymph continues to travel in only one direction, which is
upward toward the neck.
 At the base of the neck, the cleansed lymph flows into the subclavian
veins on either side of the neck.
Lymph returning to the

THE ORIGIN OF LYMPH

Lymph originates as plasma (the fluid portion of blood). The arterial blood,
which flows out of the heart, slows as it moves through a capillary bed. This
slowing allows some plasma to leave the arterioles (small arteries) and flow
into the tissues where it becomes tissue fluid.
 Also known as extracellular fluid, this is fluid that flows between the cells
but is not into the cells. This fluid delivers nutrients, oxygen, and hormones
to the cells.
 As this fluid leaves the cells, it takes with it cellular waste products and
protein cells.
 Approximately 90% of this tissue fluid flows into the small veins. Here it
enters the venous circulation as plasma and continues in the circulatory
system.

 The remaining 10% of the fluid that is left behind is known as lymph.

LYMPHATIC CAPILLARIES

In order to leave the tissues, the lymph must enter the lymphatic system
through specialized lymphatic capillaries. Approximately 70% of these
are superficial capillaries located near, or just under, the skin. The
remaining 30%, which are known as deep lymphatic capillaries, surround
most of the body‘s organs.
Lymphatic capillaries begin as blind-ended tubes that are only a single cell
in thickness. These cells are arranged in a slightly overlapping pattern, much
like the shingles on a roof. Each of these individual cells is fastened to

nearby tissues by an anchoring filament.

LYMPHATIC VESSELS

The lymphatic capillaries gradually join together to form a mesh-like
network of tubes that are located deeper in the body.
 As they become larger, and deeper, these structures become lymphatic
vessels.
 Deeper within the body the lymphatic vessels become progressively larger
and are located near major blood veins.
 Like veins, the lymphatic vessels, which are known as lymphangions, have
one-way valves to prevent any backward flow.
 Smooth muscles in the walls of the lymphatic vessels cause the angions to
contract sequentially to aid the flow of lymph upward toward the thoracic
region. Because of their shape, these vessels are previously referred to as a

string of pearls.

LYMPH NODES

Lymph nodes kill pathogens and cancer cells. They also
remove debris and excess fluids.
There are between 600-700 lymph nodes present in the average human body.
It is the role of these nodes to filter the lymph before it can be returned to the
circulatory system. Although these nodes can increase or decrease in size
throughout life, any nodes that has been damaged or destroyed, does not
regenerate.
 Afferent lymphatic vessels carry unfiltered lymph into the node. Here waste
products, and some of the fluid, are filtered out.
 In another section of the node, lymphocytes, which are specialized white
blood cells, kill any pathogens that may be present. This causes the
swelling commonly known as swollen glands.
 Lymph nodes also trap and destroy cancer cells to slow the spread of the
cancer until they are overwhelmed by it.
 Efferent lymphatic vessels carry the filtered lymph out of the node so that it

can continue its return to the circulatory system.

DRAINAGE AREAS

Lymphatic system drainage is organized into two separate, and very unequal
drainage areas. The right drainage area clears the right arm and chest. The
left drainage area clears all of the other areas of the body including both
legs, the lower trunk upper left of the chest, and the left arm.

Lymphatic Drainage Areas

Lymphatic circulation
The lymphatic system can be thought of as a drainage system needed
because, as blood circulates through the body, blood plasma leaks into
tissues through the thin walls of the capillaries. The portion of blood plasma
that escapes is called interstitial or extracellular fluid, and it contains
oxygen, glucose, amino acids, and other nutrients needed bytissue cells.
Although most of this fluid seeps immediately back into the bloodstream, a
percentage of it, along with the particulate matter, is left behind. The
lymphatic system removes this fluid and these materials from tissues,
returning them via the lymphatic vessels to the bloodstream, and thus
prevents a fluid imbalance that would result in the organism‘s death.
The fluid and proteins within the tissues begin their journey back to the
bloodstream by passing into tiny lymphatic capillaries that infuse almost
every tissue of the body. Only a few regions, including the epidermis of the
skin, the mucous membranes, the bone marrow, and the central nervous
system, are free of lymphatic capillaries, whereas regions such as the lungs,
gut, genitourinary system, and dermis of the skin are densely packed with
these vessels. Once within the lymphatic system, the extracellular fluid,
which is now called lymph, drains into larger vessels called the lymphatics.
These vessels converge to form one of two large vessels called lymphatic
trunks, which are connected to veins at the base of the neck. One of these
trunks, the right lymphatic duct, drains the upper right portion of the body,
returning lymph to the bloodstream via the right subclavian vein. The other
trunk, the thoracic duct, drains the rest of the body into the left subclavian
vein. Lymph is transported along the system of vessels
by muscle contractions, and valves prevent lymph from flowing backward.
The lymphatic vessels are punctuated at intervals by small masses of lymph
tissue, called lymph nodes, that remove foreign materials such as infectious
microorganisms from the lymph filtering through them.
In addition to serving as a drainage network, the lymphatic system helps
protect the body against infection by producing white blood cells
calledlymphocytes, which help rid the body of disease-causing
microorganisms. The organs and tissues of the lymphatic system are the
major sites of production, differentiation, and proliferation of two types of
lymphocytes—the T lymphocytes and B lymphocytes, also called T
cells and B cells. Although lymphocytes are distributed throughout the body,
it is within the lymphatic system that they are most likely to encounter
foreign microorganisms.
Lymphoid organs
The lymphatic system is commonly divided into the primary lymphoid
organs, which are the sites of B and T cell maturation, and the secondary
lymphoid organs, in which further differentiation of lymphocytes occurs.
Primary lymphoid organs include the thymus, bone marrow, fetal liver, and,
in birds, a structure called the bursa of Fabricius. In humans
the thymus and bone marrow are the key players in immune function. All
lymphocytes derive from stem cells in the bone marrow. Stem cells destined
to become B lymphocytes remain in the bone marrow as they mature, while
prospective T cells migrate to the thymus to undergo further growth. Mature
B and T lymphocytes exit the primary lymphoid organs and are transported
via the bloodstream to the secondary lymphoid organs, where they become
activated by contact with foreign materials, such as particulate matter and
infectious agents, called antigens in this context.
Thymus
The thymus is located just behind the sternum in the upper part of the chest.
It is a bilobed organ that consists of an outer, lymphocyte-rich cortex and an
inner medulla. The differentiation of T cells occurs in the cortex of the
thymus. In humans the thymus appears early in fetal development and
continues to grow until puberty, after which it begins to shrink. The decline
of the thymus is believed to be the reason T-cell production decreases with
age.
In the cortex of the thymus, developing T cells, called thymocytes, come to
distinguish between the body‘s own components, referred to as ―self,‖ and
those substances foreign to the body, called ―nonself.‖ This occurs when the
thymocytes undergo a process called positive selection, in which they are
exposed to self molecules that belong to the major histocompatibility
complex (MHC). Those cells capable of recognizing the body‘s MHC
molecules are preserved, while those that cannot bind these molecules are
destroyed. The thymocytes then move to the medulla of the thymus, where
further differentiation occurs. There thymocytes that have the ability to
attack the body‘s own tissues are destroyed in a process called negative
selection. Positive and negative selection destroy a great number of
thymocytes; only about 5 to 10 percent survive to exit the thymus. Those
that survive leave the thymus through specialized passages called efferent
(outgoing) lymphatics, which drain to the blood and secondary lymphoid
organs. The thymus has no afferent (incoming) lymphatics, which supports
the idea that the thymus is a T-cell factory rather than a rest stop for
circulating lymphocytes.
Bone marrow
In birds B cells mature in the bursa of Fabricius. (The process of B-cell
maturation was elucidated in birds—hence B for bursa.) In mammals the
primary organ for B-lymphocyte development is the bone marrow, although
the prenatal site of B-cell differentiation is the fetal liver. Unlike the thymus,
the bone marrow does not atrophy at puberty, and therefore there is no
concomitant decrease in the production of B lymphocytes with age.
Secondary lymphoid organs
Secondary lymphoid organs include the lymph nodes, spleen, and small
masses of lymph tissue such as Peyer‘s patches, the appendix, tonsils, and
selected regions of the body‘s mucosal surfaces (areas of the body lined with
mucous membranes). The secondary lymphoid organs serve two basic
functions: they are a site of further lymphocyte maturation, and they

efficiently trap antigens for exposure to T and B cells.

LYMPH NODES

The lymph nodes, or lymph glands, are small, encapsulated bean-shaped
structures composed of lymphatic tissue. Thousands of lymph nodes are
found throughout the body along the lymphatic routes, and they are
especially prevalent in areas around the armpits (axillary nodes), groin
(inguinal nodes), neck (cervical nodes), and knees (popliteal nodes). The
nodes contain lymphocytes, which enter from the bloodstream via
specialized vessels called the high endothelial venules. T cells congregate in
the inner cortex (paracortex), and B cells are organized in germinal centres
in the outer cortex. Lymph, along with antigens, drains into the node through
afferent (incoming) lymphatic vessels and percolates through the lymph
node, where it comes in contact with and activates lymphocytes. Activated
lymphocytes, carried in the lymph, exit the node through the efferent
(outgoing) vessels and eventually enter the bloodstream, which distributes

them throughout the body.

SPLEEN

The spleen is found in the abdominal cavity behind the stomach. Although
structurally similar to a lymph node, the spleen filters blood rather than
lymph. One of its main functions is to bring blood into contact with
lymphocytes. The functional tissue of the spleen is made up of two types of
cells: the red pulp, which contains cells called macrophages that remove
bacteria, old blood cells, and debris from the circulation; and surrounding
regions of white pulp, which contain great numbers of lymphocytes. The
splenic artery enters the red pulp through a web of small blood vessels, and
blood-borne microorganisms are trapped in this loose collection of cells until
they are gradually washed out through the splenic vein. The white pulp
contains both B and T lymphocytes. T cells congregate around the tiny
arterioles that enter the spleen, while B cells are located in regions called
germinal centres, where the lymphocytes are exposed to antigens and

induced to differentiate into antibody-secreting plasma cells.

MUCOSA-ASSOCIATED TISSUES

Another group of important secondary lymphoid structures is the mucosaassociated lymphoid tissues. These tissues are associated with mucosal surfaces of almost any organ, but especially those of the digestive, genitourinary, and respiratory tracts, which are constantly exposed to a wide variety of potentially harmful microorganisms and therefore require their own system of antigen capture and presentation to lymphocytes. For example, Peyer‘s patches, which are mucosa-associated lymphoid tissues of the small intestine, sample passing antigens and expose them to underlying B and T cells. Other, less-organized regions of the gut also play a role as secondary lymphoid tissue.

Diseases of the lymphatic system
The host of secondary lymphoid organs provides a system of redundancy for
antigen sampling by the cells of the immune system. Removal of the spleen,
selected lymph nodes, tonsils, or appendix does not generally result in an
excessive increase in disease caused by pathogenic microorganisms.
However, the importance of the primary lymphoid organs is clear. For
example, two autoimmune diseases, DiGeorge syndrome and Nezelof‘s
disease, result in the failure of the thymus to develop and in the subsequent
reduction in T-cell numbers, and removal of the bursa from chickens results
in a decrease in B-cell counts. The destruction of bone marrow also has
devastating effects on the immune system, not only because of its role as the
site of B-cell development but also because it is the source of the stem cells

that are the precursors for lymphocyte differentiation.