Body Fluids

OPTOMETRY · SEMESTER 1

Human Anatomy and Physiology

BODY FLUIDS COMPARTMENT

BODY FLUIDS

INTRODUCTION

Homeostasis, or the maintenance of constant conditions in the body, is a fundamental property of all living things.

In the human body, the substances that participate in chemical reactions must remain within narrows ranges of concentration.

Too much or too little of a single substance can disrupt your bodily functions.

BODY FLUIDS

INTRODUCTION CONT …

Because metabolism relies on reactions that are all interconnected, any disruption might affect multiple organs or even organ systems. Water is the most ubiquitous substance in the chemical reactions of life.

The interactions of various aqueous solutions—solutions in which water is the solvent—are continuously monitored and adjusted by a large suite of interconnected feedback systems in your body. Understanding the ways in which the body maintains these critical balances is key to understanding good health.

BODY FLUIDS

INTRODUCTION CONT …

The chemical reactions of life take place in aqueous solutions. The dissolved substances in a solution are called solutes.

In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes.

Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called an electrolyte.

For instance, sodium ions (Na+) and chloride ions (Cl-) are often referred to as electrolytes.

BODY FLUIDS

INTRODUCTION CONT …

In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis.

Osmosis is basically the diffusion of water from regions of higher concentration of water to regions of lower concentration of water, along an osmotic gradient across a semi-permeable membrane.

As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.

BODY FLUIDS

Body Water Content

Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 60 percent in adults, to as low as 45 percent in old age.

The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood.

Your brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. In contrast, teeth have the lowest proportion of water, at 8–10 percent.

BODY FLUIDS

Total Body Water

In normal adult human 70 kg
Total body water = 60% of body weight
= 42 liters (70*0.6)
However, % can change depending on
Age, sex, degree of obesity
An increase in age is accompanied by increase in fat
Hence % of body wt that is water decreases
By age of 60 TBW = 50% of body wt in men

BODY FLUIDS

Total Body Water

Women have more fat
Contain less water than men
Total body water is 45 – 50% of body wt
Children
Neonate contain more water than adults (75 – 80% TBWt)
By about 1 yr body water = 60% of body wt

BODY FLUIDS

Fluid Compartments

Intracellular Fluid

Volume = 28 L,

2/3 TBW

Interstitial Fluid

Volume = 10.5 L,

75% of ECF
Plasma
Vol = 3.5 L, 25% of ECF

BODY FLUIDS

ICF (2/3 TBW)

ECF (1/3 TBW)

Total Body Water (TBW) 42 L, 60% of Body Wt

Intracellular Fluid Compartment (ICF)

About 28 liters of the 42 liters are inside body cells
ICF constitutes
40% of total body weight (for 70kg), or
2/3 of total body water (60% of body water)

BODY FLUIDS

FLUID COMPARTMENTS

EXTRACELLUAR

FLUID

INTRACELLULAR

FLUID

INTERSTITIAL

FLUID

PLASMA

TRANSCELLULAR

FLUID
CSF
Intra ocular
Pleural
Peritoneal
Synovial

Digestive Secretions

BODY FLUIDS

Fluid Compartments

Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier.

The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes.

Extracellular fluid (ECF) surrounds all cells in the body.

Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood.

BODY FLUIDS

FLUID COMPARTMENTS

BODY FLUIDS

Fluid Compartments in the Human Body

The intracellular fluid (ICF) is the fluid within cells.
The interstitial fluid (IF) is part of the extracellular fluid (ECF) between the cells.
Blood plasma is the second part of the ECF.
Materials travel between cells and the plasma in capillaries through the IF.

BODY FLUIDS

Intracellular Fluid

The ICF lies within cells and is the principal component of the cytosol/cytoplasm.

The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 liters of fluid.

BODY FLUIDS

Intracellular Fluid CONT …

This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated.

If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities; if too much water enters a cell, the cell may burst and be destroyed.

BODY FLUIDS

Extracellular Fluid

The ECF accounts for the other one-third of the body’s water content.

Approximately 20 percent of the ECF is found in plasma.

Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes.

BODY FLUIDS

Extracellular Fluid CONT …

Gases, nutrients, and waste materials travel between capillaries and cells through the IF.

Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.

BODY FLUIDS

Extracellular Fluid CONT …

The body has other water-based ECF. These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, the synovial fluid in joints, the pleural fluid in the pleural cavities, the pericardial fluid in the cardiac sac, the peritoneal fluid in the peritoneal cavity, and the aqueous humor of the eye.

Because these fluids are outside of cells, these fluids are also considered components of the ECF compartment, but also called Transcellular Fluid.

BODY FLUIDS

Extra Cellular Fluid Compartment (ECF)

Two largest ECF compartment
Interstitial fluid makes up 75% of ECF
14*.75 = 10.5 liters
Plasma which makes up 25% of ECF
14*.25 = 3.5 liters

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Body Fluids

Blood Volume

Blood contains both ECF and ICF
ICF is the fluid within the RBC
Average blood volume
Is 8% of body wt
= 5.6 (70*0.08) liters
On the average
60% of blood vol = plasma ( 3 liters)
40% of blood vol = RBC (2 liters)
Values vary considerably in different people depending on
Sex, weight,age and other factors

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Body Fluids

BODY FLUIDS

Composition of Body Fluids

The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF.

Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein.

The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein.

BODY FLUIDS

Composition of Body Fluids CONT …

In contrast, the ICF has elevated amounts of potassium, phosphate, magnesium, and protein.

Overall, the ICF contains high concentrations of potassium and phosphate, whereas both plasma and the ECF contain high concentrations of sodium and chloride.

BODY FLUIDS

The Concentrations of Different Elements in Key Bodily Fluids

BODY FLUIDS

Composition of Body Fluids CONT …

Most body fluids are neutral in charge. Thus, cations, or positively charged ions, and anions, or negatively charged ions, are balanced in fluids.

As seen in the previous graph, sodium (Na+) ions and chloride (Cl-) ions are concentrated in the ECF of the body, whereas potassium (K+) ions are concentrated inside cells.

BODY FLUIDS

Composition of Body Fluids CONT …

Although sodium and potassium can “leak” through “pores” into and out of cells, respectively, the high levels of potassium and low levels of sodium in the ICF are maintained by sodium-potassium pumps in the cell membranes.

These pumps use the energy supplied by ATP to pump sodium out of the cell and potassium into the cell.

BODY FLUIDS

PART II

BODY FLUIDS

Fluid Movement between Compartments

Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments.

The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart.

In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by circulating albumin—at the arteriolar end of the capillary.

BODY FLUIDS

Fluid Movement between Compartments CONT …

This pressure forces plasma and nutrients out of the capillaries and into surrounding tissues.

Fluid and the cellular wastes in the tissues enter the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel.

BODY FLUIDS

Fluid Movement between Compartments CONT …

Filtration pressure squeezes fluid from the plasma in the blood to the IF surrounding the tissue cells.

The surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins.

BODY FLUIDS

Capillary Exchange

BODY FLUIDS

Capillary Exchange CONT …

Net filtration occurs near the arterial end of the capillary since capillary hydrostatic pressure (CHP) is greater than blood colloidal osmotic pressure (BCOP).

There is no net movement of fluid near the midpoint of the capillary since CHP = BCOP.

Net reabsorption occurs near the venous end of the capillary since BCOP is greater than CHP.

BODY FLUIDS

Capillary Exchange CONT …

Hydrostatic pressure is especially important in governing the movement of water in the nephrons of the kidneys to ensure proper filtering of the blood to form urine.

As hydrostatic pressure in the kidneys increases, the amount of
water leaving the capillaries also increases, and more urine filtrate is formed.

BODY FLUIDS

Capillary Exchange CONT …

If hydrostatic pressure in the kidneys drops too low, as can happen in dehydration, the functions of the kidneys will be impaired, and less nitrogenous wastes will be removed from the bloodstream. Extreme dehydration can result in kidney failure.

BODY FLUIDS

Capillary Exchange CONT …

For example, if you are sweating, you will lose water through your skin. Sweating depletes your tissues of water and increases the solute concentration in those tissues.

As this happens, water diffuses from your blood into sweat glands and surrounding skin tissues that have become dehydrated because of the osmotic gradient.

BODY FLUIDS

Capillary Exchange CONT …

Additionally, as water leaves the blood, it is replaced by the water in other tissues throughout your body that are not dehydrated. If this continues, dehydration spreads throughout the body.

When a dehydrated person drinks water and rehydrates, the water is redistributed by the same gradient, but in the opposite direction, replenishing water in all of the tissues.

BODY FLUIDS

Exchange Between Plasma and ISF

Fluid exchange between capillaries & ISF
ECF is distributed between plasma and ISF
The distribution is determined by balance between
Colloidal osmotic pressure
Hydrostatic pressure

BODY FLUIDS

Exchange Between Plasma and ISF

There are four primary forces that determine
The fluid movement through the capillary membrane
CHP capillary hydrostatic pressure
Tends to force fluid outwards from the capillary

BODY FLUIDS

A
V
CHP
Pif
CCOP
TCOP
CHP
CCOP
Pif
TCOP

Exchange Between Plasma and ISF

Pifhp interstitial fluid pressure
Plasma colloidal osmotic pressure (CCOP)
Causes osmosis of water inward through the capillary

BODY FLUIDS

A
V
CHP
Pif
CCOP
TCOP
CHP
CCOP
Pif
TCOP

Exchange Between Plasma and ISF

Interstitial fluid colloidal osmotic pressure (TCOP)
Tends to cause osmosis of water outwards from the capillary

BODY FLUIDS

A
V
CHP
Pif
CCOP
TCOP
CHP
CCOP
Pif
TCOP

Exchange Between Plasma and ISF

At arterial end outward forces
CHP = 30 mm hg

Negative Pif = 3

TCOP = 8 mm hg
TOTAL = 41 mm hg
Inward forces
PCOP = 28 mmHg
Net filtration
About (41 – 28) = 13 mm hg

BODY FLUIDS

CHP= 30

Pif = -3

CCOP= 28

TCOP = 8

CHP = 10

CCOP = 28

Pif = -3

TCOP = 8

Exchange Between Plasma and ISF

At venous end end outward forces
CHP = 10 mm hg

Negative Pif = 3

TCOP = 8 mm hg
TOTAL = 21 mm hg
Inward forces
PCOP = 28 mmHg
Net reabsorption
About 21 – 28 = 7 mm Hg

BODY FLUIDS

CHP= 30

Pif = -3

CCOP= 28

TCOP = 8

CHP = 10

CCOP = 28

Pif = -3

TCOP = 8

Exchange Between Plasma and ISF

At the arterial end
There is net filtration of fluid from capillary into the interstitial space
At venous end end
There is net reabsorption
Cause reabsorption of filtered fluid

BODY FLUIDS

CHP= 30

Pif = -3

CCOP= 28

TCOP = 8

CHP = 10

CCOP = 28

Pif = -3

TCOP = 8

Exchange Between Plasma and ISF

9/10 of the filtered fluid is reabsorbed at the venous end
The remaining 1/10 enters the lymphatic system

BODY FLUIDS

CHP= 30

Pif = -3

CCOP= 28

TCOP = 8

CHP = 10

CCOP = 28

Pif = -3

TCOP = 8

Exchange Between ECF and ICF

Distribution of fluid between the ICF and ECF
Determined mainly by
Osmotic effect of smaller solutes Na+, Cl-, and other electrolytes
Cell membrane highly permeable to water
Relatively impermeable to ions
Water moves across cell membrane to maintain osmotic equilibrium

BODY FLUIDS

PART III

BODY FLUIDS

Solute Movement between Compartments

The movement of some solutes between compartments is active, which consumes energy and is an active transport process, whereas the movement of other solutes is passive, which does not require energy.

Active transport allows cells to move a specific substance against its concentration gradient through a membrane protein, requiring energy in the form of ATP.

For example, the sodium-potassium pump employs active transport to pump sodium out of cells and potassium into cells, with both substances moving against their concentration gradients.

BODY FLUIDS

Solute Movement between Compartments CONT …

Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration.

Some molecules, like gases, lipids, and water itself (which also utilizes water channels in the membrane called aquaporins), slip fairly easily through the cell membrane; others, including polar molecules like glucose, amino acids, and ions do not.

BODY FLUIDS

Solute Movement between Compartments CONT …

Some of these molecules enter and leave cells using facilitated transport, whereby the molecules move down a concentration gradient through specific protein channels in the membrane.

This process does not require energy.

For example, glucose is transferred into cells by glucose transporters that use facilitated transport

BODY FLUIDS

Facilitated Diffusion OF GLUCOSE

Glucose molecules use facilitated diffusion to move down a concentration gradient through the carrier protein channels in the membrane.

BODY FLUIDS

Solute Movement between Compartments CONT …

BODY FLUIDS

Water Balance

On a typical day, the average adult will take in about 2500 mL of aqueous fluids.

Although most of the intake comes through the digestive tract, about 230 mL per day is generated metabolically, in the last steps of aerobic respiration.

Additionally, each day about the same volume (2500 mL) of water leaves the body by different routes; most of this lost water is removed as urine.

BODY FLUIDS

Water Balance CONT …

The kidneys also can adjust blood volume though mechanisms that draw water out of the filtrate and urine.

The kidneys can regulate water levels in the body; they conserve water if you are dehydrated, and they can make urine more dilute to expel excess water if necessary.

Water is lost through the skin through evaporation from the skin surface without overt sweating and from air expelled from the lungs.

This type of water loss is called insensible water loss because a person is usually unaware of it.

BODY FLUIDS

Regulation of Water Intake

Osmolality is the ratio of solutes in a solution to a volume of solvent in a solution.
Plasma osmolality is thus the ratio of solutes to water in blood plasma.

A person’s plasma osmolality value reflects the state of hydration.

A healthy body maintains plasma osmolality within a narrow range, by employing several mechanisms that regulate both water intake and output.

BODY FLUIDS

Regulation of Water Intake CONT …

Drinking water is considered voluntary.

So how is water intake regulated by the body? Consider someone who is experiencing dehydration, a net loss of water that results in insufficient water in blood and other tissues.

The water that leaves the body, as exhaled air, sweat, or urine, is ultimately extracted from blood plasma.

BODY FLUIDS

Regulation of Water Intake CONT …

As the blood becomes more concentrated, the thirst response—a sequence of physiological processes—is triggered.

Osmoreceptors are sensory receptors in the thirst center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood.

If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst.

The person should (and normally does) respond by drinking water.

BODY FLUIDS

Regulation of Water Intake CONT …

The hypothalamus of a dehydrated person also releases antidiuretic hormone (ADH) through the posterior pituitary gland.

ADH signals the kidneys to recover water from urine, effectively diluting the blood plasma.

BODY FLUIDS

Regulation of Water Intake CONT …

To conserve water, the hypothalamus of a dehydrated person also sends signals via the sympathetic nervous system to the salivary glands in the mouth.

The signals result in a decrease in watery, serous output (and an increase in stickier, thicker mucus output).

These changes in secretions result in a “dry mouth” and the sensation of thirst.

BODY FLUIDS

Regulation of Water Intake CONT …

Decreased blood volume resulting from water loss has two additional effects.

First, baroreceptors, blood-pressure receptors in the arch of the aorta and the carotid arteries in the neck, detect a decrease in blood pressure that results from decreased blood volume.

The heart is ultimately signaled to increase its rate and/or strength of contractions to compensate for the lowered blood pressure.

BODY FLUIDS

Regulation of Water Intake CONT …

Second, the kidneys have a renin-angiotensin hormonal system that increases the production of the active form of the hormone angiotensin II, which helps stimulate thirst, but also stimulates the release of the hormone aldosterone from the adrenal glands.

Aldosterone increases the reabsorption of sodium in the distal tubules of the nephrons in the kidneys, and water follows this reabsorbed sodium back into the blood.

BODY FLUIDS

Regulation of Water Intake CONT …

If adequate fluids are not consumed, dehydration results and a person’s body contains too little water to function correctly.

A person who repeatedly vomits or who has diarrhea may become dehydrated, and infants, because their body mass is so low, can become dangerously dehydrated very quickly.

BODY FLUIDS

Regulation of Water Intake CONT …

Endurance athletes such as distance runners often become dehydrated during long races.

Dehydration can be a medical emergency, and a dehydrated person may lose consciousness, become comatose, or die, if their body is not rehydrated quickly.

BODY FLUIDS

Regulation of Water Output

Water loss from the body occurs predominantly through the renal system.

A person produces an average of 1.5 liters (1.6 quarts) of urine per day.

Although the volume of urine varies in response to hydration levels, there is a minimum volume of urine production required for proper bodily functions.

BODY FLUIDS

Regulation of Water Output CONT …

The kidney excretes 100 to 1200 milliosmoles of solutes per day to rid the body of a variety of excess salts and other water-soluble chemical wastes, most notably creatinine, urea, and uric acid.

Failure to produce the minimum volume of urine means that metabolic wastes cannot be effectively removed from the body, a situation that can impair organ function.

The minimum level of urine production necessary to maintain normal function is about 0.47 liters (0.5 quarts) per day.

BODY FLUIDS

Regulation of Water Output CONT …

The kidneys also must make adjustments in the event of ingestion of too much fluid.

Diuresis, which is the production of urine in excess of normal levels, begins about 30 minutes after drinking a large quantity of fluid.

Diuresis reaches a peak after about 1 hour, and normal urine production is reestablished after about 3 hours.

BODY FLUIDS

Role of ADH

Antidiuretic hormone (ADH), also known as vasopressin, controls the amount of water reabsorbed from the collecting ducts and tubules in the kidney.

This hormone is produced in the hypothalamus and is delivered to the posterior pituitary for storage and release.

When the osmoreceptors in the hypothalamus detect an increase in the concentration of blood plasma, the hypothalamus signals the release of ADH from the posterior pituitary into the blood.

BODY FLUIDS

Role of ADH CONT …

ADH has two major effects. It constricts the arterioles in the peripheral circulation, which reduces the flow of blood to the extremities and thereby increases the blood supply to the core of the body.

ADH also causes the epithelial cells that line the renal collecting tubules to move water channel proteins, called aquaporins, from the interior of the cells to the apical surface, where these proteins are inserted into the cell membrane.

BODY FLUIDS

Role of ADH CONT …

The result is an increase in the water permeability of these cells and, thus, a large increase in water passage from the urine through the walls of the collecting tubules, leading to more reabsorption of water into the bloodstream.

When the blood plasma becomes less concentrated and the level of ADH decreases, aquaporins are removed from collecting tubule cell membranes, and the passage of water out of urine and into the blood decreases.

BODY FLUIDS

Aquaporins

The binding of ADH to receptors on the cells of the collecting tubule results in aquaporins being inserted into the plasma membrane, shown in the lower cell.

This dramatically increases the flow of water out of the tubule and into the bloodstream

BODY FLUIDS

diuretic

A diuretic is a compound that increases urine output and therefore decreases water conservation by the body.

Diuretics are used to treat hypertension, congestive heart failure, and fluid retention associated with menstruation.

Alcohol acts as a diuretic by inhibiting the release of ADH.
Additionally, caffeine, when consumed in high concentrations, acts as a diuretic.