Blood And Its Functions

Composition and Functions of Blood

COMPONENTS OF THE BLOOD

What are the components of blood?

Blood And Its Functions

Blood is the specialized( connective tissue) bodily fluid that delivers necessary substances to the body's cells such as nutrients, oxygen and transports waste products away from those same cells Blood accounts for 7% of the human body weight By volume the red blood cells constitute about 45% of whole blood, the plasma constitutes about 55%

The average adult has a blood volume of roughly 5 litres, composed of:

• Plasma
• Formed elements
• These formed elements of the blood are
• Erythrocytes (red blood cells)
• Adult humans have roughly 2–3 × 10>13 red blood cells
• Leukocytes (white blood cells) and 4,000–11,000 white blood cells
• Thrombocytes (platelets). 150,000–400,000 platelets in each microliter

Women have about 4 to 5 million erythrocytes per microliter (cubic millimeter) of blood and men about 5 to 6 million; people living at high altitudes with low oxygen tension will have more.

Functions of Blood

What are the functions of blood?

Function of blood

Supply of oxygen to tissues (bound to haemoglobin which is carried in red cells) bound to plasma proteins Removal of waste such as carbon dioxide, urea and lactic acid Immunological functions, including circulation of white cells, and detection of foreign material by antibodies Coagulation, which is one part of the body's self-repair mechanism Messenger functions, including the transport of hormones and the signalling of tissue damage Regulation of body pH (the normal pH of blood is in the range of 7.35 – 7.45) Regulation of core body temperature

Blood Plasma and Platelets

Blood Plasma

Is the fluid portion of the blood,

The plasma, is a remarkable solution containing an immense number of ions, inorganic molecules, and organic molecules that are in transit to various parts of the body or aid in the transport of other substances The normal plasma volume is about 5% of body weight, or roughly 3500 mL in a 70kg man Plasma clots on standing, remain fluid only if an anticoagulant is added If whole blood is allowed to clot and the clot is removed, the remaining fluid is called serum

PLASMA Vs SERUM

Blood And Its Functions

Serum has essentially the same composition as plasma except that its fibrinogen an clotting factors II, V, and VIII have been removed and it has higher serotonin content because of the breakdown of platelets during clotting The plasma consist of protein; albumin, globulin, and fibrinogen fractions The capillary walls are relatively impermeable to the plasma proteins, and the proteins are therefore exerting an osmotic force of about 25 mm Hg across the capillary wall (oncotic pressure) that pulls water into the blood

Blood Platelets

Platelets or thrombocytes are minute fragment of cells consisting of a small amount of cytoplasm surrounded by a plasma membrane Platelets are roughly disk-shaped and an average about 3μm in diameter.

They play an important role in controlling blood loss by forming platelets plugs which seal holes in small vessels The life expectancy of platelets is about 5 – 9 days Platelets arise in a unique manner by the shedding of thousands of cytoplasmic fragments from the tips of processes of megakaryocytes in the bone marrow The first detectable cell of this line is the highly basophilic megakaryoblast, followed by a promegakaryocyte stage, in which synthesis of granules begins Finally, the fully differentiated megakaryocyte, a giant cell with a large, dense, polyploid, multilobed nucleus, appears

Production and Functions of Red Blood Cell

Red Blood Cells

• Red blood cells (Erythrocytes) are the most common type of blood cell

The cells are filled with haemoglobin, a biomolecule that can bind to oxygen They take up oxygen in the lungs and release it while squeezing through the body's capillaries.

The blood's red colour is due to the colour of haemoglobin.

In humans, red blood cells develop in the bone marrow, take the form of flexible biconcave disks, lack a cell nucleus, subcellular organelles and the ability to synthesize protein, It takes about 7 days for erythrocytes to mature and live a total of about 120 days

Erythropoiesis is the process by which red blood cells (erythrocytes) are produced

• In human adults, this usually occurs within the bone marrow.
• In the early foetus, erythropoiesis takes place in the mesodermal cells of the yolk sac.

By the third or fourth month, erythropoiesis moves to the spleen and liver.

In humans with certain diseases, erythropoiesis also occurs outside the bone marrow, within the spleen or liver.

This is termed extramedullary erythropoiesis The tibia and femur cease to be important sites of haematopoiesis by about age 25; the vertebrae, sternum, pelvis and ribs, and cranium bones continue to produce red blood cells throughout life.

Erythrocytes and granulocytes belong to the myeloid lineage.

The earliest identifiable erythroid stem cells are capable of rapid bursts of cell division to form numerous daughter cells.

In the process of red blood cell maturation, a cell undergoes a series of differentiations.

The following stages of development all occur within the bone marrow.

Pluripotent haematopoietic stem cell is cells which are capable of giving rise to all blood cell types.

The first readily identifiable cell of the erythroid series is the proerythroblast, a large cell with a large euchromatic nucleus and moderately basophilic cytoplasm It also responds to erythropoietin

The proerythroblast contains small amounts of ferritin and bears some of the protein spectrin on its plasma membrane Proerythroblasts proliferate to produce smaller basophilic erythroblasts, rich in ribosomes, in which haemoglobin-RNA synthesis begins The cytoplasm becomes partially, and then uniformly, eosinophilic (the polychromatic erythroblast and orthochromatic(greyish) erythroblast respectively).

Blood And Its Functions

The nucleus becomes intensely pyknotic(condensed) and is finally extruded from the cell, leaving an anucleate reticulocyte, which enters a sinusoid Its reticular staining pattern, visible using special stains, results from residual cytoplasmic RNA which is lost within 24 hours of entering the peripheral blood circulation Reticulocyte numbers in peripheral blood are therefore a good indicator of the rate of red cell production The whole process of erythropoiesis takes 5-9 days After these stages, the cell is released from the bone marrow, and ultimately becomes an erythrocyte‘ or matures red blood cell circulating in the peripheral blood .

Regulation of Erythropoiesis

Erythropoietin is a glycoprotein (protein-sugar conjugate) that serves as the primary regulator of red blood cells (erythrocytes).

It stimulates bone marrow stem cells to differentiate into red blood cells and controls haemoglobin synthesis and red blood cell concentration.

Erythropoietin production is stimulated by reduced oxygen content in arterial blood in the kidneys.

Circulating erythropoietin binds to receptors on the surface of erythroid progenitor cells that in turn mature into red blood cells.

In infants, erythropoietin is produced mostly in the liver, but the kidneys become the primary site of erythropoietin synthesis shortly after .

Functions of Red Blood Cells

Red blood cells are highly specialized for their oxygen transport function.

Because mature RBCs have no nucleus, all their internal space is available for oxygen transport.

Because RBCs lack mitochondria and generate ATP anaerobically (without oxygen), they do not use up any of the oxygen they transport Even the shape of an RBC facilitates its function. A biconcave disc has a much greater surface area for the diffusion of gas molecules into and out of the RBC than would, say, a sphere or a cube Each RBC contains about 280 million haemoglobin molecules A haemoglobin molecule consists of a protein called globin, composed of four polypeptide chains (two alpha and two betachains); a ring like non protein pigment called a heme is bound to each of the four chains

Blood And Its Functions

At the center of each heme ring is an iron ion (Fe2+_) that can combine reversibly with one oxygen molecule, allowing each haemoglobin molecule to bind four oxygen molecules.

• Each oxygen molecule picked up from the lungs is bound to an iron ion

As blood flows through tissue capillaries, the iron–oxygen reaction reverses.

Haemoglobin releases oxygen, which diffuses first into the interstitial fluid and then into cells.

Haemoglobin also transports about 23% of the total carbon dioxide, a waste product of metabolism.

Blood flowing through tissue capillaries picks up carbon dioxide, some of which combines with amino acids in the globin part of haemoglobin.

As blood flows through the lungs, the carbon dioxide is released from haemoglobin and then exhaled.

In addition to its key role in transporting oxygen and carbon dioxide, haemoglobin also plays a role in the regulation of blood flow and blood pressure.

The gaseous hormone nitric oxide (NO), produced by the endothelial cells that line blood vessels, binds to haemoglobin.

Under some circumstances, haemoglobin releases NO. The released NO causes vasodilation, an increase in blood vessel diameter that occurs when the smooth muscle in the vessel wall relaxes.

Vasodilation improves blood flow and enhances oxygen delivery to cells near the site of NO release.

Production and Functions of White Blood Cells

White blood cells, or leukocytes, are cells of the immune system defending the body against both infectious disease and foreign materials Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell Leukocytes are found throughout the body, including the blood and lymphatic system.

The number of leukocytes in the blood is often an indicator of disease There are normally between 4×10˄9 and 11×10˄9 white blood cells in a litre of blood, making up approximately 1% of blood in a healthy adult In conditions such as leukaemia, the number of leukocytes is higher than normal, and in leukopenia, this number is much lower

Types of White Blood Cells

• There are several different types of white blood cells

They all have many things in common, but are all different One primary technique to classify them is to look for the presence of granules, which allows the differentiation of cells into the categories granulocytes and agranulocytes

Granulocytes (Polymorphonuclear Leukocytes)

Leukocytes characterised by the presence of differently staining granules in their cytoplasm when viewed under light microscopy These granules are membrane-bound enzymes which primarily act in the digestion of endocytosed particles There are three types of granulocytes: neutrophils, basophils, and eosinophils, which are named according to their staining properties

Agranulocytes (Mononuclear Leucocytes)

Leukocytes characterized by the apparent absence of granules in their cytoplasm Although the name implies a lack of granules these cells do contain non-specific azurophilic granules, which are lysosomes The cells include monocytes, macrophages and lymphocytes

Functions of White Blood Cells

Neutrophil

Neutrophils defend against bacterial or fungal infection and other very small inflammatory processes that are usually first responders to microbial infection; their activity and death in large numbers forms pus.

Neutrophils are very active in phagocytosing bacteria and are present in large amount in the pus of wounds.

These cells are not able to renew their lysosomes used in digesting microbes and die after having phagocytosed a few pathogens

Eosinophil

Eosinophils primarily deal with parasitic infections and an increase in them may indicate parasitic infection They are also the predominant inflammatory cells in allergic reactions The most important causes of eosinophilia include allergies such as asthma, hay fever, and hives; and also parasitic infections Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing inflammation

Monocyte

• They have the kidney shaped nucleus and are typically agranulated
• They also possess abundant cytoplasm

Monocytes share the vacuum cleaner (phagocytosis) function of neutrophils Are much longer lived as they have an additional role: they present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or so that an antibody response may be mounted Monocytes eventually leave the bloodstream to become tissue macrophages which remove dead cell debris as well as attacking microorganisms Neither of these can be dealt with effectively Unlike neutrophils, monocytes are able to replace their lysosomal contents and are thought to have a much longer active life

Macrophage

Once monocytes move from the bloodstream out into the body tissues, they undergo changes (differentiate) allowing phagocytosis and are then known as macrophages Lymphocyte

Lymphocytes are much more common in the lymphatic system Lymphocytes are distinguished by having a deeply staining nucleus which may be eccentric in location Are relatively small amount of cytoplasm The blood has three types of lymphocytes.

-B cells; which make antibodies that bind to pathogens to enable their destruction -T cells; these include CD4+ (helper) T cells co-ordinate the immune response (they are what become defective in an HIV infection).

-CD8+ (cytotoxic) are able to kill virus-infected and tumour cells.

Blood And Its Functions

T cells are crucial to the immune response because they possess a unique 'memory' system which allows them to remember past invaders and prevent disease when a similar invader is encountered again.

Natural Killer Cells (NK cells).

Natural killer cells are able to kill cells of the body that are infected by a virus or have become cancerous.

KEY POINTS

Blood is a specialized bodily fluid that delivers necessary substances to the body's cells such as nutrients, oxygen and transports waste products away from those same cells The fluid portion of the blood is called the plasma.

White blood cells, or leukocytes, are cells of the immune system defending the body against both infectious disease and foreign materials.

Platelets play an important role in controlling blood clotting.

EVALUATION

• What are the components of blood?
• What are the functions of blood?
• What is blood plasma?

What are the functions of leukocytes and erythrocytes?