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In this article we will discuss about:- 1. Meaning of Blood 2. Composition of Blood 3. Blood Corpuscles 4. Blood Groups 5. Coagulation 6. Functions.
Meaning of Blood:
Blood is a mobile connective tissue composed of a fluid, the plasma and the cells, the blood corpuscles. Blood is basis of life. Blood is the softest tissues in the body.
Fluids outside the cells are generally called extracellular fluids (ECF). Blood forms about 30-35 per cent of the ECF. The volume of blood in an adult person of 70 kg weight is about 5.5 litres. It is a slightly alkaline fluid having pH 7.4. pH of blood in arteries is more than in veins.
Composition of Blood:
As stated above blood is composed of a watery fluid called plasma and floating bodies termed formed elements (e.g., blood corpuscles).
Plasma:
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It is slightly alkaline non-living intercellular substance which constitutes about 55% part of the blood. It is a pale yellow but transparent and clear fluid.
Composition of Plasma:
1. Water:
Water alone forms about 90% to 92% of the plasma. Solids form about 8% of the plasma.
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2. Minerals:
These are Na+, Ca++, Mg++, HC03–, Cl–, etc.
3. Nutrients:
These include glucose, amino acids, lipids, etc. Minerals have been mentioned above.
4. Proteins:
They constitute about 6 to 8% part of plasma. These mainly include albumin for osmotic balance, globulin for defence mechanism and fibrinogen for blood clotting.
5. Defence Compounds:
Immunoglobulin’s which act as antibodies and some other substances, such as lysozyme (a polysaccharide) and properdin (a large protein) are always found in the plasma. They destroy bacteria, viruses and toxic substances that may enter into the blood from outside.
6. Excretory Substances:
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These include ammonia, urea, uric acid, creatine, creatinine, etc.
7. Dissolved Gases:
Water of blood plasma contains oxygen, carbon dioxide and nitrogen in dissolved form.
8. Anticoagulant:
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Blood plasma contains a conjugated polysaccharide, the heparin which prevents coagulation of blood inside blood vessels.
9. Hormones:
These are secreted and released in blood by endocrine glands.
10. Vitamins and Enzymes:
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Different kinds of vitamins and enzymes are present in the blood plasma.
11. Factors for clotting of blood:
These are also present in the plasma in an inactive form. Plasma without the clotting factors is called serum.
Functions of Plasma Proteins:
1. Prevention of blood loss:
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Fibrinogen and pro-thrombin play a role in blood clotting.
2. Retention of fluid in the blood:
Albumins and globulins retain water in blood plasma.
3. Body immunity:
Certain globulins called immunoglobulin’s (glycoproteins) act as antibodies in blood and tissue fluid. Thus all antibodies are immunoglobulin’s but all immunoglobins may not be antibodies.
4. Maintenance of pH:
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Plasma proteins serve as acid-base buffers. It means they maintain pH of the blood by neutralizing acids and bases.
5. Transport of certain materials:
Thyroxine (hormone) is bound to albumin or specific globulin for transport in the plasma.
6. Distribution of heat:
Plasma proteins help in uniform distribution of heat all over the body.
7. Enzymes:
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Some proteins acting as enzymes also occur in the plasma.
Blood Corpuscles:
Formed elements or blood corpuscles are of the following three types: Erythrocytes, Leucocytes and Thrombocytes. They constitute about 45% of the blood.
1. Erythrocytes (Red Blood Corpuscles or RBCs):
They are the most abundant cells in the human body.
Number:
The total number of RBCs per microliter of blood is termed as the total count of RBCs. A normal adult man and woman have 5 and 4.5 million RBCs per cubic millimetre of blood respectively.
Thus the total count of RBCs is more in man than in a woman. It is due to the fact that women undergo menstruation. Less amount of haemoglobin leads to anaemia. Anaemia may be caused by loss of blood (haemorrhage), destruction of RBCs (haemolysis or faulty formation of blood).
The increase in number of RBCs may be during muscular exercise to meet the increased demand of oxygen and at high altitudes to cope with the low oxygen content of the air. An abnormal rise in RBC count in called polycythemia.
Decrease in the number of RBCs is called erythrocytopenia which causes oxygen shortage in the blood and tissues. It is important to note that the oxygen shortage stimulates the kidney cells to secrete a hormone named erythropoietin, into the blood. Erythropoietin stimulates the bone marrow to increase the production of RBCs.
Shape:
Red blood corpuscles of all adult mammals are enucleated (non-nucleated). They are mostly biconcave and circular, however in camel and Llama RBCs are oval. Biconcave form of human RBCs is advantageous in gas exchange because biconcave disc has more surface area than a sphere.
Size:
Human erythrocytes are 7-8 µm in diameter and 1-2 µm thick near the rim.
Colour:
The RBCs look yellowish when seen singly and red when viewed in bulk. They impart red colour to the blood. Red colour is due to the presence of haemoglobin.
Structure:
Matured mammalian RBCs do not have cell organelles including nucleus, Golgi bodies, mitochondria, ribosomes, centrioles and endoplasmic reticulum. It increases the surface area of RBCs and enables these to contain more haemoglobin. Thus almost entire cytoplasm is filled with haemoglobin. In the absence of cell organelles, the consumption of oxygen is very low.
Anaerobic respiration occurs in RBCs. Haemoglobin is a conjugated protein which is made up of a protein called globin and a non protein group heme (= haeme), hence the name haemoglobin. Heme is an iron (Fe++) porphyrin complex.
A mammalian haemoglobin molecule is complex of 4 heme molecules joined with 4 globin molecules. A red blood corpuscle has some 280 million haemoglobin molecules.
Haemoglobin is the oxygen carrying pigment in most vertebrates except some ice fishes (Antarctic ice fish, Chaenocephalus) and eel larvae. Haemoglobin also occurs in the blood of some annelids and molluscs but in these invertebrates it remains in the plasma.
Quantity of Haemoglobin in RBCs:
100 ml of blood of a normal healthy individual contains about 12-16 gms of haemoglobin. The quantity of haemoglobin is less in women as they undergo menstruation.
Formation:
Formation of erythrocytes is called erythropoiesis. In the early few weeks of embryonic life, primitive nucleated RBCs are produced in the yolk sac. (one of the embryonic membranes). In later embryonic stage, RBCs are mainly produced by the liver and spleen. But from birth onwards, RBCs are produced by bone marrow.
Hemocytoblasts in red bone marrow give rise to mature RBCs. Iron and proteins are necessary raw materials for the synthesis of haemoglobin. However vitamin B12 and folic acid stimulate the maturation of RBCs. Thus iron, protein, vitamin B12 and folic acid are essential for the formation of haemoglobin and RBCs.
Deficiency of any of these nutrients can cause anaemia. Excess RBCs are stored in the spleen (graveyard of RBCs).
Life Span:
The life of a RBC is about 120 days.
Functions of RBCs:
(i) Transport of O2:
Haemoglobin of RBCs readily combines with oxygen to form oxyhaemoglobin. In the tissues oxyhaemoglobin readily gives up its oxygen. This oxygen is used for oxidation of food
(ii) Transport of CO2:
RBCs also participate in transporting carbon dioxide from tissues to lungs. Carbon dioxide combines with potassium carbonate of the red blood corpuscles to form potassium bicarbonate in the presence of an enzyme carbonic anhydrase. Carbon dioxide also combines with the amino group (—NH2) of the haemoglobin of red blood corpuscles to form carbaminohaemoglobin.
(iii) Maintenance of pH of blood:
Moreover, the haemoglobin is an excellent acid base buffer which is largely responsible for maintaining the pH of blood. Acidity of blood results haemoglobin to carry less oxygen.
Erythrocyte Sedimentation Rate (ESR):
If blood containing an anticoagulant (oxalate) is allowed to stand in a narrow vertical tube, the erythrocytes settle to the bottom half of the tube. The rate at which this occurs is called the erythrocyte sedimentation rate (ESR).
The erythrocytes settle to the bottom because their density is greater than that of plasma. ESR is very useful in diagnosing various diseases including tuberculosis. ESR in men is 0- 5 mm in 1st hour and in women it is 0-7 mm in 1st hour (Westergren Method).
Rouleaux:
In resting and slow flowing blood, the RBCs aggregate to form rouleaux (the RBCs are piled on top of each other). Fibrinogen favours rouleaux formation.
2. Leucocytes (White Blood Corpuscles or WBCs):
Leucocytes do not have haemoglobin.
Number:
The number of leucocytes per microlitre of blood is called the total leucocyte count (TLC). This varies from 6000 to 8000 mm3 of blood. Thus, they are less in number than the RBCs. The ratio of WBC: RBC in our blood is 1: 600. Rise in WBC count is termed leucocytosis. Increased TLC shows that there is acute bacterial infection.
Abnormal increase of WBCs is in malignancies like leukemia (blood cancer). Fall in WBC count is called leukopenia. In some conditions, such as folic acid deficiency, the total count of WBC decreases. The total count of WBC is useful in diagnosing various diseases. Normal or low TLC is in viral infection, malaria, typhoid or tuberculosis. The ratio of leucocyte to erythrocyte is 1.600.
Shape:
The leucocytes are rounded or irregular in shape. They can change their shape like Amoeba and are thus, capable of amoeboid movement. This enables them to squeeze out of blood capillaries into the tissues (extra vascular regions). This process is called diapedesis.
Size:
The WBCs are larger than the RBCs. Their size is from 12 to 20 pm.
Colour:
The WBCs are colourless.
Structure:
A leucocyte consists of cell membrane, nucleus and cytoplasm. The cytoplasm contains mitochondria, Golgi apparatus, centrioles besides other cell organelles.
Types:
The leucocytes are of two main types: Agranulocytes and granulocytes.
(i) Agranulocytes:
The granules are not found in the cytoplasm of these cells. The agranulocytes are of two types.
(a) Lymphocytes:
They are smaller in size containing scant cytoplasm with large rounded nucleus. They are non-motile and non-phagocytic. They produce antibodies to destroy microbes and their toxins reject grafts and kill tumour cells. They also help in healing of injuries. Lymphocytes exist in two major groups in circulation. These are B- and T- lymphocytes. Lymphocytes are 20-25% of leucocytes.
(b) Monocytes:
They are the largest of all types of leucocytes and somewhat amoeboid in shape. They have much cytoplasm. The nucleus is bean-shaped. They are motile and phagocytic in nature and engulf bacteria and cellular debris. Generally they change into macrophages after entering tissue spaces. Monocytes are 2-10% of leucocytes.
(ii) Granulocytes:
They contain granules in their cytoplasm. Their nucleus is irregular or lobed or subdivided. According to their staining property, the granulocytes are divided into three types.
(a) Eosinophil’s:
The nucleus is two lobed. They have coarse granules. Their granules take acidic stains (e.g., eosin). Their number increases in people with allergic conditions such as asthma or hay fever. They also help in dissolving blood clot. They are non-phagocytic.
They seem to play a part in the immune system. They have some similarity to lysosomes. Eosinophil’s can attach themselves to parasitic forms and cause their destruction by liberating lysosomal enzymes on their surface. Eosinophil’s are 2-3% of leucocytes.
(b) Basophils:
The nucleus is usually three lobed. They have less number of coarse granules. Their granules take basic stain (e.g., methylene blue) strongly. They release heparin, serotonin and histamine. They are probably like mast cells of connective tissue. Basophils are least (0-1%) among them.
(c) Neutrophils:
The nucleus is many lobed. They have fine granules. They stain weakly with both acid and basic stains. Neutrophils are the most numerous of all leucocytes. Certain neutrophils in female mammals possess a small spherical lobe attached to their nucleus by a stalk.
This lobe is called drum stick (= sex chromatin) or Barr body. Barr is the name of the scientist. Drumstick is formed by transformation of an X-chromosome. They eat harmful germs and are, therefore, phagocytic in nature. Neutrophils are 60-65% of leucocytes.
Formation:
Formation of leucocytes is called leucocytosis or leucopoeisis. The granulocytes and monocytes are formed only in bone marrow. Lymphocytes are produced mainly in lymph nodes; spleen, thymus, tonsils, bone marrow and Payer’s patches of small intestine.
Life Span:
The life of the granulocytes once released from the bone marrow is normally 4 to 8 hours circulating in the blood and another 4 to 5 days in the tissues. The monocytes also have a short life span of 10 to 20 hours. The lymphocytes have life spans of few days or months or even years, but this depends on the body’s need for these cells.
3. Thrombocytes (= Blood platelets):
In mammals thrombocytes are called blood platelets.
Number:
They are fewer than the RBCs and more than the WBCs in number. Blood normally contains 1, 50,000-3, 50,000 mm3 platelets of blood. Increase and decrease in the number of platelets is known as thrombocytosis and thrombocytopenia respectively.
Shape:
Blood platelets are really cell fragments rather than true cells. They are rounded or oval disc like bodies.
Size:
Platelets are 2-3 micro-metres in diameter. Thus they are much smaller than both the red and white blood corpuscles.
Colour:
Platelets are colourless.
Structure:
They are flat and non-nucleated fragments of the cells. They are simply bits of protoplasm, bounded by a membrane and contain a few cell organelles and secretary granules in the cytoplasm. They have a group of basophilic granules in the centre which give the appearance of a nucleus.
Formation:
Platelets are formed from the megakaryocytes (very large cells of the bone marrow). Formation of thrombocytes is called throinbopoiesis.
Life span:
Normal life span of blood platelets is about a week.
Function:
When an injury is caused, the blood platelets release certain chemicals which are called the platelet factors (e.g., thromboplastin).
The platelet factors help in the clotting of blood. [For differences: between Blood Plasma and Serum, between В-Lymphocytes and T- Lymphocytes, between Red Blood Corpuscles and White Blood Corpuscles, and for Haemopoiesis and Summary of Human Blood Corpuscles
Blood Groups:
There are more than 30 antigens on the surface of blood cells that give rise to different blood groups. In a blood transfusion, certain blood groups, e.g., ABO blood group, of the recipient and donor must be matched, otherwise the recipient’s immune system will produce antibodies that cause agglutination of the transfused cells and block blood circulation through capillaries.
Two types of blood groups — ABO Blood Group and Rh Blood Group (Rh Factor) are widely used all over the world.
ABO Blood Groups:
Karl Landsteiner reported first time ABO blood groups in human beings. A, В and О blood groups were discovered by Landsteiner (1900) while AB group was found out by de Castello and Steini (1902). ABO blood groups are determined by the gene I (isoagglutinin).
There are three alleles, IA, IB and I° of this gene. Proteins produced by the IA and IB alleles are called A antigen and В antigen. People with blood group A have the A antigen on the surface of their RBCs, and antibodies to antigen В in their plasma.
Persons with blood group В have В antigen on their RBCs, and antibodies against A antigen in their plasma. Individuals with AB blood group have both antigen A and antigen В on their RBCs, and no antibodies for either of the antigens in their plasma.
Type О individuals are without A and В antigens on their RBCs, but have antibodies for both these antigens in heir plasma. Individuals with blood group AB can receive blood of A, В or О group, while those with blood group О can donate blood to anyone.
This is the most important blood group for transfusion. Thus person with blood group AB is called universal recipient and person with blood group О is called universal donor. If a blood transfusion is made between an incompatible donor and recipient, reaction of antigens on the cells and antibodies in the plasma produces clots that clog capillaries.
Rh (Rhesus) Blood Group:
A protein named as rhesus antigen, is present on the surface of red blood corpuscles in many persons. It was discovered in 1940 by Landsteiner and Wiener in the blood of Rhesus monkey, hence its name. Depending on the race, 85 to 99 percent of the white population have this rhesus antigen (also called Rh factor) and are called Rh positive (Rh+).
Others who do not have this factor arc known as Rh negative (Rhr). Rh+ is dominant to Rh–. Whites Rh+ 85%, Rh– 15%, American Blacks Rh+ 95%, Rh– 5%, African Blacks Rh+ 100%.
Formation of Rh protein is controlled by a dominant gene which may be called as R. Thus, RR (homozygous) and Rr (heterozygous) persons are dominant and are Rh positive and rr (homozygous) are recessive and are Rh– negative. Both Rh+ and Rh– individuals are phenotypically normal. The problem arises during blood transfusion and pregnancy.
(i) Incompatibility during Blood Transfusion:
The first blood transfusion of Rh+ blood to the person with Rhr blood causes no harm because the Rh– person develops anti Rh factors or antibodies in his/her blood. In second blood transfusion of Rh+ blood to the Rh– person, the latter’s anti Rh factors attack and destroy the red blood corpuscles of the donor.
(ii) Incompatibility during Pregnancy:
If father’s blood is Rh+ and mother’s blood is Rh–, the foetus’ (baby in the uterus) blood is Rh+. This is a serious problem. If the Rh– blood of mother has not earlier come in contact with Rh+ blood through transfusion, her first child does not suffer (although the Rh+ blood of the foetus stimulates the formation of anti Rh factors or antibodies in the mother’s blood yet enough anti Rh factors are not produced in the mother’s blood to harm the foetus).
But in the subsequent Rh+ foetuses, the anti Rh factors (antibodies) of the mother’ blood destroy the foetal red blood corpuscles. This results in haemolytic disease of the newborn (HDN). It is called erythroblastosis foetalis (destruction of the erythrocytes of foetus).
Newborn may survive but it is often anaemic. In order to prevent HDN, Rh– mothers are injected with a defective anti Rh-antibody during all pregnancies carrying Rh+ foetus. Marriage between Rh– woman and Rh+ man is not recommended. Rh+ is dominant.
Coagulation of Blood (= Blood Clotting):
When an injury is caused to a blood vessel bleeding starts which is stopped by a process called blood clotting or blood coagulation.
This process can be described under three major steps:
(i) First Step:
At the site of an injury, the blood platelets disintegrate and release a phospholipid, called platelet factor-3 (= Platelet thromboplastin). Injured tissues also release a lipoprotein factor called thromboplastin. These two factors combine with calcium ions (Ca++) and certain proteins of the blood plasma to form an enzyme called pro-thrombinase.
(ii) Second Step:
The prothrombinase inactivates heparin (or antiprothrombin-anticoagulant) in the presence of calcium. Prothrombinase catalyzes breakdown of prothrombin (inactive plasma protein) into an active protein called thrombin and some small peptide fragments.
(iii) Third Step:
Thrombin acts as enzyme and first brings about de-polymerization of fibrinogen (a soluble plasma protein) into its monomers. Later thrombin stimulates re-polymerization of these monomers into long insoluble fibre-like polymers called fibrin.
The thin, long and solid fibres of fibrin form a dense network upon the wound and trap blood corpuscles (RBCs, WBCs and platelets) to form a clot. The clot seals the wound and stops bleeding.
Soon after the clot starts contracting and a pale yellow fluid, the serum, starts oozing out. This serum is blood plasma minus fibrinogen and blood corpuscles. Vitamin К is essential for blood clotting as it is necessary for the synthesis of prothrombin in the liver.
Bleeding Time:
When a sharp knife is used to pierce the tip of the finger or lobe of the ear, bleeding ordinarily lasts 3 to 6 minutes. However, the time depends largely on the depth of the wound and other clotting factors. Lack of clotting factors can prolong the bleeding time.
Clotting Time:
Many methods have been devised for determining clotting times. The one most widely used is glass test tube method. By this method the normal clotting time is about 3 to 8 minutes. Recent theory of blood clotting is cascade theory proposed by Macferlane. According to this theory 13 factors are required for blood clotting. VI factor is hypothetical. Actually there is no factor VI.
Role of Vitamin K in Blood Clotting:
Vitamin К is essential for blood clotting because it is necessary for the synthesis of prothrombin in the liver. If vitamin К is not sufficient in the body, blood clotting becomes inefficient.
Functions of Blood:
On the basis of the above description, the general functions of blood can briefly be summarised as below.
1. Transport of Food Materials:
Blood transports the digested food from the alimentary canal to the different body cells.
2. Transport of Respiratory Gases:
Oxygen is carried from the respiratory organs to the tissues and carbon dioxide from the tissue to the respiratory organs by blood.
3. Transport of Hormones:
Hormones are carried by blood from the endocrine glands to the places of use.
4. Transport of Excretory Matter:
Blood transports the excretory matter to the kidneys or other excretory organs.
5. Transport of Heat:
Blood allows the transfer of heat from the deeper tissue to surface of the body where it can be lost.
6. Defence against Infection:
Some white blood corpuscles are phagocytic in action; however, certain white blood corpuscles produce antitoxins to neutralize the toxins released by the foreign germs.
7. Temperature Regulation:
Blood maintains the body temperature to a constant level after distributing heat within the body.
8. Water Balance:
Blood maintains water balance to a constant level by bringing about constant exchange of water between circulating blood and tissue fluid.
9. Maintenance of pH:
Blood helps to regulate the pH of the body.
10. Prevention of Excessive Loss of Blood:
When any part of the body is injured, loss of blood is prevented by the formation of a clot.
11. Helps in Healing:
Blood maintains necessary supplies for the repair of damaged tissue. Eosinophil’s and basophils help in the healing of wounds.
12 .Maintenance of Physiological Co-operation:
Blood maintains a physiological cooperation between parts of the body by circulating from one to other parts.
