Blood is a bright-red coloured fluid connective tissue that circulates in the entire body. It consists of a straw-coloured fluid called plasma in which various cells or corpuscles are seen floating or moving. Plasma constitutes the non-living fluid part and is about 60% of the total blood. It is a slightly alkaline fluid having the pH 7.2-7.4.
The viscosity of the blood is nearly five times greater than that of water. The specific gravity of blood varies in the range of 1.035—1.075. When freshly drawn blood, to which an anti coagulating agent has been added, is placed stationary for some time, the erythrocytes start sedimenting. The rate at which these cells sediment is known as erythrocyte sedimentation rate (E.S.R.). The ESR is expressed in mm/hr and normally it varies from 4-10 mm hr.
Normally, a person has about 80-100 mg of glucose per 100 ml of blood 12 hours after a meal. But this amount rises more (not exceeding beyond 180 mg) soon after a carbohydrate rich diet. However, the persistent high levels of glucose beyond this specified threshold value in the blood causes diabetes mellitus. If the amount of glucose in the blood rises beyond 180 mg per 100 ml consistently, then glucose starts appearing in the urine. This condition is known as glucosuria.
About 50-180 mg of cholesterol is present in 100 ml of blood plasma. It is mainly used for the synthesis of new cell membranes by the body tissues, vitamin D, steroid hormones and bile salts. It is added to the blood plasma by two ways: (i) liver synthesizes cholesterol and secretes into the blood; and (ii) by the intestinal absorption of fats. Therefore, much intake of saturated fats like ghee, butter may increase the levels of cholesterol in the blood.
Cholesterol and its esters are insoluble in water and so are deposited on the walls of arteries and veins. This leads to high blood pressure and other heart disorders.
Composition of Blood
In man, the normal adult contains about 5-6 litres blood in the body constituting 5-8% of the body weight. The blood is composed of “Corpuscles”(Blood cells) and “Plasma”.
Water — 90 to 92%
Dissolved solids — 8 to 10%
(i) Proteins (Serum albumin, Serum globulin, fibrinogen) — 7%
(ii) Inorganic constituents
(Na+, Ca++, P, Mg++, Cl-, HCO3- etc.)
(iii) Organic constituents (non- protein nitrogenous substances such as urea, uric acid, ammonia, amino acids, neutral fats, glucose).
Respiratory gases (O2 and CO2)Internal secretions (Antibodies and various enzymes).
These are of mainly two types — RBC and WBC.
Erythrocytes or RBC
In man the red blood corpuscles are biconcave discs and anucleated but in fishes, amphibians, reptiles and birds are oval and nucleated. The number of RBC in the blood is about 5,400,000 per cubic mm in man and about 4,800,000 in woman. The average life span of the RBC is around 110-120 days.
The erythrocyte is made of membranous material which encloses the pigment haemoglobin and other compounds with the following break up—
(a) Water 62%
(b) Haemoglobin (nearly 100 million haemoglobin molecules) 28%
(c) Lipids of fatty acids 7%
(d) Sugars, salts, enzymes and other proteins 3%.
Haemoglobin is basically a respiratory pigment. In the embryo the RBCs are formed in the liver and spleen and after birth in the bone marrow. The formation of RBC is called haemopoiesis or erythropoiesis and the red bone marrow cells are called erythroblast.
Haemolysis: When RBCs are placed in water they swell and finally burst. Even when they swell, their haemoglobin is diffused out. This separation of cell contents from the cells (RBCs) is known as haemolysis.
Anemia: It occurs when there is a reduction in the amount of haemoglobin per unit volume (100 ml) of blood which resulting in the decrease of oxygen carrying power of blood. The normal amount of haemoglobin in male adult blood is 15.9 gm/100 ml of blood (it is considered 100%). From 70% downwards, the symptoms of anemia develop.
Leucocytes or WBC
These are larger in diameter (8-15m) and nucleated. They are non-pigmented cells having the power of amoeboid movement. Their number may be about 10,000 per cubic mm. They act as phagocytes responsible for defending the organism against invading bacteria (phagocytosis). Depending on their size, granules, staining reaction and number and shape of nuclei and WBCs are divided into three classes—
Granulocytes (Having lobated nuclei and fine granules; formed in red bone marrow).
(i) Neutrophils 65.0% - Phagoeytic;
(ii) Eosinbophils 2.8% - Non-phagocytic;
(iii) Basophils 0.2% - Non-phagocytic
Lymphocytes (Having larger size nuclei and without granules; formed in lymph nodes) 26.0%. Useful in healing of wounds.
Monocytes (Largest of WBCs and have deeply indented nuclei; formed in tonsils, spleen, thymus and mucosa of intestine) 6.0%.
The life span of granulocytes is about less than 10 days and less than 15 hours in case of lymphocytes. The excessive formation of WBC is known as Leukemia.
Functions of WBC
The main functions of WBC are to transport substances, remove dead cells and decaying tissues, fight bacteria and act as guardians of the circulatory system.
Some phgagocytic WBC may be destroyed; fragments of tissue and such other things form the “pus”.
Thus, the defence provided by the WBC is in two ways:
(i) By directly engulfing the attacking bacteria or antigen.
(ii) By the production of Antibodies.
Antibodies are special proteins of plasma which kill invading bacteria or render them ineffective. Sub-stance such as foreign proteins and micro-organisms which induce the formation of antibodies are called Antigens.
Antigens-antibodies is the basis of all kinds of vaccines. By putting a small dose of pox virus in the blood, the blood is tamed to produce pox-antibodies for a couple of years. Antibodies are very specific in their activities. Each kind of antibody is effective only against the kind of micro-organism that brought about its production or occasionally, against very similar organisms. An antibody produced as a result of infection by the mumps virus has no effect upon the organisms of diphtheria or typhoid.
Thrombocytes or Blood Platelets
They are round or oval bodies, generally non-nucleated measuring from 2 to 4 m in diameter. The platelets number about 2,50,000 in each cubic mm of human blood. They are formed in red bone marrow. They play a very important role in initiating the formation of blood clot.
Mechanism of Blood Clotting
The blood clotting is the property of plasma alone. The basic reaction for the clotting of blood is the conversion of soluble plasma protein, fibrinogen to an insoluble protein, fibrin. Fibrinogen is formed in the liver. On mixing with plasma the fibrinogen clots into fibrin in the presence of calcium and thrombin through the following series of reactions:
Prothrombin + Ca(ions) →Thrombin
(in plasma) (in plasma) → (from Platelets) Thrombin
Fibrinogen → Fibrin monomer + peptides
Fibrin monomer → Fibrin polymer Fibrin
Fibrin polymer → Insoluble fibrin clot stabilizing factor
Normally the blood does not clot in the blood vessels but when the blood is drawn outside the coagulation starts. There are certain substances or processes which can prevent the blood coagulation, these are:
(a) Heparin—Best and powerful anticoagulant
(c) Antithrombic activity
(d) Oxalates and Citrates
Inheritance of Blood
The practical importance of blood groups lies in blood transfusion and in finding out the parenthood of children. Besides A and B, there are people with other antigens M and N. These result in three blood types: M type, N type and MN type. No clumping however, takes place when these are mixed by blood transfusion. M type is very common in American Indians, while N type is very common in Australian aborigins. A man with M type cannot be the father of a child with N type of blood.
Closed Circulatory System: In such a circulatory system, the blood is confined to the vessels throughout its course from the heart. Its examples are higher animals (including man).
Open Circulatory System: In insects, the blood does not remain confined to the blood vessels but it flows freely through the body cavity and channels called lacunae and sinuseds in the tissues. The body cavity is known as hemocoel and the blood is hemolymph. Thus in insects the tissues are in direct contact with the blood.
In cockroach, there is a dorsal tubular heart having 13 segments. Blood or hemolymph circulates in the whole body due to the contractile activity of heart brought about by the contractions and relaxations of its muscular walls, assisted by the fan-shaped alary muscles. There is no respiratory pigment in the blood.
Points To Be Remembered
A mammalian heart has four chambers–2 auricles and 2 ventricles. Auricles open into ventricles, but the opening is guarded by right tricuspid and left bicuspid valves.
The rhythmic contraction and relaxation of the cardiac muscles is known as heart beat. A contraction phase of the chamber of the heart is called a systole and a relaxation phase diastole. The sequence of one systole followed by one diastole is termed as cardiac cycle and it lasts eighth-tenths of a second in a normal human adult at rest.Normally the heart beats about 72 times per minute and pumps out about 60-110 ml of blood per heart beat. The heart beat originates from the sino-auricular node (S-A node) in a specialized area between the right auricle wall close to the entrance of superior and inferior vena cavae. Therefore their area has been called pacemaker of the heart. From the S-A node, heart beat spreads over the two auricles causing their contraction and then it stimulates the another node, A-V node situated in the right auricle near the ventral part of interauricular system. The bundle of HIS continues from the A-V node and splits into two branches to form the Purkinje fibre system.
These two branches extend over the two ventricles. The impulse from the A-V node spreads along the bundle of HIS and Purkinje fibres and thus exciting the muscles of the ventricle with the result the two ventricles contract simultaneously. In heart the bundle of HIS and its branches plus the Purkinje system correspond to the telephone wires. The rate of conduction of impulse of contraction through bundle of HIS is very fast about 5 mm/sec.
First Heart Transplant in India
ndia’s first successful heart transplant operation was conducted by Dr P. Venugopal, the head of Cardiothoracic vascular surgery Dept. of the AIIMS, on 3 August, 1994.
Blood pressure may be defined as the pressure that the blood exerts against the walls of the blood vessels. Blood pressure depends on several factors such as blood volume, blood vessel space, force of the heart beat and blood viscosity, etc.
There are 3 types of blood vessels
Arteries—Carry blood away from heart to tissues.
Veins—Bring blood from tissues to the heart.
Capillaries—Connect the arteries and the veins.
Arteries: The blood in arteries flow at high pressure, created by the peristaltic movements of the muscles of the arteries. Arteries divide into thinner arteriocles.
Veins: Veins collect blood from the capillaries in the tissues and carry it to heart.
Capillaries: These are thin walled and smallest of vessels which form a very fine network of minute tubes.
Lymphatic systemIt is generally measured in terms of how high it can push a column of mercury. Blood pressure varies at different points in the circulatory system. When ventricles contract the pressure of blood in blood vessel is highest, called systolic pressure. It is normally 120 mm Hg. The pressure of blood when the ventricles are contracting, is termed as diastolic pressure and it is about 80 mm Hg in a youngman.
The red blood corpuscles never leave the blood vessels, but the plasma and leucocytes escape from blood capillaries into the tissue. This colourless portion of blood without its erythrocytes and heavy blood proteins is called lymph. The lymph carries food and O2 to cells of the body, and it takes substances from tissues to re-enter blood by means of lymphatic vessels, though some lymph enters the venous capillaries by osmosis.Lymph capillaries join to form lymph vessels or lymphatics which have thick walls and contain valves in pairs. The valves are more numerous than in veins, they allow lymph to flow away from the tissue, but prevent a back flow. Because of great permeability of lymph capillaries, colloids, tissue debris and foreign bacteria are carried away with the lymph.
The small lymph capillaries in villi of intestine are called lacteals which absorb digested fats; fats give the lymph a milky-white colour and this milky fluid is called chyle. At several places along the lymph vessels are lymph nodes (wrongly called lymph glands). There are no lymph hearts in mammals, the slowmoving lymph is propelled through the lymph vessels and nodes by body muscles, and by pressure in smaller vessels due to osmosis and by absorption of tissue fluids.
The lymphatic system differs from the blood vascular system in being an open system because it has lymph spaces between tissue cells; moreover, lymph flows in only one direction, that is from the tissues towards the heart. Hence its capillaries and lymph vessels are equivalent to veins, thus it does not form a complete circuit, as the blood vascular system does, because lymph goes from tissue cells to the veins of the blood system.