Human Blood : It is a fluid connective tissue consisting of plasma and blood corpuscles of various types.
An average adult person has about 4 to 6 litres of blood. If forms 6-10% of the body weight and some 30–35% of the extracellular fluid. The study of blood is called haematology (G. haima = blood, logos = study). A. Physical Appearance. Blood is an opaque, mobile fluid connective tissue, mesodermal in origin. It is somewhat sticky, and slightly heavier than water bulk for bulk (specific gravity 1.06). It has a saltish taste and a mild alkaline reaction (pH 7.4). Its osmotic pressure at 37°C is about 7.6 atmospheres. It is bright red when oxygenated and purple when deoxygenated. B. Composition. Blood consists of a watery fluid called plasma containing certain floating bodies termed formed elements. The latter include blood cells or corpuscles and blood platelets or thrombyocytes . The blood corpuscles are of two types : red corpuscles, or erythrocytes (RBCs) and white corpuscles, or leucocytes (WBCs). None of the corpuscles divides in the blood. The plasma and formed elements form about 55% and 45% of the volume of the blood respectively.
Plasma. The plasma is a faint yellow, slightly alkaline, somewhat viscous fluid. It is a complex mixture which is in dynamic equilibrium with the intercellular fluid bathing the cells and the intracellular fluid present within the cells. It constantly takes up and loses materials as it flows through the capillaries, yet it has constant chemical composition. It consists of about 90% water, 1% inorganic salts in true solution, and 7 or 8% proteins in colloidal state. The remaining 1 or 2% of the plasma is formed from food materials, waste products, dissolved gases, regulatory substances, anticoagulant, cholesterol and antibodies. These substances do not form an integral part of the plasma as they enter and leave at intervals. They are carried by the plasma form one place to another in the body. (i) Proteins. The plasma contains a number of proteins: serum albumen (albumin), serum globulins, properdin, prothrombin and fibrinogen. The plasma proteins serve many functions: (a) They act as a source of proteins for the tissue cell, which may synthesize their own proteins from them. (b) They serve as acid-base buffers, i.e., they maintain pH of the blood by neutralizing acids and base. (c) Albumen and globulins maintain osmotic pressure of the plasma so that the latter may retain water. Fall in the level of plasma proteins causes excessive filtering of water from the blood into the tissues. This may produce oedema (swelling) of hands and feet in persons taking protein-deficient diet. (d) Plasma proteins transport certain materials in combination with them. Thyroxin is bound to albumen or a specific globulin, insulin is combined with globulins, and fatty acids are joined to albumen for transport in the plasma. (e) Some globulins, called immunoglobulins (IG), form protective proteins, termed antibodies, in response to the entry of foreign agents, the antigens, into the body. The antibodies inactivate the antigens (microorganisms or their toxins). (f) Properdin destroys certain bacteria, neutralizes certain viruses and damages foreign red blood corpuscles. (g) Prothrombin and fibrinogen play a role in blood clotting. Inorganic Salts. The inorganic salts occur in the plasma as ions. Sodium and chloride are the principal cation and anion of the plasma. The anions bicarbonate and phosphate, and the cations potassium, magnesium, calcium, iron and manganese occur in smaller amounts. The inorganic salts are sometimes referred to as blood electrolytes. The kidneys maintain plasma electrolytes at precise concentrations – an example of homeostasis. Food Materials. The food materials present in the plasma are glucose, amino acids, fatty acids and triglycerides. Their amount depends upon the digestion of food in the alimentary canal. Normally, an adult person has 80 to 100 mg of glucose per 100 ml of blood 12 hours after a meal. If blood sugar exceeds 180 mg glucose is excreted in the urine, causing the disease diabetes mellitus or hyperglycemia. Fall in blood sugar is called hypoglycemia. Waste Products. The waste products found in the plasma are urea, uric acid, ammonia and creatinine. The kidneys remove these. Their excess causes toxic effect called uremia. Dissolved Gases. Small amounts of oxygen, carbon dioxide and nitrogen are found dissolved in the plasma. Regulatory Substances. These include hormones, vitamins and enzymes. Anticoagulant. A natural strong anticoagulant present in the plasma is a heteropolysaccharide named antiprothrombin, or heparin. It checks clotting of blood in uninjured blood vessels by preventing the conversion of prothrombin into thrombin. It is produce in the liver. Cholesterol. Liver synthesizes cholesterol and releases it into the blood. It is also absorbed into the blood from the food, such as eggs, digested in the intestine. It provides materials to the tissue cells for the synthesis of membrane lipids, vitamin D, steroid hormones and bile slats. Cholesterol normally ranges from 50 to 180 mg. per 100 ml. of blood. Rise in the level of cholesterol in the blood may cause heart trouble.
Red Blood Corpuscles (RBCs). The red blood corpuscles are the most numerous formed elements of the blood. They are the most abundant cells in the human body. A unique feature of the RBCs is the presence of a red, oxygen-carrying pigment, the hemoglobin, in their cytoplasm. Shape. The shape of RBCs are circular, biconcave, denucleated discs. Their central part is thinner than the margin. This shape provides flexibility and results in a 20 to 30% increase in surface area as compared to sphere. This favors quick diffusion of gases. Size. Human RBCs are smaller than the white corpuscles. They are 7 – 8 μm n diameter and 2 μm thick near the rim. Small size of RBC provides it greater surface area for quick diffusion of O2 into it. Number. The RBCs are far more numerous than the WBCs. A normally healthy adult man and woman have 5 and 4.5 million RBCs per cubic millimeter of blood respectively. This is called the total RBC count. The RBC count decreases in anaemia. Anemia is caused by loss of blood (hemorrhage), destruction of RBCs (hemolytic), or faulty formation of blood. The RBC count increases during 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 is called polycythemia. Decrease in the number of red blood corpuscles, termed erythrocytopenia, causes oxygen shortage in the blood and tissues. The oxygen shortage stimulates the kidney cells to secrete a hormone, named erythropoietin, into the blood. This hormone, in turn stimulates the bone marrow to increase the production of red blood corpuscles. Addition of red blood corpuscles increases the oxygen-carrying capacity of the blood. As the blood’s oxygen level becomes normal, secretion of erythropoietin stops and the production of red blood corpuscles returns to normal.
Color. The RBCs look yellowish when seen singly and red when viewed in bulk. They impart red colour to the blood. The colour is due to the presence of a solution of iron-containing pigment, hemoglobin, in them. Hemoglobin is a conjugated protein. It consists of a basic protein globin joined to a nonprotein group heme, hence the name hemoglobin. Heme is an iron-porphyrin ring. A mammalian haemoglobin molecule is a complex of 4 heme molecules joined with 4 globin molecules. There is about 15 mg of haemoglobin in 100 ml of blood. A red blood corpuscle has some 280 million haemoglobin molecules. In the lungs, due to high partial pressure of oxygen, haemoglobin takes up oxygen and changes to bright red oxyhaemoglobin. The latter carries 4 oxygen molecules loosely joined to 4Fe++ ions (Hb4 + 4O2 Hb4O8). Thus, one RBC can carry over a billion oxygen molecules (280 million × 4 = 1120 million). In the tissues, due to low partial pressure of oxygen; oxyhaemoglobin breaks up in to oxygen and deoxyhaemoglobin. In this way, the RBCs carry oxygen from the lungs to the tissues. The RBCs also carry carbon dioxide from the tissues to the lungs for elimination. It is transported in two forms : mainly in combination with the water of RBC, forming bicarbonate lons (CO2 + H2O H2CO3 H+ + HCO3–); and partly in combination with the amino group of globin, forming carbaminohaemoglobin (HbO2 + CO2 HbCO2 + H+ + O2). Structure. A red blood corpuscle is bounded by an elastic and semipermeable plasma membrane. This enables it to squeeze through capillaries having a diameter less than its own. It loses plasticity in sickle-cell anaemia. In this disorder , the RBCs block the capillaries, leading to grave consequences. An erythrocyte contains homogenous cytoplasm which loses the nucleus, endoplasmic reticulam, mitochondria, ribosome and centrioles during the development of the corpuscle. This gives a double advantage. The corpuscle has more space to hold haemoglobin. Its oxygen consumption is very low due to lack of organelles so that it can supply more oxygen carried by haemoglobin to the tissue cell. Red blood corpuscles cannot reproduce or carryout cellular metabolism due to lack of organelles. Besides haemoglobin, a red corpuscle also contains several inorganic ions, including those of sodium, potassium, calcium, magnesium, chloride and phosphate. The adult red blood corpuscles of mammals are described as enucleated(denucleated) as, when young, they have a nucleus that later disappears. Formation. Formation of red blood corpuscles is called erythropoiesis. It occurs in liver and spleen in the foetus and in the red bone marrow after birth. Proteins and irons are components of haemoglobin, and vitamin B12 and folic acid stimulate erythropoiesis. Deficiency of any of these materials may cause anaemia. Excess RBCs are stored in the spleen. Life Span and Disposal. Human RBCs remain functional in the blood for about 120 days*. The worn out RBCs are destroyed by phagocytosis in the blood itself and in the spleen and lever in particular. Their iron is returned to the red bone marrow for reuse in the synthesis of fresh haemoglobin. Their pigment is degraded to yellowish pigment bilirubin which is excreted in bile. The pale yellow colour of the plasma is mainly due to bilirubin. If bilirubin is not excreted fully, the skin and mucous membranes of the person become yellowish. This disorder is called jaundice. Special Property. In resting (drawn) and slow flowing blood, the RBCs form piles called rouleaux by adhering together due to surface tension. White Blood Corpuscles (WBCs). The white blood corpuscles lack haemoglobin. Shape. The WBCs are rounded or irregular cells. They can change their shape and are capable of amoeboid movement. This enables them to squeeze out of capillaries in to the tissues. This processes is called diapedesis. Size. The WBCs are mostly larger then the red corpuscles. They range in size from 12 to (20um). Number. The WBCs are far fewer than the RBCs. Their number varies from 5,000 to 10,000 per cubic millimeter of blood. This number is the total count of WBCs. It may increase or decrease abnormally in certain conditions. Rise in WBC count is called leucocytosis. It is a physiological response to infections (e.g.; pneumonia), inflammations such as appendicitis, and malignancies such as leukemia (blood cancer). Fall in WBC count is termed leucopenia. It occurs in conditions such as folic acid deficiency. Infection of AIDS virus. WBS count is useful