COMPOSITION OF BLOOD AND ITS FUNCTIONS

• The plasma of the blood consists of 90% of water and remaining 10% is constituted by organic and inorganic substances.
• The organic substances are plasma proteins, glucose, urea, creatinine, aminoacids, lipids, antibodies, vitamins, enzymes, Antitoxins, chlolestrol etc.
• The plasma proteins are fibrinogen, albumins and globulins.
• The inorganic substances constitute Na, K, Mg, Cl ~ HPO₄, HCO₃ etc.
• The plasma is slightly saltish and possess a pH of about 7.2.

• The plasma maintains acid base equilibrium of the blood.
• The fibrinogen of plasma brings about coagulation of blood.
• Albumins and globulins help in retention of water in the plasma, by their osmotic effect and are also protective in function.
• A fall in the level of these proteins leads to the filtering out of excessive volumes of water from the blood into the tissues, leading to edema.

• The red blood cells are biconcave circular and enucleated.
• The cell organelles like Golgi apparatus, mitochondria, RNA, centriole are present.
• The average diameter is 7.5 u . The average number of RBC is about 5 million/mm³ of blood.
• The number of R.B.C. increases in acute exercises and high altitutes.
• The red blood cells consists of a respiratory pigment called Haemoglobin.
• It is a conjugated protein composed of a protein called globin, united to a prosthetic group called haeme. Haeme is a porphyrin derivative and consists of four iron atoms.
• The amount of haemoglobin will be 14-16 gm/100 ml of blood.
• The oxygen combines with haemoglobin to form oxyhaemoglobin in the form of which it is transported to different tissues of the body.
• Oxyhaemoglobin is dissociated at tissue level to release oxygen into tissues and retaining haemoglobin in the blood.
• Carbon dioxide is also transported in the blood in the form of carbamino compounds when combines with heamoglobin.
• The life span of R.B.C. is 120 days.
• The spleen helps in trapping the old cells and broken fragments.
• The dead cells are disposed by macrophages present in the liver, bone marrow and spleen.
• The haemoglobin of dead R.B.C. is also broken down into haeme and globin.
• The iron is liberated from the haeme and once again used in the formation of R.B.C.
• The development of R.B.C. is called erythropoiesis. In the foetus the liver, red bone marrow and spleen are centres for the formation of R.B.C.
• After birth they are developed exclusively from red bone marrow.
• The red bone marrow consists of precursors of R.B.C. like proerythroblasts or haemo erythroblasts, which transform into erythrocytes. The vitamins B₁₂ and folic acid are necessary for erythrocyte production.
• Decrease in the total count of RBC causes anaemia and the increase in RBC causes polycythemia.

• The white blood cells are larger than R.B.C. but they are few in number.
• They are nucleated blood cells. The ratio of red cells to white blood cells is 600 to 1.
• The number of leucocytes varies from 5000 to 9000/mm³ of blood.
• The number increases at times of stress and pain.
• The increase in the number of leucocytes is called Leucocytosis.
• In certain diseases the leucocytes decrease in number.
• The reduction in the leucocyte count below the normal level is called Leucopenia.
• Abnormal increase in the number of leucocytes with plenty of immature cells is called Leukemia.
• The W.B.C. are of two types basing on the nature of cytoplasm and nucleus.They are
  1. Granula leucocytes and
  2. Agranular leucocytes.

• Granular leucocytes are provided with granular cytoplasm and lobed nucleus.
• Hence they are called granularpolymorphonuclear leucocytes.
• They constitute upto 64.7% of the total leucocytes. The granular leucocytes are divided into three types basing on staining properties.

• The neutrophils constitue about 62% of the total leucocytes.
• The nucleus is many lobed. They are phagocytic in function.
• They protect the body against infection, hence they are called Microscopic policemen.

• They consitute upto 2 .3%. The nucleus is bilobed.
• They help in destruction of foreign bodies and toxins produced from them.
• They are stained with acidic dye like eosin.
• During infection their number increases and this condition is called eosinophilia.
• Decrease in the total count of eosinophils is called eosinopaenia.

• These cells constitute upto 0 .4%. The cytoplasm consists of bluish-black granules of variable size.
• They are stained with alkaline dye like methylene blue. Nucleus is bilobed or ribbon-like.
• It is beleived that they are transported to tissues where they become mast cells.

• The lymphocytes are round, nongranular with large nucleus.
• They constitute about 27% of the total leucocytes.
• The lymphocytes are of two types namely (a) B-lymphocytes and (b) T- lymphocytes.
• They protect the body against infection by producing antibodies.

• These are larger leucocytes with a horse-shoe shaped nucleus that is eccentrically situated.
• These cells constitute 5.3% of the leucocytes and are phagocytic in function.
• They clean the blood from debris. Hence they are called Internal scavangers.
• The development of leucocytes is called Leucopoiesis.
• In the embryonic stage the leucocytes are derived from the mesodermal cells.
• In the adult the granulocytes are produced exclusively from the red bone marrow.
• The lymphocytes and monocytes are developed from the lymphoid tissues of the body and to some extent from the bone marrow.
• The primitive white blood cells present in the bone marrow are called Myelocytes.
• These are precursors of white blood cells. The life span of W.B.C. is one day.

• Platelets are small colourless bodies which are spherical, oval or rod shaped.
• They measure upto 2.5 u- in diameter. Their number varies from 2,50,000 to 5,00,000/mm³.
• The gaint cells or megakaryocytes in red bone marrow are broken down into pieces called blood platelets.
• The life span of platelets is few days. They are destroyed in the spleen.
• The blood platelets produce thromboplastin that helps in coagulation of blood.
• They also help to close the vascular wounds by forming platelet plug.