Explanation Composition of blood: Blood is a connective tissue and consists of a fluid matrix called plasma in which, the cellular elements- red blood cells, white blood cells and blood platelets remain suspended. Plasma comprises 55% - 60% and cellular elements about 40 %- 45% of blood. Plasma: Plasma is an alkaline, straw coloured liquid. Plasma is non- - living and does not contain fibres. Plasma is composed of water (90 %- 92%), proteins like albumin, globulin and fibrinogen (7% - 8%), inorganic salts like sodium chloride and sodium bicarbonate (1%) and traces of other substances like glucose, amino acids, antibodies, clotting factors, hormones and urea. Plasma from which the protein fibrinogen is removed is called serum. Plasma is the transporting medium and transfers substances like nutrients, hormones and vitamins to the cells. Cellular elements: The cellular elements of the blood are the red blood corpuscles or erythrocytes (RBC), white blood corpuscles or leucocytes (WBC) and blood platelets or thrombocytes. Red Blood Corpuscles: Red Blood Corpuscles are biconcave disc like cells without a nucleus. The cells are minute in size, about seven micrometre in diameter and have a tendency to form a row or rouleaux, which facilitates their movement in capillaries. Red blood cells contain the respiratory red pigment haemoglobin, which can readily combine with oxygen to form oxyhaemoglobin. Oxyhaemoglobin is an unstable compound, which readily gives off oxygen to the tissues. Haemoglobin imparts the red colour to the cells. Red blood corpuscles are produced in the marrow of long bones. The average life of red blood cells is about 120 days, after which, it is destroyed in the spleen or liver. The bone marrow produces new red blood corpuscles regularly to replace those destroyed. An adult human male has about five million red blood cells per cubic millimetre of blood whereas the adult human female has around four and a half million red blood cells per cubic millimetre of blood. White blood corpuscle: White blood corpuscles are colourless, so they are called leukocytes. White blood corpuscles are nucleated, contain no haemoglobin and are amoeboid. They produce pseudopodia by means of which they can squeeze through the pores of the capillary walls to enter the infected tissues and engulf the disease causing bacteria. The passage of white blood cells, through the unruptured wall of a blood vessel, by changing its shape is called diapedesis. The number of white blood corpuscles is approximately 4000 - 8000 per cubic millimetre of blood. They play an important role in the body's defence mechanism. White blood cells are visible under the microscope when they are stained. White blood cells are classified as granulocytes and agranulocytes, according to the presence or absence of granules in their cytoplasm. Granulocytes are white blood cells with granulated cytoplasm. Eosinophils, basophils and neutrophils are the three types of granulocytes. . Eosinophils: The cytoplasm of the eosinophils has coarse granules, which stain dark red with eosin (an acid dye). The nucleus has two lobes connected by a thin strand. It represents 2%-5% of the total white blood corpuscles, but the number increases in individuals with allergic conditions like hay fever and asthma. Eosinophils have anti-histamine properties. Basophils: The cytoplasm of Basophils has granules, which stain with basic dyes, such as Methylene Blue. The nucleus is large and lobed. Basophils represent about 0.5% of total white blood corpuscles. It produces histamine and heparin. Heparin is an anti-clotting substance that is found in damaged tissues and is involved in inflammation. Inflammation stimulates repair of damaged tissue. Neutrophils: The cytoplasm of neutrophils has granules that stain with neutral dyes. Nucleus has 3 - 4 lobes. It comprises about 70% of the total white blood corpuscles. Neutrophils exhibit diapedesis and move to infected areas of the body to engulf and digest the disease causing bacteria. Agranulocytes: Agranulocytes have non-granular cytoplasm. The different types of agranulocytes are lymphocytes and monocytes. Lymphocytes: Lymphocytes are formed in the lymph nodes. The cells have a large nucleus with a slight dent on one side. It is smallest of the white blood corpuscles. It comprises about 25% of the total white blood corpuscles. Lymphocytes produce antibodies that help in the defence of the body. Monocytes: Monocytes are large cells with a kidney or bean shaped nucleus. It comprises 4% of the total white blood corpuscles. Monocytes move and engulf bacteria. Blood Platelets: Blood platelets are cell fragments of giant cells called megakaryocytes, produced in the bone marrow. They are colourless, minute structures floating in the blood plasma. They are non- nucleated and irregular in shape. There are approximately 0.25 million platelets per cubic millimetre of blood. Their life span is 5-9 days, after which, it is destroyed in the spleen and liver. It helps in the clotting of blood. Functions of the blood The blood performs important functions, which includes transportation, prevention of infection, temperature regulation and formation of clots. Transport: The blood acts as a transport system and ensures that all the cells are supplied with nutrients. It transports the respiratory gases oxygen and carbon dioxide. Oxygen is carried to the cells and carbon dioxide is transported from the cells to the lungs. It removes excretory products from the cells to the kidneys, skin, lungs and liver for excretion. It carries hormones to specific targets. Temperature regulation: Blood distributes the heat produced in the liver and muscles evenly around the body to maintain a uniform body temperature. Prevention of infection: The microbes that enter the body are destroyed by white blood cells. The toxins produced by these microbes are neutralised by antitoxins produced by the white blood cells. The white blood cells produce antibodies that help in the defence of the body in the following ways. The antibodies cover the surface of the pathogens, which facilitates the amoeboid white blood cells to ingest the pathogens. Antibodies make the surface of the pathogens sticky, which makes them clump together and thus retard their movements. This results in their confinement to one infected site. Antibodies can also dissolve the surface of the pathogens and thus destroy them. White blood cells exhibit phagocytic characteristics, that is, they are capable of engulfing disease causing bacteria and thus protect the body. Formation of Clots: The blood forms clots where the blood vessels are damaged to prevent excessive loss of blood and entry of germs.