The body consists of many tissues. Each tissue is made up of many cell. Similarly, blood also has made up of many type of cells, that float in the liquid plasma. Therefore, blood is called as Fluid tissue. Adult has 5 liters of blood.
When there is an injury or a cut on the body, blood comes out. When the finger is pricked with a needle blood oozes out. When a small quantity of fresh blood is allowed to settle in a test-tube or on a glass plate, a part of it becomes solid. This is called clot. Rest of the blood is in fluid form and floats above the clot. This fluid part is called serum. The solid part of blood consists of cells called Corpuscles.
Functions of blood:
Blood performs several functions in the body. They are given below.
1. It transports oxygen from lungs to various parts of the body.
2. It transports carbon dioxide from various parts of the body to lung.
3. It transports water, salts and digested food materials from intestine to various parts of the body.
4. It transports waste material from various parts of the body to kidney and skin.
5. It transports substances called hormones from endocrine glands to other body parts. Hormones control the functions of these body parts.
6. It helps in maintaining uniform temperature, water and salt content of the body.
7. It helps in defending the body from disease causing germs.
Blood has two components. They are
1. Plasma
2. Corpuscles or Cells.
Plasma contains nearly 90% of water. It transports hormones, urea, proteins, nutrients, salts and waste products.
Blood corpuscles are of three types. They are
1. Red blood Corpuscles or cells or erythrocytes.
2. White blood corpuscles or leucocytes.
3. Thrombocytes or blood platelets.
The erythrocytes are round, flat, disc like and biconcave cells. They have no nuclei. They are red in colour due to the presence of a red colouring substance called haemoglobin. Haemoglobin carries oxygen and carbon dioxide. Hence, it takes part in respiration. In man the number of erythrocytes is 4.5 to 5.5 millions per cubic millimeter. The red blood cell are produced in the bone marrow. Erythrocytes do not live long in the body. In man they live for 100 to 125 days and then they degenerate, disintegrate and disappear. New red blood cells are formed in the place of dead cells.
Haemoglobin is a large molecule. It contains Protein molecule and 4 atoms of iron. The leucocytes are comparatively larger. They are irregular in shape and look like amoebae. Each leucocyte has a nucleus. Their number is lesser than that of red blood cells. In man the number of leucocytes is 4 to 11 thousands per cubic millimeter. The white blood cells capture and destroy harmful and disease producing organisms that invade or enter the body. Therefore, they protect the body from the harmful effects of those germs. In the battle between the leucocytes and the invading germs, some of the leucocytes are also get killed. The other leucocytes remove the dead germs and dead leucocytes from the body in the form of pus. Thus, leucocytes act as scavengers also. Hence the leucocytes are called policemen and scavengers of the body.
The third type of cells in the blood is blood platelets or thrombocytes. The throbocyte has no nucleus. The thrombocytes are helpful in the formation of blood clot. Clot formation prevents the loss of blood due to bleeding during injuries.
When we breath. air enters the lungs. Lungs are supplies with blood. Haemoglobin present in red blood cells combines with oxygen. This occurs in lungs. This blood is called oxygenated blood. This is supplies to various parts of the body. Heart pumps blood to various parts of the body. In the tissues, blood gives out oxygen and collects carbon dioxide. The blood with less amount of oxygen is called deoxygenated blood. The oxygenated blood flows through arteries. The deoxygenated blood flows through veins.