The Basic Facts About Blood

Blood is a living tissue composed of cellular elements and a watery fluid called plasma. Blood volume is the total amount of blood circulating within the body. It represents about 8% of body weight. In females volume averages 4-5 litres, in males 5-6 litres.

The cellular parts comprising of red cells, white cells and platelets made up nearly 45% of the volume of blood. The Plasma, which makes up the remaining 55% is 92% water. Blood has three main functions. These are Transport, Defence against disease, and Regulation of body Temperature.

Red blood cells (Erythrocytes) are the most common type of formed element in blood. Red blood cells are manufactured in the bone marrow of some bones including the Ribs, Vertebrae and some limb bones. RBCs, as they are called are produced at a very brisk pace of about 9000 million per hour. This is so because they have a short life of about four months. One reason for this is because they do not have a nucleus. RBCs are red because of the pigment Haemoglobin which carries oxygen. Haemoglobin is a protein, and contains iron. Old red cells are broken down in the Liver, Spleen and Bone marrow. Some of the iron from the haemoglobin is stored, and used for making new haemoglobin. Some is turned into bile pigment and excreted. RBCs also carry some Carbon Dioxide molecules from the cells to the lungs, but about 70% of the Carbon Dioxide dissolves within the plasma as Bicarbonate ions. The design of the red blood cell makes it ideal for oxygen and carbon dioxide transport. It is disc-shaped, indented in the centre and flexible enough to squeeze through the smallest capillary.

White blood cells (Leukocytes) are made in the bone marrow and in the Lymph Nodes. WBCs as they are called have a nucleus, which is often quite large and lobed. They can move around and can squeeze out through the walls of blood capillaries into all parts of the body. Their role is to fight infection, and to clear up any dead body cells.

Platelets are small fragments of cells, with no nucleus. They are made in bone marrow. Platelets help in the formation of blood clots. When platelets come into contact with a damage tissue, they stick to the edges of the damaged area, and then to each other, forming a plug. Larger wounds however need a larger barrier than this. Blood plasma contains several substances, which are involved in blood clotting. There are thirteen of these blood-clotting factors. If any one of them is defective, then blood will not clot. For example, clotting disorder due to a missing factor V111 is referred as Haemophilia. Two of these blood-clotting factors are Prothombin and Fibrinogen, which are soluble proteins dissolved in the blood plasma. If a tissue is damaged, it releases a chemical called Thromboplastin. This converts Prothrombin to Thrombin. Thrombin acts on Fibrinogen, converting it to the protein Fibrin. Fibrin is insoluble and forms fibres across the wound. Blood cells and platelets get caught up in the fibres, forming a clot.