Blood: plasma and formed elements

Every cell needs a supply of food and oxygen, and needs its waste taken away. Simple animals like sponges and coelenterates just let the surrounding water flow through their bodies. More complex animals use body fluids: blood and lymph.

Blood is a special connective tissue: a fluid matrix, the plasma, carrying formed elements (cells and cell pieces). Plasma makes up about 55% of blood. It is a straw-coloured, slightly thick fluid: 90-92% water and 6-8% proteins. The main proteins are fibrinogen (for clotting), globulins (for defence) and albumins (to keep the osmotic balance, holding water in the blood). Plasma also carries minerals, glucose, amino acids, lipids and inactive clotting factors. Plasma without the clotting factors is called serum.

The formed elements make up about 45% of blood:

• Red blood cells (RBCs), also called erythrocytes: the most numerous, about 5 to 5.5 million per mm³ of blood in an adult man. They are made in the red bone marrow. In most mammals they have no nucleus, and are biconcave discs (thinner in the middle). They are full of the red pigment haemoglobin (12-16 g per 100 mL of blood), which carries oxygen. They live about 120 days and are then destroyed in the spleen, the "graveyard of RBCs".
• White blood cells (WBCs), also called leucocytes: 6000 to 8000 per mm³. They have a nucleus, are colourless, and usually live a short time.
• Platelets (thrombocytes): tiny cell pieces broken off from large cells (megakaryocytes) in the bone marrow. They release substances needed for clotting. Too few platelets cause excessive bleeding.

WBCs are of two groups:

• Granulocytes (with granules in their cytoplasm): neutrophils, the most abundant WBCs (60-65%), which swallow and destroy germs (phagocytic); eosinophils (2-3%), which fight infections and are linked to allergic reactions; and basophils, the fewest (0.5-1%), which release histamine, serotonin and heparin and are involved in inflammation.
• Agranulocytes (no granules): lymphocytes (20-25%), of two kinds, B and T, which are responsible for the body's immune responses; and monocytes (6-8%), which are also phagocytic.