Is estimated to be 80 000 miles –

more than three times the distance

around the world.

Translation

Clotting of the blood

If a blood vessel is ruptured it is important that the resultant loss of blood is quickly arrested. If not, the pressure of the blood in the circulatory system could fall dangerously low. At the same time it is important that clotting does not occur during the normal circulation of blood. If it does, the clot might lodge in some blood vessel, cutting off the blood supply to a vital organ and possibly resulting in death from thrombosis. For this reason the clotting process is very complex, involving a large number of stages. Only under the very specific conditions of injury are all stages completed and clotting occurs. But the chances of clotting taking place in other circumstances is reduced.

Cellular fragments in the blood called platelets (thrombocytes) are involved in the clotting or coagulation of the blood. At the site of a wound the damaged cells and ruptured platelets release thromboplastins. The platelets attract clotting factors which create a cascade effect whereby each activates the next in the chain. At the end of these chain reactions factor X is produced which in the presence of calcium ions and vitamin K causes the inactive plasma protein, prothrombin, to become converted to its active form, thrombin. This in turn converts another plasma protein, the soluble fibrinogen, to fibrin, its insoluble form. The fibrin forms a meshwork of threads in which red blood cells become trapped. These dry to form a clot beneath which repair of the wound takes place. The clot not only prevents further blood loss, it also prevents entry of bacteria which might cause infection.

Defence against infection - phagocytosis

Two types of white cell, the neutrophils and monocytes, are capable of amoeboid movement. Both types carry out phagocytosis. White cells carry out phagocytosis for two reasons: to protect the organism against pathogens and to dispose of dead, dying or damaged cells and cellular debris.

In protecting against infection the phagocyte is attracted to chemicals produced naturally by bacteria. The recognition is aided by the presence of opsonins - plasma proteins which attach themselves to the surface of the bacteria. The phagocytes have specific proteins on their surface that bind to these chemo-attractants. This causes the phagocyte to move towards the bacteria. The phagocyte strongly adheres to a bacterium on reaching it. The bacterium is enveloped and a vacuole, called a phagosome, forms. Lysosomes within the phagocyte migrate towards the phagosome into which they release lytic enzymes that break down the bacterium. The breakdown products are finally absorbed by the phagocyte.

Phagocytosis causes inflammation at the site of infection. The hot and swollen area contains many dead bacteria and phagocytes which are known as pus. Inflammation results when histamine is released as a result of injury or infection. This causes dilation of blood capillaries from which plasma, containing antibodies, escapes into the tissues.