Clotting and lymph

When you get a cut, the bleeding soon stops. That is coagulation (clotting), which prevents too much blood being lost. If you look at the dark reddish lump that forms, it is a clot: a network of thin threads called fibrin, with dead and damaged blood cells trapped in it.

How fibrin forms, in a chain of steps:

• An injury makes platelets release certain factors, and damaged tissue releases some too. These start a cascade of reactions among inactive factors in the plasma.
• The cascade forms an enzyme complex, thrombokinase.
• Thrombokinase changes inactive prothrombin into active thrombin.
• Thrombin changes the soluble plasma protein fibrinogen into insoluble threads of fibrin.

Calcium ions play a very important role in clotting.

Lymph. As blood flows through the tiny capillaries, some water and small dissolved substances squeeze out into the spaces between the cells. The large proteins and most of the blood cells stay behind in the blood. This leaked fluid is tissue fluid (interstitial fluid). It has the same minerals as plasma, and all exchange of food and gases between the blood and the cells passes through it.

The lymphatic system, a network of fine vessels, collects this fluid and drains it back into the major veins. The fluid in these vessels is lymph. It is colourless (it has no red blood cells) and carries lymphocytes, which are important for immunity. Lymph also carries nutrients and hormones, and fats from digested food are absorbed into lymph through the lacteals inside the villi of the small intestine.