PAIN AND BLOOD
Red blood cells in tissue are the most dramatic and least important aspects of injury, assuming that the bleeding stops. The colour of a black eye or a bruise is caused by the haemoglobin in blood that has leaked into tissue, and the gaudy rainbow of colours that follows is a sign of the breakdown of the haemoglobin. Far more important are the white cells that migrate from blood vessels into damaged tissue even if the vessels are not cut open. They set about engulfing the debris of damage, and recognize and counteract the presence of abnormal components by way of the immune system. This invasion occurs in all sick tissue but is particularly dramatic if bacteria have started to multiply in the damaged tissue when the white cells form a barrier to create a pus-filled abscess. The fluid of the blood, called the serum, also plays a crucial role. Elaborate interaction of proteins in the serum with damaged cells forms the clot which blocks leaking blood vessels. Other active components of serum move into damaged tissue and are evident in the swelling of poorly tissue.
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