Chapter 07

Blood Vessels

Apart from inert appendages like nails and hair, all parts of the body require a constant supply of oxygen.  Just as important, though not quite as urgent, is the need to supply nutrients and to remove the waste products of metabolism.  In complex organisms, these transport functions are served by the vascular system.  Some tissues can withstand total interruption of blood supply for some hours.  Others, like the brain and kidneys, cannot.  Cooling, by reducing metabolic activity, increases the tolerance time.

Blood is pumped by the heart into the aorta and pulmonary artery, that from the former being distributed to the body generally and that from the latter only to the lungs.  The major arteries divide into branches which in turn repeatedly subdivide into vessels of (finally) microscopic size.  The terminal arterial vessel is called an arteriole and it feeds one or more capillaries.  The distal ends of capillaries drain into venules many of which join together to form small veins which are tributaries of larger ones.  Blood from the general (systemic) circulation is finally collected in the superior and inferior venae cavae, both of which drain into the right atrium (auricle) of the heart; four pulmonary veins (the veins from the lungs) empty into the left atrium.

The real business of the blood circulation is carried on at capillary level.  Capillaries are microscopic blood vessels with exceedingly thin walls which permit the free passage of dissolved substances such as oxygen, carbon dioxide, salts and water but are, in normal circumstances, largely impervious to cells and proteins.  Protein rich fluids which accumulate in the intercellular spaces are removed by lymphatic vessels, which eventually empty into veins.

Two potentially deleterious consequences can arise, namely, loss of blood supply interruption of blood flow to a part, and blood loss from tearing or cutting of the walls of blood vessels.  In cases where remediable action can be undertaken, failure to recognise these developments or to treat them in good time will have serious consequences, unnecessarily endangering life and limb.

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