Humoral Regulation of Circulation:
The term “humoral regulation” means regulation by substances in the body fluids such as hormones, ions, etc. Among the most important of these factors are the following.
(1) Norepinephrine and Epinephrine- The adrenal medulla secrete both epinephrine and norepinephrine. Norepinephrine has vasoconstrictor effects in almost all vascular beds of the body and epinephrine has similar effects in some, but not all beds. For instance, epinephrine causes vasodilation in both skeletal and cardiac muscle.
(2) Angiotensin- Angiotensin is the most powerful vasoconstrictor substance known. As little as one ten-millionth of a gram can increase the arterial pressure of a human being as much as 10-20 mmHg under some conditions. A decrease in arterial pressure or a decrease in the quantity of sodium in the body fluids will cause the kidneys to secrete the substance renin. The renin in turn acts on one of the plasma proteins, the renin substrate, to split away the vasoactive peptide angiotensisn. The angiotensin in turn causes marked constriction of the peripheral arterioles. It also causes constriction of the renal tubules, thereby causing the kidneys to retain both water and salt, thus increasing the body fluid volume, which helps to raise the arterial pressure. Hence, an initial decrease in arterial pressure or decrease in sodium causes a compensatory build-up of body fluid and sodium as well as an increase in arterial pressure, thereby compensating for the original deficit.
(3) Histamine- Histamine is released by essentially every tissue of the body that becomes damaged. Most of the histamine is probably derived from eosinophils and mast cells in the damaged tissues.
Histamine has a powerful vasodilator effect on the arterioles and also has a very potent effect on increasing capillary porosity, allowing leakage of both fluid and plasma proteins into the tissues. Though the role of histamine in the normal regulation of circulation is unknown, in many physiological conditions the intense arteriolar dilation and increased capillary porosity caused by histamine cause tremendous quantities of fluid to leak out of the circulation into the tissues to cause edema.
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