Circulation in a particular region or organ is directly proportional to the degree of local activity.
It is adjusted on two lines:
(1) By regulating the general circulation, and
(2) By adjusting the local blood vessels.
Generally, vasodilatation of an active part is accompanied with vasoconstriction of other inactive parts. In this way, blood is shifted from the inactive to the active regions without any fall of general blood pressure.
Methods of Study:
A Plethysmograph or an Oncometer is an airtight or water-tight vessel, connected with a suitable recording instrument. An organ (heart, spleen, liver, kidney, etc., in animals) or a limb (in human beings) is introduced into the vessel. The volume changes of the part cause corresponding pressure changes in the instrument which are recorded.
Increased volume is caused by increased blood flow due to vasodilatation. Decreased volume is due to vasoconstriction. The size and shape of the oncometer differ according to the part or the organ whose volume changes are to be measured. Cardiometer is a special type of oncometer.
ii. Venous Occlusion Plethysmography (Fig. 7.95):
The principle is same as above, with the difference that, the venous outflow is stopped by occluding the veins (by raising the pressure on veins to about 50-70 mm of Hg, viz., a little below the diastolic pressure of the subject) for short periods. Rise of volume is due to accumulation of arterial blood. From these data the minute flow can be roughly estimated.
iii. By Noting Skin Temperature:
Done by a thermo-couple connected to a galvanometer. Fall of temperature shows less blood flow (vasoconstriction). Rise of temperature shows more blood flow (vasodilatation).
Through the above methods, a rough idea about the circulatory status of a particular organ can be studied.
But for quantitative evaluation of different regional flows, the following methods are generally followed:
(a) Electromagnetic flowmeter,
(c) Bubble flowmeter,
(f) Nitrous oxide method, and
(g) Isotope clearance methods.
The following regional circulations are of special physiological importance:
(a) Coronary circulation,
(b) Cerebral circulation,
(c) Pulmonary circulation,
(d) Hepatic circulation,
(e) Splenic circulation,
(f) Renal circulation.
(g) Capillary circulation,
(h) Cutaneous circulation, and
(i) Skeletal muscle circulation.