[Microcirculatory changes in infancy and old age]

An account of the microcirculatory unit according to Zweifach is followed by an explanation of the marked differences in capillary and tissue perfusion in children and aged. Prolonged hypertonia of the metarteriolar smooth muscle cells of the precapillary sphincters depresses capillary perfusion, even though the preferential circulation remains, resulting in marked changes in cellular metabolism in the tissues. If the capillary circulation is excluded for a sufficiently long period, less oxygen and substrates will be transported to the tissues concerned, intravasal red cell clumping will exacerbate hypoxia, and drainage of metabolic residues will be reduced. Microcirculatory vasoconstriction may be the result of heat, touch, noise, hormonal factors, medicaments, toxic, nervous, psychic factors, etc. Whit time, continuous damage to the microcirculation leads to involution of the microcirculation in favour of the preferential circulation, creation of an increasingly large number of arteriovenous short-circuits, and hyperstomia of arteriovenous anastomoses. Depressed capillary circulation leads to a reduced flow of liquids through the vessel walls owing to a diminished diffusion surface with consequent progressive lessening of tissue metabolic activity. Involution of the microcirculation is thus one of the main steps in the ageing process. A cell's metabolic pattern is dependent on the environment in which it lives, as well as its stock of enzymes and differentiation stage. This environment is greatly changed by alterations of the microcirculation. The gradual depression of the microcirculation during ageing, coupled with the consequent diminution of perfusion of tissues and interstitial liquids are thus of significance in ageing itself, in the slowing down of tissue change, and increased entropy to the energy levelling coincident with death.