Circulatory and Respiratory Functions of the Blood

Abstract

The mean circulatory pressure (MCP) is a dynamic measurement which can be extrapolated from the rate of rise in venous pressure when the activity of the heart is suddenly stopped and the pressure is equilibrated from the aorta to the rightatrium. As mentioned, an accurate measurement of MCP is difficult to obtain in laboratory animals because of numerous compensatory mechanisms that are elicited at the time of induced cardiac arrest. (For review, see [ 1 ].) The question naturally arises: to what extent is such a radical intervention representative of the actual events at the level of the microcirculation, accounting for the fact that the distribution of fl ow is tightly bound to metabolic demands of the tissues and/or individual organs? The only substitute for such “flow averaging“ is to examine the actual phenomenon itself, i.e., the capillary flow at this critical juncture in the life of the experimental animal or the human subject. (As discussed in Chap. 21 , the methods for direct observation of the microvascular beds during normal conditions as well as in critical illness exist and have provided valuable new insights into microvascular dynamics.) However, such information will not able to give us an answer about the actual conditions, unless we are acquainted with the true nature of underlying phenomenon. We will now examine two separate groups of “near-death” circulatory phenomena: the first pertaining to venous and the latter to arterial circulations. Please note that in this context the term “venous circulation” encompasses the entire compartment occupied by the venous blood including the pulmonary vasculature. By analogy, the “arterial” compartment occupies the rest of the circuit containing the arterial blood.