Normal cardiac output, oxygen delivery and oxygen extraction.

The total amount of blood flow circulating through the heart, lungs and all the tissues of the body represents the cardiac output. Most individual tissues determine their own flow in proportion to their metabolic rate. The skin is a notable exception where the priority is thermal rather than metabolic. Renal blood flow and metabolic rate are related but plasma flow determines metabolic rate rather than metabolic rate determining blood flow. Brain, heart, skeletal muscle and the splanchnic area all vary their blood flows according to local tissue metabolic rate. Summation of peripheral blood flows constitutes venous return and hence cardiac output. Cardiac output is therefore, largely, determined by the metabolic rate of the peripheral tissues; the heart 'from a flow standpoint, plays a "permissive" role and does not regulate its own output'. This peripheral tissue, largely metabolic, determination of cardiac output has been known for many years. Evidence will be presented that blood flow is scaled according to a tissue specific ratio of oxygen delivery (DO2) to oxygen consumption (VO2). For the brain DO2 is approximately three times VO2, for heart muscle DO2 is 1.5 to 1.6 times VO2 and is very similar for skeletal muscle for moderate exercise. Brain, heart and skeletal muscle have the ability to sustain appropriate blood flow in the face of varying blood pressure within limits--the phenomenon known as 'autoregulation'. "Autoregulation, in regard to arterial blood pressure, has been observed" also "in the kidney" and "modest autoregulation" was observed "in the intestines and liver but not in skin". Guyton et al. have suggested that the term 'auto-regulation' should also include variation of blood flow in proportion to metabolic rate and the compensatory changes in blood flow which occur in the face of varying arterial oxygen content (CaO2). This article gives examples of the very precise compensation for CaO2 change in the form of sustained tissue specific DO2:VO2 ratios (corresponding with tissue specific oxygen extraction, E = VO2/DO2). The adequacy of this adjustment for brain, exercising skeletal muscle and heart is particularly striking; skeletal muscle will, for example when CaO2 is reduced, steal blood supply from nonexercising tissues sustaining its own oxygen delivery at normal levels.