Blood Supply to Organs

Before turning to open circulations, we consider how blood moves through the capillaries inside the three-dimensional volume of an organ. We often express blood flows to organs as volumes of blood flowing per minute into or out of an organ as a percentage of the total cardiac output. We can also calculate blood flow per unit mass of tissue. Both these values indicate where flow goes as a percentage of the cardiac output, but these simple numbers do not reveal the pattern of how blood traverses capillaries. As a rule, simplified global descriptions are inadequate to explain happenings at microscopic levels of resolution. In the heart, blood flow per unit mass of tissue is not uniform in space. Nor is blood flow constant in time over periods longer than a few beats. This finding is important in medicine where radiologists may perceive defects in deposition of thallium in x-ray films of coronary flows and attribute these to pathology. We may do surgery to repair a normal heart. What is abnormal and what is normal? Often we cannot tell.

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