Precision of Blood Volume and Extracellular Fluid Volume Regulation

By studying Figure 29-12, one can see why the blood volume remains almost exactly constant despite extreme changes in daily fluid intake. The reason for this is the following: (1) a slight change in blood volume causes a marked change in cardiac output, (2) a slight change in cardiac output causes a large change in blood pressure, and (3) a slight change in blood pressure causes a large change in urine output. These factors work together to provide effective feedback control of blood volume.

The same control mechanisms operate whenever there is a loss of whole blood because of hemorrhage. In this case, fluid is retained by the kidneys, and other parallel processes occur to reconstitute the red blood cells and plasma proteins in the blood. If abnormalities of red blood cell volume remain, such as occurs when there is deficiency of erythropoietin or other factors needed to stimulate red blood cell production, the plasma volume will simply make up the difference, and the overall blood volume will return essentially to normal despite the low red blood cell mass.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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