The oxygen-carrying capacity of whole blood is determined by its concentration of hemoglobin. If the hemoglobin concentration is below normal—in a condition called anemia—the oxygen content of the blood will be abnormally low. conversely, when the hemoglobin concentration rises above the normal range—as occurs in polycythemia (high red blood cell count)—the oxygen-carrying capacity of blood is increased accordingly. This can occur as an adaptation to life at a high altitude.
The production of hemoglobin and red blood cells in bone marrow is controlled by a hormone called erythropoietin, produced by the kidneys. The secretion of erythropoietin—and thus the production of red blood cells—is stimulated when the amount of oxygen delivered to the kidneys is lower than normal. Red blood cell production is also promoted by androgens, which explains why the hemoglobin concentration in men is from 1 to 2 g per 100 ml higher than in women.
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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.