Perfusion of the lungs is one of the functions of the circulatory system. The circulatory system uses a pump (the heart) and a network of vessels to transport blood around the body. Circulatory systems are the subject of Chapter 49, so here we will discuss only one aspect of perfusion: how blood transports respiratory gases.
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The protein hemoglobin in red blood cells binds, transports, and releases O2 to the tissues of the body.
Factors influencing the affinity of hemoglobin for oxygen include its chemical composition, the PO2 in the blood plasma, the blood pH, and the presence of 2,3-
Most of the carbon dioxide produced by body cells is transported from tissues to alveoli in the form of bicarbonate ions dissolved in blood plasma.
The liquid part of blood, the plasma, carries some O2 in solution, but its ability to transport this nonpolar molecule is limited. The blood plasma of a human can contain in solution about 3 mL of O2 per liter of plasma, which is inadequate to support even basal metabolism. However, the blood of vertebrates and many invertebrates, contains molecules that bind and release O2 and thus augment its transport capacity. These molecules pick up O2 where PO2 is high and release it where PO2 is lower. There are many O2 transport molecules in the animal kingdom, but in vertebrates this role is played by hemoglobin, a protein contained in red blood cells (RBCs). The percent of the blood volume consisting of RBCs is called the hematocrit. The hematocrit of human males averages 45 percent and that of females 40 percent. Hemoglobin increases the capacity of blood to carry 60 times more oxygen than it could carry in solution, making high rates of metabolism possible.