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The Composition of Blood
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Blood circulates within the cardiovascular system to provide a vehicle for the distribution of respiratory gases, nutrients, water, electrolytes, hormones, antibodies, drugs, metabolic waste, and heat throughout the body. Most of the major negative feedback control systems of the body defend normal blood composition (e.g., blood glucose concentration, blood volume, and arterial O2 partial pressure). Blood is composed of cellular elements (i.e., red blood cells, white blood cells, and platelets), which are suspended in blood plasma. A person who weighs 70 kg has approximately 5 L of blood, of which about 2 L is occupied by the formed cellular elements and 3 L is plasma.
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This chapter will discuss mainly the physiologic regulation of the red blood cell mass and the mechanisms that prevent bleeding.
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Plasma is the part of the extracellular fluid that is contained within the cardiovascular system. It is a dilute solution, which by weight is approximately 92% water, 7% protein, and 1% small dissolved solutes (e.g., ions, urea, glucose, amino acids, and lipids). The normal plasma concentrations of selected ions and small molecules are almost the same as those in the interstitial fluid because of the free exchange of water and small solutes across most blood capillaries (Table 3-1). In contrast, most capillaries are impermeable to plasma proteins. The resulting difference in protein concentration between the plasma and the interstitial fluids creates a colloid osmotic (oncotic) pressure gradient that opposes the filtration of plasma out of the capillaries (Chapter 4). Albumin is the most abundant type of plasma protein and, therefore, is the greatest contributor to the plasma oncotic pressure. The major types of plasma protein and their functions are listed in Table 3-2.
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