key concept 49.4 Circulatory System Functions Depend on Blood and Blood Vessels

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Blood is a connective tissue, and like all connective tissues, it consists of cells suspended in an extracellular matrix. There are a variety of blood cell types with different functions, and the extracellular matrix has a complex composition that is highly regulated. The unusual feature of blood is that the extracellular matrix is a liquid, so blood is a fluid tissue that circulates in a system of closed vessels through the bodies of vertebrates and invertebrates that have closed circulatory systems. The properties of the different classes of vessels reflect their functions.

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  • Red blood cells are generated in bone marrow and, until they are destroyed in the spleen, they transport respiratory gases.

  • Platelets and blood proteins are responsible for blood clotting.

  • Smooth muscle cells in arteries and arterioles control blood flow to specific tissues and also control blood pressure.

  • Extracellular fluid exchange between the blood and the interstitial space occurs in capillary beds and depends on hydrostatic pressure pushing water out of the capillaries and osmotic pressure pulling water back into the capillaries.

  • One-way valves and muscle contraction help return venous blood to the heart.

The cells of the blood can be separated from the fluid matrix, called blood plasma, by centrifugation (Figure 49.11). If a sample of blood is spun in a centrifuge, all the cells move to the bottom of the tube, leaving the clear, straw-colored blood plasma on top. The packed-cell volume, or hematocrit, is the percentage of the blood volume made up by red blood cells (RBCs). Normal hematocrit is about 42 percent for women and 46 percent for men, but these values can vary considerably. They are usually higher, for example, in people who live and work at high elevations, because their exposure to low oxygen stimulates the production of more RBCs. Between the blood plasma and the packed RBCs there is a very thin layer—less than 1 percent of the blood volume—called the buffy coat that contains most of the white blood cells and platelets. Platelets are pinched-off fragments of a class of bone marrow cells. Here we consider the RBCs and the platelets. White blood cells, or leukocytes, are cells of the immune system, discussed in Chapter 41.

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Figure 49.11 The Composition of Blood Blood consists of a complex aqueous solution (the blood plasma) and numerous cell types and cell fragments. The hematocrit (arrow) is a measure of the volume of RBCs as a percentage of total blood volume. At the surface of the packed cells is the thin buffy coat that contains leukocytes and platelets.