The pumping of the heart sends blood by way of arteries to the capillaries where exchange takes place across thin capillary walls (Fig. 11.6). Blood that has passed through the capillaries returns to the heart via veins. Capillary walls are largely composed of one layer of epithelial cells connected by tight junctions. Capillaries are extremely numerous. The body most likely contains a billion capillaries, and their total surface area is estimated at 6,300 square meters. Therefore, most cells of the body are near a capillary.
In the tissues of the body, metabolically active cells require oxygen and nutrients and give off wastes, including carbon dioxide. During capillary exchange-not including the gas-exchanging surfaces of the lungs-oxygen and nutrients leave a capillary, and cellular wastes, including carbon dioxide, enter a capillary. Certainly, arterial blood contains more oxygen and nutrients than venous blood, and venous blood contains more wastes than arterial blood.
The internal environment of the body consists of blood and tissue fluid. Tissue fluid is simply the fluid that surrounds the cells of the body. In other words, substances that leave a capillary pass through tissue fluid before entering the body’s cells, and substances that leave the body’s cells pass through tissue fluid before entering a capillary.
The composition of tissue fluid stays relatively constant because of capillary exchange. Tissue fluid is mainly water. Any excess tissue fluid is collected by lymphatic capillaries, which are always found near blood capillaries.
Figure 11.6 Capillary exchange. At the arterial end of a capillary, blood pressure is higher than osmotic pressure; therefore, water tends to leave the bloodstream. In the midsection of a capillary, smallmolecules follow their concentration gradients: Oxygen and nutrients leave the capillary, while wastes, including carbon dioxide, enter the capillary. At the venous end of a capillary, osmotic pressure is higher than blood pressure; therefore, water tends to enter the bloodstream.
Water and other small molecules can cross through the cells of a capillary wall or through tiny clefts that occur between the cells. Large molecules in plasma, such as the plasma proteins, are too large to pass through capillary walls.
Three processes influence capillary exchange-blood pressure, diffusion, and osmotic pressure:
Blood pressure, which is created by the pumping of the heart, is the pressure of blood against a vessel’s (e.g., capillary) walls.
Diffusion, as you know, is simply the movement of substances from the area of higher concentration to the area of lower concentration.
Osmotic pressure is a force caused by a difference in solute concentration on either side of a membrane.
To understand osmotic pressure, consider that water will cross a membrane toward the side that has the greater concentration of solutes, and the accumulation of this water results in a pressure. The presence of the plasma proteins, and also salts to some degree, means that blood has a greater osmotic pressure than does tissue fluid. Therefore, the osmotic pressure of blood pulls water into and retains water inside a capillary.
Notice in Figure 11.6 that a capillary has an arterial end (contains arterial blood) and a venous end (contains venous blood). In between, a capillary has a midsection. We will now consider the exchange of molecules across capillary walls at each of these locations.
Arterial End of Capillary
When arterial blood enters tissue capillaries, it is bright red because the hemoglobin in red blood cells is carrying oxygen. Blood is also rich in nutrients, which are dissolved in plasma.
At the arterial end of a capillary, blood pressure, an outward force, is higher than osmotic pressure, an inward force. Pressure is measured in terms of mm Hg (mercury); in this case, blood pressure is 30 mm Hg, and osmotic pressure is 21 mm Hg. Because blood pressure is higher than osmotic pressure at the arterial end of a capillary, water and other small molecules (e.g., glucose and amino acids) exit a capillary at its arterial end.
Red blood cells and a large proportion of the plasma proteins generally remain in a capillary because they are too large to pass through its wall. The exit of water and other small molecules from a capillary creates tissue fluid. Therefore, tissue fluid consists of all the components of plasma, except that it contains fewer plasma proteins.
Midsection of Capillary
Diffusion takes place along the length of the capillary, as small molecules follow their concentration gradient by moving from the area of higher to the area of lower concentration. In the tissues, the area of higher concentration of oxygen and nutrients is always blood, because after these molecules have passed into tissue fluid, they are taken up and metabolized by cells. The cells use oxygen and glucose in the process of cellular respiration, and they use amino acids for protein synthesis.
As a result of metabolism, tissue cells give off carbon dioxide and other wastes. Because tissue fluid is always the area of greater concentration for waste materials, they diffuse into a capillary.
Venous End of Capillary
At the venous end of the capillary, blood pressure is much reduced to only about 15 mm Hg, as shown in Figure 11.6. Blood pressure is reduced at the venous end because capillaries have a greater cross-sectional area at their venous end than their arterial end. However, there is no reduction in osmotic pressure, which remains at 21 mm Hg and is now higher than blood pressure. Therefore, water tends to enter a capillary at the venous end. As water enters a capillary, it brings with it additional waste molecules. Blood that leaves the capillaries is deep maroon in color because red blood cells now contain reduced hemoglobin-hemoglobin that has given up its oxygen and taken on hydrogen ions.
In the end, about 85% of the water that left a capillary at the arterial end returns to it at the venous end. Therefore, retrieving fluid by means of osmotic pressure is not completely effective. The body has an auxiliary means of collecting tissue fluid; any excess usually enters lymphatic capillaries.
Figure 11.7 Lymphatic capillaries. Lymphatic capillaries lie near blood capillaries. The black arrows show the flow of blood. The yellow arrows show that lymph is formed when lymphatic capillaries take up excess tissue fluid.
Lymphatic vessels are a one-way system of vessels. Notice that lymphatic capillaries have blind ends that lie near blood capillaries (Fig. 11.7). Lymphatic vessels have a structure similar to that of cardiovascular veins, except that their walls are thinner and they have more valves. The valves prevent the backward flow of lymph as lymph flows toward the thoracic cavity. Lymphatic capillaries join to form larger vessels that merge into the lymphatic ducts. Lymphatic ducts empty into cardiovascular veins within the thoracic cavity.
Lymph, the fluid carried by lymphatic vessels, has the same composition as tissue fluid. Why? Because lymphatic capillaries absorb excess tissue fluid at the blood capillaries. The lymphatic system contributes to homeostasis in several ways. One way is to maintain normal blood volume and pressure by returning excess tissue fluid to the blood.
Edema is localized swelling that occurs when tissue fluid accumulates. Edema can be caused by several factors: an increase in capillary permeability; a decrease in the uptake of water at the venous end of blood capillaries due to a decrease in plasma proteins; an increase in venous pressure; or insufficient uptake of tissue fluid by the lymphatic capillaries. Another cause of edema is blocked lymphatic vessels. One dramatic cause of a blockage is the parasitic infection of lymphatic vessels by a small worm. An affected leg can become so large that the disease is called elephantiasis.