Capillaries
Capillaries
The Italian Marcello Malpighi was the first to describe capillaries in 1661, thus confirming the existence of the minute links between the arterial and venous systems that Harvey knew must exist but could not see. Malpighi studied capillaries of a living frog’s lung, which is still one of the simplest and most vivid preparations for demonstrating capillary blood flow.
Capillaries are present in enormous numbers, forming extensive networks in nearly all tissues (Figure 33-16). In muscle there are more than 2000 per square millimeter (1,250,000 per square inch), but not all are open at the same time. Indeed, perhaps less than 1% are open in resting skeletal muscle. But when muscle is active, all capillaries may open to bring oxygen and nutrients to the working muscle fibers and to carry away metabolic wastes.
Capillaries are extremely narrow, averaging in mammals about 8 m in diameter, which is only slightly wider than the red blood cells that must pass through them. Their walls are formed by a single layer of thin endothelial cells, held together by a delicate basement membrane and connective tissue fibers.
Capillary Exchange
Capillaries are quite permeable to small ions, nutrients, and water Blood pressure within a capillary tends to force fluids out through the capillary walls and into the surrounding interstitial space. Because larger molecules such as plasma proteins cannot pass through the capillary wall, an almost protein-free filtrate is forced out. This fluid movement is important in irrigating the interstitial space, in providing tissue cells with oxygen, glucose, amino acids, and other nutrients, and in carrying away metabolic wastes. For capillary exchange to be effective, fluids that leave the capillaries must at some point reenter the circulation. If they did not, fluid would quickly accumulate in tissue spaces, causing edema. The delicate balance of fluid exchange across the capillary wall can be accounted for by the two opposing forces of hydrostatic (blood) pressure and osmotic pressure (Figure 33-17).
In a capillary, the blood pressure that pushes water molecules and solutes across the capillary wall is greatest at the arteriolar end of the capillary and declines along its length as blood pressure falls (Figure 33-17). Opposing the blood hydrostatic pressure is an osmotic pressure created by the proteins that cannot pass across the capillary wall. This colloid osmotic pressure, which is about 25 mm Hg in mammalian plasma, tends to draw water back into the capillary from the tissue fluid. The result of these two opposing forces is that water and solutes tend to be filtered out of the arteriolar end of the capillary where hydrostatic pressure exceeds osmotic pressure, and to be drawn in again at the venous end where osmotic pressure exceeds hydrostatic pressure.
The actual situation is a bit more complicated because there is a small hydrostatic pressure in the interstitial fluid, and a small amount of protein does leak through the capillary wall. The protein tends to accumulate at the venule end of the capillary, building up a small osmotic pressure there. Although actual calculation of the pressure differences must take into account interstitial fluid hydrostatic and osmotic pressures, the principle of capillary fluid shift is as we have presented it.
The amount of fluid filtered across the capillary wall fluctuates greatly among different capillaries. Usually outflow exceeds inflow, and the excess fluid, called lymph, remains in the interstitial spaces between tissue cells. This excess is picked up and removed by lymph capillaries of the lymphatic system and eventually returned to the circulatory system via larger lymph vessels (see the following text).
The Italian Marcello Malpighi was the first to describe capillaries in 1661, thus confirming the existence of the minute links between the arterial and venous systems that Harvey knew must exist but could not see. Malpighi studied capillaries of a living frog’s lung, which is still one of the simplest and most vivid preparations for demonstrating capillary blood flow.
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| Figure 33-16 Capillary bed. Precapillary sphincters (muscles encircling an opening) control blood flow through capillaries. |
Capillaries are present in enormous numbers, forming extensive networks in nearly all tissues (Figure 33-16). In muscle there are more than 2000 per square millimeter (1,250,000 per square inch), but not all are open at the same time. Indeed, perhaps less than 1% are open in resting skeletal muscle. But when muscle is active, all capillaries may open to bring oxygen and nutrients to the working muscle fibers and to carry away metabolic wastes.
Capillaries are extremely narrow, averaging in mammals about 8 m in diameter, which is only slightly wider than the red blood cells that must pass through them. Their walls are formed by a single layer of thin endothelial cells, held together by a delicate basement membrane and connective tissue fibers.
Capillary Exchange
Capillaries are quite permeable to small ions, nutrients, and water Blood pressure within a capillary tends to force fluids out through the capillary walls and into the surrounding interstitial space. Because larger molecules such as plasma proteins cannot pass through the capillary wall, an almost protein-free filtrate is forced out. This fluid movement is important in irrigating the interstitial space, in providing tissue cells with oxygen, glucose, amino acids, and other nutrients, and in carrying away metabolic wastes. For capillary exchange to be effective, fluids that leave the capillaries must at some point reenter the circulation. If they did not, fluid would quickly accumulate in tissue spaces, causing edema. The delicate balance of fluid exchange across the capillary wall can be accounted for by the two opposing forces of hydrostatic (blood) pressure and osmotic pressure (Figure 33-17).
 |
| Figure 33-17 Fluid movement across the wall of a capillary. At the arterial end of the capillary, hydrostatic (blood) pressure exceeds colloid osmotic pressure contributed by plasma proteins, and a plasma filtrate is forced outside the capillary. At the venous end, colloid osmotic pressure exceeds the hydrostatic pressure, and fluid is drawn inside the capillary. In this way plasma nutrients are carried into the interstitial space where they can enter cells, and metabolic end products from the cells are drawn into the plasma and carried away. |
In a capillary, the blood pressure that pushes water molecules and solutes across the capillary wall is greatest at the arteriolar end of the capillary and declines along its length as blood pressure falls (Figure 33-17). Opposing the blood hydrostatic pressure is an osmotic pressure created by the proteins that cannot pass across the capillary wall. This colloid osmotic pressure, which is about 25 mm Hg in mammalian plasma, tends to draw water back into the capillary from the tissue fluid. The result of these two opposing forces is that water and solutes tend to be filtered out of the arteriolar end of the capillary where hydrostatic pressure exceeds osmotic pressure, and to be drawn in again at the venous end where osmotic pressure exceeds hydrostatic pressure.
The actual situation is a bit more complicated because there is a small hydrostatic pressure in the interstitial fluid, and a small amount of protein does leak through the capillary wall. The protein tends to accumulate at the venule end of the capillary, building up a small osmotic pressure there. Although actual calculation of the pressure differences must take into account interstitial fluid hydrostatic and osmotic pressures, the principle of capillary fluid shift is as we have presented it.
The amount of fluid filtered across the capillary wall fluctuates greatly among different capillaries. Usually outflow exceeds inflow, and the excess fluid, called lymph, remains in the interstitial spaces between tissue cells. This excess is picked up and removed by lymph capillaries of the lymphatic system and eventually returned to the circulatory system via larger lymph vessels (see the following text).
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