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Capillary Pressures and Capillary Exchange

Pearson
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In capillaries, hydrostatic pressure is exerted by blood. Thus, capillary hydrostatic pressure is equivalent to the blood pressure in the capillaries. Capillary hydrostatic pressure is also called filtration pressure because it forces fluid out of the capillaries. Because of friction encountered in the capillaries, the capillary hydrostatic pressure is lower at the venule end of the capillary bed then the arteriole end. In theory, the hydrostatic pressure of the interstitial fluid in the tissue spaces opposes the hydrostatic pressure in the capillaries. Normally, however there is very little fluid in the tissue spaces because it is quickly picked up by the lymphatic capillaries, so the hydrostatic pressure of the interstitial fluid is very low. Net hydrostatic pressure equals the hydrostatic pressure in the capillary minus the hydrostatic pressure in the interstitial fluid. The net hydrostatic pressure forces fluid out of the capillary. Let’s determine the net hydrostatic pressure at the arteriole end of the capillary bed. Taking the capillary hydrostatic pressure of 35 mm of mercury minus the interstitial fluid hydrostatic pressure of 1 mm of mercury gives us 34 mm of mercury for the net hydrostatic pressure at the arteriole end of the capillary bed. Now, let’s determine the net hydrostatic pressure at the venule end of the capillary bed. Here, 15 mm of mercury minus 1 mm of mercury gives us 14 mm of mercury for the net hydrostatic pressure at the venule end of the capillary bed. Now let's look at osmotic pressure. Osmotic pressure is the pull on water exerted by large nondiffusible solutes like proteins. The higher the solute concentration, the more the solution pulls, or holds, water. For each beaker shown, note the relative concentrations of solutes in the sac and in the bathing solutions. The movement of a solvent such as water through a membrane from the dilute solution to a more concentrated solution is called osmosis. Let’s observe the net movement of water in each beaker. This beaker contains a sac with 100 solute molecules and bathing solution with 80 solute molecules. This beaker contains a sac with 1000 solute molecules and bathing solution with 80 solute molecules. This beaker contains a sac with 100 solute molecules and bathing solution with 200 solute molecules. Because of its high content of plasma proteins, capillary blood has a relatively high osmotic pressure, which tends to draw fluid into the capillary like the first sac on the previous screen.
In capillaries, hydrostatic pressure is exerted by blood. Thus, capillary hydrostatic pressure is equivalent to the blood pressure in the capillaries. Capillary hydrostatic pressure is also called filtration pressure because it forces fluid out of the capillaries. Because of friction encountered in the capillaries, the capillary hydrostatic pressure is lower at the venule end of the capillary bed then the arteriole end. In theory, the hydrostatic pressure of the interstitial fluid in the tissue spaces opposes the hydrostatic pressure in the capillaries. Normally, however there is very little fluid in the tissue spaces because it is quickly picked up by the lymphatic capillaries, so the hydrostatic pressure of the interstitial fluid is very low. Net hydrostatic pressure equals the hydrostatic pressure in the capillary minus the hydrostatic pressure in the interstitial fluid. The net hydrostatic pressure forces fluid out of the capillary. Let’s determine the net hydrostatic pressure at the arteriole end of the capillary bed. Taking the capillary hydrostatic pressure of 35 mm of mercury minus the interstitial fluid hydrostatic pressure of 1 mm of mercury gives us 34 mm of mercury for the net hydrostatic pressure at the arteriole end of the capillary bed. Now, let’s determine the net hydrostatic pressure at the venule end of the capillary bed. Here, 15 mm of mercury minus 1 mm of mercury gives us 14 mm of mercury for the net hydrostatic pressure at the venule end of the capillary bed. Now let's look at osmotic pressure. Osmotic pressure is the pull on water exerted by large nondiffusible solutes like proteins. The higher the solute concentration, the more the solution pulls, or holds, water. For each beaker shown, note the relative concentrations of solutes in the sac and in the bathing solutions. The movement of a solvent such as water through a membrane from the dilute solution to a more concentrated solution is called osmosis. Let’s observe the net movement of water in each beaker. This beaker contains a sac with 100 solute molecules and bathing solution with 80 solute molecules. This beaker contains a sac with 1000 solute molecules and bathing solution with 80 solute molecules. This beaker contains a sac with 100 solute molecules and bathing solution with 200 solute molecules. Because of its high content of plasma proteins, capillary blood has a relatively high osmotic pressure, which tends to draw fluid into the capillary like the first sac on the previous screen.