Think About It

If the albumen concentration on side A were half what it was in the original experiment, would the fluid on that side reach a higher or lower level than before? Explain.

The equilibrium between osmosis and filtration will be an important consideration as we study fluid exchange through blood capillaries in chapter 20. Blood plasma also contains albumins. In the preceding discussion, side A is analogous to the bloodstream and side B is analogous to the tissue fluid surrounding the capillaries (although tissue fluid is not distilled water). Water

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108 Part One Organization of the Body

Osmotic pressure

Hydrostatic pressure

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Osmotic pressure

Hydrostatic pressure

Osmosis Figure Mcgraw

Figure 3.15 Osmosis. The dashed line represents a selectively permeable membrane dividing the chamber in half. The large particles on side A represent any solute, such as albumen, too large to pass through the membrane. The small particles are water molecules. (a) Water diffuses from side B, where it is relatively concentrated, to side A, where it is less concentrated. Fluid level rises in side A and falls in side B. (b) Net diffusion stops when the weight (hydrostatic pressure) of the fluid in side A balances the osmotic pressure. At this point, water passes at equal rates from A to B by filtration and from B to A by osmosis. The two processes are then in equilibrium.

Figure 3.15 Osmosis. The dashed line represents a selectively permeable membrane dividing the chamber in half. The large particles on side A represent any solute, such as albumen, too large to pass through the membrane. The small particles are water molecules. (a) Water diffuses from side B, where it is relatively concentrated, to side A, where it is less concentrated. Fluid level rises in side A and falls in side B. (b) Net diffusion stops when the weight (hydrostatic pressure) of the fluid in side A balances the osmotic pressure. At this point, water passes at equal rates from A to B by filtration and from B to A by osmosis. The two processes are then in equilibrium.

leaves the capillaries by filtration, but this is approximately balanced by water moving back into the capillaries by osmosis.

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