Osmolarity osmosis

Plasma is freely filtered from the glomerulus so that everything in the plasma, except for the plasma proteins, is filtered. Therefore, the initial osmolarity of the filtrate is no different from that of the plasma and is about 300 mOsm/1 (see Figure 19.5). Approximately 125 ml/min of the plasma is filtered. As the filtrate flows through the proximal tubule, 65% of the filtered Na+ ions are actively reabsorbed, and 65% of the filtered Cl ions and water are passively reabsorbed. Because the water follows the sodium by way of osmosis, no change takes place in the osmolarity of the filtrate — only a change in volume. At the end of the proximal tubule, approximately 44 ml of filtrate with an osmolarity of 300 mOsm/1 remain in the tubule. 

Osmosis, water movement across a semipermeable membrane driven by differences in osmotic pressure, is an important factor in the life of most cells. Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules. This permeability is due partly to simple diffusion of water through the lipid bilayer and partly to protein channels (aquaporins see Fig. 11-XX) in the membrane that selectively permit the passage of water. Solutions of equal osmolarity are said to be isotonic. Surrounded by an isotonic solution, a cell neither gains nor loses water (Fig. 2-13). In a hypertonic solution, one with higher... 

Weigh each component and dissolve in high quality water (cell culture grade, double glass distilled or reverse osmosis, and filtration, i.e., 18 M 2) in a 1 L volumetric flask, but withhold the BSA for addition later. Bring the volume up to 1 L. Measure the osmolarity (290 5). 

The steroid hormone aldosterone, synthesized in the zona glomerulosa of the adrenal cortex, also plays an important role in maintaining blood osmolar-ity. It binds its receptors in the cytoplasm of epithelial cells of the distal colon and the renal nephron, followed by translocation of the hormone-receptor complex to the nucleus and activation of transolption of ion transport genes to increase Na reabsorption and secretion. Water follows Na+ movement by osmosis. These transporters include the luminal amiloride-sensitive epilheUal Na+ channel, the luminal channel, the serosal Na, K+-ATPase, the Na+/H+exchanger, and the NaVCT cotransporter. 

Osmosis and osmotic pressure. Osmosis is the movement of solvent from a dilute solution to a more concentrated solution through a semipermeable membrane. The pressure that must be applied to the more concentrated solution to stop this flow is the osmotic pressure. The osmotic pressure, like the pressure exerted by a gas, may be treated quantitatively by using an equation similar in form to the ideal gas equation tt = MRT. By convention the molarity of particles that is used for osmotic pressure calculations is termed osmolarity (osmol). 

Blood plasma has an osmolarity equivalent to a 0.30 M glucose solution or a 0.15 M NaCl solution. This latter is true because in solution NaCl dissociates into Na+ and Cl and thus contributes twice the number of solute particles as a molecule that does not ionize. If red blood cells, which have an osmolarity equal to blood plasma, are placed in a 0.30 M glucose solution, no net osmosis will occur because the osmolarity and water concentration inside the red blood cell are equal to those of the 0.30 M glucose solution. The solutions inside and outside the red blood cell are said to be isotonic iso means "same," and tonic means "strength") solutions. Because the osmolarity is the same inside and outside, the red blood cell will remain the same size (Figure 18.20b). 

The process of transport of water through a semiperme-able membrane due to a concentration difference is called osmosis and the final pressure difference between both sides of the membrane is called osmotic pressure. Basically, the osmotic pressure should be expressed in Pascal, but in practice the words osmolality or osmolarity are used to indicate osmotic pressure, both with osmole as unit. 

After a prestress, to take up all excess membrane area, a small tension is applied ( 0.3 mN m ) and the vesicle is transferred into an adjacent microchamber where the solution is at 10 % higher osmolarity (hyperosmotic) than that in the vesicle interior. Water then leaves the vesicle by osmosis. The subsequent change and rate of change of vesicle volume due to water efflux, at constant vesicle membrane area, is thus measured from the increase in the projection length of the membrane in the pipet. The result is a plot of relative volume change V/V versus time (Figure 9.14) from which the permeability coefficient is derived. 

Factors Regulating Movement. The body requires water. To ensure that this requirement is fulfilled, the sensation of thirst creates a conscious desire for water. The sensation of thirst is caused by nerve centers in the hypothalamus of the brain which monitors the concentration primarily of sodium In the blood. When the sodium concentration, and hence the osmolarity of the blood, increases above the normal 310 to 340 mg/100 ml (136 to 145 mEq/liter), cells in the thirst center shrink. They shrink because the increased osmotic pressure of the blood pulls water out of their cytoplasm. This shrinking causes more nervous impulses to be generated in the thirst center, thus creating the sensation of thirst. Increased osmolarity of the blood is primarily associated with water loss from the extracellular fluid. As water is lost the sodium concentration of the remaining fluid increases. When water is drunk, it moves across the membrane lining the gut into the blood thereby decreasing the sodium concentration—osmolarity—of the blood. In turn, the cells of the hypothalamus take on water and return to their normal size. This time water moves back into these cells via osmosis in the opposite direction. 

---