Osmolarity hyperosmotic

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. 

The chyme that passes into the intestine from the stomach typically is not hyperosmotic, but as its macromolecular components are digested, the osmolarity of that solution increases (e.g. glucose is osmotically more active than sucrose). 

Use of hyperosmotics in the United States is limited to intravenous preparations. Hyperosmotics cause increased blood serum osmolarity which pulls water from tissues into the bloodstream. By increasing the osmotic gradient between plasma and the eye, vitreal dehydration occurs, which results in reduced ocular volume and corresponding lowered lOP. The results are relatively rapid (15 minutes to 2 hours) and short in duration (6 to 8 hours). Hyperosmotics are indicated when there is a need for rapid temporary reduction in high lOP. 

Figure la. The effect of hyperosmotic media on cell volume. Cells suspended in 0.3 osmolar sodium chloride (upper curve), 0.6 osmolar sodium chloride and nonelectrolyte (middle curve), 0.6 osmolar sodium chloride (lower curve). 

To understand the mechanism of concentration of urine in the kidney under the influence of antidiuretic hormones, it is necessary to briefly review some elementary concepts in kidney physiology. Although the osmolarity of the glomerular filtrate is equal to that of plasma, the osmotic pressure of the interstitial tissue in the renal medulla and papilla is greater than that of plasma. Vasopressin changes the permeability of the limiting membrane and facilitates the reabsorption of water from the tubular lumen (isosmotic) toward the interstitial tissue of the hyperosmotic medulla and papilla. 

If two solutions contain the same number of particles they may be said to be iso-osmotic, or simply isosmotic, with respect to each other. If one solution exhibits a greater osmolarity than another solution it is hyperosmotic with respect to the less concentrated solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic or hyposmotic, with respect to the more concentrated solution. Iso-, hyper- and hypo-osmolarity should always be stated with respect to another solution. 

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