1. Ion and Water Transport in the nephron.

• The proximal tubule returns much of the water and dissolved chemicals to the bloodstream. The proximal tubule is very permeable to water and moderately permeable to most dissolved substances including urea. Sodium and glucose are cotransported across the cells of the proximal tubule into the blood, thus returning large amounts of glucose to the bloodstream. If such a mechanism weren't present, then there would be no way to prevent loss of the small glucose molecule into the urine. (Eckert, Fig. 14-28)

• The descending part of the Loop of Henle is very permeable to water and thus water diffuses into the interstitial fluid, concentrating the still dilute filtrate, as the filtrate passes down the tubule. Most of the water moves into the interstitial fluid at the start because the filtrate is most dilute there and diffusion depends upon the difference in water concentration between the filtrate (high water conc.) and the interstitial fluid (lower water concentration). But as the urine moves deeper in the descending tubule, water movement from the tubule is less, but not zero. The filtrate becomes more concentrated, but the interstitial fluid is also more concentrated than at the top of the tubule. Water still diffuses from the filtrate to the interstitial fluid.

• The ascending portion of the Loop of Henle is quite impermeable to water, becoming particularly so in the upper portion with a thickened wall. Sodium and chloride passively move out of the filtrate in the thin portion of the ascending tubule and actively in the thick portion and the distal tubule and into the interstitial fluid surrounding the nephron. The water is left behind, making the filtrate more dilute again, even more dilute than when it entered the descending tubule but with MUCH LESS VOLUME.

• The gradient of osmolarity in the interstitial fluid results from the "vasa recta" capillary network. (Blood flow enters the glomerulus from the afferent arteriole and leaves via the efferent arteriole, and finally goes into the "vasa recta" network of capillaries which twist and branch around the ascending and descending tubules. The blood enters the network at the ascending tubule and leaves from the descending tubule, thus the blood moves in the OPPOSITE DIRECTION TO THE URINE.) Sodium ions pumped out of the ascending tubule enter the blood stream, thus increasing the concentration of sodium in the blood. When the blood reaches the inner medulla, sodium concentration in the blood is high, causing sodium to flow out of the blood and into the interstitial fluid. Since the inner medullary portion of the Loop of Henle is impermeable to sodium, sodium stays in the interstitial fluid and creates the gradient of high osmolarity in the medullary portion of the interstitial fluid and lower osmolarity in the outer medullary and cortex regions of interstitial fluid.

• Now we have set up a gradient of interstitial fluid osmolarity as a result of the asymmetry in water permeability and sodium chloride movement between descending and ascending tubules. Now, the collecting duct goes "down" again, water progressively diffuses out of the collecting duct into the more "concentrated" interstitial fluid and the result is concentrated urine with low volume. (Eckert, Fig. 14-34)

3. Renin-Angiotensin-Aldosterone System.

• This system is helps maintain arterial blood pressure when blood volume decreases, for example if there is blood loss.

• The afferent arteriole acts as a pressure receptor, causing secretion of renin when blood flow is low. Furthermore, low salt concentrations detected by the macula densa also cause renin secretion. This leads to a complicated chain of events, including Angiotensin II causing arteriolar vasoconstriction and increased blood pressure. The adrenal gland releases Aldosterone, causing reduction in excretion of sodium chloride and thus a decrease in water excretion. (You should know what the following do: Macula densa, Renin, Angiotensin I and II, ACE, Aldosterone, ADH.)

3. Diuretics cause increased in the output of urine, sometimes by preventing sodium transport from the tubules. (You should know how the four major types of diuretics act and examples of molecules that represent these different types of diuretics.)

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.