The Filtration Membrane: Extending from each podocyte are thousands of foot-like processes termed ___ that wrap around glomerular capillaries. The spaces between the these of each podocyte are the ____. A thin membrane, the slit membrane, extends across each filtration slit; it permits the passage of molecules with a diameter smaller than 0.006 − 0.007 μm, including water, glucose, vitamins, amino acids, very small plasma proteins, ammonia, urea, and ions. Less than 1 percent of albumin, the most plentiful plasma protein, passes the slit membrane because, with a diameter of 0.007 μm, albumin is slightly too big to get through.
Renal Autoregulation of GFR: occurs when stretching triggers contraction of smooth muscle cells in the walls of afferent arterioles. As arterial blood pressure rises, the walls of the afferent arterioles are stretched. In response, smooth muscle fibers in the wall of the afferent arteriole contract, which narrows the arteriole's lumen. As a result, renal blood flow decreases, thus reducing GFR to its previous level. Conversely, when arterial blood pressure drops, the walls of the afferent arterioles are stretched less. As the smooth muscle cells in the wall relax, the afferent arterioles dilate, renal blood flow increases, and GFR increases. Renal Autoregulation of GFR:
___ is so named because part of the renal tubules—the macula densa of the juxtaglomerular apparatus—provides feedback to the glomerulus (Figure 24.10). When GFR is above normal due to elevated systemic blood pressure, filtered fluid flows more rapidly along the renal tubules. As a result, the proximal convoluted tubule and nephron loop have less time to reabsorb Na+, Cl−, and water. Macula densa cells are thought to detect the increased delivery of Na+, Cl−, and water in the filtered fluid and respond by inhibiting nitric oxide (NO) release from cells in the juxtaglomerular apparatus. When NO is released, it stimulates afferent arterioles to dilate; when NO release is inhibited, afferent arterioles constrict, resulting in decreased blood flow into the glomerular capillaries, and decreased GFR. On the other hand, as blood pressure falls (resulting in decreased GFR), filtered fluid flows more slowly along the renal tubules (allowing time for reabsorption), and the release of NO from the juxtaglomerular apparatus is no longer inhibited by the macula densa cells. As the level of NO increases, afferent arterioles dilate, increasing blood flow into the glomerulus, and increasing GFR. Macula densa cells also inhibit release of renin from juxtaglomerular cells when increased Na+ and Cl− is detected in filtered fluid. As you will see in Concept 24.7, this links renal autoregulation of GFR with hormonal regulation of tubular reabsorption and secretion.
Collecting ducts deep in the renal medulla are permeable to urea, allowing it to diffuse from the tubular fluid into the interstitial fluid of the medulla. As urea accumulates in the interstitial fluid, some of it diffuses into the tubular fluid in the descending and thin ascending limbs of the nephron loops, which also are permeable to urea (Figure 24.19a). However, while tubular fluid flows through the thick ascending limb, distal convoluted tubule, and cortical portion of collecting duct, urea remains in the lumen because cells in these segments are impermeable to urea. As water reabsorption continues via osmosis in the presence of ADH, the concentration of urea in the tubular fluid further increases. More urea diffuses into the interstitial fluid of the renal medulla, and the cycle repeats. The constant transfer of urea between the renal tubule and interstitial fluid of the medulla is termed urea recycling. In this way, reabsorption of water from the tubular fluid of the collecting ducts promotes the buildup of urea in the interstitial fluid of the renal medulla, which in turn promotes water reabsorption. The solutes left behind in the lumen thus become very concentrated, and a small volume of concentrated urine is excreted. Countercurrent flow also allows solutes and water to passively exchange between the blood of the vasa recta and interstitial fluid of the renal medulla. Note in Figure 24.19b that just as tubular fluid flows in opposite directions in the nephron loop, blood flows in opposite directions in parallel ascending and descending limbs of the vasa recta. Blood entering the vasa recta is fairly dilute. As it flows down the descending limb into the renal medulla, where the interstitial fluid becomes increasingly concentrated, Na+, Cl−, and urea diffuse into the blood from the interstitial fluid and water flows out of the blood, resulting in increasingly more concentrated blood. As the concentrated blood flows up the ascending loop of the vasa recta, the interstitial fluid becomes increasingly less concentrated. As a result, Na+, Cl−, and urea diffuse from the blood back into interstitial fluid, and water diffuses from interstitial fluid back into the vasa recta. Blood leaving the vasa recta is only slightly more concentrated than when it entered the vasa recta. The nephron loop establishes the osmotic gradient in the renal medulla, but the vasa recta maintains that osmotic gradient. About two-thirds of body fluid is ___ or cytosol, the fluid within cells. The other third, called ____, is outside cells and includes all other body fluids. Extracellular fluid includes interstitial fluid between tissue cells, synovial fluid in joints, cerebrospinal fluid in the nervous system, aqueous humor and vitreous body in the eyes, endolymph and perilymph in the ears, plasma in blood, lymph in lymphatic vessels, and pleural, pericardial, and peritoneal fluids between serous membranes. An increase in blood volume, as might occur after you finish a supersized drink, stretches the atria of the heart and promotes release of atrial natriuretic peptide. Atrial natriuretic peptide increases natriuresis, the urinary loss of Na+. As Na+ is excreted, followed by Cl− and water, blood volume decreases. An increase in blood volume also slows release of renin from kidney juxtaglomerular cells. When renin level drops, less angiotensin II is formed. Decline in angiotensin II increases glomerular filtration rate and reduces Na+, Cl−, and water reabsorption in the kidney tubules. In addition, less angiotensin II leads to less aldosterone, which decreases Na+ and Cl− reabsorption in the collecting ducts. As more Na+ and Cl− ions are excreted in the urine, more water is lost in urine, which decreases blood volume. By contrast, when someone becomes dehydrated, higher levels of angiotensin II and aldosterone promote urinary reabsorption of Na+ and Cl−, and water by osmosis with the solutes, and thereby conserve the volume of body fluids.
The major hormone that regulates water loss is antidiuretic hormone (ADH). An increase in the osmotic pressure of body fluids not only stimulates the thirst mechanism, as previously discussed, but also stimulates release of ADH (see Figure 24.17). ADH promotes water reabsorption by the collecting ducts. As a result, water is reabsorbed from the tubular fluid and a small volume of concentrated urine is produced. By contrast, intake of water decreases the osmotic pressure of body fluids. Within minutes, ADH secretion shuts down, collecting ducts become less permeable to water, and more water is lost in the urine.
Michelle Provost-Craig, Susan J. Hall, William C. Rose 8th EditionElaine N. Marieb 12th EditionElaine N. Marieb, Suzanne M. Keller 12th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis