12 3 OsmoRegulation 11 17 I have updated the reading to clarify a few confusing points and align it better with the activity These changes are noted in blue Between mammalian taxa the ability of the kidney to concentrate urine is affected by the loop of Henle As filtrate travels up the ascending limb Na is passively and actively reabsorbed causing the filtrate to become more dilute than the interstitium the single effect This is multiplied by countercurrent exchange between the descending limb and the ascending limb One might expect that a longer loop of Henle results in more concentrated urine as the countercurrent multiplier will span a larger distance and have a greater effect If larger kidneys have longer loops it is reasonable to expect that these animals would produce more concentrated urine which is generally true However there are exceptions For example some desert rodents are highly adapted to retain water and produce extremely concentrated urine They have much smaller kidneys and shorter loops of Henle than large mammals but they produce much more concentrated urine Read the following brief paper by Greenwald and Stetson 1988 https utexas instructure com courses 1099761 files 35307177 download verifier gQQnPVhTzDIDYXuNKN83anG9HHOHNc0BRQnTm5pe wrap 1 https utexas instructure com courses 1099761 files 35307177 download verifier gQQnPVhTzDIDYXuNKN83anG9HHOHNc0BRQnTm5pe wrap 1 to learn more about hypothetical and observed differences that may account for this apparent contradiction From lecture Within a single individual kidney function also varies The filtration secretion and reabsorption functions of the kidney are dynamically regulated by intrinsic factors from within the kidney like blood pressure in the afferent arteriole and extrinsic factors from outside the kidney like hormones in the bloodstream Intrinsic regulation allows the kidney to maintain GFR within a fairly wide range of cardiovascular conditions such as changes in blood pressure or volume Outside of this tolerable range extrinsic regulation allows the kidney to alter secretion and reabsorption to bring cardiovascular conditions back to normal This allows the animal to osmoregulate and ionoregulate in response to changes in environment diet and other factors that affect endocrine function Recall that filtration occurs when water and small solutes are pushed between the endothelial cells of the glomerular capillaries and the foot processes of the podocytes excluding anything larger than 14 nm in diameter These are squeezed out of the capillaries into Bowman s capsule due to the blood pressure in the glomerular capillaries Factors that regulate filtration do so by changing the glomerular filtration rate GFR or the flow rate of filtered blood through the glomerulus GFR is typically expressed in units of mL blood min m2 For example in humans normal GFR ranges from 90 to 120 mL min 1 73 m2 GFR is a combination of three forces 1 Blood hydrostatic pressure in the glomerular capillaries 2 Hydrostatic pressure in Bowman s capsule from the interstitial fluid that fills the capsule 3 Osmotic pressure due to large protein in the blood or oncotic pressure The table below illustrates the relative direction of each pressure the effect of each and examples of magnitude of each Recall that GFR can be regulated intrinsically by the mesangial cells in the glomerulus see 12 2 These are contractile cells that wrap around the glomerular capillaries to regulate GFR When these cells contract the basement membrane that underlies them also contracts reducing the surface area for filtration of water and small solutes out of the glomerular capillaries thereby reducing GFR Another intrinsic regulatory mechanism is changing the diameter of the afferent arteriole myogenic regulation When the smooth muscle of the afferent arteriole constricts its resistance to blood flow increases As a result less blood will flow into the glomerular capillaries which will decrease glomerular capillary pressure This decreases GFR per unit time less blood will be filtered by the glomerulus due to afferent arteriole constriction Conversely afferent arteriole dilation will increase GFR due to lower resistance and increased blood flow in the glomerular capillaries Constriction and dilation of the efferent arteriole will have the opposite effect If resistance to blood flow out of the glomerulus increases due to efferent constriction GFR will increase If resistance to blood flow out of the glomerulus decreases due to efferent dilation GFR will decrease Afferent arteriole diameter is regulated by a negative feedback loop that involves the juxtaglomerular apparatus of the distal tubule As you can see in the figure below the nephron is oriented such that the ascending limb approaches the glomerulus positioning the distal tubule in the fork between the afferent and efferent arterioles The distal tubule has a densely packed set of cells in this fork called the macula densa that respond to increases or decreases in GFR by constricting or dilating the afferent arteriole respectively in part through the nearby juxtaglomerular cells Together the macula densa and juxtaglomerular cells comprise the majority of the juxtaglomerular apparatus When GFR is too high more solutes are filtered into the nephron The juxtaglomerular apparatus responds to this by decreasing GFR Recall that the distal tubule reabsorbs NaCl The macula densa cells have apical NKCC channels that import Na and basolateral Na K ATPases that actively export Na which results in Na reabsorption from the filtrate to the interstitium When NaCl in the filtrate increases more Na enters the macula densa cells than can be pumped out and the cell swells This activates stretch sensitive membrane channels that let ATP passively escape the cell This is converted into adenosine which acts as a paracrine short distance vasoconstrictor that constricts the nearby afferent arteriole thereby decreasing GFR back to normal The macula densa cells also control GFR by releasing hormone like lipids called prostaglandins When GFR is too low the macula densa cells release prostaglandins These act as paracrine messengers on the nearby juxtaglomerular cells which line the afferent arteriole When the juxtaglomerular cells are stimulated by prostaglandins they release a vasoconstrictive enzyme called renin that acts on the efferent arteriole By constricting the efferent arteriole this system brings GFR back up to normal This balance of
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