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Lecture 17

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1    Lecture 17: Glomerular Filtration, Tubular Absorption, Tubular Secretion, Excretion, Micturition and Ion Balance 1. Regulation of Glomerular Filtration Rate Myogenic regulation of glomerular filtration rate (GFR) occurs via vascular smooth muscle found around the afferent glomerular arteriole. This is stretch-sensitive smooth muscle which, in response to an increase in blood pressure, will contract and reduce the diameter of the afferent arteriole (vasoconstriction). This increases the resistance to flow and therefore decreases blood flow which in turn decreases glomerular hydrostatic pressure and therefore GFR does not increase. Thus, even though mean arterial pressure increases GFR is kept constant. The second mechanism to regulate GFR (keep it constant in the face of fluctuating blood pressure) involves vasoconstrictor and vasodilator signals released from the macula densa cells of the distal tubule which are in close physical proximity to the glomerulus. Vasoconstrictor substances are released in response to an increase in filtrate flow through the distal tubule, and a 2    corresponding increase in sodium and chloride content of the fluid surrounding the cells of the macula densa. This means the distal tubule and the glomerulus act in a feedback loop, with glomerular filtration affecting the distal tubule (release from the macula densa), and the macula densa cells of the distal tubule altering flow into the glomerular capillaries. Vasoconstrictor substances are released in order to counter an increase in filtration while vasodilator substances will counter a decrease in filtration. The actual nature of these macula densa-produced vasoconstrictor and vasodilator substances is currently unknown. The third mechanism that helps toregulate GFR in the face of a change in blood pressure involves mesangial cells. These are modified smooth muscle cells located on the glomerular capillaries, that contract and spread out in response to an increase in mean arterial pressure. When they contract they cover parts of the filtration slits and decrease the surface area available for filtration thereby reducing GFR (to counter an increase in GFR caused by an increase in glomerular hydrostatic pressure when mean arterial pressure increases). 3    2. Tubular Reabsorption Tubular reabsorption is the selective movement of substances from inside the kidney tubules back into the blood. The general strategy of the kidney is to filter everything that can be filtered - that is to say, small molecules and ions (things such as proteins are too large to cross the filtration barrier) - and then reabsorb back that which isn’t going to be excreted in the urine. Most reabsorption occurs in the proximal and distal tubules - though it also occurs between the Loop of Henle and the vasa recta capillary bed that surrounds it as well. Reabsorption in the proximal tubule is unregulated, whilst in the distal tubule, reabsorption is under hormonal control. Most of the substances that are filtered and then reabsorbed are reabsorbed at very high rates. In other words, most of what is filtered is reabsorbed. Forexample,99.2% of all filtered water is reabsorbed as is 100% of glucose (at least in normal healthy conditions). Most ions are reabsorbed + at a rate of about 99%, though only about 86.1% of K ions are reabsorbed. Only 50% of the filtered urea is reabsorbed. Although urea isthe nitrogenous waste that weare excreting, half of it is reabsorbed in order to help establish the medullary osmotic gradient that facilitates the formation of concentrated urine. Reabsorption processes can be either passive or active. When active, the active process can either be on the apical membrane (toward the kidney tubule) or on the basolateral membrane (i.e., next to the peritubular fluid and the blood). 4    3. Tubular Secretion and Excretion Tubular secretion is the selective transfer of substances from the capillary blood into the tubules. This is usually an active process with the active transport occurring either across the apical or the basolateral membrane of the kidney tubular cell. 5    The amount of a substance that isexcreted is the amount filtered, plusthe amount secreted into the tubule, minus everything that is reabsorbed. For any given solute, if the amount excreted is greater than the filtered load (which is, in itself, the glomerular filtration rate multiplied by the amount of a solute in the plasma), then there has been a net secretion of that particular substance. If the amount excreted is less than the filtered load then there has been net reabsorption. We can calculate the rate of excretion of a substance by multiplying the concentration of said substance in the urine, by the urine flow rate. Excretion Rate = U x VX U = concentration of a substance in the urine X V = urine flow rate 6    4. Micturition Micturition is the voiding of urine or urination. Fluid from the kidney tubules ultimately drains into the renal pelvis and travels through the ureter into the bladder where it is stored until the bladder is emptied during urination. The bladder is lined with a muscle called the detrusor muscle. The voiding of urine through the urethra is regulated by an internal urethral sphincter and an external urethral sphincter. The micturition reflex begins as the bladder fills. This causes the detrusor muscles to stretch, activating stretch receptors. These receptors send a signal up to the brainstem leading to the following effects: 1) An increase in parasympathetic activity causes the detrusor muscle to contract (forcing urine out of the bladder), 2) A decrease in sympathetic activity to the internal sphincter (causing it to relax and open) and 3) Adecrease in somatic neuron activity to the external sphincter causing it to relax and open. Contraction of the detrusor muscle and relaxation of the external urethral sphincter can be overridden voluntarily (i.e., toilet training). 7    5. Sodium Regulation Sodium (like all ions) is filtered at the glomerulus and then reabsorbed back into the blood. In the proximal tubule, sodium reabsorption occurs via unregulated bulk diffusion (no hormonal control). In the distal tubule sodium reabsorption is under hormonal control via, for example, aldosterone (which increases the rate of Na reabsorption), atrial natriuretic peptide (which does the opposite) and the renin-angiotensin system (which is primarily a blood pre
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