10. Case problems with pictures in topic "Phуsiology of excretion"

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Case problem 10.1. What would happen if a significant number of glomerular capillaries were clogged, as can happen in someone with very high blood glucose concentrations for a long period of time (for example, in untreated diabetes mellitus)?

Figure 10.1. The renal corpuscle. (a) Anatomy of the renal corpuscle. (b) Inset view of podocytes and capillaries. (c) Glomerular filtration membrane. Whereas most plasma proteins are turned back, some polypeptides are filtered. This creates a filtrate in Bowman's capsule that is essentially protein free. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.1. The glomerular filtration rate would be greatly decreased. This would result in a decrease in the removal of toxic substances from the blood. As you will learn, kidney disease is a common and troubling consequence of long-term untreated diabetes mellitus.

Case problem 10.2. What would be the effect of an increase in plasma albumin (the most abundant plasma protein) on glomerular filtration rate (GFR)?

Figure 10.2. Forces involved in glomerular filtration. The symbol π denotes the osmotic force due to the presence of protein in glomerular capillary plasma. (Note: The concentration of protein in Bowman’s space is so low that πBS, a force that would favor filtration, is considered zero). Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.2. GFR will decrease because the increase in plasma osmotic force from albumin will oppose filtration.

Case problem 10.3. Describe the immediate consequences of a blood clot occluding the afferent arteriole or the efferent arteriole.

Figure 10.3. Control of GFR by constriction or dilation of afferent arterioles (AA) or efferent arterioles (EA). (a) Constriction of the afferent arteriole or (c) dilation of the efferent arteriole reduces PGC, thus decreasing GFR. (b) Constriction of the efferent arteriole or (d) dilation of the afferent arteriole increases PGC, thus increasing GFR. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.3. A blood clot occluding the afferent arteriole would decrease blood flow to that glomerulus and greatly decrease GFR in that individual glomerulus. A blood clot in the efferent arteriole would increase PGC and, therefore, GFR. If this only occurred in a few glomeruli, it would not have a significant effect on renal function because of the large number of total glomeruli in the two kidneys providing a safety factor.

Case problem 10.4. How would you calculate the filtered load and excretion rate of glucose?

Figure 10.4. The relationship between plasma glucose concentration and the rate of glucose filtered (filtered load), reabsorbed, or excreted. The dotted line shows the transport maximum, which is the maximum rate at which glucose can be reabsorbed. Notice that as plasma glucose exceeds its threshold, glucose begins to appear in the urine. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.4. Filtered load = GFR × Plasma glucose concentration. Excretion rate = Urine glucose concentration × Urine flow rate.

Case problem 10.5. Referring to part (c), what would be the effect of a medication that blocks the Na+ channels in the apical membrane of the cortical collecting duct?

Figure 10.5. Mechanism of Na+ reabsorption in the (a) proximal tubule, (b) ascending limb of the loop of Henle, and (c) cortical collecting duct. The sizes of the letters denote high and low concentrations. “X” represents organic molecules such as glucose and amino acids that are cotransported with Na+. The fate of the K+ that the Na+/K+-ATPase pumps transport is discussed in the later section dealing with renal K+ handling. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.5. It would decrease sodium reabsorption from the tubular fluid. This will result in an increase in urinary sodium excretion. The osmotic force of sodium will carry water with it, thus increasing urine output. Examples of such diuretics are triamterene and amiloride.

Case problem 10.6. What effect would an ACE inhibitor have on renin secretion and angiotensin II production? What effect would an angiotensin II receptor blocker (ARB) have on renin secretion and angiotensin II production?

Figure 10.6. Summary of the renin-angiotensin system and the stimulation of aldosterone secretion by angiotensin II. Angiotensinconverting enzyme (ACE) is located on the surface of capillary endothelial cells. The plasma concentration of renin is the ratelimiting factor in the renin–angiotensin system; that is, it is the major determinant of the plasma concentration of angiotensin II. (aa = Amino acids). Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.6. An ACE inhibitor will decrease angiotensin II production. The resultant increase in Na+ and water excretion would decrease blood pressure, leading to a reflexive increase in renin secretion. An ARB would also decrease blood pressure and therefore increase renin secretion. However, with an ARB, angiotensin II would increase because angiotensin-converting-enzyme activity would be normal.

Case problem 10.7. What would be the effect of denervation (removal of sympathetic neural input) of the kidneys on Na+ and water excretion?

Figure 10.7. Pathways by which decreased plasma volume leads, via the renin-angiotensin system and aldosterone, to increased Na+ reabsorption by the cortical collecting ducts and hence to decreased Na+ excretion. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.7. Under normal conditions, the redundant control of renin release, as indicated in this figure, as well as the participation of vasopressin, would allow the maintenance of normal sodium and water balance even with denervated kidneys. However, during severe decreases in plasma volume, like in dehydration, the denervated kidney may not produce sufficient renin to maximally decrease Na+ excretion.

Case problem 10.8. Explain how this figure illustrates the general principle of physiology that the functions of organ systems are coordinated with each other.

Figure 10.8. Pathways by which Na+ and water excretion decrease in response to severe sweating. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Answer 10.8. The adaptation to a hot environment depends on the ability to lose heat from the body by sweating. The ability to detect a decrease in plasma volume by low-pressure baroreceptors in the heart and an increase in osmolarity by osmoreceptors in the brain sets in motion a coordinated response to minimize the loss of body water and ions including Na+. This includes a decrease in GFR in the kidneys and an increase in secretion of aldosterone from the adrenal cortex. The decreased GFR decreases the amount of water and ions entering the filtrate in the kidneys, thereby decreasing losses in the urine. The increased concentration of plasma aldosterone increases renal Na+ reabsorption. The increased synthesis of vasopressin in the hypothalamus and its release from axons in the posterior pituitary leads to an increase in vasopressin in the blood that signals the kidneys to increase water reabsorption. Therefore, the coordination of organs from the nervous system (the brain), endocrine system (posterior pituitary), circulatory system (heart), and urinary system (kidneys) minimizes the loss of water and Na+ during sweating until the deficits of both can be replaced by increased ingestion and absorption in the gastrointestinal tract.

Good luck with your studies!