Assignment 3 HP2 PDF

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Assignment 3 (20 marks total) 1.

Explain how net filtration pressure behind the GFR is determined. Describe the influence fluid pressure within the Bowman’s capsule has on GFR? (3 marks) Net filtration pressure behind the GFR is determined by the hydrostatic pressure created by the pressure of the blood flowing through the capillary. The hydrostatic pressure in the glomerulus (about 55mmHg) is greater than both the colloid osmotic pressure and the hydrostatic fluid pressure on Bowman’s capsule, which causes the movement of fluid out of the glomerulus into Bowman’s capsule. The hydrostatic pressure remains greater than the opposing pressures all throughout the length of the capillary, allowing filtration to take place along the entire glomerulus. The net filtration pressure behind the GFR is determined by adding everything that opposes filtration (capillary colloid osmotic pressure and fluid pressure within Bowman’s capsule) and then subtracting that answer from the capillary’s hydrostatic pressure which favors filtration. This can be seen in the following equation.

GFR=PH −π − Pfluid Fluid pressure within the Bowman’s capsule opposes filtration so it influences GFR by decreasing it. The fluid in the enclosed space creates a hydrostatic fluid pressure that counteracts the filtration pressure from the capillaries. This decreases the amount of fluid that can get into the capsule decreasing the net filtration and GFR. 2.

Explain the renal threshold. Draw a graph of transport rate v. [substrate]plasma and indicate where renal threshold and transport maximum occur on the graph. (3 marks) The renal threshold is the plasma concentration of a certain solute at which it begins to be excreted in urine. This threshold is the concentration at which the saturation of the transport carriers occurs and as a result, the transport maximum is reached. The transport maximum is the constant rate at which the solute will be transported once the saturation point is reached. After the renal threshold occurs, the solute can no longer be reabsorbed by mediated transport because of the high amounts present, so it begins to appear in the urine.

3.

Diagram and compare the renal clearances for inulin and penicillin if plasma concentration = 5 mg/100 mL for both, GFR = 100 mL/min, and 100 mL of plasma is reabsorbed. (4 marks) In inulin, clearance is equal to GFR because the amount excreted is equal to the amount filtered in the glomerulus, which leads to the conclusion that no inulin is reabsorbed or secreted. Inulin clearance then equals 100mL/min. In penicillin, the amount excreted is greater than the GFR, this indicates that there was a net secretion of penicillin into the filtrate. Penicillin clearance will be 140mL/min. The clearance of penicillin is calculated with the following formula Clearance of penicillin= excretion rate / concentration in plasma Clearance of penicillin= (7mg/min)/ (0.05mg/mL) Clearance of penicillin = 140mL/min

4.

Explain how the osmolarity of the blood is changed as it passes through the vasa recta. Be sure to state the sources of any solutes or water that enters the blood. (4 marks) The vasa recta is a capillary bed surrounding the loop of Henle. It goes down into the medulla and then up like the arrangement of the loop of Henle, however, it goes in the opposite direction. When the loop of Henle is descending, the capillary is ascending and when the loop of Henle is ascending, the capillary is descending; this anatomical organization allows them to act as a countercurrent exchange. The vasa recta takes in the solutes and water that are being lost from the loop of Henle. This is done to maintain the osmolarity of the medullary interstitium. The vasa recta begins its flow where the ascending loop of Henle ends, and at this point the blood has a normal plasma osmolarity of 300mOsM. The ascending loop of Henle is permeable to solutes but not to water and as it moves up from the bottom of loop it pumps out chloride, potassium, and sodium. At the same time, the vasa recta is descending and the solutes that are diffusing out from the loop of Henle move into the vasa recta. This causes the osmolarity of the vasa recta to increase. As the vasa recta continues descending, more solutes are added so the osmolarity keeps increasing until, at the bottom of the loop, the osmolarity is the same as the one of the interstitial fluid. Then, the vasa recta starts to ascend and is in close proximity with the descending loop of Henle. The descending loop of Henle is only permeable to water and because of the high concentration of solutes in the vasa recta, this capillary pulls on all the water from the descending loop and this decreases the blood’s osmolarity in the capillary. As the vasa recta gets closer to the beginning of the descending loop of Henle, more water is added to the blood, decreasing its osmolarity even more, and by the end of the vasa recta and the beginning of the descending loop of Henle, the blood has a normal plasma osmolarity; the same as the one with which it had begun (300 mOsM). The reason why the blood in the vasa recta takes all the water and solutes that are being reabsorbed from the loop of Henle is to maintain the osmolarity of the medullary interstitium.

5.

Describe or diagram the action of atrial natriuretic peptide. (6 marks) Atrial natriuretic peptide (ANP) is a peptide hormone, the most important signal molecule in normal physiology. It opposes the effects of vasopressin and aldosterone. It promotes natriuresis (sodium excretion) and diuresis (water excretion) to decrease blood volume and decrease blood pressure. It is usually co-secreted with brain natriuretic peptide and together they make the natriuretic peptides group. The stimulus for the release of ANP is increased blood volume and pressure, which cause an increased atrial stretch. In response, specialized myocardial cells in the atria of the heart act as the integrating center and produce the hormone. It is first made as a large prohormone and then it is cut into active hormone portions. The hormone is then dissolved in plasma, because of its hydrophilicity, and transported to its target where it binds to membrane-receptor enzymes that activate the cGMP second messenger pathway to create a response. ANP will act on different target tissues and cells to ultimately get to the same goal: lower the blood volume to decrease blood pressure. The main target for ANP are the kidneys, specifically the renal tubule and the afferent arteriole. In the renal tubule, it will decrease sodium reabsorption; this keep sodium in

the lumen and if sodium does not leave neither does water as it will not have a gradient to follow. In the afferent arteriole, it causes vasodilation and overrides the myogenic response of the glomerulus. Vasodilation of this vessel causes increase flow, which results in an increase GFR and in a greater volume of water and salt excreted. The increased flow through the afferent arteriole also causes a decrease in the release of renin, which ultimately decreases the production of aldosterone and vasopressin which would increase blood pressure and volume even more. ANP also acts on other endocrine and nervous system integrating centers to decrease blood pressure. It has an effect on the hypothalamus, adrenal cortex, and medulla oblongata. ANP in the hypothalamus affects the release of vasopressin by the posterior pituitary gland, and a decrease in vasopressin secretion decreases water reabsorption. In the adrenal cortex, ANP inhibits the secretion of aldosterone, which decreases sodium reabsorption. This causes a decrease in fluid reabsorption as well. Lastly, ANP acts on the medulla oblongata (nervous system integrating center). Its response is to decrease the sympathetic nervous system output, which causes blood pressure to decrease. As portrayed above, ANP has a pronounced effect on different tissues and cells which together help create the wanted response: a decrease in blood pressure which will mainly occur by decreasing blood volume....