NROSCI/BIOSC 1070 and MSNBIO 2070 Exam # 2 October 25, 2013 Total POINTS: 100 20% of grade in class 1) During exercise, plasma levels of Renin increase moderately. Why should Renin levels be elevated during exercise? Discuss the physiological mechanisms responsible for the elevations in Renin levels during exercise. (5 points). Page 1 of 10
2) A variety of mechanisms contribute to regulating blood flow to tissues, including skeletal muscle. Paracrine factors such as adenosine and K + increase in muscle during both exercise and following hemorrhage. However, during exercise the muscle arterioles dilate, whereas they constrict following hemorrhage. Describe the physiological mechanisms responsible for the opposite changes in muscle blood flow during exercise and following hemorrhage, despite the fact that both conditions result in an increase in the levels of paracrine factors that can promote vasodilation. (10 points). Page 2 of 10
3) During an experiment on an anesthetized animal, which is artificially ventilated to maintain stable blood oxygenation, 25% of the blood volume is removed at the time of the vertical dashed line below. Draw the resulting changes in the parameters indicated. You need not be as concerned about the absolute magnitude of the changes as by the relative timing of the changes and whether the values decrease or increase. On the next page, describe the physiological mechanisms responsible for the changes in parameters you indicate on the diagram. (30 points). Question Continues on Page 4 Page 3 of 10
Briefly describe the physiological mechanisms responsible for the changes in the parameters that occur following hemorrhage, which you diagrammed on page 3. a) Changes in cardiac output: Cardiac output drops 25% following the hypovolemia, but rapid recovery occurs due to the baroreceptor reflex (increased heart rate and contractility, decreased venous compliance). This is partially offset by an increased afterload. Slower recovery occurs as fluid is absorbed from the interstitial space (increased preload). b) Changes in mean arterial pressure: There would be an immediate fall in mean arterial pressure, due to decreased cardiac output, followed by a recovery. The fast recovery involves activation of the baroreceptor reflex, with resultant increase in peripheral resistance and decrease in venous compliance. The increase in precapillary resistance, along with the decrease in venous pressure (a consequence of hypovolemia), will cause capillary hydrostatic pressure to fall, thereby favoring reabsorption of fluid from the interstitial space (this occurs over a time span of 30-60 min). Recovery of mean arterial pressure will also be aided (also on a slower time scale) by activation of the renin-angiotensin system. c) Changes in blood volume: There will be an immediate 25% decrease in blood volume, followed by a slow recovery as fluid is reabsorbed from the interstitial space. It should be recognized that this is a self-limiting process. Since the fluid being reabsorbed is largely protein-free there is a gradual decrease in plasma oncotic pressure, decrease in interstitial fluid hydrostatic pressure, and increase in interstial oncotic pressure as reabsorption progresses, thereby slowing down fluid movement. Fluid transfer ceases when the net hydrostatic and oncotic pressure gradients are equal. Thus complete restitution of blood volume by this mechanism is not possible. d) Changes in heart rate: Rapid activation of the baroreceptor reflex (fall in carotid sinus pressure) will cause an immediate increase in heart rate. This response will be slowly attenuated as the blood volume expands and cardiac output increases. e) Changes in hematocrit: Since it is whole blood that is lost, there is no immediate change in hematocrit. Hematocrit slowly decreases as fluid is reabsorbed from the interstitial space f) Changes in interstitial fluid pressure: There is a slow decrease reflecting the progression of fluid transfer from the interstitial space to the vascular space. Page 4 of 10
4) In the bottom diagram on page 3, it is evident that interstitial fluid pressure is slightly negative under normal physiological circumstances. Discuss why the pressure in the interstitial fluid is normally slightly negative unless a physiological perturbation occurs. (5 points). Page 5 of 10
5) Based on the schematic diagram of a nephron below, in the table on the next page label the names of the segments (a-f) and indicate the approximate percentage of the filtered load of Na + that is absorbed in each segment and the major Na + transporter on the apical membrane of that segment involved in the transport. (15 points). Question Continues on Page 7 Page 6 of 10
a) Segment name (0.5 pts each) Approximate percentage of the filtered load of Na + that is absorbed (1 pt each) Major Na + transporter on the apical membrane of that segment involved in the transport (1 pt each) b) c) d) e) f) Page 7 of 10
6) The following questions relate to filtration fraction. a) What is meant by the renal filtration fraction and how would you determine its value by measuring the clearance of certain substances? (7 points). b) Which of the following numbers best represents a normal value for filtration fraction: 5%, 10%, 20%, 40%, 60%? (2 points). c) You measure filtration fraction and determine it to be significantly elevated. List 3 separate possible causes of elevated filtration, and propose measures that can be used to distinguish among them. (6 points). Page 8 of 10
7) Glomerular filtration rate (GFR) is stable across a wide range of aortic blood pressures. a) Describe two distinct mechanisms that contribute to this stability of GFR. (5 points). b) In response to very large changes in aortic pressure, what happens to GFR? Why does GFR change in response to such large changes in aortic blood pressure? (5 points). Page 9 of 10
8) At the level of the adrenal cortex, there are two major stimuli for aldosterone secretion. Describe these two major stimuli and why they make physiological sense in the context of the actions of aldosterone. (10 points). Page 10 of 10