Note: During any ONE run the ph remains constant. It may be at any one of the above levels but it never change during a single run.

Transcription

1 1 BGYC34 (2007) PhysioEx Lab 10 AcidBase Balance Marking Scheme Part 1 Complete PhysioEx lab #10. Handin all of the pages associated with the lab. Note that there are 9 activities to be completed. You DO NOT need to hand in the histology review supplement. There are 10 questions in part 1 for which marks are assigned. Convert the mark out of 10 to a mark out of 5. Add the mark out of 5 to the mark out of 8 from part 2. The final mark is out of 13. Activity 1: Normal Breathing At 20 seconds, ph = 7.38 or 7.39 or 7.40 or 7.41 or 7.42 At 40 seconds, ph = 7.38 or 7.39 or 7.40 or 7.41 or 7.42 At 60 seconds, ph = 7.38 or 7.39 or 7.40 or 7.41 or 7.42 Note: During any ONE run the ph remains constant. It may be at any one of the above levels but it never change during a single run. Did the ph level of the blood change at all during normal breathing? If so, how? No, the ph level of the blood does not change (during any single run) during normal breathing. Was the ph level always within the normal range for the human body? Yes, the ph level remained within the normal range (7.38 to 7.42) during normal breathing. Did the PCO 2 level change during the course of normal breathing? If so, how? No, the PCO 2 level remained constant (40 mmhg) during normal breathing. Activity 2a: Hyperventilation Run 1 At 20 seconds, ph = approximately 7.45 At 40 seconds, ph = approximately 7.54 At 60 seconds, ph = approximately 7.67

2 2 Did the ph level of the blood change at all during this run? If so, how? Yes, the ph level of the blood increased over time. Was the ph level always within the normal range for the human body? No, the ph level was not in the normal range for the human body. It increased above normal values. If not when was the ph value outside of the normal range and what acidbase imbalance did this ph value indicate? The ph value began to rise above the normal range between 10 and 20 seconds. This is a respiratory alkalosis. (1 MARK) Did the PCO 2 level change during the course of this run? If so, how? Yes, the PCO 2 level progressively decreased over time (during the hyperventilation). If you observed an acidbase imbalance during this run, how would you expect the renal system to compensate for this condition? You would expect the renal system to compensate for the respiratory alkalosis by decreasing HCO 3 reabsorption and decreasing H + ion secretion. (1 MARK) How did the hyperventilation trace differ from the trace for normal breathing? Did the tidal volumes change? During hyperventilation, tidal volume was markedly increased and there was small increase in breathing frequency. What might cause a person to hyperventilate? Two of the major causes of hyperventilation are anxiety and fever. Some drugs (such as cocaine) will also induce hyperventilation. Activity 2b: Hyperventilation Run 2 What happened to the trace after the 20 second mark when you stopped the hyperventilation? Did the breathing return to normal immediately? Explain your observation. After the period of hyperventilation and the return to normal breathing there was approximately one normal breath before breathing stopped for a few (around ten)

3 3 seconds. This occurred because PCO 2 was low enough to shut off breathing due to the removal of the CO 2 stimulus from the central and peripheral chemoreceptors. (1 MARK) Activity 3: Rebreathing At 20 seconds, ph = approximately 7.35 At 40 seconds, ph = approximately 7.30 At 60 seconds, ph = approximately 7.25 Did the ph level of the blood change at all during this run? If so, how? Yes, blood ph decreased over time. Was the ph level always within the normal range for the human body? No, the ph level was not always within the normal range; it was lower. If not when was the ph value outside of the normal range and what acidbase imbalance did this ph value indicate? The ph value began to fall out of the normal range at about 2025 seconds. This was a respiratory acidosis. (1 MARK) Did the PCO 2 level change during the course of this run? If so, how? Yes, the PCO 2 level increased over the course of this run. If you observed an acidbase imbalance during this run, how would you expect the renal system to compensate for this condition? In order to compensate for a respiratory acidosis, the kidney would increase the rate of HCO 3 reabsorption and increase the rate of H + ion secretion. (1 MARK) How did the rebreathing trace differ from the trace for normal breathing? Did the tidal volumes change? The rebreathing trace showed greater levels of breathing compared to normal. Tidal volume was elevated as was breathing frequency (albeit the tidal volume change was more obvious than the frequency change). Give examples of respiratory problems that would result in ph and PCO 2 patterns similar to what you observed during rebreathing. Lung disease or airway obstruction could lead to the retention of CO 2 which would increase PCO 2 and decrease ph.

4 4 Activity 4: Renal Response to Normal AcidBase Balance At normal PCO 2 and ph levels, what level of H + was present in the urine? The [H + ]in the urine was normal. There is no quantitative answer to be given. What level of [HCO 3 ] was present in the urine? The [HCO 3 ] in the urine was normal. There is no quantitative answer to be given. Why does the blood ph change as the PCO 2 changes? This question seems out of place. As PCO 2 increases, blood ph decreases because CO 2 is being hydrated to a proton and bicarbonate ion. Activity 5: Renal Response to Respiratory Alkalosis What level of [H + ] was present in the urine at each of these PCO 2 /ph levels? At a PCO 2 level of 35 mmhg, urine [H + ] was normal. As PCO 2 was lowered to 30 and 20 mmhg, urine [H + ] was lowered (the kidney was retaining protons). What level of [HCO 3 ] was present in the urine at each of these PCO 2 /ph levels? At a PCO 2 level of 35 mmhg, urine [HCO 3 ] was normal. As PCO 2 was lowered to 30 and 20 mmhg, urine [HCO 3 ] was increased (the kidney is getting rid of bicarbonate). Recall that it may take hours or even days for the renal system to respond to disruptions in acidbase balance. Assuming that enough time has passed for the renal system to fully compensate for respiratory alkalosis, would you expect PCO 2 levels to increase or decrease? Would you expect blood ph levels to increase or decrease? The kidney can fully compensate for the ph changes (i.e., bring arterial ph back to normal) but PCO 2 (and HCO 3 ) levels will remain low until the respiratory disturbance (alkalosis) is removed. Blood PCO 2 levels would remain at the low level induced by the respiratory alkalosis. The renal compensation would not change this. (1 MARK)

5 5 Recall our discussion on renal compensation for respiratory disorders that occurred in lecture 20.See the slides on renal compensation for a respiratory alkalosis and the note that a new steady state PCO 2 level is achieved. Recall your activities in the first experiment on respiratory acidosis or alkalosis. Which type of breathing resulted in PCO 2 levels closest to the ones we experimented with in this activity normal breathing, hyperventilation or Rebreathing? The reduced PCO 2 levels resemble those observed during hyperventilation which, of course, would induce a respiratory alkalosis. Explain why this type of breathing resulted in alkalosis. Hyperventilation causes an increase in CO 2 excretion. Since CO 2 is hydrated to produce a proton and a bicarbonate ion, less CO 2 in the blood means less hydrogen ions and therefore an increase in blood ph. Activity 6: Renal Response to Respiratory Acidosis What level of [H + ] was present in the urine at each of these PCO 2 /ph levels? As PCO 2 was raised to lowered to 60, 75 and 90 mmhg, urine [H + ] was elevated (the kidney was secreting protons). What level of [HCO 3 ] was present in the urine at each of these PCO 2 /ph levels? As PCO 2 was raised to 60, 75 and 90 mmhg, urine [HCO 3 ] was lowered (the kidney was retaining bicarbonate). Recall that it may take hours or even days for the renal system to respond to disruptions in acidbase balance. Assuming that enough time has passed for the renal system to fully compensate for respiratory acidosis, would you expect PCO 2 levels to increase or decrease? Would you expect blood ph levels to increase or decrease? Ultimately the kidney can compensate for changes in blood ph and restore ph to normal. However, the kidney cannot restore PCO 2 and HCO 3 levels to normal. Both PCO 2 and HCO 3 levels remain elevated until the respiratory acidosis ceases. (1 MARK) Note, see the slides on renal compensation for a respiratory acidosis. The renal compensation occurs along a PCO 2 isobar so PCO 2 does not change during the renal compensation. A new steady state PCO 2 is achieved that is higher than normal. HCO 3 levels are also higher.

6 6 Recall your activities in the first experiment on respiratory acidosis or alkalosis. Which type of breathing resulted in PCO 2 levels closest to the ones we experimented with in this activity normal breathing, hyperventilation or Rebreathing? The elevated PCO 2 levels resemble those observed during rebreathing which would induce a respiratory acidosis. Explain why this type of breathing resulted in acidosis. Rebreathing or hypoventilation cause CO 2 retention therefore leading to a decrease in ph. Activity 7: Respiratory Response to Normal Metabolism What is the respiratory rate? 15 breaths per minute. Are the blood ph and PCO 2 values within the normal ranges? Yes. Activity 8: Respiratory Response to Increased Metabolism How did respiration change? As metabolic rate increased, breathing increased. How did blood ph change? As metabolic rate increased, blood ph decreased, despite the increase in breathing. How did PCO 2 change? As metabolic rate increased blood PCO 2 increased despite the increase in breathing. How did [H + ] change? As metabolic rate increased blood [H + ] increased despite the increase in breathing. How did [HCO 3 ] change? As metabolic rate increased blood [HCO 3 ] decreased. Explain why these changes took place as metabolic rate increased?

7 7 As metabolic rate increased, CO 2 levels increased because CO 2 is a metabolic waster product. This caused an increase in H + levels and hence a decrease in ph. Breathing increased (due to stimulation of chemoreceptors) in an attempt to restore blood CO 2 levels to normal but in this case the increase in breathing was not able to compensate for the elevated metabolism. (1 MARK) Which metabolic rates caused ph levels to decrease to a condition of metabolic acidosis? Metabolic rates of 60+ caused a metabolic acidosis. What were the ph values at each of these rates? 7.34, 7.26 and 7.25 were the ph levels at metabolic rates of 60, 70 and 80, respectively. By the time the respiratory system fully compensated for the acidosis, how would you expect the ph values to change? Once the respiratory system fully compensated for the acidosis you would expect ph to increase back to normal levels. (1 MARK) Activity 9: Respiratory Response to Decreased Metabolism How did respiration change? As metabolic rate decreased, breathing decreased. How did blood ph change? As metabolic rate decreased, ph increased. How did PCO 2 change? As metabolic rate decreased, PCO 2 decreased. How did [H + ] change? As metabolic rate decreased blood [H + ] decreased. How did [HCO 3 ] change? As metabolic rate increased, blood [HCO 3 ] increased. Explain why these changes took place as metabolic rate decreased?

8 8 As metabolism decreased, there was less CO 2 produced therefore less H + ions and a higher ph. Breathing decreased because the reduction in CO 2 and the increase in ph reduced the chemoreceptor stimulus for breathing. The changes in breathing in this case were not sufficient to restore these values to normal levels during the period of reduced metabolic rate. (1 MARK) Which metabolic rates caused ph levels to increase to a condition of metabolic acidosis? Metabolic rates of 20 and 30 raised ph above 7.45 which is the cutoff (see the introduction) for a metabolic alkalosis. What were the ph values at each of these rates? 7.45, 7.48, 7.52 were the values at metabolic rates of 40, 30 and 20, respectively. By the time the respiratory system fully compensated for the alkalosis, how would you expect the ph values to change? ph would return to normal levels once the respiratory system fully compensates for the alkalosis.

9 9 Part 2 (10 marks) Answer the following questions. Staple the answers to the back of the PhysioEx pages. 1) Drugs such as SIDS or DIDS block the activity of chloridebicarbonate exchangers. Illustrate, on a phbicarbonate (Davenport) diagram, the effects (on acidbase balance) of adding SIDS or DIDS to the proximal tubule epithelial cells in the kidneys. Explain your diagram. (2 marks). See the lecture 1920 notes for Davenport diagrams. If the activity of the chloridebicarbonate exchanger is blocked in the proximal tubule then there is going to be little or no bicarbonate reabsorption from the kidneys back into the blood. This would lead to a reduction in blood bicarbonate levels; hence a metabolic acidosis. A Davenport diagram such as the one below should be shown. The key point is that there should be some indication that the position has shifted from point 1 (normal blood values) to point 2 (along the pco 2 isobar; i.e., no change in pco 2 ) showing a reduction in ph and a reduction in bicarbonate levels). Give 1 mark for the diagram and 1 mark for the explanation. [HCO 3 ] mmhg mm pco 2 isobars ph HOWEVER, some students may offer an alternate explanation that is also worth full marks. Even if there is no bicarbonate reabsorption in the proximal tubule there still could be bicarbonate synthesis in the distal tubule and even the proximal tubule. Furthermore, there could also be bicarbonate reabsorption in the distal tubule. This doesn t normally occur since all bicarbonate is normally reabsorbed in the proximal tubule. In this case there would be no acidbase disturbance and the graph would simply show normal acidbase status at point 1. In this case give 1 mark for the diagram and 1 mark for the explanation.

10 10 2) Assume that the initial resting values for pco 2, [HCO 3 ] and ph are 40 mmhg, 25 mm and 7.4, respectively. If a metabolic acidosis occurs such that ph falls to 7.3, what would be the corresponding [HCO 3 ]? (2 marks) Use the HendersonHasselbach equation (below) and the following information to determine the answer. α = ; pk= 6.1 ph = pk + log ([HCO 3 ] / α pco 2 ) Given that it is a metabolic acidosis, ppco 2 does not change. Therefore, use the values of PCO 2 = 40 mmhg and ph = 7.3 in the calculation. ph = pk + log ([HCO 3 ] / α pco 2 ) ph = ph + log [HCO 3 ] log α pco = log [HCO 3 ] log (0.0301) (40) 1.2 = log [HCO 3 ] log = log [HCO 3 ] log [HCO 3 ] = Give 1 mark if the logical progression reaches this far. [HCO 3 ] = antilog [HCO 3 ] = 19.1 mm Give 1 mark if the correct answer is also given. NOTE: I expect that most of the students will show their work. However, I did not ask for this. If they just give the correct answer with no calculations they still get the full two marks. 3) Why do diarrhea and vomiting lead to metabolic acidosis and alkalosis, respectively? (2 marks) Diarrhea leads to a large loss of HCO 3 in the watery feces. (0.5 marks) This HCO 3 would normally have been reabsorbed into the blood across the intestine. Because bicarbonate is being lost in the diarrhea its level in the blood falls leading to a metabolic acidosis. (0.5 marks) Vomiting leads to a loss of H + ions in the vomit. (0.5 marks)

11 11 The H + ions that are secreted into the stomach can be reclaimed, into the blood, once the chyme (from the stomach) enters the intestine. Since they are no longer in the stomach contents they cannot be reabsorbed into the blood. Therefore blood H + levels fall leading to a metabolic alkalosis. (0.5 marks) 4) Rank the following in order from the least effective buffer to the most effective buffer. (2 marks) The following list goes from the most effective buffer (top of the list) to the least effective buffer (bottom of the list) Whole blood from a person with polycythaemia Whole blood from a normal person Whole blood from an anaemic person Blood plasma from a normal person = Blood plasma from an anaemic person Blood serum from a normal person Distilled water Give the full 2 marks if the list is correct. If two possibilities are switched around (just amongst the two of them) then remove 0.5 marks. If three possibilities are misplaced remove 1 mark. If more than 3 possibilities are misplaced then deduct all marks. The following is an explanation but the students didn t need to provide it in order to get marks. Blood is a better buffer than plasma because of the presence of haemoglobin in the red blood cells (this was covered in the lectures). The more red blood cells that are present in the blood, the more buffering capacity it will have. Polycythaemia is an increase in the number of red cells; anaemia is a decrease in the number of red blood cells. Plasma is blood without the red blood cells. There is no reason for the plasma to be different from a normal versus anaemic person. Serum is plasma with the clotting proteins removed. Since proteins act as buffers, serum is a worse buffer than plasma. Distilled water would have no buffering capacity.