GED1
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physiology please explain vasoconstriction, dilatation with regards to tissue fluid formation
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(Original post by GED1)
if the mean arterial blood pressure remains constant, are these following facts correct?all answers are appreciated thanks
a) constriction of arterioles in one organ will result in a fall in the mean capillary blood pressure in that organ
b) widespread constriction of arterioles throughout the body will cause a rise in cardiac output
c) constriction of arterioles in one part of the body diverts blood flow to those parts of the body where the arteriolar tone is unchanged
d) constriction of arterioles in one organ will result in a decrease in blood flow to that organ
e) constriction of arterioles in one organ will results in a decrease in the rate of formation of tissue fluid in that organ.
looks like c, d and e are correct
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GED1
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(Original post by AortaStudyMore)
looks like c, d and e are correct
could you possibly explain why? thanks
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macpatgh-Sheldon
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Hi,

AortaStudyMore is, of course, the expert on the vascular system; however, and I might be wrong, but I think only c and d are correct.

a) INCORRECT: Blood pressure in general and for a specific organ will INCREASE following vasoconstriction, since the arteriolar resistance will increase, thus tending to impede blood flow; also blood pressure is a function of cardiac output and peripheral resistance.

b) INCORRECT: Cardiac output, being the product of stroke volume and heart rate, is determined by these two factors ONLY. Stroke volume can increase if venoconstriction reduces venous pooling and therefore increases venous return to the right atrium; this increase in stroke volume would be related to a positive inotropic effect, but would be caused as a result of the Frank-Starling Law. Arteriolar constriction should not have any effect on stroke volume - however, having said that, arteriolar constriction will tend to increase blood pressure (see 1 above); this will tend to reduce heart rate via the nerve impulses to the cardiac centre from the carotid sinus in attempt to normalize the b.p.

c) CORRECT: Of course, narrower vessel mean reduced blood flow (the rate of flow along an artery approximates to Pouiselle's Law, whereby flow rate is directly proportional to the fourth power of the radius of a tube (here a blood vessel).

d) CORRECT: As in (c) above.

e) AMBIGUOUS: The formation of interstitial fluid is related to the "hydrostatic pressure minus the osmotic pressure" since hydrostatic pressure .(generated by the blood pressure) tends to force fluid out of blood vessels, while a high osmotic pressure (low water potential) tends to draw water into the vessel. Vasoconstriction will tend to increase the hydrostatic pressure at the organ concerned, tending to increase outward flow of fluid. This is illustrated by the ultrafiltration in the renal glomerulus, where the wider afferent arteriole leading onto the narrower efferent arteriole increases the hdrostatic pressure, forcing fluid into the Bowman's capsule. This consideration would predict that vasoconstriction, by increasing local pressure within the vessel, might increase the formation of interstitial fluid. However, it is a known fact that a severe drop in blood pressure sufficient to reduce renal blood flow significantly can cause pre-renal failure by reducing the GFR (in general, urine output of less than 30 cm3/hour with other conditions steady should be taken seriously); from this viewpoint, one would expect a reduction of tissue fluid formation with reduced blood flow to an organ (in agreement with the answer by AortaStudyMore) UNFAIR Q, I would say!

Hope this helps.

M.
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(Original post by macpatelgh)
Hi,

AortaStudyMore is, of course, the expert on the vascular system; however, and I might be wrong, but I think only c and d are correct.

a) INCORRECT: Blood pressure in general and for a specific organ will INCREASE following vasoconstriction, since the arteriolar resistance will increase, thus tending to impede blood flow; also blood pressure is a function of cardiac output and peripheral resistance.

b) INCORRECT: Cardiac output, being the product of stroke volume and heart rate, is determined by these two factors ONLY. Stroke volume can increase if venoconstriction reduces venous pooling and therefore increases venous return to the right atrium; this increase in stroke volume would be related to a positive inotropic effect, but would be caused as a result of the Frank-Starling Law. Arteriolar constriction should not have any effect on stroke volume - however, having said that, arteriolar constriction will tend to increase blood pressure (see 1 above); this will tend to reduce heart rate via the nerve impulses to the cardiac centre from the carotid sinus in attempt to normalize the b.p.

c) CORRECT: Of course, narrower vessel mean reduced blood flow (the rate of flow along an artery approximates to Pouiselle's Law, whereby flow rate is directly proportional to the fourth power of the radius of a tube (here a blood vessel).

d) CORRECT: As in (c) above.

e) AMBIGUOUS: The formation of interstitial fluid is related to the "hydrostatic pressure minus the osmotic pressure" since hydrostatic pressure .(generated by the blood pressure) tends to force fluid out of blood vessels, while a high osmotic pressure (low water potential) tends to draw water into the vessel. Vasoconstriction will tend to increase the hydrostatic pressure at the organ concerned, tending to increase outward flow of fluid. This is illustrated by the ultrafiltration in the renal glomerulus, where the wider afferent arteriole leading onto the narrower efferent arteriole increases the hdrostatic pressure, forcing fluid into the Bowman's capsule. This consideration would predict that vasoconstriction, by increasing local pressure within the vessel, might increase the formation of interstitial fluid. However, it is a known fact that a severe drop in blood pressure sufficient to reduce renal blood flow significantly can cause pre-renal failure by reducing the GFR (in general, urine output of less than 30 cm3/hour with other conditions steady should be taken seriously); from this viewpoint, one would expect a reduction of tissue fluid formation with reduced blood flow to an organ (in agreement with the answer by AortaStudyMore) UNFAIR Q, I would say!

Hope this helps.

M.
Pre-capillary (afferent) arteriole vasodilation raises capillary hydrostatic pressure, so (e) is right. The more fluid you have in the capillaries, the greater the pressure is due to gravity pulling the fluid down on top of itself (kind of like how water pressure is greater at the bottom of the ocean than at the surface). So conversely you would assume that vasoconstriction would reduce the rate at which tissue fluid is formed. You are right though, the questions are all a bit **** tbh
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Jpw1097
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(Original post by macpatelgh)
Hi,

AortaStudyMore is, of course, the expert on the vascular system; however, and I might be wrong, but I think only c and d are correct.

a) INCORRECT: Blood pressure in general and for a specific organ will INCREASE following vasoconstriction, since the arteriolar resistance will increase, thus tending to impede blood flow; also blood pressure is a function of cardiac output and peripheral resistance.

b) INCORRECT: Cardiac output, being the product of stroke volume and heart rate, is determined by these two factors ONLY. Stroke volume can increase if venoconstriction reduces venous pooling and therefore increases venous return to the right atrium; this increase in stroke volume would be related to a positive inotropic effect, but would be caused as a result of the Frank-Starling Law. Arteriolar constriction should not have any effect on stroke volume - however, having said that, arteriolar constriction will tend to increase blood pressure (see 1 above); this will tend to reduce heart rate via the nerve impulses to the cardiac centre from the carotid sinus in attempt to normalize the b.p.

c) CORRECT: Of course, narrower vessel mean reduced blood flow (the rate of flow along an artery approximates to Pouiselle's Law, whereby flow rate is directly proportional to the fourth power of the radius of a tube (here a blood vessel).

d) CORRECT: As in (c) above.

e) AMBIGUOUS: The formation of interstitial fluid is related to the "hydrostatic pressure minus the osmotic pressure" since hydrostatic pressure .(generated by the blood pressure) tends to force fluid out of blood vessels, while a high osmotic pressure (low water potential) tends to draw water into the vessel. Vasoconstriction will tend to increase the hydrostatic pressure at the organ concerned, tending to increase outward flow of fluid. This is illustrated by the ultrafiltration in the renal glomerulus, where the wider afferent arteriole leading onto the narrower efferent arteriole increases the hdrostatic pressure, forcing fluid into the Bowman's capsule. This consideration would predict that vasoconstriction, by increasing local pressure within the vessel, might increase the formation of interstitial fluid. However, it is a known fact that a severe drop in blood pressure sufficient to reduce renal blood flow significantly can cause pre-renal failure by reducing the GFR (in general, urine output of less than 30 cm3/hour with other conditions steady should be taken seriously); from this viewpoint, one would expect a reduction of tissue fluid formation with reduced blood flow to an organ (in agreement with the answer by AortaStudyMore) UNFAIR Q, I would say!

Hope this helps.

M.
(Original post by AortaStudyMore)
looks like c, d and e are correct
I would say (a) is also correct. Vasoconstriction of arterioles reduces blood flow and hence reduces capillary hydrostatic pressure downstream, therefore mean capillary blood pressure would decrease. This also explains why (e) is correct. I don't see what is wrong with the question. The reason why a severe drop in BP (such as in hypovolaemic shock or anaphylactic shock) causes pre-renal acute kidney injury is because the fall in BP stimulates the baroreflex which augments sympathetic activity. This increased sympathetic activity to the kidney causes constriction of the renal vasculature, which combined with the reduction in BP itself, reduces blood flow to the kidney which in turn reduces GFR - potentially causing AKI.

(b) is incorrect. Ignoring compensatory mechanisms such as the baroreflex (since the widespread constriction of arterioles in itself could be a compensatory mechanism), widespread constriction of arterioles increases the total peripheral resistance which increases mean arterial pressure. The increase in mean arterial pressure would decrease stroke volume as it increases afterload (the load against which the heart must contract to eject blood during systole) which would decrease cardiac output.

(c) is correct. Constriction of arterioles supplying a particular organ reduces the blood flow to that organ, and therefore the blood is diverted to other organs. This is what happens during exercise, for example. There is constriction of arterioles supplying the GI tract, skin (at least initially), kidney and inactive skeletal muscles which diverts blood from these organs to active skeletal muscle (which is vasodilated due to elevated levels of vasodilators such as adenosine, K+, CO2, lactate, inorganic phosphate, H+ and perhaps hypoxia itself). Therefore, (d) is also correct.

(e) is correct for the same reason (a) is correct. Constriction of arterioles reduces downstream capillary hydrostatic pressure. This increases filtration of fluid which increases the rate of formation of interstitial fluid.
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(Original post by Jpw1097)
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I would say (a) is also correct. Vasoconstriction of arterioles reduces blood flow and hence reduces capillary hydrostatic pressure downstream, therefore mean capillary blood pressure would decrease. This also explains why (e) is correct. I don't see what is wrong with the question. The reason why a severe drop in BP (such as in hypovolaemic shock or anaphylactic shock) causes pre-renal acute kidney injury is because the fall in BP stimulates the baroreflex which augments sympathetic activity. This increased sympathetic activity to the kidney causes constriction of the renal vasculature, which combined with the reduction in BP itself, reduces blood flow to the kidney which in turn reduces GFR - potentially causing AKI.

(b) is incorrect. Ignoring compensatory mechanisms such as the baroreflex (since the widespread constriction of arterioles in itself could be a compensatory mechanism), widespread constriction of arterioles increases the total peripheral resistance which increases mean arterial pressure. The increase in mean arterial pressure would decrease stroke volume as it increases afterload (the load against which the heart must contract to eject blood during systole) which would decrease cardiac output.

(c) is correct. Constriction of arterioles supplying a particular organ reduces the blood flow to that organ, and therefore the blood is diverted to other organs. This is what happens during exercise, for example. There is constriction of arterioles supplying the GI tract, skin (at least initially), kidney and inactive skeletal muscles which diverts blood from these organs to active skeletal muscle (which is vasodilated due to elevated levels of vasodilators such as adenosine, K+, CO2, lactate, inorganic phosphate, H+ and perhaps hypoxia itself). Therefore, (d) is also correct.

(e) is correct for the same reason (a) is correct. Constriction of arterioles reduces downstream capillary hydrostatic pressure. This increases filtration of fluid which increases the rate of formation of interstitial fluid.
yh I guess you're right about (a)
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