this is the 4th lecture in the series of the kidney and the second on the topic of osmoregulation. the last lecture basically looked at the mechanics behind osmoregulation: varying the the "motivation" (the osmotic difference between the tubules and the hypertonic interstitium) and the "permission" (the permeability of the tubules to solute and water). this section looked at the actual neuroendocrine feedback mechanisms in place which regulate fluid volume in response to changes in blood pressure or other signals. the first section introduced the importance of homeostatically maintaining the glomerular filtration rate (the rate at which blood is filtered from the glomerulus into the tubule) and renal plasma flow (the amount of blood that actually flows into the glomerulus). if the RPF and by extension GFR are too high, there is a danger that important nutrients will be simply excreted and not reabsorbed due to the fast perfusion rate through the nephritic tubules. in the reverse case, with a slow GFR / RPF, there is the danger that unwanted solutes will be reabsorbed.
GFR and RPF are thus maintained homeostatically by several different mechanisms against sharp increases or decreases in blood fluid volume and pressure. tubuloglomerular feedback is one such mechanism in which the afferent arteriole is stimulated to constrict in response to elevated blood pressure via a pathway that involves the macula densa sensing more solute particles-- causing the GFR and RPF to decrease back to normal levels. "myogenic" regulation is another, more reflexive feedback mechanism in which blood pressure changes stimulate baroreceptors in the afferent arteriole, which then constricts or dilates, depending if the blood pressure rises or falls, respectively.
we then look at the regulation of reabsorption, which occurs through multiple mechanisms. the first one mentioned is that of starling forces: the balance between hydrostatic and oncotic pressure in the capillaries. the efferent arteriole constricts and causes a pressure gradient such that while the pressure in the afferent arteriole and glomerulus is 60mmHg, the pressure in the efferent arteriole and the capillaries is 20mmHg. this pressure drop aids reabsorption in two ways: it lowers the hydrostatic pressure (less fluid coming into the capillaries), and raises the oncotic pressure (the pressure drop causes more fluid to be filtered, leaving more solutes in the plasma in the capillaries). both of these changes support reabsorption of solutes.
the next few mechanisms are all different molecules / hormones that counter blood pressure drops / increase reabsorption via different mechanisms. angiotensin II is one such molecule, and is created in response to renin release: renin cleaves angiotensin I from angiotensinogen in the kidney and liver, which is then converted to angiotensin II via angiotensin converting enzyme (ACE) in the lungs. angiotensin II has several effects on the kidney which all increase reabsorption. first, it works directly and quickly on the proximal tubules, constricting both the afferent and efferent arterioles.
second, it stimulates the release of aldosterone from the adrenal cortex, which increases the reabsorption in the distal tubules. it does so by upregulating the amount of Na/K pumps and sodium channels in the tubular membrane, thereby facilitating sodium (and thus water, which osmotically follows) reabsorption. aldosterone is also secreted in response to high potassium levels, since upregulating the Na/K pumps would increase potassium excretion.
third, it stimulates the release of ADH from the hypothalamus, which increases the collecting duct's permeability to water and thus aids reabsorption. ADH can also be released in response to blood pressure drops via the sympathetic nervous system, but generally only in response to large fluid drops such as in the case of hemorrage. (ADH is described in greater detail in lecture 3). finally, angiotensin II elicits the thirst impulse in the hypothalamus, causing an increase of fluid volume.
the next mechanism for increasing reabsorption is via the sympathetic nervous system. this pathway starts at the baroreceptors in the aortic and carotid arch, which can sense a drop in blood volume/pressure, stimulating the hypothalamus, which then triggers sympathetic activity from the medulla and spinal cord, which ultimately stimulate constriction of the afferent and efferent arteriole (recall that this increases reabsorption as in the case of angiotensin II above). this pathway can also be triggered by emotional cues such as fright and can produce a response within seconds, whereas the renin/angiotensin response is more on the order of minutes.
we then look at the factors that have the opposite effect: increasing excretion and decreasing reabsorption, generally in response to an increase in fluid volume (ingesting large amounts of liquid), increased Na+ concentration (which causes increased retention of fluid) or increased blood pressure. the first mechanism is via the atrial natriuretic peptide, which is released from the granules of the atrial baroreceptor cells in response to elevated blood pressure. in the kidney, ANP dilates the afferent arteriole, increasing GFR, and also inhibits the release of the molecules mentioned above: angiotensin II, ADH, and aldosterone. "pressure natriuresis" is the other mechanism by which excretion can be increased, and is the process in which increased blood pressure causes increased excretion of solute and water. this is accomplished via two mechanisms: first, the sodium channels and Na/K pumps are downregulated and endocytos-ed in the proximal tubule, decreasing its ability to reabsorb. second, nitric oxide is released from the endothelium of the vasa recta, which causes it to vasodilate- this increases the hydrostatic pressure and decreases the oncotic pressure, which via starling forces decreases the capillaries' absorptive capacity.
questions
1. volume of blood and extracellular fluid is regulated by...
2. sodium reabsorption is regulated by...
3. describe what happens when the GFR deviates too low or high from the homeostatic condition.
4. what regulates RBF (renal blood flow) and GFR?
5. what is the usual problem regarding regulation of renal filtration?
6. describe the tubuloglomerular feedback mechanism when body fluid volume is increased.
7. what does the macula densa release that constricts the afferent arteriole?
8. describe the myogenic mechanism for regulating renal blood flow.
9. what is the pressure drop between the afferent arterioles and the vasa recta and what is it caused by?
10. why is the pressure drop necessary?
11. what is the filtration fraction and how is it derived?
12. what are the two conditions in the capillaries necessary for reabsorption?
13. describe how the efferent arteriole influences reabsorption.
14. what is glomerulotubular balance?
15. where is fine regulation of Na+ levels and extracellular volume is carried out by?
16. what are the neuroendocrine factors that increase reabsorption and decrease excretion?
17. where and in response to what is renin released in the nephron?
18. describe the production of angiotensin II.
19. what does angiotensin II do in the kidneys?
20. describe the actions of angiotensin II on the proximal tubules of the nephron.
21. describe the actions of angiotensin II on the distal tubules of the nephron.
22. how does Captopril work?
23. describe the effect of aldosterone on the the distal tubules.
24. aldosterone is a ... secreted by the ...
25. aldosterone is released in response to...
26. sympathetic reflexes are triggered by...
27. describe how baroreceptor stimulation can lead to sympathetic stimulation of arteriole and tubular cells.
28. describe how sympathetic activity facilitates reabsorption.
29. describe the response time of sympathetic stimulation vs. that of the neuroendocrine stimulation.
30. describe ADH's role in blood volume/pressure regulation.
31. what are the two factors that decrease reabsorption / increase excretion?
32. where is atrial natruretic peptide released from?
33. how does ANP increase excretion?
34. what is pressure natriuresis?
35. what is the mechanism that it uses?
36. what role does nitric oxide play in pressure natriuresis?
37. what is the pressure natriuresis equilibrium point?
38. what are the two factors that counterbalance each other in the vasoconstriction / vasodilation of the vasa recta?
answers
1. the amount of sodium that is excreted
2. neuroendocrine factors.
3. when GFR is too low, unwanted waste products might be reabsorbed. if GRF is too high, nutrients might be excreted.
4. changes in the resistance of the afferent arteriole.
5. renal failure, where a drop of blood pressure causes too significant a drop in the renal filtration rate.
6. increased body fluid volume increases GFR and RBF, which increases solute concentration in the tubule. macula densa senses increased solute concentration and stimulates afferent arteriole to constrict, which decreases GFR and RBF to normal levels.
7. adenosine.
8. the afferent arterioles have barorecptors that are stretched when blood pressure rises, causing them to constrict reflexively, thus lowering the RBF to a normal rate.
9. 60mmHg in the afferent arterioles and 20mmHg in the vasa recta / peritubular capillaries, caused by efferent arteriole capillaries.
10. because higher pressure is necessary for filtration (hence the higher afferent arteriole pressure) and lower pressure is necessary in the capillaries for reabsorption.
11. the filtration fraction is generally 20% and is derived from the glomerular filtration rate / renal plasma flow.
12. capillary hydrostatic pressure is low, and oncotic pressure is high.
13. when the efferent arteriole constricts, the hydrostatic pressure in the capillaries drop and the oncotic pressure (because the filtration fraction increases, leaving more solute in the capillaries) increases. both of these conditions increase reabsorption.
14. a mechanism that ensures that 2/3 of the filtrate is reabsorbed in the proximal tubule despite changes in the GFR.
15. neuroendocrine control of reabsorption in the proximal and distal tubules.
16. angiotensin II, aldosterone, ADH, sympathetic nervous system.
17. in response to blood pressure drop (sensed by baroreceptors in afferent arteriole), or decreased solute concentration sensed by macula densa, renin is released by JGA cells.
18. in the liver and kidney, renin cleaves angiotensin I from angiotensinogen. angiotensin I is converted to angiotnesin II by angiotensin coverting enzyme (ACE) mostly in lungs, but also in heart and kidney.
19. releases aldosterone and promotes reabsorption of Na and water.
20. angiotensin II has a direct, fast effect on the proximal tubules: it constricts the afferent arteriole, the efferent arteriole, and stimulates the proximal tubule to reabsorb more Na+ and water.
21. angiotensin II has a slower more indirect effect on the distal tubules: it stimulates release of aldosterone from adrenal cortex, which increases Na+ and water reabsorption in distal tubules. it also stimulates the hypothalamus to secrete ADH as well as to provoke the thirst response, leading to fluid intake and increased blood pressure.
22. it is an angiotensin converting enzyme-inhibitor, which prevents ADH and aldosterone release, causing less reabsorption and thus lowering fluid volume and blood pressure.
23. increases Na+ reabsorption and K+ secretion. upregulates Na/K pump and Na channels in tubular membrane cells.
24. mineralocorticoid secreted by the adrenal cortex.
25. angiotensin II, high K+ levels in blood, ACTH released from anterior pituitary gland during stress response
26. emotional signals or drops in blood pressure.
27. aortic and carotid baroreceptors stimulate hypothalamus, which triggers sympathetic activity from medulla and spinal cord, activating the pre ganglionic neurons which innervate the prevertebral ganglia, and the post ganglionic neurons which innervate the tubular and arteriolar cells.
28. sympathetic activity stimulates constriction of afferent and efferent arterioles and stimulates renin release, thereby increasing reabsorption (see question 20)
29. sympathetic stimulation produces a response within seconds whereas the response of neuroendocrine stimulation occurs on the order of minutes.
30. baroreceptors can also stimulate ADH release in response to low blood pressure, which stimulates reabsorption in the collecting ducts and increases fluid volume and blood pressure.
31. atrial natriuretic peptide and pressure natriuresis.
32. released from granules in atrial baroreceptor cells.
33. increase GFR by dilating the afferent arteriole. decreases secretion of renin, ADH, and angiotensin II, inhibiting reabsorption of Na+ and water.
34. pressure natriuresis is the compensatory process in which increased blood pressure stimulates increased excretion of solute and water, thereby lowering blood pressure.
35. sodium channels and sodium / potassium pumps are downregulated and removed endocytically in proximal tubule cells, thereby decreasing reabsorption / increasing excretion.
36. nitric oxide is relased from the endothelium of the vasa recta in response to elevated blood pressures; this causes the hydrostatic pressure of the capillaries to go up and the oncotic pressure to go down, decreasing reabsorption and increasing excretion.
37. it is the point on the curve of salt excretion vs. blood pressure where the salt excretion (y axis) equals salt intake -- any deviation from this point will be eventually brought back by the homeostatic mechanisms discussed.
38. nitric oxide and angiotensin II.
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