Lecture 10: Regulation of Na Balance and Extracellular Fluid Volume (Bolser)

Question 1

what does Na balance determine?

Answer 1

the size of the ECF (extracellular fluid) and long term regulation of blood pressure

Question 2

what determines filtered load of Na?

Answer 2

GFR (glomerular filtration rate)

Question 3

what regulates Na excretion?

Answer 3

autoregulation of GFR buffers changes in Na excretion that might occur as a result of large changes in blood pressure

Question 4

what is tubuloglomerular feedback?

Answer 4

it involves the macula densa - “the salt detectors.”

when they notice that Na is increased, it triggers a DECREASE in GFR

Question 5

what is glomerulotubular balance

Answer 5

in NORMAL conditions, the rate of proximal tubular Na reabsorption is adjusted to match the filtration rate by reabsorption of a relatively constant fraction of filtered Na.

It buffers the Na excretion when actue changes in GFR change the filtered load of Na.

Question 6

mechanisms of glomerulotubular balance

Answer 6

1. an increase in the filtered load of organic acids cause a proportional increase in Na reabsorption
2. peritubular capillary starling forces - Increased GFR leads to less water leaving the glomerular capillaries and entering the peritubular capillaries resulting in an increase in peritubular capillary oncotic pressure and a decrease in peritubular capillary hydrostatic pressure. this favors fluid flow from interstitium into the capillaries (reabsorption).
3. loss of glomerulotubular balance during increased Na uptake. GFR increases but percentage of Na reabsorbed drops so the increased Na load is excreted.

Question 7

what is most important about glomerulotubular balance?

Answer 7

even with wildly changing BP and transient increases in filtered load, starling forces are activated so that sudden changes in filtered load are compensated and the net effect is that 2/3 of Na are reabsorbed.

Question 8

what allows Na loads to be excreted with a increase in BP?

Answer 8

hydrostatic pressure increases

fluid uptake into peritubular capillaries decreases

tubular Na reabsorption decreases

Na excretion increases

Question 9

what allows Na loads to be excreted with an increase in extracellular fluid volume?

Answer 9

oncotic pressure decreases

fluid uptake into peritubular capillaries decreases

tubular Na reabsorption decreases

Na excretion increases

Question 10

what effect does aldosterone have on the distal tubule and collecting duct

Answer 10

it acts on principal cells to increase Na lumen permeability to increase Na reabsorption

Question 11

aldosterone does what 3 things to increase Na permeability

Answer 11

1. increases permeability of the luminal border of the cell to Na
2. increased quantity of Na, K-ATPase
3. increased amount of ATP to the ATPase to increase enzyme activity

Question 12

what 4 things regulate aldosterone release?

Answer 12

1. low Na in diet + low Na plasma = increase aldosterone release
2. increase K in plasma = increase aldosterone release
3. increased plasma angiotensin II = increase aldosterone release
4. ACTH necessary for basal release

Question 13

a decrease in plasma volume does what to Na excretion?

Answer 13

decreased plasma volume causes
|
increase renin secretion
|
increase plasma angiotensin
|
increase aldosterone secretion 
|
increase plasma aldosterone
|
increase tubular reabsorption of Na
|
DECREASED Na excretion

Question 14

what effect does an increase in plasma K have in K excretion?

Answer 14

increased plasma K
|
increase aldosterone secretion
|
increase plasma aldosterone
|
increase tubular secretion of K
|
Increase K excretion

Question 15

what is the main way K is secreted?

Answer 15

through principal cells in collecting ducts

Question 16

what can cause increases in plasma potassium?

Answer 16

metabolic acidosis, trauma and/or dietary issues they trigger the following cascade:

increase K plasma
|
increase aldosterone secretion
|
increase plasma aldosterone
|
increase tubular secretion of K
|
Increase K excretion

Question 17

what controls the release of renin

Answer 17

1. afferent arteriole baroreceptors detect ECF volume
2. macula densa - detect decreases in NaCl tubules stimulates renin
3. increased renal sympathetic nerve activity increases renin release by decreasing renal blood flow

Question 18

what is atrial natriuretic peptide (ANP)

Answer 18

it is a hormone released from atria in response to increased ECF volume.

Question 19

in what ways does ANP increase Na excretion?

Answer 19

increasing GFR,

inhibiting Na reabsorption in collecting duct

inhibits renin and aldosterone secretion

Question 20

when is the sympathetic nervous system important in regulating Na reabsorption?

Answer 20

when ECF (extracellular fluid) volume falls

Question 21

what effect does the sympathetic nervous system have in increasing Na reabsorption?

Answer 21

it enhances the release of renin and influences peritubular capillary hydrostatic pressure through an increase in the filtration fraction. this promotes a greater proportion of the renal plasma being diverted into Bowman’s capsule before it reaches the peritubular capillaries. this results in a decrease in peritubular capillary hydrostatic pressure which favors Na reabsorption.

Question 22

what is the filtration fraction formula?

Answer 22

FF = GFR / RPF

Question 23

what effect does an increase in GFR have on Na?

Answer 23

it INCREASES Na excretion

Question 24

what are diuretics?

Answer 24

compounds that alter urine flow primarily by influencing Na reabsorption in various locations of the nephron

Question 25

wht kind of diuretic is the strongest? what is an example of this type of diuretic?

Answer 25

loop diuretics

example of a loop diuretic is lasix (aka furosemide)

Question 26

what do loop diuretics do?

Answer 26

cause Na to remain in diluting segments

Question 27

what is the action of lasix?

Answer 27

inhibits Na reabsorption in thick ascending limb

as a result more Na is delivered to distal nephron and tubular fluid entering the cortex is not diluted. in the presence of ADH less water can be reabsorbed.

less Na is transported into the medullary interstitium, so the medullary concentration gradient is reduced, resulting in less water reabsorbed in the medullary collecting ducts

the end result is diuresis and natriuresis (Na excretion)

Question 28

action of thiazides

Answer 28

1. inhibition of Na-Cl cotransporter in the distal tubule

this results in more Na and water being delivered to the colecting ducts and a diuresis and naturesis

Question 29

what are some examples of potassium-sparing diuretics?

Answer 29

amiloride and spironolactone

Question 30

action of potassium-sparing diuretics

Answer 30

Na channel blockers prevent Na from entering cells thereby reducing reabsorption and inducing diuresis and natriuresis

aldostereone receptor blockers prevent hte actions of aldosterone, resulting in same effects

both types of diuretic reduced the amount of K that is secreted by reducing the availability of intracellular Na.