Lecture #18: Renal Regulation of Ion Concentrations Flashcards Preview

Human Physiology 1 -- Zach H. > Lecture #18: Renal Regulation of Ion Concentrations > Flashcards

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1

What is the normal intracellular concentration of potassium?

140 mEq/L

2

What is the clinical significance of excess extracellular potassium?

> An increase of 3 to 4 mEq/L can lead to cardiac arrhythmias.

> Higher concentrations can lead to cardiac arrest or fibrillation.

3

Why must the kidneys be able to rapidly adjust extracellular potassium concentration?

Kidneys must adjust potassium excretion rapidly and precisely in response to wide variations in intake.
- mainly involves distal and collecting tubules
- intake of potassium from a single meal can be as high as 50 mEq
* extracellular fluid contains 2% of total body potassium

4

What is the total amount of potassium in the extracellular and intracellular body compartments?

Extracellular = 4.2 mEq/L x 14L = 59 mEq

Intracellular = 140 mEq/L x 28 L = 3920 mEq

5

What is the overall effect of aldosterone secretion on potassium excretion?

Increase in extracellular potassium stimulates increase in aldosterone secretion.

6

What part of the renal tubule is responsible for potassium reabsorption and what part is responsible for potassium secretion?

Potassium Reabsorption:
- proximal tubule
- ascending limb of Henle

Potassium Secretion:
- late tubule
- collecting duct

7

Describe the mechanism by which principal cells secrete potassium.

> NOTE: potassium secretion by principal cells is stimulated by potassium concentration and aldosterone.

> Na/K ATPase pumps sodium from intracellular into renal interstitial fluid of the kidney and pumps potassium into the principal cell. This creates a concentration gradient so that sodium can be reabsorbed from the tubular lumen into the principle cell through the ENaC channel. Potassium is then secreted into the tubular lumen because of the concentration gradient maintained by the Na/K ATPase.



***Figure 29-3 show this mechanism well.

8

What cell in the kidney tubule system reabsorbs sodium and secretes potassium?

Principal Cells

9

What factors stimulate principal cells to secrete potassium?

> potassium concentration

> aldosterone




** Note that potassium secretion by principal cells is stimulated by potassium concentration and aldosterone. **

10

Describe the relationship between tubular flow rate and potassium secretion.

An increase in tubular flow rate leads to an increase in potassium secretion rate. Look at figure 29-9 because the relationship curves are different depending on high, low, or normal potassium diet.

11

Describe and explain why high sodium intake has little effect on potassium excretion.

*This is answered by figure 29-10

Increased sodium intake decreases aldosterone secretion and, therefore, potassium excretion.

However, increased sodium intake also increases GFR and decreases proximal tubular reabsorption of sodium. This leads to an increase in distal tubular flow rate and increase in potassium excretion.

Therefore, high sodium diet leads to little change in potassium excretion.

**The increase in sodium intake leads to decreased aldosterone, which inhibits potassium secretion in cortical collecting ducts. Increased sodium intake also causes increased GFR and decreased proximal tubular sodium reabsorption, which both lead to an increased distal tubular flow rate, thus increasing potassium secretion in cortical collecting ducts. The effect of these two lead to an unchanged potassium excretion in the cortical collecting ducts.

12

What is the overall effect of aldosterone secretion on potassium excretion?

Increase potassium excretion

13

What is the normal extracellular concentration of potassium ion?

Precisely regulated at 4.2 mEq/L (+/- 0.3 mEq/L)

14

How does plasma pH effect the amount of plasma calcium bound to plasma proteins?

> Changes in plasma pH on calcium binding:
- Acidosis -> less calcium is bound to the plasma proteins.

- Alkalosis -> more calcium is bound to the plasma proteins.


* low pH = less calcium binding
* high pH = more calcium binding

15

List the effects that parathyroid hormone (PTH) have.

1) stimulates bone reabsorption

2) stimulates activation of vitamin D

3) indirectly increases tubular calcium reabsorption

16

How much filtered calcium is reabsorbed, and where in the kidney tubule does this occur?

>Reabsorption in proximal tubule:
- About 99% of filtered calcium is reabsorbed!
*65% via paracellular route
*20% via transcellular route

>Reabsorption in loop of Henle:
- restricted to thick ascending limb
*50% through paracellular route
-- passive diffusion and slight + charge of tubular lumen
*50% via transcellular route stimulated by PTH

>Reabsorption in distal tubule:
- almost entirely via active transport
- calcium-ATPase pump in basolateral membrane
- stimulated by PTH

**Factors that regulate tubular calcium reabsorption**
> increased levels PTH (decreases calcium excretion)
> plasma concentration of phosphate (decrease Ca excretion)
> metabolic acidosis (decreases Ca excretion)

17

List the factors that decrease calcium reabsorption.

1) decreased levels of PTH

2) metabolic alkalosis

3) increased level plasma concentration of Ca

18

What mechanism controls phosphate excretion?

Overflow Mechanism
* [phosphate] all filtered phosphate is reabsorbed

* [phosphate] > 0.1 -> excess is secreted

- 75-80% reabsorbed in proximal tubule
> transcellular pathway
- 10% reabsorbed in distal tubule

19

What is the role of PTH in phosphate reabsorption?

> PTH promotes bone reabsorption ->
- increased [phosphate] in extracellular fluid

> PTH -> decreases transport maximum for phosphate by renal tubules ->
- greater loss of phosphate in urine

20

What effects do insulin and catecholamines have on extracellular potassium levels?

Insulin -> stimulates potassium uptake by cells.

Catecholamines:
-> beta-adrenergic stimulation (epinephrine):
- stimulates potassium uptake by cells
-> beta-adrenergic receptor blockers -> hyperkalemia

21

What are the relationships of Conn's syndrome and Addison's disease to aldosterone secretion and potassium levels?

> Hypokalemia:
- excess secretion of aldosterone (Conn's syndrome)
* low potassium/high aldosterone = Conn's syndrome

> Hyperkalemia:
- deficiency in aldosterone secretion (Addison's disease)
* high potassium/low aldosterone = Addison's disease

22

What effect does metabolic acidosis have on extracellular potassium concentration?

increase in [extracellular K+]

* increase [H+] -> reduction in activity of Na/K-ATPase pump, which causes a decrease in cellular uptake of K+, thus causing an increase in [extracellular K+]

23

What effect does metabolic alkalosis have on extracellular potassium concentration?

decreased [extracellular K+]



24

What effects do cell lysis, strenuous exercise, and increased extracellular fluid osmolarity have on extracellular potassium concentration?

Cell lysis, strenuous exercise, and increased extracellular fluid osmolarity all cause hyperkalemia, which is an increase in extracellular potassium concentration.

25

Describe the role of intercalated cells in controlling potassium levels.

Intercalated cells reabsorb K+ during K+ depletion.
- possible through a H/K-ATPase
- secrete H+ into tubular lumen

26

In which portion of the kidney tubule does the most potassium reabsorption occur?

proximal tubule

27

Which of the following cells play a major role in the secretion of potassium?
a) intercalated cells
b) principal cells
c) chief cells
d) podocytes

b) principal cells

28

Which of the following plays a major role in stimulating potassium secretion by the kidney tubule?
a) aldosterone
b) angiotensin II
c) sodium ion
d) PTH

a) aldosterone

29

What percentage of calcium is stored in the bones?

99%

30

What percentage of potassium is excreted in a normal human?

1%