Renal regulation of K, Ca, Mg

Question 1

K concentrations

Answer 1

-cells are very sensitive to changes in K concentrations
-increases in levels (hyperkalemia) can cause cardiac arrest, decreases (hypokalemia) can also be critical

Question 2

Normal plasma K levels

Answer 2

4.2 mEq/L
-if increased more than 3-4 mEq/L can cause cardiac arrest

Question 3

Extracellular vs. intracellular K levels

Answer 3

Extracellular: 2%
Intracellular: 98%

Question 4

Steps to K adjustments

Answer 4

1.Transport K between extra and intracellular K stores (fast response)
2. Adjusting the changes in plasma K concentrations is by renal excretion (slow response)

Question 5

Insulin release

Answer 5

-causes shift of K into the cell

Question 6

Aldosterone impact on K

Answer 6

-shifts K into the cells

Question 7

Beta-adrenergic stimulation

Answer 7

-shifts K into the cells
-Epi increases K uptake

Question 8

Alkalosis

Answer 8

shifts K into the cells

Question 9

Acidosis

Answer 9

-shifts K out of cells
-due to reduction of Na/K pump function

Question 10

Cell lysis

Answer 10

-shift K out of cells
-occurs due to severe muscle injury

Question 11

Strenuous exercise

Answer 11

-shift K out of the cells
-due to skeletal muscles will release K

Question 12

Increased extracellular fluid osmolarity

Answer 12

-shift K out of cells
-due to cells becoming dehydrated and intracellular K goes up and cells will send K out

Question 13

What effects K+ excretion?

Answer 13

1.filtration rate- decreased GFR will lead to hyperkalemia
2. Tubular reabsorption (mainly proximal tubules)
3. K secretion in late distal and collecting tubules (important for day-to-day adjustments)
>principle cells important for secretion. If intake of K is high, K secretion increases even more than the filtered amount

Question 14

K Excretion

Answer 14

-Secretion by principle cells which make up the majority of epithelial cells in late distal and collecting tubules

Question 15

Steps of K excretion

Answer 15

1. Uptake from interstitium into the cells by Na/K pump
2. Diffusion of K into the lumen (membrane is permeable to K by using K channels)

Question 16

Factors of K excretion

Answer 16

-Na/K pump
-concentration gradient
-permeability of luminal membrane

Question 17

Factors affecting K secretion

Answer 17

1.Increased extracellular K
2.Increased aldosterone
3.increased tubular flow rate
4.acidosis (increased extracellular H+)

Question 18

Increased extracellular K

Answer 18

-important factor and works very quickly when plasma concentration of K is increased
-3 mechanisms:
1.stimulates Na/K pump, moves K into epithelial cells and diffusion into tubules
2.Reduces leakage of intracellular K to the interstitium
3.stimulates aldosterone secretion and K secretion

Question 19

Increased aldosterone

Answer 19

-aldosterone enhances the function of Na/K
-increases permeability of apical cell membrane of K
-extracellular concentrations of K and aldosterone have a positive feedback loop

Question 20

Increased tubular flow rate

Answer 20

-occurs when volume expansion, high Na intake, treatment with diuretics
-when high tubular flow rate, secreted K is quickly flushed and net K secretion is stimulated
-important when Na intake is high. High Na causes less aldosterone release and less K secretion, but since the tubular flow rate is increased, more K secretion is stimulated which helps to excrete excess K

Question 21

Acidosis

Answer 21

-short term: reduces K secretion because Na/K pump is inhibited
-chronic: increases K secretion because Na, Cl, water reabsorption is reduced and tubular flow increases and leads to more K secretion

Question 22

Extracellular K and aldosterone feedback loop

Answer 22

1.Increased extracellular K signals more aldosterone release
2.More aldosterone increases K excretion
3.More K excretion reduces extracellular K and that reduces aldosterone

Question 23

Calcium normal concentration `

Answer 23

2.4mEq/L

Question 24

Hypocalcemia vs. hypercalcemia

Answer 24

-Hypo= reduced nerve and muscle excitability
-Hyper= suppresses neuromuscular excitability and may cause cardiac arrhythmia

Question 25

3 forms of calcium in plasma

Answer 25

1.calcium ions (50% active form)
2.Protein-bound (40%)
3.Non-ionized form (10%)

Question 26

Calcium storage

Answer 26

-99% stored in bone, 1% inside cells, 0.1% in extracellular fluid

Question 27

Dietary calcium excreted

Answer 27

-90% excreted in feces

Question 28

Acidosis impact on calcium

Answer 28

Acidosis reduces the binding of calcium to plasma proteins

Question 29

Parathyroid hormone regulation of calcium

Answer 29

1.Stimulates bone reabsorption
2.Activates vitamin D and interstitial reabsorption of calcium
3.Directly increases renal calcium reabsorption

Question 30

Calcium excretion, filtration and reabsorption distributions

Answer 30

-calcium is not secreted by kidneys so:
Excretion rate=filtration -reabsorption

Filtration: all ionized calcium
Reabsorption: 99% (65% proximal, 25-30% in loop of Henle, 4-9% in distal and collecting tubules)

**kidneys regulate plasma calcium concentrations by adjusting reabsorption

Question 31

Calcium reabsorption in proximal tubules

Answer 31

80% paracellular (dissolved in water)

20% transcellular reabsorption

**independent of parathyroid hormone

Question 32

Mechanism of transcellular calcium reabsorption

Answer 32

-transcellular within the proximal tubules
1.diffusion at apical membrane due to electrochemical gradient (calcium channels)
2. Calcium pumps at the basolateral membrane
3. Calcium-sodium counter transport at basolateral membrane (3Na in, 1Ca out)

Question 33

Calcium reabsorption in loop of Henle

Answer 33

-only in thick ascending limb
-50% paracellular (diffusion due to the more positive charge in the lumen)
-50% transcellular (regulated by PTH)

Question 34

Calcium reabsorption in distal tubules

Answer 34

-mainly active transport
-mechanisms similar to those in proximal tubules

-affected by PTH and vitamin D (both stimulate calcium reabsorption)

Question 35

How does calcium excretion change when there is increased volume or arterial pressure?

Answer 35

With increased volume or arterial pressure:
-sodium and water reabsorption is reduced and since calcium reabsorption in proximal tubules is dependent on water reabsorption, calcium reabsorption will be reduced too

Question 36

What is the effect of phosphate on calcium reabsorption?

Answer 36

Phosphate increases PTH and therefore, increases
calcium reabsorption

Question 37

Magnesium storage

Answer 37

-50% in bones
-49% in cells
-1% in extracellular fluid

-more than half the Mg in plasma is bound to proteins

Question 38

Mg excretion

Answer 38

-Mg intake is usually high, and kidneys need to excrete about half of absorbed Mg
>10-15% of filtered Mg is excreted

Question 39

How is Mg regulated?

Answer 39

-regulation mainly by adjusting the reabsorption

Question 40

Distribution of Mg reabsorption

Answer 40

Loop of Henle: 65%

Proximal Tubules: 25%

Question 41

What increases Mg excretion?

Answer 41

-increased extracellular Mg
-increase extracellular Ca
-increases extracellular volume

Question 42

Phosphate excretion

Answer 42

-follows an overflow pattern. The phosphate transport max is 0.1 mM/min so there is usually a higher intake so there is continued urine phosphate excretion

Question 43

Phosphate reabsorption distribution

Answer 43

Proximal tubules: 75-80%
Distal tubules: 10%

Question 44

How much phosphate is excreted?

Answer 44

10%

Question 45

Reabsorption of Phosphate in proximal tubules

Answer 45

-transcellular transfer by co-transport of Na

Question 46

Phosphate change of transport maximum

Answer 46

-a low phosphate intake leads to increased transport maximum over time

Question 47

Regulation of phosphate

Answer 47

regulated by PTH
>PTH decreases the transport maximum of phosphate and therefore increases its excretion