Physiology 2: Reabsorption and Secretion of Potassium, Calcium

Question 1

Definition and effects of HYPERkalemia

Answer 1

Increased blood potassium leads to increased cell excitability and decreased Em.

ECG shows:
tall-thin T-waves in ECG
increased PR interval
depressed ST segment

Question 2

Definition and effects of HYPOkalemia:

Answer 2

decreased blood potassium and leads to decreased cell excitability and increased Em.

ECG shows:
increased QT interval
Inverted or Flat T-waves
Depressed ST segment
Appearance of U-waves

Question 3

What is the primary way to control K+ levels?

Answer 3

K+ excretion is controlled by adjusting the rate of tubular K+ secretion, not the rate of K+ reabsorption in the distal nephron.

Question 4

Which cells in the distal nephron mediate K+ reabsorption?

Answer 4

Intercalated cells

Question 5

By what means is K+ reabsorbed in distal nephron?

Answer 5

Since K+ enters against electrochemical gradient, K+ must be ACTIVE transport.

Believed to have a K+/H+ ATPase and/or a K+ATPase (different than the NaK-ATPase).

Question 6

What cells in the distal nephron mediate K+ secretion?

Answer 6

Principal cells, and controlled in-part by Aldosterone

Question 7

How is K+ moved into the cells across the basolateral membrane in Principal Cells?

Answer 7

NaK-ATPase pump, moving 2K in and 3Na out.

Question 8

Effects of Increase plasma K+ levels:

Answer 8

Enhances Na-K-ATPase pump activity
Stimulates insulin release - promotes K+ uptake in muscles and liver
Stimulates epinephrine release from adrenal medulla that promotes K uptake in muscles and liver via B2-receptors
Stimulates the release of aldosterone from the adrenal cortex which increase plasma K excretion
Enhances K-H exchange in muscles, RBCs and liver.

Question 9

What happens to [K] in plasma in the face of Alkalosis?

Answer 9

As pH increases, plasma [K] increases

Question 10

What happens to [K] in plasma in the face of acidosis (decreasing pH)?

Answer 10

As pH decreases (Acidosis), plasma [K] decrease.
Acidosis causes entry of extracellular H into cells in exchange for K leading to mild HYPERkalemia and reduced K secretion.

Question 11

Hyperkalemia effect on aldosterone?

Answer 11

Stimulation of Aldosterone secretion.

Question 12

Where in the kidney is the reabsorption of Ca++ the greatest? By what means?

Answer 12

Proximal tubule (~70%).
Passive process and follows the establishment of a favorable concentration gradient.

Question 13

How is Ca++ absorption facilitated in the loop of Henle?

Answer 13

A positive lumen potential is established by the Na-K-Cl2 pump by recycling of K into the lumen.

Question 14

Consequences of a loop diuretic on Ca absorption?

Answer 14

Decreases because blocking of Na-K-Cl2 pump, decreases Na+ and Ca++ reabsorption.

Question 15

Actions/effects of Parathyroid Hormone (PTH) on calcium?

Answer 15

increases resorption of bone which increases plasma Ca++ and phosphate
increases Ca++ reabsorption in the dotal tubule
increases phosphate excretion (decreases reabsorption)

Question 16

Where in the kidney is phosphate reabsorption greatest?

Answer 16

Proximal tubule (~80%)

Question 17

Importance of phosphate that remains in the tubule?

Answer 17

It is a titratable acid, an important tubular H+ acceptor that helps to excrete the daily acid load

Question 18

Net Effect of PTH:

Answer 18

Decreases phosphate reabsorption by decreasing the activity of the 2Na-phosphate co-transporter and decreasing the tubular maximum (™) for phosphate = increase in phosphate excretion.