Homeostasis Series (7) - Ecf [K+]

Front 1

What percentage of total body potassium is intracellular? Extracellular?

Back 1

98% intracellular

2% extracellular

Front 2

Is potassium primarily an intracellular or extracellular ion?

Back 2

Potassium is a primarily intracellular cation, though a fraction is present in the ECF uniformly distributed through the intravascular and interstitial compartments

Front 3

How is the distribution of the K+ between the intracellular and extracellular compartments normally maintained?

Back 3

Na+/K+ ATPase, which pumps 3 sodium ions out of the cell for every 2 potassium ions it pumps into the cell.

Front 4

What is the physiologic role of potassium?

Back 4

Potassium is a critical determinant of the resting membrane potential of cells, which result from the leak of potassium from their interior.

The repolarization of both neurons and myocytes is also critically dependent on the efflux of potassium out of the cell to restore the resting potential.

Front 5

What is the normal range of potassium in the plasma?

Back 5

3.5 – 5.0 mmol/L

Front 6

The ECF sodium concentration varies greatly with changes to total body water. Why is the same not true for the ECF potassium concentration?

Back 6

The ECF plasma concentration is so low that changes to total body water don’t affect it as much (compare 3.5-5.0 mmol/L for potassium to 135-145 mmol/L for sodium)

Slight changes in the distribution of potassium between the intracellular and extracellular compartments, however, can result in wide swings in ECF [K+] due to the massive intracellular K+ stores.

Front 7

Which two factors determine ECF potassium concentration?

Back 7

1) Total body potassium

2) Internal distribution of potassium (intracellular vs. extracellular potassium)

Front 8

What/where is the body’s potassium reservoir?

Back 8

Intracellular compartment, where most of the body’s potassium resides

Front 9

REGULATION OF TOTAL BODY POTASSIUM

Back 9

Good luck!

Front 10

Which two factors determine total body potassium?

Back 10

1) Intake
2) Output

Front 11

What is the main source of potassium in the North American diet?

Back 11

Protein-containing foods, particularly meat

Front 12

What is the source of all of the body’s potassium under normal conditions?

Back 12

The diet

Front 13

What is/are the route/routes through which potassium is eliminated from the body?

Back 13

1) Physiologic elimination via the kidney and GI tract
2) Pathologic elimination via the GI tract and sweat

Front 14

Under physiologic conditions, what percentage of potassium excretion occurs via the kidneys/urine?

What percentage occurs via the GI tract/stool?

Back 14

90% urine

10% stool

Front 15

What is the ONLY factor that can be physiologically altered so as to alter total body potassium?

Back 15

Amount of K+ secreted into the renal lumen

Front 16

POTASSIUM REABSORPTION

Back 16

Good luck!

Front 17

Which two regions of the nephron are primarily responsible for potassium reabsorption?

Back 17

1) Proximal convoluted tubule

2) Thick ascending limb of the loop of Henle

Front 18

What percentage of filtered potassium is reabsorbed in the PCT?

Back 18

70%

Front 19

How does the body regulate the amount of potassium reabsorbed in the PCT?

Back 19

It doesn’t; reabsorption is fixed

Front 20

What percentage of filtered potassium is reabsorbed in the thick ascending loop of Henle?

Back 20

20%

Front 21

What is the mechanism by which potassium is reabsorbed in the thick ascending loop of Henle?

Back 21

Coupled with Na+ and Cl- reabsorption via the Na+/K+/2Cl- symporter on the apical membrane of the tubular cell

Front 22

How does the body regulate the amount of potassium reabsorbed in the thick ascending loop of Henle?

Back 22

It doesn’t; reabsorption is fixed.

Front 23

POTASSIUM SECRETION

Back 23

Good luck!

Front 24

Which region of the nephron is primarily responsible for potassium secretion?

Back 24

Late DCT/collecting duct

Front 25

What are the two principle cell types of the late DCT/collecting duct?

Back 25

Principle cells and intercalated cells

Front 26

What is the mechanism by which potassium is secreted in the principle cells?

Back 26

Potassium secretion is coupled inescapably to sodium reabsorption via the Na+/K+ ATPase on the basolateral membrane of the principle cell.

Na+ reabsorption is the primary action of the principle cell, with K+ secretion occurring as a kind of byproduct.

The degree of sodium reabsorption thus determines the degree of potassium secretion.

The degree of sodium reabsorption is itself determined by the hormone aldosterone, which increases the permeability of the apical membrane of the principle cells to sodium, thereby increasing sodium reabsorption and increasing sodium secretion.

Front 27

Increased aldosterone levels result in __________ Na+ reabsorption and __________ potassium secretion by the principle cells.

Total body potassium __________.

Back 27

Increased Na+ reabsorption

Increased K+ secretion (and therefore decreased total body potassium)

Front 28

Increased aldosterone levels result in __________ Na+ reabsorption and __________ potassium secretion by the principle cells.

Total body potassium __________.

Back 28

Decreased Na+ reabsorption

Decreased (and therefore increased total body potassium)

Front 29

What is the primary role of the intercalated cell?

Back 29

Active transport of protons against their concentration gradient into the lumen for excretion from the body.

These protons are byproducts of bicarbonate production by the intercalated cells.

Front 30

What is the relationship between proton secretion and potassium secretion by the intercalated cell?

Back 30

Increased H+ secretion results in decreased K+ secretion

Proton secretion is the intercalated cell's priority, so tough luck for K+!!!

Front 31

Summarize the physiologic regulation of total body potassium.

Back 31

The only mechanism the body has of regulating total body potassium is through regulating potassium secretion by the principle cells.

This is done in a roundabout manner, as secretion of potassium by the principle cells occurs only secondary to sodium reabsorption.

In times of increased ECF K+, then, the body moves to secrete more K+. This is accomplished through INCREASING sodium reabsorption via the hormone aldosterone.

Aldosterone is released directly by the adrenal cortex in times of hyperkalemia. This effect occurs independently of the RAAD system.

Front 32

What is the the body's move to decrease total body potassium?

Back 32

Increase K+ secretion by principle cells by increasing Na+ reabsorption by increasing aldosterone release from the adrenal cortex.

Front 33

What is the body's move to increase total body potassium?

Back 33

Decrease K+ secretion by principle cells by decreasing Na+ reabsorption by decreasing aldosterone release from the adrenal cortex.

Front 34

The kidney handles sodium, hydrogen, and potassium ions in a closely linked manner.

What is the hierarchy of importance of these processes for the kidney?

Back 34

Na+ reabsorption takes priority, then hydrogen ion secretion, then finally K+ secretion

Front 35

Why is potassium secretion so low on the list of the kidney's priorities?

Back 35

K+ is predominantly the property of the ICF. Individual cells regulate their own internal composition. The kidney is responsible for regulating the ECF.

Front 36

INTERNAL DISTRIBUTION OF POTASSIUM

Back 36

Good luck!

Front 37

Name three factors determining the distribution of potassium between the intracellular into the extracellular compartments.

Back 37

1) Insulin – insulin powers up the Na+/K+ pump, driving K+ into cells and reducing the ECF [K+]

2) Beta-adrenergic stimulation – activation of beta-adrenergic receptors powers up the Na+/K+ pump, driving K+ into cells and reducing the ECF [K+]

3) pH – K+ is exchanged for H+ ions entering or exiting the cell in order to maintain ECF electroneutrality

Front 38

What effect does insulin have on ECF [K+]?

Back 38

Decreases ECF [K+]

Front 39

What effect does beta-adrenergic stimulation have on ECF [K+]?

Back 39

Decreases ECF [K+]

Front 40

What effect does acidosis have on ECF [K+] and why?

Back 40

Acidosis increases ECF [K+]

During acidosis, H+ ions enter cells for intracellular buffering. K+ exits cells in exchange for the H+ ions in order to maintain the electroneutrality of the ECF.

Front 41

What effect does alkalosis have on ECF [K+] and why?

Back 41

Alkalosis decreases ECF [K+]

During alkalosis, H+ ions leave cells (and their intracellular buffers). K+ ions enter cells in exchange for the H+ ions in order to maintain the electroneutrality of the ECF.

Front 42

SUMMARY

Back 42

Good luck!

Front 43

Which two broad factors determine ECF [K+]?

Back 43

1) Total body K+
2) Internal distribution of K+

Front 44

Which two factors determine total body K+?

Back 44

1) Intake
2) Output

Front 45

Which two factors determine K+ excretion by the kidney?

Back 45

1) Reabsorption (PCT, loop of Henle)
2) Secretion (late DCT, collecting duct)

Front 46

How does the body regulate total body potassium?

Back 46

Weak regulation of K+ secretion in kidneys via release/restriction of aldosterone as direct response to changes in ECF [K+]

Front 47

Name three factors determining the internal distribution of potassium between the intracellular and extracellular compartments.

Back 47

1) Insulin

2) Sympathetic stimulation

3) pH