test 10

Question 1

Potassium content control

Answer 1

 Tightly controlled
 Usually changes less than ± 0.3 mEq/liter
 Cell functions very sensitive to changes
 Resting membrane potentials
 98% located intracellular volume with only 2% extracellular
 Daily intake usually ranges between 50 mEq/liter to 200 mEq/liter
 Small changes in extracellular K+ can easily lead to hyper or hypokalemia
 Only 5 to 10% of intake removed by feces (rest removed by kidneys
 Movement between intra and extracellular compartments possible (first line of defense against changes in ECF concentration

Question 2

Factors that shifts K+ into cells (Potential hypo)

Answer 2

• increase in Insulin
• increase in Aldosterone
(also es K+ secretion)
• increase in Β-adrenergic stimulation
• Alkalosis (high H+ reduces action of Na/K ATPase = less K+transfer into cells)

Question 3

Factors that shifts K+ out of cells (Potential hyper)

Answer 3

• Insulin deficiency (diabetes mellitus)
• Aldosterone deficiency (Addison’s disease)
• Β-adrenergic blockade
• Acidosis
• Cell lysis
• Strenuous exercise
• Increased extracellular fluid osmolarity

Question 4

Renal Excretion of Potassium determined by

Answer 4

 Rate of potassium filtration
 Rate of potassium reabsorption
 Rate of potassium secretion

Question 5

K+ filtration

Answer 5

 180 liter/day x 4.2 mEq/liter = 756 mEq/day

Question 6

K+ reabsorption

Answer 6

 Consistent Reabsorption
 65% proximal tubule
 25 to 30% in loop (mainly thick ascending segment)

Question 7

K+ secretion

Answer 7

 Distal tubule & cortical collecting tubule

 Approximately 1/3 of excreted potassium

Question 8

Distal Tubule & Cortical Collecting Tubule: High potassium intake

Answer 8

 Distal tubule & cortical collecting tubule increase potassium secretion
 Very strong mechanism – rate of potassium excretion can exceed amount of potassium being filtered

Question 9

Distal Tubule & Cortical Collecting Tubule: Low potassium intake

Answer 9

 Secretion rate decreases
 Can decrease secretion to point where there is net reabsorption
 Excretion can fall to 1% of filtered potassium (756 mEq/day x 0.01 = 8 mEq/day)

Question 10

Principal Cells – Potassium Secretion

Answer 10

 Make up 90% of cells in late distal and cortical collecting tubule
 Secretion driven by Na-K ATPase in basolateral border of cells
 Move K+ into cell setting up concentration gradient
 Concentration gradient drives diffusion from cell into tubular lumen
 Tubular membrane contains special channels for K+ diffusion
 Usually provide high permeability for K+ movement out of the cell
 BK: High conductance Big K channels
 ROMK: Renal outer medullary K channels

Question 11

Intercalated Cells – Potassium Reabsorption

Answer 11

 Reabsorb potassium especially during potassium depletion
 Could be related to H-K ATPase
 Located tubular membrane
 Pumps H+ from tubular cell into lumen (secretion)
 Pumps K+ from tubular lumen into cell (reabsorption)
 K+ diffuses from cell into interstitial space via basolateral membrane
 Major effect only during potassium depletion

Question 12

Type A intercallated cells

Answer 12

K+ reabsorption

H+ secretion

Question 13

Type B intercalated cells

Answer 13

K+ secretion

H+ reabsorption

Question 14

Three factors that CONTROL rate of K+ secretion

Answer 14

 Activity of Na-K ATPase
 Electrochemical gradient for K+ movement from the blood to the tubular lumen
 Permeability of tubular membrane to K+

Question 15

Stimulation of Potassium Secretion

Answer 15

 Increased extracellular [K+]
 Increased [aldosterone]
 Increased tubular flow rate
 Increased [H+] will DECREASE potassium secretion

Question 16

Increased Plasma Potassium

Answer 16

 Always a certain level of secretion even at normal [K+
 Increased [K+] stimulates action of Na-K ATPase
 More K+ moved into cell from interstitial space which increases gradient from cell interior to tubular lumen
 [K+] of renal interstitial fluid increases (increased plasma concentration) which decreases amount of K+ diffusing from cell interior into interstitial space
 Increase [K+] in plasma stimulates release of aldosterone

Question 17

Increased Aldosterone

Answer 17

• increases rate of sodium reabsorption by late distal tubule and collecting duct
   Increases activity of Na-K ATPase
   So an increase in sodium reabsorption will also increase potassium secretion
   Increases tubular membrane permeability for potassium

Question 18

Plasma Potassium & Aldosterone

Answer 18

 Great example of negative feedback control system
 Factor being controlled (potassium) has feedback effect on controller (aldosterone)
 Small change in plasma [K+] produced huge change in aldosterone concentration

Question 19

Regulation of Potassium Excretion

Answer 19

• Anything that affects our ability to produce aldosterone will have a big effect on potassium excretion!!
• High aldosterone (primary aldosteronism) -> Hypokalemia -> hypernaturemia
• Low aldosterone (Addison’s disease) -> Hyperkalemia

Question 20

Increased K+ intake with intact aldosterone feedback

Answer 20

 Big change in intake (x7 increase) small change [K+] (4.2 to 4.3 mEq/liter)

Question 21

Increased K+ intake with blocked aldosterone feedback

Answer 21

 Big change in intake (x7 increase) big change in [K+] (3.8 to 4.7 mEq/liter)

Question 22

Relationship between tubular flow rate and potassium secretion

Answer 22

• greatly affected by potassium intake
   Higher the intake, the greater the effect created by tubular flow
   Increased distal tubular flow rate will increase potassium secretion
   Increased tubular flow rate can be caused by volume expansion; high sodium intake; specific diuretics

Question 23

Mechanism for Tubular Flow Effect

Answer 23

 As potassium diffuses into tubular lumen, the increase in luminal concentration will decrease the gradient thus decreasing the movement of potassium
 Increased tubular flow carries potassium away thus helping to preserve the gradient. The higher the flow the better the gradient is preserved, the more potassium is secreted

Question 24

Preserving K+ Excretion With Changing Na+ Intake: Assume high Na+ intake

Answer 24

 Aldosterone secretion decreases which will produce a decrease K+ secretion
 BUT since sodium reabsorption is decreased, overall distal tubular flow is increased which results in an increase in K+ secretion
- THE TWO OFF SET EACH OTHER

Question 25

Acute Acidosis Decreases K+ Secretion

Answer 25

 Acidosis (increase H+) reduces potassium secretion  Reduces the activity of Na-K ATPase – decreases driving force for moving potassium from cell interior to tubular lumen  Prolonged acidosis produces increased potassium excretion  Result of decreased reabsorption of sodium chloride and water in proximal tubule and increased distal tubular flow  Alkalosis (decrease H+) increases potassium secretion

Question 26

Plasma Calcium

Answer 26

 Total calcium in plasma: 5 mEq/liter  50% in ionized form  40% bound to plasma protein  Normal ion concentration: 2.4 mEq/liter (1.2 mmol/liter)  Hypocalcemia: increases muscle and nerve excitability (hypocalcemic tetany)  Hypercalcemia: depressed neuromuscular excitability which can lead to cardiac arrhythmias

Question 27

Calcium Stores in Body

Answer 27

 99% of calcium stored in bone  HUGE reservoir – if plasma concentration drops, body will move calcium from the bone – if plasma concentration rises, body will move calcium back into the bone  1% present in intracellular space and cell organelles  0.01% present in extracellular fluid

Question 28

Hypokalemia ECG

Answer 28

U wave

Question 29

Hyperkalemia ECG

Answer 29

peaked T wave

Question 30

Parathyroid Hormone (PTH)

Answer 30

 PTH most important control agent for calcium  90% excreted via gastrointestinal tract (feces) (≈900 mg/day)  10% excreted via kidneys (urine) (≈100 mg/day)

Question 31

PTH regulation accomplished through 3 actions:

Answer 31

 Stimulation of bone resorption of calcium (Ca2+ release)  Stimulation of vitamin D which stimulates calcium reabsorption by intestines  Direct stimulation of renal tubule reabsorption of calcium

Question 32

As extracellular calcium concentration falls:

Answer 32

 Parathyroid gland directly stimulated to increase secretion of PTH  Increased PTH concentration stimulates bone to increase release of bone salts (resporption) which includes the release of large amounts of calcium

Question 33

As extracellular calcium concentration increases

Answer 33

 Parathyroid gland decreases PTH secretion |  Decreased PTH concentration decreases salt resporption to point where calcium will be added to the bone

Question 34

Calcium Excretion

Answer 34

 Freely filtered, reabsorbed BUT NOT secreted - only filter very small %  Proximal tubule: 65% filtered load reabsorbed  Loop of Henle: 25 to 30% filtered load reabsorbed  Distal tubule / Collecting tubule: 4 to 9% filtered load reabsorbed  Normally only 1% of filtered load is excreted  Changes as plasma concentration changes (i.e. intake changes)

Question 35

Proximal Tubule Reabsorption of Ca++

Answer 35

 80% of amount reabsorbed carried by water via paracellular pathway  20% of amount reabsorbed via a transcellular pathway - PTH has no effect (just follows Na and H20)

Question 36

Thick Ascending Loop – Ca++ Reabsorption

Answer 36

 Paracellular pathway accounts for 50% of reabsorption in loop (passive)  Transcellular pathway accounts for 50% of reabsorption in loop (active stimulated by PTH)

Question 37

Distal Tubule – Ca++ Reabsorption

Answer 37

 Almost all transport via Transcellular pathway (active)  Increased [PTH] increases Ca++ reabsorption  Reabsorption also increased by Vitamin D and calcitonin

Question 38

increase Ca++ Reabsorption

Answer 38

- increase [PTH] (primary controller) - decrease in EC fluid volume - decrease in BP - increase plasma phosphate - metabolic acidosis - vitamin D

Question 39

Decrease Ca++ reabsorption

Answer 39

- decrease [PTH] - increase in EC fluid volume - increase in BP - decrease plasma phospate - metabolic alkalosis

Question 40

Phosphate

Answer 40

 Normal tubular maximum of 0.1 mMol/minute  If filtered load under Tmax, all phosphate reabsorbed  If filtered load over Tmax, phosphate is excreted  Plasma threshold level approximately 0.8 mMol/liter  Normal plasma concentration around 1 mMol/liter – Large intake of phosphate each day (milk & meat)

Question 41

Phosphate Reabsorption

Answer 41

 Proximal Tubule: 75 to 80% of filtered phosphate reabsorbed  Enters cells from lumen via Na-Phosphate co-transport mechanism  Loop of Henle: Very small amounts  Distal Tubule: 10% of filtered phosphate reabsorbed  Collecting Tubule: Very small amounts  Approximately 10% of filtered phosphate is excreted

Question 42

Tmax for phosphate

Answer 42

 Tmax can change based on intake |  Low intake, Tmax will increase over time

Question 43

PTH effect on Phosphate

Answer 43

 As PTH increases bone resorption of calcium, phosphate is also resorbed  increase [PTH] decreases the Tmax for phosphate so less phosphate is reabsorbed and more is excreted

Question 44

Magnesium

Answer 44

 >50% stored in bone  Most of what is left is located in the intracellular volume  <1% located in extracellular volume  TOTAL plasma magnesium = 1.8 mEq/liter BUT >50% is bound to plasma proteins so free ionized is 0.8 mEq/liter  Daily intake ≈ 250 to 300 mg/day BUT only 50% is actually absorbed by the gastrointestinal tract (125 to 150 mg/day)  Renal excretion of magnesium is ≈ 10% to 15% of filtered load

Question 45

Magnesium Reabsorption

Answer 45

 Proximal Tubule: 25% of filtered load  Loop of Henle: Primary site of reabsorption – 65% of filtered load  Distal Tubule / Collecting Tubule: <5% of filtered load

Question 46

increase [magnesium] results in

Answer 46

- decrease reabsorption and increase excretion

Question 47

increase in EC fluid volume results in what for Mg

Answer 47

- decrease reabsorption and increase excretion

Question 48

increase in [Ca2+] results in

Answer 48

- decrease reabsorption and increase excretion