Lecture 18: Renal regulation of ion concentrations-Exam 3

Question 1

Amount of potassium in body compartments

Answer 1

Extracellular: 4.2 mEq/L x 14L=59 mEq
Intracellular: 140 mEq/L x 28L= 3920 mEq

Question 2

Major factors responsible for K+ excretion

Answer 2

• Direct influence on distal tubules and collecting ducts via increase in extracellular [K+]
• Effect of aldosterone secretion on K+ excretion

Question 3

Sites of K+ reabsorption and secretion

Answer 3

756 mEq/day K+ filtered
Reabsorption: 65% in proximal tubule, 27% in ascending limb
Secretion: 4% in late tubule and collecting duct
Excretion: 12% in urine

Question 4

Mechanisms by which K+ intake raises K+ excretion

Answer 4

Increase K+ intake causes increased plasma [K+] which causes directly an increase in K+ secretion
Increased plasma [K+] causes increased aldosterone levels which indirectly lead to an increase in K+ secretion

Question 5

Effects of sodium intake

Answer 5

Increase Na+ intake —>Increase aldosterone—>inhibit K+ secretion in cortical collecting tubules
Increase Na+ intake causes an increase in GFR which increases distal tubular flow rate
Increase Na+ intake decreases proximal tubular Na+ reabsorption which increases distal tubular flow rate
Increase distal tubular flow rate activates K+ secretion in cortical collecting tubule
In summary sodium intake does not affect K+ excretion

Question 6

Insulin effects on K+ regulation

Answer 6

Stimulates K+ uptake by cells

Question 7

Aldosterone Effects on extracellular K+ regulation

Answer 7

Increase K+ uptake by cells
Stimulates active reabsorption of Na+ by principal cells via Na+K+ ATPase pump
Increases permeability of luminal membrane for K+
Increases extracellular K+—>Aldosterone secretion
Hypokalemia–> excess secretion of aldosterone (Conn’s syndrome)
Hyperkalemia–> deficiency in aldosterone secretion (Addison’s disease) caused by cell lysis, strenuous exercise increased extracellular fluid osmolarity

Question 8

Catecholamine effects on extracellular K+

Answer 8

Beta adrenergic stimulation (epinephrine) stimulates K+ uptake by cells
Beta adrenergic receptor blockers cause hyperkalemia

Question 9

Metabolic conditions and [extracellular K+]

Answer 9

Metabolic acidosis= Increase in [extracellular K+]: increase H+ causes reduction in activity of pump which leads to decrease in cellular uptake of K+
Metabolic alkalosis= decrease in [extracellular K+]

Question 10

Characteristics of principal cells

Answer 10

90% cells in the late distal tubule and cortical collecting tubules
Secrete K+
Control K+ secretion through activity of ATPase pump, electrochemical gradient and permeability of luminal membrane

Question 11

Characteristics of intercalated cells

Answer 11

Reabsorb K+ during K+depletion

Question 12

Extracellular concentration of calcium

Answer 12

50% of total plasma calcium is in the ionized form
Acidosis= less calcium bound to plasma proteins
Alkalosis= more calcium bound to plasma proteins

Question 13

Maintenance of constant extracellular [Ca2+]

Answer 13

GI tract important in calcium ions homeostasis
Large amount of calcium ions excretion occurs in the feces
Almost all calcium is stored in bone
PTH regulators of bone uptake and release of calcium ions
Parathyroid glands are stimulated by low calcium ions level and increase secretion of PTH

Question 14

PTH effects

Answer 14

Stimulates bone reabsorption
Stimulates activation of vitamin D
Indirectly increases tubular reabsorption of calcium ions

Question 15

Calcium reabsorption in tubules

Answer 15

• In the proximal tubule:99% of filtered calcium is reabsorbed
• In Loop of Henle: reabsorption is restricted to thick ascending limb
• In distal tubule: Reabsorption is almost entirely via active transport with the ca2+atpase in basolateral membrane and is stimulated by PTH

Question 16

Factors that decrease tubular calcium excretion

Answer 16

Increase levels of PTH
Plasma concentration of phosphate
Metabolic acidosis

Question 17

Compensation mechanisms

Answer 17

Decrease [Ca2+]—>Increase PTH=>Increase renal calcium reabsorption and calcium release from bones
Increase PTH—>Increase activation of vitamin D3=>Increase intestinal calcium ions reabsorption

Question 18

Phosphate excretion

Answer 18

Controlled by overflow mechanism
Phosphate transport maximum for reabsorption=0.1mM/min
[phosphate]All filtered phosphate is reabsorbed
[Phosphate]>0.1=> Excess is secreted

Question 19

Role of PTH in phosphate reabsorption

Answer 19

75-80% phosphate in reabsorbed in proximal tubule
10% phosphate is reabsorbed in distal tubule
PTH promotes bone reabsorption=>increase [Extracellular phosphate]
PTH decreases transport maximum for phosphate by renal tubules=greater loss of phosphate in urine

Question 20

Extracellular potassium

Answer 20

Normally regulates at 4.2 mEq/L (0.3 mEq/L)
Increase of 3-4 mEq/l leads to cardiac arrhythmias
Higher concentration can lead to cardiac arrest or fibrillation
Extracellular fluid contains 2 % of total body potassium
Food intake can be up to 50 mEq
Kidneys must adjust K+ excretion rapidly and precisely