Aims
• Potassium, phosphate, calcium, and
magnesium excretion
• Readings: Sherwood, Chapter 14 &
Guyton, Chapter 29.
Potassium
• 98% in cells only 2% in
extracellular fluid.
• Most excretion of K+ is
done by the kidneys
~90%.
• Extracellular K+ levels are
precisely controlled.
• Too much (hyperkalemia)
• Too little (hypokalemia).
• Cells serve as an
“overflow” for K+.
Guyton’s Textbook of Medical Physiology 29-1
Regulation of Potassium distribution
(Extracellular vs. Intracellular)
• Increase Intracellular K+
• Insulin & Aldosterone
• Catecholamines (Epinephrine)
• Metabolic alkalosis
Guyton’s Textbook of Medical Physiology table 29-1
Regulation of Potassium distribution
(Extracellular vs. Intracellular)
• Increase Extracellular K+
• Metabolic acidosis
• Cell lysis
• Strenuous exercise
Guyton’s Textbook of Medical Physiology table 29-1
Regulation of Potassium distribution
(Extracellular vs. Intracellular)
• Increased extracellular fluid osmolarity.
– Causes water to leave the cell resulting in an
increase in the cellular concentration of K+.
– This results in an increase in K+ diffusion out
of the cell.
Renal Potassium Excretion
Amount of K+ excreted is determined by:
Rate of K+ filtration.
Usually fairly constant at ~750 mEq/day.
Guyton’s Textbook of Medical Physiology 29-2
Renal Potassium Excretion
Amount of K+ excreted is determined by:
Rate of K+ reabsorption.
65% of the filtered K+ is
reabsorbed in the proximal tubule.
25-30% of the filtered K+ is
reabsorbed in the thick ascending
limb of the loop of Henle .
Remember this is due to 1-Na+-2Cl-1K+ cotransporter in the lumenal
membrane.
Guyton’s Textbook of Medical Physiology 29-2
Renal Potassium Excretion
Amount of K+ excreted is determined by:
Rate of K+ secretion.
The cause of most daily variation
in K+ excretion!
In the late distal and cortical
collecting tubules K+ can be
reabsorbed or secreted.
Guyton’s Textbook of Medical Physiology 29-2
Renal Potassium Excretion
During K+ depletion the
___________________________
cells of the distal and cortical
collecting tubules can reabsorb
K+.
Actively transported with H+ via
a H+/K+ ATPase
Guyton’s Textbook of Medical Physiology 29-2 and 27-11
Renal Potassium Excretion
Normally about
1/3 of the K+
intake is secreted
into the distal and
cortical collecting
tubules.
Guyton’s Textbook of Medical Physiology 29-2
Renal potassium excretion
High K+ diets can
cause more K+ to be
excreted than was in
the glomerular filtrate.
Due to elevated
secretion.
Guyton’s Textbook of Medical Physiology 29-2
Renal Potassium Excretion
Low K+ diets can
cause net K+
reabsorption.
The K+ excretion can
fall to 1% of K+ in the
glomerular filtrate.
Guyton’s Textbook of Medical Physiology 29-2
Mechanism of Potassium Secretion
• Principal cells (90% of distal and cortical
collecting tubule epithelial cells)
– Na+-K+ ATPase (basolateral membrane)
– K+ specific channels (luminal membrane)
Guyton’s Textbook of Medical Physiology 27-11
Sherwood’s Human Physiology 14-24 5th Ed. & 14-21 6th Ed.
Regulation of Potassium Secretion by
Principal Cells
Urinary K+ excretion
• Increased extracellular
fluid K+ concentration
stimulates K+ secretion.
Extracellular K + concentration
Guyton’s Textbook of Medical Physiology 29-4
• Stimulates Na+-K+ ATPase
resulting in K+ diffusing out
of the cell and into the
tubule lumen.
• Stimulates aldosterone
secretion by adrenal
cortex.
Urinary K+ excretion
Regulation of Potassium Secretion by
Principal Cells
• Aldosterone stimulates
K+ secretion.
• Stimulates Na+-K+
ATPase resulting in K+
diffusing out of the cell
and into the tubule
lumen.
• Increases the luminal
membrane’s K+
permeability.
Extracellular K + concentration
• (negative feedback
control)
Guyton’s Textbook of Medical Physiology 29-4
Regulation of Potassium Secretion by
Principal Cells
Guyton’s Textbook of Medical Physiology 29-7
Summary of Aldosterone Control of
Tubular Reabsorption & Secretion
• Aldosterone Regulation
– Increased plasma K+
– Decreased Na+ conc.
– Decreased ECF volume
– Decreased arterial
pressure
Sherwood’s Human Physiology 14-25 5th Ed. &
14-22 6th Ed.
Regulation of Potassium Secretion by
Principal Cells
• Increased distal tubular flow rate
____________________________________
K+ secretion.
• High flow rate washes K+ away resulting in
the K+ concentration gradient driving the
transport of K+.
• Under high Na+ intake conditions blood
pressure ↑ (and thus filtration and flow rate ↑)
and aldosterone secretion is inhibited so this
maintains a constant K+ secretion.
– Remember aldosterone causes Na+ reabsorption
and K+ secretion.
Regulation of Potassium Secretion by
Principal Cells
• Acute acidosis decreases K+ permeability.
– Decreases the activity of the Na+-K+ ATPase.
Control of renal Calcium Excretion
• Ca++ ______________________ secreted in
the kidneys.
– Renal Ca++ excretion = Ca++ filtered - Ca++ reabsorbed.
– Normally 99% of the filtered Ca++ is reabsorbed.
– PTH is the main regulator of Ca++ reabsorption.
• Increased PTH increases Ca++ reabsorption in thick
ascending loop and distal tubules.
Control of Renal Calcium Excretion
Guyton’s Textbook of Medical Physiology table 29-2
Control of Renal Calcium Excretion
Guyton’s Textbook of Medical Physiology 29-10
Calcium Handling in the Nephron
Costanzo’s Physiology 6-31
Regulation of Renal Phosphate
Excretion
• Overflow mechanism
– Normally the renal tubules have a transport
maximum for reabsorbing phosphate.
– Normally our diet supplies us with much more
phosphate than we can reabsorb.
• Thus it is excreted.
Regulation of Renal Phosphate
Excretion
• Changes in the amount of ingested
phosphate can change the transport
maximum for reabsorbing phosphate.
– Low phosphate diet can increase transport
maximum for reabsorbing phosphate.
– ______________________ can decease the
transport maximum for reabsorbing
phosphate.
Phosphate Handling in the Nephron
Costanzo’s Physiology 6-30
Magnesium Excretion
• Normally about 10% of the renal filtrate of
magnesium is excreted.
• Regulation of magnesium excretion is
achieved by changing the tubular
reabsorption.
– 25-30% of magnesium reabsorption occurs in
the proximal tubule.
– 60-65% of magnesium reabsorption occurs in
the loop of Henle.
– ~5% of magnesium reabsorption occurs in the
distal and collecting tubules.
Magnesium Handling in the Nephron
Costanzo’s Physiology 6-31
Renal Clearance
• Volume of plasma that is completely cleared
of a given substance by the kidneys per unit
time.
– Useful way to quantify the excretory function of
the kidney
• Cs = Us x V / Ps (Us x V is urinary excretion rate)
– Cs is the clearance rate of a substance
– Us is the urine concentration of that substance
– V is the urine flow rate
– Ps is the plasma concentration of the substance
Renal Clearance
= inulin, (filtered, not reabsorbed, not secreted)
< inulin, (filtered, reabsorbed, not secreted)
> inulin, (filted, not reabsorbed, secreted)
Next Time
• Renal Regulation of Acid/Base Balance &
Micturition
• Readings; Sherwood, Chapters 14 & 15
Objectives
1. Describe the regulators of K+ distribution.
1. Intracellular vs . Extracellular
2. Describe renal K+ excretion.
1. Rate determinants
2. Location of reabsorption & secretion and cells involved.
3. Mechanism and regulation of K+ secretion.
3. Describe renal Ca++ excretion.
1. Factors that increase or decrease.
2. Location of reabsorption.
4. Describe renal PO4- excretion.
1. Overflow mechanism.
2. Location of reabsorption.
5. Describe renal Mg++ excretion.
1. Location of reabsorption.