k + homeostasis. the need: ecf k + concentration is critical for the function of excitable cells...
TRANSCRIPT
K+ Homeostasis
K+ Homeostasis
The need:ECF K+ concentration is critical for the function of excitable cells
However, about 98% of is in K+ ICFICF concentration of K+ – 120-140 mmol/LICF volume – twice the ECF volumeBUTThe kidney has to regulate K+ handling based on K+ level in ECFIncrease in in the body will have a smaller effect on ICF than ECF
K+ Homeostasis
The need:
ECF K+ concentration needs to be maintained
Extracellular & Intracellular K+
K+ K+
Equilibrium between Intracellular and extracellular K+
• Helps stabilise ECF K+
Glomerular Filtration of K+
Freely filtered at the glomerulus
Tubular Handling of K+
Fate of filtered K+
Percentage of filtered load reabsorbed /secreted
Variable
60-70 %
20 %
Variable
K+ Reabsorption in the Proximal Tubule
K+ dependent on Na+
reabsorption
Na+ reabsorption ismostly a constant fractionof filtered load
K+ reabsorption is alsoa constant fractionof filtered load
Na+
Na+
K+
K+
Na+
Na+
TubularLumen
Capillary
water
K+ K+
K+K+
water
K+ Reabsorption in Loop of Henle
Thick ascending limb
Absorbed with Na+Percentage absorbed constant
Na+
Na+
K+
K+
K+2Cl-
TubularLumen Capillary
Na+
K+ Handling in the Distal Tubule
Principal cells – regulated by aldosterone K+ is secreted in
exchange for Na+ at the luminal surface
Na+
Capillary
K+
Na+
K+
Na+
Lumen
K+ Handling in the Distal Tubule
Intercalated cells
H+ is secreted in exchange for K+ at the luminal surface K+
Capillary
H+
Lumen
K+ Handling by Kidney
K+ reabsorption / secretion1. Linked to reabsorption or secretion of other ions
Mostly Na+ H+ to a lesser extent
2. Depends on aldosterone Secretion is regulated mostly by renin-angiotensin
mechanism Serum K+ also has a direct effect on aldosterone secretion
K+ Handling by Kidney
Regulatory mechanism for Plasma K+
1. Intracellular K+ - depends on several factors dietary K+ content Acid base status Insulin – increases influx of K+
2. Increased plasma K+ stimulates aldosterone secretionHowever, aldosterone is mainly a Na+ regulator
K+ handling by the kidney is determined more by other factors than plasma K+ itself
K+ Excretion
K+ excretion by kidney is increased by
1. Aldosterone 2. Alkalosis3. Increased delivery of Na+ to the distal tubule4. Increased dietary K+
5. Exit of from K+ cells – damage to cells, lack of insulin High K+ levels result in increased filtration of K+
ICF and ECF K+ Equilibrium
Factors affecting K+ flux between ICF and ECF
1. Acid base status2. Insulin3. Sympathetic neurotransmitters4. Exercise5. Damage to cell membrane
Ca2+ Handling by the Kidney
Glomerular Filtration of Ca2+
Ca2+ In blood1. Free – 55-60%
Ionised Complexed with organic anions
2. Bound to protein – 40-45%
Only the free Ca2+ is filterable
Tubular Handling of Ca2+
Fate of filtered Ca2+
Reabsorption onlyNo secretion
8 %
65 %
25 %
2 %
Proximal tubular Reabsorption of Ca2+
Passive Paracellular Due to concentration
gradient created by Na+
reabsorption
Na+ dependent reabsorption
Na+
Na+
K+
K+
Na+
Na+
TubularLumen
Capillary
water
Ca2+ Ca2+
water
Thick Ascending Limb Reabsorption of Ca2+
Passive Paracellular Due to concentration
gradient created by Na+
reabsorption
Na+ dependent reabsorption
Na+
Na+
K+
K+
K+2Cl-
TubularLumen Capillary
Na+
Ca2+ Ca2+
Reabsorption of Ca2+
Approximately 90% of Ca2+ reabsorption is Na+ dependent
Distal Tubular Reabsorption of Ca2+
Active transcellular Regulated by parathormone Ca2+
TubularLumen Capillary
Na+Ca2+
Regulation of Tubular Reabsorption of Ca2+
Na+ reabsorption Parathormone – sensitive to ionised calcium level in blood Acid base status – acidosis inhibits reabsorption and alkalosis
enhances reabsorption
Phosphate Handling by the Kidney
Glomerular Filtration of Phosphate
Phosphate In blood1. Free – 90-95%
2. Bound to protein – 5-10%
The free phosphate is freely filtered
Tubular Handling of Phosphate
Fate of filtered phosphate
Reabsorption almostentirely in the proximaltubule
85 %
15 %
Proximal Tubular Reabsorption of Phosphate
Symport system coupled to Na+ reabsorption Has tubular maximum Parathormone inhibits reabsorption
The phosphate that is excreted in urine performs an important function – buffering secreted H+
Magnesium Handling by the Kidney
Glomerular Filtration of Magnesium
Magnesium in blood1. Free – 50-60 %
2. Bound to protein – 40-50 %
The free magnesium is filtered
Tubular Handling of Magnesium
Fate of filtered magnesium
Highest reabsorption in the thick ascendinglimb
30 %
5 %
60 %
5 %
Tubular Reabsorption of Magnesium
Passive Mostly paracellular route Dependent on Na+ reabsorption
Urate Handling by the Kidney
Renal Handling of Urate
Freely filtered at the glomerulus Proximal tubule
Passively reabsorbed in the early part Actively secreted in the mid proximal tubule Passively reabsorbed in the late segment
Secretion is regulated by plasma levels of urateReabsorption > secretion
