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Regulated absorption and secretion in the DCT/CD (Acid base (absorption…
Regulated absorption and secretion in the DCT/CD
Acid base (absorption and secretion of H+ & bicarbonate)
HCO3-
100% filtered & 100% reabsorbed (80-90% reabsorbed in PCT, 10-20% in LOH)
H+
Produced in excess by metabolism
Some unregulated secretion in PCT (so some removed)
Secreted in exchange for Na. Some protons bind to non bicarbonate buffers e.g. phosphate. Excreted in urine.
Ammino acid + proton gives ammonium which then secreted by Na-NH4 antiporter
Captured by non bicarbonate buffers
New pumps in DCT
H+ ATPase
Actively pumps H+ into filtrate in DCT
H+ K+ ATPase
Actively pushes H+ into urine in exchange for K+ in the DCT
Intercalated cells
In distal nephron
Regulate acid excretion
Type A
Work during acidosis
Active on apical membrane
Protons from cabronic acid dissociation pumped into urine and bind to non bicarbonate buffers
BRAND NEW BICARBONATE IS SYNTHESISED AND ABSORBED
(from dissociation of carbonic acid)
Type B
Work during alkalosis
Active on basolateral membrane
Secrete HCO3- into filtrate in exchange for chloride ion
Absorption of "new" H+
Na and K
Sodium
100% filtered
Approx 90% reabsorbed (65-70% in PCT, 20-25% in LOH)
Potassium
100% filtered
100% reabsorbed (65-70% in PCT, 30-35% in LOH)
Via hormonally regulated SECRETION of K+ and REABSORPTION of Na+ in the
principal cells
:
Regulated by aldosterone - produced in adrenal gland
Aldosterone effects
Acute effects
Increased activity of existing channels and pumps
Long term effects
Synthesis of new channels and pumps
Ca and phosphate
Calcium
50% bound to albumin so only free 50% is filtered
Approx 70% reabsorbed in PCT
Selective reabsorption of Ca in DCT and CD under parathyroid hormone control
Normally only 1-2% filtered Ca is excreted
Phosphorous
Inorganic phosphate 100% filtered
Dietary excess excreted in kidney (most animals intake excess phosphorous)
Reabsorption in PCT by Na cotransport. Inhibited by parathyroid hormone
NOT reabsorbed in DCT and CD