Lecture 29 Renal Regulation Flashcards
Osmoregulation (Water Balance)
- involves regulation of body fluid osmolarity (concentration) and total fluid volume
To maintain steady state
water gain = water loss
urine is a major avenue of water loss (~ 1.5 L/day)
kidneys conserve water, control volume and concentration of urine excreted
. Regulation of ECF Osmolarity
- ECF osmolarity affects H2O movement in and out of cells
normal ECF osmolarity = 290 mOsm
↑ ECF osmolarity → ↓ ICF volume
↓ ECF osmolarity → ↑ ICF volume
hypothalamus
ADH
osmoreceptors respond to high plasma osmolarity
neurosecretory cells produce ADH (vasopressin), secreted by posterior pituitary
high ADH levels
↑ permeability of CD to H2O
→ ↑ H2O reabsorbed from CD
→ concentrated urine, less H2O lost
low ADH levels
↓ permeability of CD to H2O
→ ↓ H2O reabsorbed from CD
→ dilute urine, more H2O lost
(e.g. diabetes insipidus)
Negative feedback control
↑ ECF osmolarity → ↑ ADH secretion → ↑ H2O reabsorption from CD → ↓ ECF osmolarity
Regulation of ECF Volume
- ECF volume affects blood pressure
- kidneys help control ECF volume
- Na+ and Cl
- are the most abundant ECF solutes
- total amount of Na+ in the ECF affects ECF volume
↑ Na+ in ECF → ↑ ECF osmolarity → ↑ ADH → ↑ H2O reabsorption → ↑ ECF volume
kidneys help control ECF volume via:
- regulation of H2O reabsorption/ excretion - controlled by ADH
- regulation of solute reabsorption/ excretion
Fluid imbalances may involve change in
osmolarity, volume, or both.
e.g., hypertonic dehydration: ↑ ECF osmolarity and ↓ ECF volume
isotonic dehydration: ↓ ECF volume with normal ECF osmolarity
. Electrolyte Balance: Na+ and K+ Regulation
- most Na+ and K+ filtered into nephrons is reabsorbed in the PCT
- regulated reabsorption and secretion of Na+ and K+ in the DCT and upper CD
aldosterone
secreted by the adrenal cortex
- stimulates Na+ reabsorption and K+ secretion in principle (P) cells of DCT and CD
- activates apical Na+ and K+ channels and basolateral Na+-K+ pumps
What does ICF volume depend on?
what decreases ICF volume?
ECF osmolarity
Increase in ECF concentration decreases ICF volume
aldosterone secretion is stimulated by
- high plasma [K+]
2. renin-angiotensin-aldosterone system
renin-angiotensin-aldosterone system
responds to low BP and low [Na+]
juxtaglomerular apparatus
renin
angiotensin II effects
juxtaglomerular apparatus
granular (juxtaglomerular) cells - sense BP in afferent arteriole
macula densa - senses [Na+] in tubular fluid
renin
in blood
in capillaries
- enzyme secreted into blood by granular cells
in blood, renin converts angiotensinogen to angiotensin I
in capillaries, angiotensin converting enzyme (ACE) converts ANG I to ANG II
angiotensin II effects
- vasoconstriction → peripheral resistance → BP
2. stimulates aldosterone secretion → Na+ reabsorption → plasma volume → BP
Renal Acid-Base Regulation
Kidneys control excretion of metabolic (non-CO2) acids and bases
- normally secrete H+ and reabsorb HCO3-
- rates of H+ secretion and HCO3-
- reabsorption are adjusted to respond to alterations in
pH and [HCO3-] of the plasma
- net result is regulation of plasma [HCO3-] and pH
Negative feedback control
Normal pH and [HCO3-]
normal pH = 7.4 and [HCO3-] = 24 mM
decrease [HCO3-] and/or in pH -> increase in H+ secretion and increase in HCO3- reabsorption → increase [HCO3-], increase pH
increase [HCO3-] and/or in pH → decrease in H+ secretion and HCO3- reabsorption → decrease [HCO3-], decrease in pH
Mechanism of bicarbonate reabsorption
1-4
- HCO3
- in tubular fluid (PCT and DCT) combines with H+ to form CO2 + H2O
(catalyzed by carbonic anhydrase in the tubule) - CO2 diffuses into the tubule epithelial cells
- CO2 is converted to H+ + HCO3
- (via carbonic anhydrase inside the cell) - HCO3
- is transported to ECF,
H+ is pumped back out to the tubule lumen
