Lecture 29 Renal Regulation

Question 1

Osmoregulation (Water Balance)

Answer 1

• involves regulation of body fluid osmolarity (concentration) and total fluid volume

Question 2

To maintain steady state

Answer 2

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

Question 3

. Regulation of ECF Osmolarity

Answer 3

• ECF osmolarity affects H2O movement in and out of cells
  normal ECF osmolarity = 290 mOsm
  ↑ ECF osmolarity → ↓ ICF volume
  ↓ ECF osmolarity → ↑ ICF volume

Question 4

hypothalamus

ADH

Answer 4

osmoreceptors respond to high plasma osmolarity

neurosecretory cells produce ADH (vasopressin), secreted by posterior pituitary

Question 5

high ADH levels

Answer 5

↑ permeability of CD to H2O
→ ↑ H2O reabsorbed from CD
→ concentrated urine, less H2O lost

Question 6

low ADH levels

Answer 6

↓ permeability of CD to H2O
→ ↓ H2O reabsorbed from CD
→ dilute urine, more H2O lost
(e.g. diabetes insipidus)

Question 7

Negative feedback control

Answer 7

↑ ECF osmolarity → ↑ ADH secretion → ↑ H2O reabsorption from CD → ↓ ECF osmolarity

Question 8

Regulation of ECF Volume

Answer 8

• 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

Question 9

kidneys help control ECF volume via:

Answer 9

1. regulation of H2O reabsorption/ excretion - controlled by ADH
2. regulation of solute reabsorption/ excretion

Question 10

Fluid imbalances may involve change in

Answer 10

osmolarity, volume, or both.
e.g., hypertonic dehydration: ↑ ECF osmolarity and ↓ ECF volume
isotonic dehydration: ↓ ECF volume with normal ECF osmolarity

Question 11

. Electrolyte Balance: Na+ and K+ Regulation

Answer 11

• 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

Question 12

aldosterone

Answer 12

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

Question 13

What does ICF volume depend on?

what decreases ICF volume?

Answer 13

ECF osmolarity

Increase in ECF concentration decreases ICF volume

Question 14

aldosterone secretion is stimulated by

Answer 14

1. high plasma [K+]

2. renin-angiotensin-aldosterone system

Question 15

renin-angiotensin-aldosterone system

Answer 15

responds to low BP and low [Na+]
juxtaglomerular apparatus
renin
angiotensin II effects

Question 16

juxtaglomerular apparatus

Answer 16

granular (juxtaglomerular) cells - sense BP in afferent arteriole
macula densa - senses [Na+] in tubular fluid

Question 17

renin
in blood
in capillaries

Answer 17

• 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

Question 18

angiotensin II effects

Answer 18

1. vasoconstriction →  peripheral resistance →  BP

2. stimulates aldosterone secretion →  Na+ reabsorption →  plasma volume →  BP

Question 19

Renal Acid-Base Regulation

Answer 19

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

Question 20

Negative feedback control

Normal pH and [HCO3-]

Answer 20

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

Question 21

Mechanism of bicarbonate reabsorption

1-4

Answer 21

1. HCO3
   - in tubular fluid (PCT and DCT) combines with H+ to form CO2 + H2O
   (catalyzed by carbonic anhydrase in the tubule)
2. CO2 diffuses into the tubule epithelial cells
3. CO2 is converted to H+ + HCO3
   - (via carbonic anhydrase inside the cell)
4. HCO3
   - is transported to ECF,
   H+ is pumped back out to the tubule lumen