Renal Physiology Lecture 2: Water Balance

Question 1

How much of our body weight is water?

Answer 1

• 50-60%

Question 2

Intake of water in food/ beverage

Answer 2

~2100 mL/d

Question 3

Metabolism use of water

Answer 3

~ 200-300 mL/d

Question 4

Water losses

Answer 4

• most in urine
• some in feces
• sweating
• insensible
  
  • respiratory
  • skin diffusion

Question 5

What is water balance critcal for?

Answer 5

maintaining blood volume & pressure

Question 6

Why are kidneys essential in water balance?

Answer 6

• conserve or excrete water to match intake
• fluid leaving kidneys is lost from the body
  
  • must be replaced
• Adjustment of sodium & water reabsorption at renal tubules to control how concentrated urine is

Question 7

Kidney function with excess water

Answer 7

kidneys produce large volumes of dilute urine

Question 8

Kidney function with dehydration

Answer 8

Kidneys produce a small volume of concentrated urine

Question 9

What hormone controls water balance?

Answer 9

Vasopressin

• aka antidiuretic hormonne → ADH

Question 10

When is vasopressin released?

Answer 10

released in response to low blood volume/pressure or increased plasma osmolarity

• drives renal reabsorption

Question 11

Vaspression target

Answer 11

Distil tubule and collecting duct

• increased expression and insertion of water channels (aquaporins)
  
  • so they become more permeable to water so less water in the tubule which means less water in urine so more concentrated

Question 12

Process for vasopressin

Answer 12

Question 13

Purpose of more aquaporins being expressed via ADH

Answer 13

Aquaporins, especially AQP2 mediate renal water reabsorption in response to vasopression

Question 14

How do aquaporins get inserted?

Answer 14

Vasopressin binds V2 receptors at the distal tubule & collecting duct, activates a cAMP second messenger system and inserts AQP2 water pores in membrane and water is absorbed by osmosis into blood

• inserted in apical & basolateral membrane
• located in intracellular storage vesicles

Question 15

What happens when vasopression is absent?

Answer 15

Aquaporins are removed by endocytosis from membrane and are stored is vesicles with tubular epithelium cells

Question 16

What stimulate vasopressin release?

Answer 16

• High plasma osmolality is the more potent stimulus for vasopressin release
  
  • monitored by hypothalamic osmoreceptors
    
    • firing rate increases as osmolarity increases (silent if it drops)
• Decreased blood volume/ pressure
  
  • large decreases stimulate vasopressin release
  • information relayed by arterial baroreceptors

Question 17

What stimulates thirst?

Answer 17

• hypothalamic osmoreceptors stimulate thirst
  
  • when plasma osmolarity rises
• Full satiety when depleted water is replenished by gut reabsorption
• avoidance behaviour prevents dehydration
  
  • such as avoiding the sun