5. Control of Water Balance

Question 1

What are the units for osmolarity?

Answer 1

osmoles/litre (mosmol/l)

Question 2

What happens to a hyperosmotic cell in solution?

Answer 2

• Water moves in

* Cell swells

Question 3

What happens to a hypoosmotic cell in solution?

Answer 3

• Water moves out

* Cell shrinks

Question 4

What does it mean by the body operating in a ‘constant osmolarity environment’?

Answer 4

• If you increase salt concentration, osmolarity increases

* However, water follows the salt so osmolarity returns to the original level

Question 5

What are the 3 reasons for urine production?

Answer 5

• Removal of excess volume (to prevent oedema + hypertension)
• Removal of excess water (prevent ECF from becoming hypoosmotic - cell swelling)
• Removal of excess salt (prevent ECF from becoming hyperosmotic - cell shrinkage)

Question 6

What is the most prevalent solute in plasma/ECF?

Answer 6

Sodium

Question 7

How do we get rid of water?

Answer 7

• Urine output - 1500ml/day (controllable)
• Sweat - 450ml/day (uncontrollable, depends on fever, climate and activity)
• Respiration - 350ml/day (uncontrollable, varies with physical activity)
• Faeces - 100ml/day (uncontrollable, depends on solidity)

Question 8

Where does water removal occur in the tubular system?

Answer 8

All regions apart from the ascending limb

Question 9

How much water passes through the glomerular fenestrae?

Answer 9

125ml/min

Question 10

Where is the most water reabsorbed and why?

Answer 10

• PCT

* Most solute pumping occurs here, water follows

Question 11

How does urine maintain its concentration (above normal plasma osmolarity)?

Answer 11

Osmolarity increases from 290-1200mosmol/l from the cortex to the inner medulla

Question 12

What is the difference in osmolarity between the ascending limb and interstitial fluid, and how does this affect the osmolarity of fluid at the end of the Loop?

Answer 12

• 200mosmol/l => 400mosmol/l
• Created by active pumping of salts into the interstitial fluid
• When water moves out from the descending limb, more salt leaves at the bottom of the loop, generating a further difference of 200mosmol/l
• Therefore, the tubular fluid that leaves the Loop of Henle to the DCT is hypoosmolar (as a lot of salts have been lost)

Question 13

How does urea play a part in generating a concentration gradient in the nephron?

Answer 13

• The bottom of the Loop is permeable to urea
- urea enters the tubular fluid making it hyperosmolar (UT-A2)
- urea enters the descending vasa recta in this area too (UT-B1)
• The collecting duct is permeable to urea, which is removed (UT-A1, UT-A3)

Question 14

What urea transporter problems have been observed in humans?

Answer 14

• Point mutations in UT-A2 - reduced blood pressure

* Loss of function mutations in UT-B - reduction in urine concentrating ability

Question 15

Describe the renal medullary blood flow

Answer 15

• All tubular cells require oxygen and nutrients
• Ordinary blood supply - loss of hyperosmotic IF gradient, as excess salt would move into capillaries
• Vasa recta is permeable to water and solutes
• In the descending limb, water diffuses out/solutes diffuse into the limb from the vasa recta
• Reverse happens in the ascending limb

Question 16

What is vasopressin and what does it do?

Answer 16

• aka ADH, 9aa peptide hormone
• Synthesised in the hypothalamus
• Packaged into granules then secreted from neurohypophysis (PPG)
• Binds to specific receptrs on basolateral membrane of the principal cells
• Insertion of aquaporins into luminal membrane (stored in vesicles unless vasopressin acts)
• Also stimulates urea transport from inner medullary collecting duct into Loop of Henle

Question 17

What triggers ADH release (and inhibition)?

Answer 17

• Hypothalmic osmoreceptors respond to an increase in plasma osmolarity >300mosmol/l
• Baroreceptors - secondary signal, stimulated by a fall in BP/blood volume
• Ethanol inhibits ADH, less water reabsorption, increased urine volume

Question 18

What happens to water, sodium and urea in the nephron when you drink a lot of water?

Answer 18

• Decreased ADH
• Decreased water permeability of collecting duct
• Sodium can be reabsorbed but not water
• Urea not recycled as much
- this lowers the concentration gradient, which reduces water reabsorption elsewhere

Question 19

What happens to water and urea in the nephron when you are dehydrated?

Answer 19

• More ADH
• Increased water permeability of collecting duct
• Increased urea recycling
- this increases the concentration gradient, which increase water reabsorption elsewhere

Question 20

What is Diabetes insipidus?

Answer 20

• No/insufficient ADH production
• Mutant ADH receptor
• Mutant aquaporin
• Less water reabsorbed
• Polyuria (>30L/day) and polydipsia (unremitting thirst)