US Lecture 5 - Water Balance

Question 1

What is osmolarity?

Answer 1

Measure of the solute concentration in a solution - depends on the number of dissolved solutes present. The greater the number of dissolved particles, the greater the osmolarity

Question 2

How much more water and salt than we need do we consume to replace that which was lost?

Answer 2

20-25%

Question 3

Why do we need to get rid of excess volume, water and salt?

Answer 3

Volume: oedematous and BP rises Water: Dilute salt in body > decreased osmolarity so cells will SWELL Salt: Too high salt levels > cells will SHRINK

Question 4

What is the most abundant component of the plasma and ECF?

Answer 4

Water

Question 5

What is the most prevalent solute in the plasma ad ECF?

Answer 5

Sodium

Question 6

What does the level of salt determine and what is water balance used for?

Answer 6

Salt determines ECF volume and water balance regulates plasma osmolarity

Question 7

Of the total body water, how much is intracellular fluid and how much is ECF?

Answer 7

Question 8

How is water removed from the body?

Answer 8

Skin and sweat: variable and uncontrollable (fever, climate, activity) - 450ml/day Faeces: uncontrollable (diarrhoea) - 100ml/day Urine output: variable and controllable - 1500ml/day

Question 9

How much of the filtered load reaches the descending loop of Henle, the distal convoluted tubule and the collecting duct?

Answer 9

Question 10

How is the urine concentrated above normal plasma osmolarity?

Answer 10

A region of hyperosmolar interstitial fluid is produced

Question 11

How is the region of hyperosmolar interstitial fluid established?

Answer 11

Countercurrent mechanism

Descending loop impermeable to salts but permeable to water

Ascending loop impermeable to water but ‘permeable’ (pumps out) salts

Urea permeability of the bottom of the loop and the collecting duct

Question 12

What is the function of the vasa recta?

Answer 12

It is permeable to water and solutes - water diffuses out of the descending limb and solutes diffuse into the desc. limb In the ascending limb the opposite happens, so O2 and nutrients are delivered without the loss of gradient

Question 13

What is Vasopressin?

Answer 13

Peptide hormone - 9 a.a. Synthesised in hypothalamus, packaged into granules Secreted from posterior pituitary Binds to specific receptors on basolateral membrane of principal cells in the collecting ducts

Question 14

What does VP cause in the collecting duct?

Answer 14

Causes the insertion of aquaporins in the cell membranes, increaing water permeability - AQ2 mainly

Question 15

What does VP stimulate in the CD?

Answer 15

Stimulates urea transport from CD into thin asc. limb of LoH and interstitial tissue by increasing the membrane localisation of UTA1 and UTA3 in the CCD

Question 16

What is the trigger for ADH release?

Answer 16

Plasma osmolarity above 300mOs - regulated by osmoreceptors in the hypothalamus Marked fall in blood volume and BP - monitored by baroreceptors/stretch receptors

Question 17

What substance inhibits ADH release?

Answer 17

Ethanol, leading to dehydration as urine volume increases

Question 18

What is the water permeability like in the nephron?

Answer 18

Question 19

What happens when there is a decrease in plasma osmolarity? - Water Load

Answer 19

Decreased plasma osmolarity (detected by hypothalamic osmoreceptors) -> decreased ADH release -> Decrease CD permeability -> increase urine flow rate

Question 20

What happens when there is low ADH/water diuresis?

Answer 20

Solute reabsorption without water reabsorption lowers urine osmolarity to 50mosmol/l SO: large volume of dilute urine

Question 21

What happens when there is increased plasma osmolarity? - Dehydration

Answer 21

^ Plasma osmolarity -> ^ thirst and ^ ADH release (both detected by hypothalamic receptors) -> ^ ADH release -> ^ CD permeability -> Decrease urine flow rate

Question 22

What tends to lower plasma osmolarity?

Answer 22

Increased water intake AND decreased fluid loss

Question 23

What happens when there is high ADH/maximal antidiuresis?

Answer 23

Osmotic equilibration in the cortex and medulla leads to high urine osmolarity - so SMALL volume of concentrated urine

Question 24

What is the negative feedback loop of increased plasma osmolarity?

Answer 24

Question 25

What are the 3 classes disorders of water balance?

Answer 25

No/insufficient production of ADH No detection of ADH (mutant ADH receptor) No response to ADH signal (AQ mutant)

Question 26

Give an example of a water balance disorder and of its symptoms

Answer 26

Diabetes insipidus - unremitting thirst and excretion of large amount of watery urine

Question 27

Why is the hyperosmolar environment required for retention of water?

Answer 27

Water is reabsorbed by osmosis so need to generate a hyperosmolar env compared to the fluid in the collecting duct

Fluid starts with osmolarity approaching that of plasma and contains solutes that need to be excreted - removal of water would increase the osmolarity, needing to maintain osmotic gradient that allows this to happen as the fluid passes along the latter part of the nephron

Question 28

What is the importance of the urea permeability in the CD in response to ADH?

Answer 28

Increased urea permeability causes urea to move down its conc gradient into the interstitium and increase the interstitial osmolarity so more water can be reabsorbed

Question 29

Identify 2 triggers of ADH secretion

Answer 29

Hyperosmolarity identified by osmoreceptors

Low BP identified by baroreceptors