management of electrolyte and water balance

Question 1

salt intake and excretion

Answer 1

intake = 10g
sweat - 0.25g
urine - 9.5 g
faeces - 0.25g

Question 2

what hormone controls the amount of sodium lost in urine or reabsorbed into the plasma

Answer 2

aldosterone

Question 3

what is aldosterone

Answer 3

a steroid hormone produced in the adrenal cortex from cholesterol
- plasma conc 0.1-0.15

Question 4

what is aldosterone produced in response to

Answer 4

in response to a fall in NaCl intake or an increase in the potassium concentration in the interstitial fluid

Question 5

what is aldosterone stimulated and inhibited by

Answer 5

stimulated by angiotensin 2 and inhibited by ANP

Question 6

what stimulates aldosterone release if Na+ loss is less severe

Answer 6

ACTH - corticotrophin

Question 7

main functions of aldosterone

Answer 7

sodium retention
potassium excretion
secondary retention of water - increased ECF

Question 8

Stretch receptors

Answer 8

We have specialized cells that sit around the afferent arteriole that can detect changes in pressure
- they are stretch receptors.

Question 9

macula densa

Answer 9

Macula densa - this has a sensing capability so it can detect changes in sodium levels
and also osmolality

Question 10

what does aldosterone work with

Answer 10

renin - angiotensin system
- detects changes in BP and blood volume
a fall: in BP in afferent arteriole or in Na+ conc in the filtrate
Triggers the renin angiotensin system

Question 11

renin angiotensin system

Answer 11

• decreased plasma volume
• decreased arterial BP - detected by macula densa cells
• renin granular cells
• renin will active the angiotensinogen by chopping it into bits and it becomes angiotensin 1
• ACE converts angiotensin 1 to 2
• angiotensin 2 is a powerful vasoconstrictor
• adrenal cortex secrete aldosterone
• renal tubule increase sodium reabsorption and decrease sodium excretion

Question 12

another hormone affecting sodium reabsorption

Answer 12

atrial natriuretic peptide (ANP)

Question 13

how does aldosterone increase sodium and decrease potassium

Answer 13

• main site of action DCT and CD
• insertion of sodium channels (ENaC)
• activate Na+/H+ - more sodium pumped out of the cell by active transport - increasing conc gradeint for sodium to go back in
• activation of Na+/H+

Question 14

ANP

Answer 14

• produced in heart - stored in atrial cells
• trigger - stretching of atrium (high ECV) from increased blood volume
• ANP release into circulation and goes to kidneys
• increased NaCl and water excretion
• blood volume will decrease

Question 15

actions of ANP

Answer 15

• inhibit NaCl reabsorption in medullary CD
• inhibits ADH stimulated water reabsorption in collecting duct
• inhibits ADH secretion from posterior pituitary
• inhibit angiotensin 2 and aldosterone

Question 16

BNP

Answer 16

b type
- lower BP, relax BV, reduce workload on the heart
- BNP effective in diagnosing congestive heart failure
- B type found in the brain and also highly concentrated in the ventricles

Question 17

how can congestive heart failure happen

Answer 17

Congestive heart failure can happen if you have a problem with Your ventricles- they can’t push the blood out as they should do.
- So if you’ve got any issues with contractile apparatus in the heart or if you’ve got a problem with the valves - can get increased pressure in the Atria which can then back up into the pulmonary system.
- Eventually We’ll get a backup in the pulmonary system and that can then further lead to a backup within the systemic circulation and begin increasing pressure in the systemic circulation from the pulmonary circulation, which can then lead to right side failing

Question 18

other factors affecting sodium reabsorption/ secretion - urodilatin

Answer 18

• same gene as ANP
• similar AA sequence
• secreted by DCT and CD cells
• inhibits sodium reabsorption
• much more potent than ANP

Question 19

other factors affecting sodium reabsorption/ secretion - sympathetic nerves

Answer 19

• secrete catecholamines - Adr/ NAdr
• stimulate sodium, water reabsorption in PCT, LOH, DCT, CD

Question 20

other factors affecting sodium reabsorption/ secretion - DOPAMINE

Answer 20

dopamine synthesised by PCT cells has opposite effects to Adr/NAdr
- acts in PCT

Question 21

other factors affecting sodium reabsorption/ secretion - uroguanylin

Answer 21

produced in small intestine
also inhibit sodium and water reabsorption

Question 22

total body water

Answer 22

32 - 40 litres - 70% body weight

Question 23

by what mechanism does the body tell us that we need more/ less water

Answer 23

thirst and sweating

Question 24

control of ECV by thirst/ sweating

Answer 24

Na+/H20 intake = Na+/H20 excretion

sweat :
- hypo-osmotic fluid
- causes decreased ECV
- increases body fluid osmolality

Question 25

what happens when prolonged/ severe sweating

Answer 25

decreased plasma volume: decreased GFR - increased plasma aldosterone (salt reabsorb in CD) - decreased sodium excretion also an increase in plasma osmolality at the same time: - increased ADH - water absorption - decreased water excretion

Question 26

thirst- blood volume and plasma osmolality

Answer 26

decreased blood volume - detected by baroreceptors increased plasma osmolality - detected by osmoreceptors causes angiotensin 2 release - stimulates thirst centre in hypothalamus - DRINK angiotensin 2 also causes release of aldosterone - helps to rebalance with increase in sodium reabsorption and water

Question 27

diuretics

Answer 27

substances that cause enhanced excretion of water and solutes

Question 28

uses of diuretics

Answer 28

hypertension pulmonary oedema oedema chronic heart failure

Question 29

common mechanism of diuretics

Answer 29

inhibition of sodium reabsorption in the nephron

Question 30

non clinical diuretics - pressure diuresis

Answer 30

-increased ECV and BP Autoregulation in cortex but not in the medulla - washout of sodium/ solutes decrease osmotic gradient and decrease max urine osmolality - less reabsorption - more diuresis - EVC returns to normal

Question 31

non clinical diuretics H20 diuresis

Answer 31

increased water intake thus inhibiting ADH excretion of water - more diuresis

Question 32

clinical diuretics - osmotic diuretics

Answer 32

e.g. mannitol act on PCT - enters tubule fluid by filtration - exerts osmotic pressure in the tubule as it cannot be reabsorbed - inhibits reabsorption of solute and water by altering osmotic drivign forces - doesn't act on specific membrane proteins - alters water reabsorption in PCT and descending LOH

Question 33

carbonic anhydrase inhibitors

Answer 33

e.g. acetazolamide - act primarily on PCT where enzyme carbonic anhydrase is most abundant - also acts on principal and intercalating cells in the collecting duct - carbonic anhydrase causes carbon dioxide and water to combine to form carbonic acid which dissociates in protons and bicarbonate ions - bicarbonate reabsorbed with sodium - inhibition of carbonic anhydrase means less formation of bicarbonate, which is driven by Na+, therefore less Na+ reabsorbed

Question 34

loop diuretics

Answer 34

e.g. furosemide and bumetanide - act on the loop of Henle - enter tubule lumen by secretion - block Na+-K+-2Cl- symport on apical membrane ascending limb - prevent sodium reabsorption - disrupt countercurrent multiplier as ascending limb impermeable to water - impair kidneys role of diluting and concentrating urine

Question 35

when are loop diuretics used

Answer 35

they are the most potent diuretics - used for decreasing ECV - used in treatment of high BP and pulmonary oedema - chronic heart failure

Question 36

problems with loop diuretics

Answer 36

hypokalaemia (low potassium in plasma) - postural hypotension

Question 37

thiazide diuretics

Answer 37

e.g. bendroflumethiazide - enters tubule fluid by filtration/ secretion - inhibit sodium reabsoprion - blocks Na+/Cl- symport on distal tubule cell apical membrane - thus inhibits water reabsorption

Question 38

problems with diuretics

Answer 38

if overused can lead to dehydration - can cause electrolyte imbalances

Question 39

amiloride drug

Answer 39

sodium channel blockers (ENaC) - less water reabsorption potassium-sparing diuretic

Question 40

spironolactone drug

Answer 40

blocks aldosterone receptors in cytoplasm potassium sparing diuretic

Question 41

fluid rebound with diuretics

Answer 41

- important that patients continue taking fluids with diuretics - if not they will produce small amounts of concentrated urine - this will be sensed by the nephrons to activate the renin -angiotensis system - the rise in plasma osmolality also stimulates a release of ASH which will also act to retain water