Water and Sodium

Question 1

Osmolality of most cells?

Answer 1

290

Question 2

High IC electrolytes

Answer 2

potassium (NOT sodium)

Question 3

High EC electrolytes

Answer 3

sodium (NOT potassium)

Question 4

What happens if you give a person intravenous hypertonic saline?

Answer 4

Blood pressure goes up, goes up more than isotonic because blood is more concentrated so water moves into blood from interstitium

Question 5

What happens if you give IV saline?

Answer 5

BP increases, short time effect -> no fluid shift, no change in osmolality

Question 6

What happens if you give intravenous 5% dextrose solution?

Answer 6

No sodium, osmolality is the same so is the same as giving normal saline
Metabolises sugar quickly, so essentiality water remains which is hypotonic

Question 7

Function of the proximal tubule

Answer 7

70% solutes and water reabsorbed

Fluid resorption mostly isosmotic

Question 8

Function of the loop of henle

Answer 8

Makes urine dilute by reabsorbing salt
Descending limb water permeable - salt stays in
Thin ascending limb water impermeable - salt diffuses out
Thick ascending limb reabsorbs Na (NaKCl2)

Question 9

Function of the distal tubule

Answer 9

Active solute reabsorption (Na/K)

Question 10

Function of collecting duct

Answer 10

ADH sensitive
If no ADH – CD is water insensitive hence dilute urine
In presence of ADH – water is reabsorbed

Question 11

3 things needed to make dilute urine

Answer 11

Adequate fluid delivery
Na reabsorption in TAL
impermeable CD for water

Question 12

4 things are needed to make a concentrated urine

Answer 12

Not too much fluid delivery
Na reabsorption in TAL
Normal ADH production,
Normal medullary hypertonicity

Question 13

Hormonal regulation of water

Answer 13

ADH
Renin-angiotensin-aldosterone
ANP

Question 14

ADH actions and mechanism of action

Answer 14

Direct vasoconstrictor
NaCl reabsorption in TAL loop of Henle
Water retention in collecting ducts
Receptor binding activates cAMP which stimulates water channel (aquaporin 2) incorporation into apical membrane

Question 15

ANP actions

Answer 15

Polypeptide released from cardiac myocytes
Increases urinary excretion of Na and water
Inhibits Na resorption by collecting duct
Inhibits renin production and aldosterone secretion

Question 16

How could you lose too much water through your kidneys?

Answer 16

No ADH
Kidney insensitive to ADH
NaKCl2 channel blocked
Other solutes carrying water through the kidney 
Alcohol, caffeine 
Drugs inhibiting ADH
Excess ANP

Question 17

Clinical features of dehydration

Answer 17

Postural hypotension, tachycardia, low skin turgor, sunken eyes, dry mouth, thirst

Question 18

Biochemical features of dehydration

Answer 18

Serum osmolality high
Serum Na usually high
Serum K / Mg / Ca – high/low or normal
Serum urea high
Hb high

Question 19

Dehydration treatment

Answer 19

Give back the lost fluid
– if true dehydration – water !
– if salt and water – saline!
Get other solutes right too

Question 20

What is DI?

Answer 20

Central (lack of secretion) or nephrogenic (failure to respond) to ADH (AVP)