Biochemistry- Na+ and water

Question 1

Does water follow sodium or does sodium follow water?

Answer 1

Water follows sodium.

Question 2

What hormones regulate sodium and therefore regulate water?

Answer 2

Mineralocorticoids- e.g. aldosterone (main one)

Cortisol

Question 3

What does a mineralocorticoid do?

Answer 3

Activity means sodium retention occurs and therefore water is retained.

Question 4

What are the consequences of too little mineralocorticoid activity?

Answer 4

Little/no sodium is retained therefore water is lost.

Question 5

Which hormone controls water? Where is it released from?

Answer 5

ADH- released from posterior pituitary.

Question 6

What is the function of ADH?

Answer 6

It acts on renal tubules to cause

• water reabsorption
• antidiuretic affect

Question 7

If there is an increase in ADH, what will the urine be like?

Answer 7

concentrated.

Question 8

If there is a decrease in ADH, what will the urine be like?

Answer 8

Diluted.

Question 9

How can ADH’s affect be assessed?

Answer 9

Look at urine osmolarity.
A high urine osmolarity- urine is concentrated
A low urine osmolarity- urine is dilute.

Question 10

How can disorders to do with sodium concentration be classified?

Answer 10

Decreased Na- Too much water
-Too little sodium

Increased Na- Too little water
-too much sodium.

Question 11

What are the causes of too much water in the body?

Answer 11

Increased intake of water

Decreased urine output e.g. SIAD.

Question 12

What are the causes of too little sodium in the body?

Answer 12

Increased sodium loss- kidneys e.g. in Addisons disease
Gut
Skin

Or decreased sodium intake (rare).

Question 13

What are the causes of too little water in the body?

Answer 13

Diabetes insipidus (problem with ADH secretion)
Decreased water intake.

Question 14

What are the causes of too much sodium in the body?

Answer 14

Some IV medications (given as sodium salts)
Near- drowning in the sea
Infants given high salt feeds

Question 15

A 24 year-old student presents with a six month history of malaise, tiredness, poor appetite and one stone weight loss. She has developed a craving for salty foods – crisps in particular. She has had a number of dizzy spells particularly while in warm places.

She is thin. She has low BP which falls further on standing. You have the impression that she is tanned, and you find increased pigmentation in her mouth and hand creases.

Her bloods show a low sodium [122] and a high potassium [5.8].

Answer 15

Addisons disease.

Question 16

What is Addison’s disease?

Answer 16

Basically primary adrenal insufficiency meaning you can’t make enough mineralocorticoid. This means she can’t retain sodium and therefore can’t retain water.

Question 17

What symptoms will someone with Addisons disease have?

Answer 17

She will be clinically dehydrated- causing symptoms of dizziness (due to postural hypotension)
Also get hyper pigmentation (as the pituitary produces lots of ACTH to try and stimulate the adrenals- ACTH contains a sequence for melanocyte stimulating hormone in it).

Question 18

Typical presentation of too much water

Answer 18

Patient often in hospital with other illness
Routine biochemistry shows decrease in [Na]
On examination, volume status usually unremarkable
Patient usually has no symptoms specifically due to low sodium
Investigations for causes of sodium loss e.g. Addison’s negative
Patient presumed to have too much water
Most of these patients have ‘inappropriate’ ADH secretion

Question 19

What is SIAD?

Answer 19

Syndrome of innapropriate antidiuresis.

Question 20

What are the non-osmotic stimuli for ADH release?

Answer 20

Hypovolaemia/hypotension
Pain
Nausea and vomiting.

Question 21

Analysing the example of too much water:

Answer 21

ADH is secreted to a non-osmotic stimuli. This causes water retention- often this occurs slowly and the water is distributed over all body compartments- therefore patients volume status will be unremarkable clinically.
It will first present when U&E’s are done and sodium is low
Diagnosis of water excess is a diagnosis of exclusion once they’ve excluded that sodium isn’t too low as in not enough is being secreted (Addisons).

Question 22

A 29 year-old man is admitted to ITU following a cycling accident in which he sustained a severe head injury. During his ITU stay his urine output is in excess of 12 litres daily; his IV fluid requirement is correspondingly large.

Serum sodium is 167 mmol/L on admission to ITU and slowly falls as fluid replacement ‘catches up’. He is in addition commenced on desmopressin (exogenous ADH) which produces a sharper fall in sodium.

Answer 22

A huge head injury transected his pituitary stalk. His posterior pituitary is no longer connected and ADH cannot be excreted.
Therefore water is not reabsorbed in its usual way.
He is essentially peeing out pure water.
Giving him exogenous ADH will help to restore water balance (desmopressin).
Sodium level is high reflecting the water deficit.

Question 23

What does hypovolaemia and low sodium imply?

Answer 23

Sodium deficit is extremely low (too little sodium)

Question 24

What clinical sign shows hypervolaemia?

Answer 24

Most often seen as oedema.

Question 25

In the capillaries, what does the hydrostatic pressure do?

Answer 25

Pushes water out.

Question 26

In the capillaries, what does the oncotic pressure do?

Answer 26

Pulls water in.

Question 27

If you have a circulating volume depletion, which hormone is switched on?

Answer 27

Aldosterone- to retain sodium and therefore water.

Question 28

Consider the example of decreased sodium concentration:

Answer 28

The cause has to be either decreased sodium or increased water.
To distinguish between these two you can check the patients volume status. If they are dry- too little sodium. If normal- too much water.
If too little sodium then it has to be decreased production or increased loss. Decreased production (Addisons disease).
If you suspect decreased production- check ACTH and cortisol levels.

Question 29

How should you treat the following scenarios:

1) Too little sodium
2) Too much water
3) Too little water
4) Too much sodium

Answer 29

1) Give sodium
2) Fluid restrict
3) Give water
4) Get rid of excess sodium using diuretics then replace lost water.

Question 30

Tests used to diagnose primary adrenal insufficiency?

Answer 30

Random cortisol

Short synacthen test-

Question 31

Describe a short synacthen test?

Answer 31

Synacthen is a synthetic analogue of ACTH administered intravenously. If the adrenal glands are working properly they should respond to this and produce cortisol.
The cortisol is measured at 0, 30 and then 60 minutes.
If there is a long SSR- then do a long SSR.

Question 32

Describe a long synacthen test and what it is used for?

Answer 32

Used to help distinguish between primary or secondary adrenal failure.
If you give lots of synacthen over the course of a few days and the problem is outwit the adrenal gland, they should start to perk up and work (usually a pituitary problem). If the problem is with the adrenal gland, prolonged exposure to the hormone will still not cause them to work.