Physiology: Sodium and Water Balance

Question 1

What is syndrome of inappropriate ADH (SIADH)?

Answer 1

Non-osmotic stimuli common in disease e.g. hypovolaemia/hypotension, pain, nausea/vomiting, stimulate ADH release which is inappropriate for the osmolar state

Question 2

How might syndrome of inappropriate ADH present?

Answer 2

Patient in hospital with other illness retains more water due to excess ADH, occurs slowly so no hyponataemia symptoms but incidental finding on bloods

Question 3

Name the body compartments which 5% dextrose is able to enter

Answer 3

Plasma, interstitial fluid, intracellular fluid compartment

Question 4

Why does water follow sodium?

Answer 4

Water follows solute by osmosis; as sodium concentration is greater than any other solute, in practice water follows sodium

Question 5

If plasma osmolarity decreases, ADH will ________

Answer 5

Decrease

Question 6

When is low/high sodium very serious?

Answer 6

If Na+ is very low or very high, which can cause symptoms

Symptomatic hypo/hypernatraemia indicates very high or low Na+ - life-threatening (e.g. altered consciousness, confusion, N+V, fitting)

If Na+ has fallen or risen rapidly to current level, it may be serious even if concentration falls within normal reference range

Question 7

Why are loop diuretics effective in the management of oedema?

Answer 7

An oedematous patient has too much water and sodium - treat with loop diuretics which cause loss of sodium (and water)

Question 8

Describe the clinical presentation of hypernatraemia

Answer 8

Coughing, shortness of breath

Tiredness

Pulmonary oedema

Pleural effusion

Ascites

Swelling in ankles and legs

Question 9

_______ is present and can move between all body compartments, whereas ______ is confined to the ECF

Answer 9

Water, sodium

Question 10

Describe the effect of decreased levels of ADH

Answer 10

Decreased plasma osmolarity → decreased ADH → less aquaporins in DCT and CD cells of the kidney → less water reabsorbed in kidney → large volume of dilute urine (low osmolarity)

Question 11

ADH is released by the ______ pituitary

Answer 11

Posterior

Question 12

Name the main mineralocorticoid

Answer 12

Aldosterone

Question 13

If there is too much mineralocorticoid activity, there will be ____ of sodium

Answer 13

Gain

Question 14

_____natraemia can be caused by too little sodium or too much water

Answer 14

Hyponatraemia

Question 15

If plasma osmolarity increases, ADH will ________

Answer 15

Increase

Question 16

Why will the effects of too much/too little sodium will produce more clinical evidence of changes than water gain or loss?

Answer 16

Sodium is confined to the ECF, whereas water loss or gain is distributed through all body compartments

Question 17

_______natraemia can be caused by too much sodium or too little water

Answer 17

Hypernatraemia

Question 18

Name the body compartments 0.9% saline is able to enter

Answer 18

Plasma, interstitial fluid

Question 19

Name some causes of hypernatraemia

Answer 19

↑ Na+ intake

↓ Na+ loss

↑ H2O loss e.g. diabetes insipidus

↓ H2O intake e.g. very young or very old - quite common

Question 20

Describe the clinical presentation of hyponatraemia

Answer 20

Increased pulse

Dry mucous membranes

Soft/sunken eyeballs

Decreased skin turger

Decreased consciousness

Decreased urine output

Postural decrease in blood pressure

Question 21

Describe the effect of increased levels of ADH

Answer 21

Increased plasma osmolarity → increased ADH → more aquaporins in DCT and CD cells of the kidney → more water reabsorbed in kidney → small volume of concentrated urine (high osmolarity)

Question 22

Name the body compartments plasma/blood is able to enter

Answer 22

Plasma

Question 23

What is mineralocorticoid activity?

Answer 23

Steroids with mineralocorticoid activity result in Na+ reabsorption in renal tubules in exchange for K+/H+, increasing Na+ levels

Question 24

Oedema stimulates the release of ___ and ________, which ends up making the oedema worse

Answer 24

ADH, aldosterone

Question 25

Name some causes of hyponatraemia

Answer 25

↑ Na+ loss

↓ Na+ intake

↓ H2O excretion e.g. SIADH - most common

↑ H2O intake (compulsive water drinking)

Question 26

Why is sodium confined to the ECF?

Answer 26

Due to Na+/K+ transporter in the plasma membrane

Question 27

What causes oedema?

Answer 27

Oedema signifies effective circulating (vascular) volume depletion due to an altered balance of Starling forces at capillary level resulting in increased flow of fluid from the vascular system into the interstitium