Physiology: Sodium and Water Balance Flashcards
What is syndrome of inappropriate ADH (SIADH)?
Non-osmotic stimuli common in disease e.g. hypovolaemia/hypotension, pain, nausea/vomiting, stimulate ADH release which is inappropriate for the osmolar state
How might syndrome of inappropriate ADH present?
Patient in hospital with other illness retains more water due to excess ADH, occurs slowly so no hyponataemia symptoms but incidental finding on bloods
Name the body compartments which 5% dextrose is able to enter
Plasma, interstitial fluid, intracellular fluid compartment
Why does water follow sodium?
Water follows solute by osmosis; as sodium concentration is greater than any other solute, in practice water follows sodium
If plasma osmolarity decreases, ADH will ________
Decrease
When is low/high sodium very serious?
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
Why are loop diuretics effective in the management of oedema?
An oedematous patient has too much water and sodium - treat with loop diuretics which cause loss of sodium (and water)
Describe the clinical presentation of hypernatraemia
Coughing, shortness of breath
Tiredness
Pulmonary oedema
Pleural effusion
Ascites
Swelling in ankles and legs
_______ is present and can move between all body compartments, whereas ______ is confined to the ECF
Water, sodium
Describe the effect of decreased levels of ADH
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)
ADH is released by the ______ pituitary
Posterior
Name the main mineralocorticoid
Aldosterone
If there is too much mineralocorticoid activity, there will be ____ of sodium
Gain
_____natraemia can be caused by too little sodium or too much water
Hyponatraemia
If plasma osmolarity increases, ADH will ________
Increase
Why will the effects of too much/too little sodium will produce more clinical evidence of changes than water gain or loss?
Sodium is confined to the ECF, whereas water loss or gain is distributed through all body compartments
_______natraemia can be caused by too much sodium or too little water
Hypernatraemia
Name the body compartments 0.9% saline is able to enter
Plasma, interstitial fluid
Name some causes of hypernatraemia
↑ Na+ intake
↓ Na+ loss
↑ H2O loss e.g. diabetes insipidus
↓ H2O intake e.g. very young or very old - quite common
Describe the clinical presentation of hyponatraemia
Increased pulse
Dry mucous membranes
Soft/sunken eyeballs
Decreased skin turger
Decreased consciousness
Decreased urine output
Postural decrease in blood pressure
Describe the effect of increased levels of ADH
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)
Name the body compartments plasma/blood is able to enter
Plasma
What is mineralocorticoid activity?
Steroids with mineralocorticoid activity result in Na+ reabsorption in renal tubules in exchange for K+/H+, increasing Na+ levels
Oedema stimulates the release of ___ and ________, which ends up making the oedema worse
ADH, aldosterone
Name some causes of hyponatraemia
↑ Na+ loss
↓ Na+ intake
↓ H2O excretion e.g. SIADH - most common
↑ H2O intake (compulsive water drinking)
Why is sodium confined to the ECF?
Due to Na+/K+ transporter in the plasma membrane
What causes oedema?
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
