crrm2.10: water balance21/02/08
Learning Outcomes
Explain how ADH action on the kidney controls whole body water balance
- Water balance is the maintenance ofextracellular osmolality
- Osmolarity is sensed by osmoreceptors in the hypothalamus in the third ventricle of the brain
- Thirst centres in the brain can be stimulated or repressed to regulate water intake
- ADH can be produced to control renal water excretion
- Effective circulating volume is sensed by baroreceptorswhich can stimulate the paraventricular neuron to release ADHunder certain circumstances (e.g. haemorrhage)
- ADH is synthesised in supraoptic and paraventricular neurons in the hypothalamus
- It is transported along their axons to the posterior pituitaryfrom which it is released
- Increased ADH levels proportionally concentrateurine conserving bodily water
- ADH acts on V2 receptors in the kidney
- Osmoreceptors are very sensitive to increases in osmolality and are the major osmolality controller
- Above 280mOsm levels of ADH secretion rise rapidly to maximum at 300mOsm
- Baroreceptors are not very sensitive and require at least a 10% decrease in blood volume to trigger ADH secretion
- However, they are capable of inducing an ADH output4 times higher than the osmoreceptors
- Heightened emotions, stress, high temperature (?), exercise and pain all stimulate ADH secretion
Describe changes in volume and osmolarity of the intracellular and extracellular fluid compartments under various pathophysiological conditions in which salt and/or water balance are perturbed
- Plasma sodium concentration is a good clinical measure of osmolality
- Sodium is the major plasma cation (140mM)
- Each sodium ion will have a corresponding anion
- Doubling the sodium concentration in a blood sample gives a good estimate of osmolality
- Lowered osmolality is measured as a decreased plasma sodium concentration – hyponatraemia
- Elevated osmolality is measured as an increased plasma sodiumconcentration – hypernatraemia
- Both conditions are caused by disturbances of salt balance, water intake or water output
- Hyponatraemia is defined as a plasma sodium concentration under135mM
- Caused by hypoaldosteronism, polydipsia, renal disease, vomiting, SIADH (see below)
- Hypernatraemia is defined as a plasma sodium concentration over 145mM
- Caused by dietary net salt gain, lack of access to water, insensible water loss, diabetes
Define how disorders in ADH secretion affect water balance
- Syndrome of inappropriate ADH secretion (SIADH) is overproduction of ADH causing hyponatraemia
- Caused by post-operative pain, intracranial trauma, tumours, drugs, pulmonary disease
- Ecstasy causes SIADH and can result in death due to water intoxication
- Diabetes insipidushas two forms:
- Central –impaired ADH secretion due to hypothalamic damage e.g. head trauma
- Nephrogenic –impaired ADH response from sickle cell anaemia, lithium therapy
- Water restriction allows clinical determination of the form of diabetes insipidus
- Exogenous ADH (e.g. nasal spray) will induce urinary concentration in central DI only
- Changes in effective circulating volume reset the ‘osmostat’:
- Depleted ECV increases ADH release sensitivity tolerating a lower osmolality to maintain blood volume
- Expanded ECV decreases ADH release sensitivity tolerating a higher osmolality to reduce blood volume
- Pregnancy and morphine also act to increase ADH release sensitivity (lower osmolality)
- Alcoholism also acts to decrease ADH release sensitivity (higher osmolality)