5. Osmosis

Definition: osmosis describes the process of the net movement of water molecules due to diffusion between areas of different concentration.

osmolarity is the number of osmoles (or mosmoles) of solute per litre of solution, Osm l/1, and is influenced by temperature

Osmolality is the number of osmoles per kilogram of solution, Osm kg–1 and, because it is temperature-independent, removes a source of potential inaccuracy.

Osmolality= [2 x Na+] + [ Glucose] + Urea all in mmol l1 at around 290 mosmol k

this is the difference between the measured osmolality and the calculated osmolarity of the sample. Its clinical relevance lies in the fact that it may identify an osmotically active substance, such as ethyl alcohol, that is not normally present in plasma, and so can help to differentiate the cause of a metabolic acidosis.

: an effective concentration gradient of water can be produced between two compartments separated by a semi-permeable membrane movement of water into such a compartment will increase the pressure and/or volume of the compartment. opposed by increasing the pressure in the compartment,

which utilizes one or more of the colligative properties of a solution. depend on the osmolality and are depression of freezing point, elevation of boiling point, reduction in vapour pressure and exertion of osmotic pressure 1 mol of a solute which is added to 1 kg of water will depress the freezing point by 1.86 C

: the oncotic pressure is the contribution made to total osmolality by colloids (hence the alternative term ‘colloid osmotic pressure’, COP). The plasma oncotic pressure, at 25–28 mmHg, is only about 0.5% that of total plasma osmotic pressure, but it is significant because it is the major factor in the retention of fluid within capillaries. Albumin is responsible for about 75% of the total COP.

colloid osmotic pressure can be measured by an oncometer, which comprises a semi-permeable membrane which separates the plasma sample from a saline reference solution. The change to the oncotic pressure can readily be transduced and measured.

in contrast to osmolality, which measures all the particles in a solution, tonicity refers only to those particles which exert an osmotic force Urea and glucose are freely permeable and so are not included.

Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH Diabetes insipidus (DI)

non-osmotic release of ADH with consequent water retention and hypotonicity intracranial tumours and pulmonary malignancy and infection via water restriction and, in chronic cases, with the use of demeclocycline (a tetracycline) which blocks ADH action in the kidney.

: this increases conservation of water and sodium in the distal renal tubules via a mechanism mediated by cAMP Osmoreceptors in the supraoptic nuclei of the hypothalamus have a mean threshold of 289  2.3 mosmol kg1. Above this plasma level, ADH release is stimulated. (The kidneys should be able to produce a urine osmolality of at least 1,000 mosmol kg1.)

: this also has many causes and can be neurogenic (with deficiency of ADH synthesis or impaired release) or nephrogenic (with renal resistance to the action of ADH). It is characterized by massive diuresis and hypovolaemia. Neurogenic DI is treated with desmopressin (an ADH analogue) in a dose tailored to allow a mild diuresis to avoid the complication of water intoxication. Chlorpropamide potentiates the effects of endogenous ADH and also sensitizes distal tubules.

Glycine intoxication (TUR syndrome) with hyponatraemia: this may follow excessive absorption of irrigating fluid during transurethral procedures (usually prostatectomy). Treatment is with administration of normal saline and judicious diuretic. Rapid restoration of normal sodium (for example, by the use of hypertonic saline) is associated with central pontine myelinosis.

this follows excessive intake of water, usually self-inflicted (29% of the finishers in one Hawaiian Ironman Triathlon were hyponatraemic), but is also associated with iatrogenic infusion of large volumes of glucose solution. The decrease in plasma osmolality inhibits ADH secretion, but it can still cause potentially fatal electrolyte disturbance

Hyperosmolar states: the commonest hyperosmolar state is that of hyperglycaemic non-ketotic hyperosmolar coma, secondary to type 2 diabetes and precipitated by any dehydrating illness or reduction in insulin activity. (The serum osmolality is typically >320 mOsmol kg1) Hyperosmolarity can also be iatrogenic following, for example, the administration of mannitol to neurosurgical patients.