5. Osmosis

Question 1

Definition

Answer 1

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

Question 2

Osmolarity

Answer 2

osmolarity is the number of osmoles (or mosmoles) of solute
per litre of solution,

Osm l/1, and is influenced by temperature

Question 3

Osmolality

Answer 3

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.

Question 4

Estimation of osmolality

Answer 4

Osmolality= [2 x Na+] + [ Glucose] + Urea all in mmol l1

at around 290 mosmol k

Question 5

Osmolar gap:

Answer 5

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.

Question 6

Osmotic pressure

Answer 6

: 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,

Question 7

Measurement of osmotic pressure

Answer 7

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

Question 8

Oncotic pressure

Answer 8

: 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.

Question 9

colloid osmotic pressure

Answer 9

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.

Question 10

Tonicity:

Answer 10

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.

Question 11

Conditions That Result in Derangements of Osmolality

Answer 11

Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH

Diabetes insipidus (DI)

Question 12

Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH

Answer 12

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.

Question 13

ADH

Answer 13

: 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.)

Question 14

Diabetes insipidus (DI)

Answer 14

: 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.

Question 15

(TUR syndrome)

Answer 15

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.

Question 16

Water intoxication:

Answer 16

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

Question 17

Hyperosmolar states:

Answer 17

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.