Methods of movement of ions and molecules between compartments
Movement of a substance from an area of high concentration to one of low concentration. This results from random movement of the fluid's constituent particles.
Carrier molecules transport substances from high to low concentrations. (Does not require energy).
Protein pumps within the membranes use energy, which is usually supplied by ATP. These pumps can move substances against concentration gradients.
Water moves across membranes from solutions of low concentration to those of high concentration.
Osmolality: the number of osmoles per kilogram solvent. The osmolality of plasma is maintained at 280-305 mosmol/kg.
It may be estimated by mosmol/kg = glucose + urea + (2 x sodium) (mmol/L)
Osmolarity: the number of osmoles per litre solution.
Osmoles: the molecular weight of a substance divided by the number of freely moving particles liberated in solution.
Tonicity: this is the effective osmolarity of a solution. It is a measure of those particles which are capable of exerting an osmotic force across the cell membrane.
Factors controlling volume and concentration of body fluids
Sodium is the major cation present in the extracellular fluid (ECF). Sodium and its associated anions account for about 85% of ECF osmolality and for about 90% of the ECF tonicity. Under usual physiological conditions, the distribution of total body water between the ECF and intracellular fluid (ICF) is effectively determined by the sodium in the ECF.
Hypernatraemia refers to a serum sodium concentration that is above normal. Serum sodium levels greater than 155 mmol/L are clinically significant. Hypernatraemia always implies hypertonicity of all body fluids.
1. Extrarenal causes
a. Decreased fluid intake. An oral intake of 700 ml/day is required.
b. Increased skin losses (e.g. profuse sweating).
c. Increased gastrointestinal losses.
2. Renal causes
a. Osmotic diuresis. Decreased antidiuretic hormone (ADH) effect.
b. Central diabetes insipidus (failure of ADH synthesis or release) (due to tumour, sarcoidosis, trauma).
c. Nephrogenic diabetes insipidus (renal diseases, hypercalcaemia, hyperkalaemia, lithium ingestion, urinary tract obstruction).
a. Central nervous system disorders
b. Extracellular volume depletion
c. Abnormal urine output
a. Free water may be administered orally, which is the preferred route, or intravenously as a 5% dextrose solution. Infusion of a fluid with a tonicity less than 150 mOsm/L is dangerous and may lead to acute haemolysis.
c. Thiazide diuretics and other drugs, such as chlorpropamide, enhance the renal tubular effects of ADH and also contribute to the stimulation of ADH release.
Hyponatraemia refers to a serum sodium concentration of less than 135 mmol/L. It is usually a problem of too much water, rather than too little sodium.
a. Pseudohyponatraemia is a laboratory artifact that occurs in the setting of hyperlipidaemia or hyperproteinaemia.
b. Hypertonic hyponatraemia results from the shift of water from the ICF to ECF, which is caused by the presence of osmotically active particles (e.g. glucose) in the ECF space. Serum sodium concentration is reduced, but the osmolality of the ECF is normal.
c. True hyponatraemia (hypotonic hyponatraemia) is clinically significant when the serum sodium concentration is lower than 125 mmol/L and the serum osmolality is lower than 250 mOsm/kg.
1. Decreased renal water excretion
2. Decreased glomerular filtration rate
3. Increased proximal tubular reabsorption
4. Decreased renal perfusion pressure in diseases such as congestive heart failure, cirrhosis and nephrotic syndrome stimulates proximal tubular reabsorption
5. Increased collecting tubular reabsorption of water is induced by non-osmotically stimulated ADH secretion syndrome of inappropriate ADH secretion (SIADH). This syndrome is associated with the following disorders:
b. Central nervous system diseases
c. Pulmonary diseases
d. Drug-induced SIADH (chlorpropamide, clofibrate)
1. Fluid restriction. All patients who are severely hyponatraemic should reduce free water intake to approximately 700 ml/day.
2. Inhibition of water reabsorption (frusemide).
3. Hypertonic infusion. Infusion of 3% sodium chloride rapidly raises the tonicity of the ECF. The amount of Na+ needed to raise the serum sodium is calculated using the following equation:
(normal serum Na+ ) – (current serum Na+ ) x total body water.
Related examination questions
a) may increase intracellular fluid volume
b) may be seen in SIADH
c) may incease the secretion of atrial natriuretic peptide
d) may increase the plasma osmolality
e) of acute onset may be associated with cerebral oedema
a) should be corrected with hypertonic saline
b) always implies a disturbance of total body water
c) is associated with abnormal aldosterone secretion
d) cannot be interpreted without clinical data
e) is associated with advanced carcinoma of the bronchus