Home » Water and electrolytes and acid-base homeostasis
The concentration of hydrogen ions in both extracellular and intracellular compartments is extremely tightly controlled; very small changes may lead to major cell dysfunction. Hydrogen ion concentration has traditionally been expressed as pH, the negative logarithm of [H+], but this may contribute to complacency about the real magnitude of changes in [H”]
Basic principles
CARBOHYDRATE AND FAT METABOLISM
HYPOPHOSPHATAEMIA
Significant hypophosphataemia may occur in a number of clinical situations, either due to redistribution into cells, to renal losses, or to decreased intake, and may cause:
• Muscle weakness-diaphragmatic weakness, decreased cardiac contractility, skeletal muscle rhabdomyolysis
• Left-shifted oxyhaemoglobin dissociation (reduced 2,3- diphosphoglycerate (2,3-DPG)).
• Confusion, hallucina
Plasma magnesium levels are normally maintained within the range 0.7-1.1 mmol litre:”. Like potassium, magnesium is principally an intracellular cation.
Regulation of magnesium balance is mainly via the kidney. Primary disturbance of magnesium balanceis uncommon, hypo- or hypermagnesaemia usually developing on a background of more obvious fluid and electrolyte disturbances. Disturbance in magnesium balanc
Regulation of serum potassium concentration
The usual dietary intake varies between 80 and 150 mmol daily depending upon fruit and vegetable intake. Most of the body’s potassium (3500 mmol in an adult man) is intracellular. Serum potassium levels are controlled by:
• Uptake of K+ into cells
• Renal excretion
• Extra-renal losses (e.g. gastrointestinal)
Uptake of potassium into cells is governed by t
These are best thought of as disorders of body water content. As discussed above, sodium content is regulated by volume receptors; water content is adjusted to maintain, in health, a normal osmolality and (in the absence of abnormal osmotically active solutes) a normal sodium concentration. Disturbances of sodium concentration are caused by disturbances of water balance.
HYPONATRAEMIA
Hyponatraemia is one of
Deficiency of sodium and water causes shrinkage both of the interstitial space and of the blood volume and may have profound effects on organ function.
CLINICAL FEATURES
SYMPTOMS are variable. Thirst, muscle cramps, nausea and vomiting, and postural dizziness may occur. Severe depletion of circulating volume causes hypotension and impairs cerebral perfusion, causing confusion and eventual coma.
SIGNS can be d
Water accounts for 50–60% of total body weight. This percentage varies with age, sex and body build, being less in women and the obese, as fat has a lower water content. Changes in total body water from one day to the next is, in an individual, reflected by changes in body weight. In a healthy, 70-kg male, total body water is approximately 42 litres, distributed between several compartments.
Two-thirds
Introduction
In health, the volume and biochemical composition of both extracellular and intracellular fluid compartments in the body remains remarkably constant. Many different disease states result in changes of control either of extracellular fluid volume, leading to clinical abnormalities such as oedema and hypotension or hypertension, or of the electrolyte composition of extracellular fluid. A sensible
