These figures may differ slightly according to local policies. The serum sodium level defined as вЂnormalвЂ™ is 135-145mg/dl the normal potassium level is 3.5-4.5mg/dl and the normal magnesium level is 0.6-1.0mg/dl (Metheny, 1996). For example, cardiac arrhythmias are triggered by low potassium and low magnesium levels (Docherty, 2002 Smith, 2000). If there is too much or too little of any of these electrolytes this can cause problems. In addition, вЂ electrolyte balanceвЂ™ is essential, in other words the correct concentration of various ions in the body, namely sodium, potassium and magnesium. To attain the correct balance of ECF and ICF a patient must also take in the correct amount of fluid (Docherty and McIntyre, 2002). Regulation of ECF osmolarity is achieved by balancing the intake and output of sodium with that of water. In addition to regulating total volume, the osmolarity of bodily fluids is closely monitored and regulated because variations may cause damage to cellular structure (swelling or shrinking), disrupting normal cellular function (Edwards, 2001 Metheny, 1996). These receptors trigger ADH release, thus conserving water at the kidneys (Tortora and Grabowski, 2002 Metheny, 1996). The aorta and carotid arteries also have receptors that are sensitive to a reduction in blood pressure (related to the pressure in the left ventricle). They stop ADH secretion, which leads to increased excretion of water through renal filtration.

Sensors detect stretching of the atria of the heart indicating excessive returning volume of ECF (increased venous return). Normal levels of sodium in the ECF will attract and maintain the optimum amount of water (Tortora and Grabowski, 2002 Edwards, 2001).ДЄDH release is also influenced in the following circumstances. When sodium concentration in the blood decreases the adrenal cortex is stimulated into secreting the hormone aldosterone, which instructs the distal nephrons of the kidney to retain more sodium. This in turn results in more concentrated urine and an increase in water returned to the ECF, thus correcting the volume depletion (Tortora and Grabowski, 2002 Edwards, 2001) (Fig 1). In response, the posterior pituitary gland releases antidiuretic hormone (ADH, sometimes called vasopressin) into the bloodstream, resulting in the kidneys retaining water. The hypothalamus is an osmoreceptor - a sensory end organ that reacts to changes in osmotic pressure and has an effect on the pituitary gland. When fluid volume decreases, the concentration of sodium in the blood will increase (increased osmolarity, the amount of solute per unit volume), which in turn stimulates the hypothalamus (Tortora and Grabowski, 2002). Carriage of waste products to the appropriate organ for excretion - generally through the liver and renal system.

Body fluid compartments images