DISORDERS OF WATER HOMEOSTASIS
Water is necessary to maintain the volumes of body compartments, for excretion of waste products and as a medium in which biochemical reactions occur. Water intake is variable and can depend, to some extent, on social habits but is supplied in the diet, from food as well as water and as a product of oxidative metabolism. Its loss is variable although an almost fixed amount, called the insensible loss, occurs from the GIT, skin and lungs. An average 70 kg man has 42 dm3 of water distributed between various body compartments (Figure 8.4). Water accounts for 60% of body weight in men but only 55% in women given they have a higher proportion of fat. In disease, patients can be dehydrated, where water loss caused by vomiting and diarrhea exceeds gain,or overhydrated, with an accumulation of water in body compartments. The clinical features of dehydration and over hydration are listed in Table 8.1. A reduced extracellular fluid (ECF) volume causes a decline in blood circulation with decreased excretion of wastes and reduced oxygen and nutrient supply to the cells. Humans deprived of fluid intake die after a few days because the reduced total body fluid leads to a circulatory collapse.
The kidneys regulate water balance by varying the output of urine from 0.5 to 15 cm3 min–1 to match water intake. When there is an excess of water, the kidneys lose water rapidly but in times of shortage it is conserved. The total body water is therefore kept constant. Water loss from the kidneys can be regulated by the hormone arginine vasopressin also called antidiuretic hormone (ADH). Antidiuretic hormone acts by altering the permeability to water of the collecting ducts in the kidneys. Osmoreceptor cells in the hypothalamus detect an increase or decrease in osmolality between the intracellular fluid (ICF) and ECF. An increase in the osmolality of the ECF stimulates the receptors and these, in turn, stimulate the release of ADH from the posterior pituitary gland . Antidiuretic hormone then stimulates the kidneys to retain water and produce a more concentrated urine. The retention of water helps return the osmolality of the ECF back to normal. If the osmolality of the ECF is low, the osmoreceptors are not stimulated and ADH is not released. This results in water loss from the kidneys in dilute urine. The loss of water helps to increase the osmolality of the ECF back to normal values. A low blood or ECF volume can be detected by baroreceptors in the aortic arch and carotid sinus . These receptors also stimulate a release of ADH and, indeed, this mechanism can override the release of ADH by osmolality to maintain blood volume and therefore circulation. Antidiuretic hormone interacts with a second hormone, aldosterone to maintain the normal volume and concentration of the ECF. Aldosterone, a steroid hormone, is produced by the adrenal cortex and released in response to a low ECF volume or blood pressure. It stimulates retention of Na+ together with water in the kidneys returning the ECF volume back to normal.
There are distinctive signs and symptoms associated with loss of water from body compartments. For example, loss of water from the ICF results in cell dysfunction that presents clinically as confusion, lethargy and coma. Loss of water from the ECF decreases blood pressure, leading to renal shutdown and shock. A reduction in total body water (ICF and ECF) produces a combination of both effects.
All body fluids contain electrolytes (Table 8.2). The regulation of water content by ADH helps to maintain normal electrolyte concentrations within the body. The concentration of Na+ and K+ in the ICF and ECF are maintained largely by the activity of the plasma membrane Na+/K+-ATPase . This enzyme acts as an energy-dependent pump that expels Na+ from the cell in exchange for an intake of K+ to maintain both at physiological concentrations. The concentrations of these ions are maintained within narrow ranges and, since water can flow freely through most membranes, the concentrations of Na+ and K+ are responsible for maintaining the appropriate osmolalities of these compartments. The movement of water from one compartment to another is mainly responsible for determining their volumes.
Homeostatic mechanisms exist to minimize changes in body water and electrolyte composition and are particularly important in maintaining the volume of the ECF. Water will remain in the extracellular compartment only if its osmolality is sufficiently high.
The assessment of fluid and electrolyte disorders in patients is a significant workload in the hospital pathology laboratory. In most cases, clinical tests to determine the concentrations of electrolytes in blood must be interpreted in conjunction with a clinical examination which involves taking the patient’s clinical history, looking for signs and symptoms of hydration or dehydration and assessing kidney function.