David Corkill

Introduction

This chapter explores some of the physiological mechanisms that support normal body function, the failure of which may necessitate nursing intervention in the ED patient. The basic physiological concepts of homeostasis are examined and a number of homeostatic mechanisms described. The importance of temperature regulation is discussed and the homeostatic mechanisms that maintain normal levels of fluids and electrolytes, glucose, blood pressure and haemostasis are outlined. The mechanisms of oxygen and carbon dioxide transport to and from the cells are discussed. Finally, the breakdown of homeostatic mechanisms is considered in a discussion of physiological shock.

Homeostasis

A single-celled organism, such as an amoeba, requires warmth, oxygen, nutrients and fluids in order to survive and must be able to rid itself of waste products. It interacts directly with the outside world in order to achieve this (Fig. 23.1). The human body is a highly complex collection of millions of cells, very few of which are in direct contact with the outside world, and yet each individual cell has the same survival requirements as the amoeba – a constant supply of fluids, nutrients, oxygen and warmth in order to live and the ability to remove waste products. The external environment (the ‘outside world’) of the cells in the body is the interstitial fluid that surrounds them (see Fig. 23.2 for body fluid compartments) and this fluid must be kept supplied with all the components that the cells might need. Individual cells need to maintain a constant environment within relatively narrow limits in order to function optimally and this constant state must be maintained whatever is happening to the body as a whole. The term ‘homeostasis’, first used by an American physiologist, Walter Cannon, in 1932, refers to the physiological mechanisms that maintain the body in a relatively constant state despite changes in the environment. The word comes from the Greek and means ‘standing the same’, something of a misnomer since physiological function is never static but constantly fluctuating. Homeostasis is essential if the metabolic activities that occur constantly in all cells are to continue.

Throughout the body there are many self-regulating homeostatic mechanisms that aim to maintain an internal ‘steady state’. Most homeostatic mechanisms within the body work by ‘negative feedback’, where a deviation from normal will cause a response to restore the steady state – thus too little of something will cause more to be produced, too much of something will trigger mechanisms to reduce the amount. Once steady state is reached, the homeostatic mechanisms are switched off. An example of a see-saw is commonly used to illustrate this concept (Fig. 23.3). In order to function, homeostatic mechanisms require specialized receptors to detect deviations from the ‘steady state’; they also require a control centre to receive and process the information and the ability to stimulate appropriate body organs and structures to redress the imbalance.

The homeostatic mechanisms involved in temperature regulation, fluid and electrolyte balance, oxygen and carbon dioxide transport and maintenance of blood glucose and blood pressure will be examined in more detail.

Temperature control

Maintenance of a constant core body temperature, within the internal organs, is essential for optimal functioning of cellular enzymes. Humans are homeothermic and normally maintain a constant core temperature of 37 C regardless of the external temperature. The skin temperature may be several degrees different from the core temperature and varies between areas of the body, as those who always seem to have cold feet and hands will know. Body temperature is usually lower, by about 0.5°C at night and is 0.5–1°C higher in women during the second half of the menstrual cycle as a result of normal circadian rhythms. Children have higher core temperatures than neonates and the elderly, and core temperature can rise by up to 2°C during strenuous exercise. Despite all these normal variations, the body must maintain a careful balance between heat gained and heat lost. A summary of factors influencing heat gain and loss is given in Box 23.1.

Fluid and electrolyte balance

Water is the basis of all body fluids, e.g., plasma, tissue fluids and lymph, and accounts for approximately 60 % of total body weight. Body water contains many electrolytes, substances that dissolve and dissociate into ions (develop electrical charges). The main electrolytes in the body are sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺) and magnesium (Mg²⁺), all of which are positively charged anions, and the negatively charged cations chloride (Cl^–), bicarbonate (HCO₃⁻), protein (Pr⁻) and phosphate (PO₄²⁻).

Fluid is either inside the cells (intracellular) or outside the cells (extracellular). Extracellular fluid includes blood plasma, interstitial or tissue fluid that bathes the cells (see above), and small amounts of transcellular fluid, found in body cavities such as intraocular, peritoneal and pleural fluid, cerebrospinal fluid and digestive juices. Figure 23.2 shows how these fluid compartments compare.

Intracellular fluid contains more positively charged potassium and magnesium and negatively charged protein and phosphate than extracellular fluid (which contains more positively charged sodium ions and negatively charged chloride ions) (Fig. 23.5). The ions are prevented from diffusing into other compartments by the selective permeability of the cell membranes and by the presence of a pumping mechanism within cell walls which actively pumps out sodium and exchanges it for potassium. This difference between intra- and extracellular fluids is essential in nerve and muscle cells (excitable tissues), since nerves would be unable to relay messages and muscles unable to contract without it.

The interstitial fluid which bathes cells throughout the body must be maintained in a stable state as it provides the cells with nutrients and maintains the correct temperature for them to function effectively and receives their waste products. Disturbances in the electrolyte content and the concentration, osmolality and osmolarity of the extracellular fluid will affect the intracellular fluid and will impair cell and body function as a result. Normal cell function relies on fluid and electrolyte homeostasis.

Fluids normally enter the body only through the mouth. Thirst is a stimulus triggered when osmoreceptors in the hypothalamus detect a fall in the osmotic pressure of plasma passing over them. Fluid and electrolyte balance by intake alone would be inefficient, since either too much or too little may be ingested for any number of reasons. The body regulates levels of both water and electrolytes at the point of exit, mainly by the action of hormones on the distal tubules of the kidney.

Water balance is coordinated by the thirst centre in the hypothalamus, which controls the release of antidiuretic hormone (ADH). When the concentration of extracellular fluid rises as a result of a fluid intake below body requirements, osmoreceptors in the anterior hypothalamus sense the change and trigger impulses to allow the release of ADH from the posterior pituitary gland. ADH acts on the distal tubules of the kidney so that water is reabsorbed into the circulation. The mechanism is switched off once extracellular osmolarity returns to normal. This is another good example of negative feedback.

The hormone aldosterone, secreted from the adrenal cortex, is responsible for maintaining sodium levels in the body. A fall in blood sodium levels or a rise in serum potassium is detected by specialized cells in the adrenal cortex and increases the release of aldosterone, which acts to reabsorb sodium from the renal tubules and to reduce its excretion in saliva, gastric juices and the skin. Aldosterone production is also stimulated by a fall in the extracellular fluid volume via the renin-angiotensin system activated within the kidney. Potassium balance is closely linked with sodium and when sodium is reabsorbed, potassium is generally excreted. The body is inefficient at conserving potassium and blood levels are not indicative of total body potassium as most of this electrolyte is intracellular.

Optimal kidney function is vital for maintaining fluid and electrolyte homeostasis, and damage through whatever causes (trauma, disease, old age, etc.) will reduce the efficiency of the homeostatic system.

Fluid and sodium balance are closely linked and hormonal responses are triggered by both changes in extracellular fluid volumes and changes in plasma osmolality. A diagrammatic representation is given in Fig. 23.6.

Calcium levels in the body are regulated by the secretion of parathyroid hormone from the four parathyroid glands. The hormone is released directly in response to low extracellular fluid concentrations of calcium and stimulates the release of calcium from bone and its reabsorption from the kidney tubules. In addition, vitamin D is activated and increases the amount of calcium absorbed in the gut from food. When calcium is reabsorbed, phosphate is lost. High calcium levels stimulate the release of calcitonin from the thyroid gland. Calcitonin inhibits the release of calcium from bone and increases its excretion through the kidney until levels return to normal and the mechanism is switched off (Fig. 23.7).