The mechanisms of feedback loops are explored. These are essential to maintaining homeostasis. The chapter will concentrate on how normal temperature, blood pressure and respiration are maintained and controlled by the body.

Individual behavioural, environmental, social and emotional factors also have an effect on how the internal body balance is maintained. These factors are explored under psychosocial and environmental subject knowledge.

The knowledge needed by the nurse for making a comprehensive assessment of temperature, blood pressure and respirations is explored. The nursing management of some common problems that threaten homeostasis are also included in this section.

Professional issues that influence the role of the nurse in the assessment of homeostasis are identified. The discussion includes an exploration of how the issues of consent, professional accountability and health promotion apply to the nurse’s role when undertaking observations and measurements.

Suggestions are made for portfolio development and experiential exercises in which assessment of homeostasis can be practised in a safe environment. Consolidation of knowledge gained from the chapter and through your practice experience is facilitated through case study work.

On pages 172–173 there are four case studies, each one relating to one of the branches of nursing. You may find it helpful to read one of them before you start the chapter and use it as a focus for your reflections while reading.

Before outlining specific knowledge needed for clinical decision making regarding body temperature, blood pressure and respiration, it is important to understand the internal mechanisms involved in maintaining balance in all body systems and also to consider potential external influences.

Before discussing control loops, it is useful to consider an everyday occurrence. Sian is running a bath for her toddler, Tom. She runs in hot and cold water and tests the temperature with her hand. If it is too cold she turns off the cold tap and runs in more hot, if it gets too hot she runs in more cold. Eventually she ends up with a bath full of water at the right temperature. What Sian has just done is an illustration of how the body controls all of our physiology through feedback loops. The elements of a feedback loop are:

This situation described is an example of negative feedback. In negative feedback, the response that is triggered by the detector negates the deviation from the desired point. Negative feedback loops are the most common mechanism for controlling body functions. Positive feedbacks tend to amplify the trigger rather than reduce it, thus moving the body further away from homeostasis. They are rarely found in healthy individuals (Seeley et al 2008). However, many diseases are characterized by positive feedback, including rheumatoid arthritis, asthma and heart failure. Positive feedback also has a role to play in mental health settings; panic attacks and phobias have elements of positive feedback, where, for example, awareness of sweating and a pounding heart increase anxiety levels even further.

The hypothalamus is a key control and integration centre in the brain and is part of the limbic system, which helps to maintain homeostasis. The hypothalamus regulates eating and drinking, sleeping and waking, body temperature, hormone balances, heart rate, sex drive and emotions (Seeley et al 2008). It also directs the master gland of the brain – the adenohypophysis (pituitary gland) – which is responsible for regulating hormonal balance within the body. The brain is therefore a major organ of adaptation and can respond to changes in the external and internal environment quickly and flexibly to maintain health (Shmaefsky 2007). How homeostasis is specifically maintained is the major focus of this chapter, but although homeostasis is concerned with the stability and maintenance of internal physiological systems, it is also important to consider such balance as being within a context of some external influence.

Large amounts of heat energy are needed to convert liquid water into water vapour, a process known as evaporation. If you wet your hands the liquid water strips heat out of the tissues of your hand and uses this heat energy to turn from liquid to vapour. We experience two things: firstly, our hands chill and secondly, they dry. Sweat is composed mainly of water and as it evaporates the energy used in the process of evaporation cools the body. Water is constantly being lost from the body; insensible water loss (i.e. too small or gradual to be perceived) is approximately 500 mL/day as a result of evaporative loss from the skin and the respiratory passages. This loss can be increased as a result of exercise or sweating. This is why on a hot day the cooling of the body through cool sponges may be soothing. In nursing the use of cooling liquids to reduce temperature is recognized with some caution. In children, whose surface areas are large, there is a risk of too rapid cooling leading to discomfort and in most patients there is a risk of shivering, which is counterproductive as it is a mechanism which generates heat.

In conduction, heat loss or gain is brought about by contact between the surface of the skin and some other object that is either warmer or cooler, for example jumping into a hot bath after being outdoors in the winter for 2 hours in the freezing cold, or having a lovely chilled drink on a hot day. Heat loss to cold objects can be decreased by insulation and clothing, for example wearing thick socks and boots when walking in snow.

Radiation is a process whereby heat is transferred from one heat source to another via infrared radiation without direct contact. For example, when you are cold it is possible to gain heat by standing in the sunshine. The human body also loses heat via radiation. The amount of heat lost through radiation is directly proportional to the amount of skin exposed. Babies and small children have a relatively large skin surface area when compared to their size so they tend to lose heat rapidly via radiation. Conversely, they can also be re-warmed by radiation too, and baby resuscitation equipment often includes overhead infrared warming lamps.

As air moves over the skin heat is lost through convection. When there is little movement of air, the still air forms an insulating barrier and reduces the amount of heat lost; when air movement increases heat loss also increases. Fleecy clothing works by trapping a warm layer of air against the skin, thus reducing the heat lost by convection. The use of electric fans to cool clients with fevers is an example of this principle.

Body temperature is regulated by a feedback loop. As discussed above, heat is constantly being produced by metabolic activity. In order for this heat production to become and remain homeostatically balanced, there has to be a heat loss of the same proportion. This is obtained by balancing the scales between heat input and heat output (Fig. 7.1).

Thermoreceptors located in the periphery and central nervous system respond to a lowering of temperature and transmit impulses to the preoptic area of the brain, located in the hypothalamus, stimulating the heat promoting centre. As a consequence there is sympathetic nervous system stimulation resulting in vasoconstriction of the blood vessels in the skin, and shivering is increased (Fig. 7.2). The sympathetic nervous system response also triggers the release of epinephrine and thyroxine, both of which increase metabolic activity throughout the body, thus increasing heat production.

Sympathetic stimulation resulting in the release of epinephrine and thyroxine increases the basal metabolic rate by mobilizing the fat stores from adipose tissue and as a consequence generating heat. This process is known as chemical thermogenesis or non-shivering thermogenesis, and its function is vital in the newborn because their heat loss is large due to their large surface area, and in the early years the shivering response is poorly developed. This adipose tissue is called ‘brown fat’ and humans have only a small quantity and can therefore only raise their body heat production by 10–15%.

We also see goose bumps developing. In mammals and birds this response results in raising the fur or feathers to trap a layer of warm air against the body. As humans are largely hairless this mechanism has little effect in heat conservation and is largely redundant.

The other important set of responses to low temperatures are the non-physiological behavioural responses such as finding shelter, dressing in warm clothing, turning up the heating in a house. These factors are important to consider as it is often a failure of behaviour rather than a failure of physiology that results in unplanned and life-threatening heat loss.

A rise in temperature is recognized by peripheral and central thermoreceptors; they relay this information to the preoptic area of the hypothalamus and the heat losing centre is stimulated. As a result, there is parasympathetic stimulation; cutaneous vasodilatation, diaphoresis (sweating) and decreased metabolism ensue and heat loss is increased. Shivering is inhibited.

In addition to the physiological mechanisms, nurses must remember the important role played by a person’s behaviour. Cooling behaviours include taking off clothes, seeking shade and a cool environment, drinking fluids, and active cooling, for example jumping in a swimming pool.

Blood pressure is needed to drive blood around the circulatory system, ensuring delivery of oxygen and nutrients and the removal of wastes. This is known as perfusion. Blood flow varies in different regions of the body and in different organs; thus we might talk of renal perfusion or cerebral perfusion. The body uses a range of compensatory mechanisms to maintain adequate blood pressure and hence perfusion. If the needs of an organ or region of the body increase then the delivery of blood to that area will also have to increase to meet the needs.

The pumping action of the heart is one of the main factors that generates blood pressure. The speed with which the heart contracts each minute (heart rate) multiplied by the amount of blood it expels per contraction (stroke volume) is referred to as the cardiac output (i.e. cardiac output = heart rate × stroke volume), and should this increase, a corresponding increase in blood pressure will be observed (Seeley et al 2008).

Another factor that must be borne in mind is the degree of friction created when blood travels rapidly through the blood vessels. This is referred to as peripheral resistance. The widening (vasodilatation) or narrowing (vasoconstriction) of a blood vessel will result in either a reduction or increase in this friction of blood against the walls of the blood vessels and, as a consequence, a decrease or increase in the blood pressure. A division of the autonomic nervous system called the sympathetic nervous system regulates the size of blood vessels. The middle layer in the arteriolar wall, called the tunica media, is in a state of partial contraction as a result of continual activity by the sympathetic division of the autonomic nervous system. This is referred to as sympathetic tone and the tone derives from a group of cells in the vasomotor centre within the medulla oblongata in the brain. An increase or decrease in vasomotor centre (VMC) activity will lead to a corresponding increase or decrease in blood pressure (Fig. 7.3).

Pulse rate (Table 7.2) indicates the speed with which the heart is beating, and it varies with age. A rapid pulse is called a tachycardia and may be triggered by excitement, anxiety or exertion; however, it is also indicative of problems such as haemorrhage or fever. Conversely, a slow pulse is referred to as bradycardia. In health, a well-trained athlete may exhibit bradycardia because their heart muscle is very efficient. Bradycardia can be indicative of heart and neurological disease and is also caused by stimulation of the parasympathetic nervous system.

The normal pulse is regular and the time between each beat is constant. If the normal rhythm of ventricular contraction alters, the interval between beats is interrupted. This can be due to a heart beat occurring earlier (premature) or later or a missed beat. An irregular pulse pattern is called an arrhythmia or dysrhythmia. Some dysrhythmias are there all the time and are continuous; others come and go and are intermittent, and may be triggered by certain events, or even something as simple as a cup of coffee! Sinus arryhthmia is a change in rhythm more commonly found in children and is associated with an increase in heart rate on inspiration and a decrease on expiration (Montague et al 2005). Other changes are often associated with cardiovascular disease and can be life-threatening if cardiac output is seriously affected (see Ch. 4, ‘Resuscitation and emergency care’).

Pulse strength can be separated into two important factors:

Pulse volume is the degree of distension felt in the arterial wall in response to the pressure wave exerted on ventricular contraction. It usually relates to the amount of blood pumped out with each contraction.

Tension is specifically related to blood pressure. More force is needed when compressing the artery to feel for a pulse when the blood pressure is high. Conversely, less force is required when the blood pressure is low. When automated equipment is used to measure the pulse rate, such as a pulse oximeter, it is important to manually check rhythm and strength as the equipment will not detect this.

The process of pulmonary ventilation relies upon two characteristics of gases such as air. Firstly, gases move from areas of high pressure to areas of lower pressure along pressure gradients. Secondly, there is an inverse relationship between volume and pressure – i.e. if you increase the volume the pressure goes down and if you decrease the volume of a fixed amount of gas the pressure rises. This universal property of gases is termed Boyle’s law (Cree & Rischmiller 2001). In the respiratory system there is a continuous movement of air into and out of the lungs along pressure gradients. Upon inspiration, contraction of the diaphragm and the intercostal muscles cause the volume of the chest cavity, and hence the lungs, to increase. This causes the pressure inside the lungs to drop below atmospheric pressure. This leads to a movement of air from the atmosphere to the lungs during inspiration. However, on expiration, when the diaphragm and the intercostal muscles return to their normal position, the pressure gradient is reversed and air moves out of the lungs.

Once the tissues of the body have used the O₂ it is essential not only that the waste product CO₂ is removed, but also that O₂ concentrations are replenished. Gas exchange in the lungs is the process whereby there is a movement of O₂ into the blood in the pulmonary blood capillaries, while CO₂ moves outwards into the lung alveoli. This movement of gases is brought about by their movement along concentration gradients; this is where gases move by diffusion from areas of higher concentration to areas of lower concentration (Cree & Rischmiller 2001). In mixtures of gases, concentration is often measured as partial pressure in mmHg or kPa rather than in grams or litres. Figure 7.5 refers to the partial pressures of O₂ (pO₂) and CO₂ (pCO₂ ). Because the pO₂ in the alveoli is greater than in the pulmonary blood vessels, O₂ moves down the concentration gradient. The diffusion of O₂ from alveoli to blood capillaries allows deoxygenated blood to be converted to oxygenated blood. The diffusion of CO₂ from the blood capillary to the alveoli works on exactly the same principle as O₂ diffusion. Due to the relatively high partial pressure of CO₂ (pCO₂) in the pulmonary blood capillary and the low pCO₂ in the alveoli, CO₂ diffuses down its concentration gradient (see Fig. 7.5).