Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 67

Unlike many animals, humans do not have a layer of fur to minimise heat losses. The thin layer of hair that we possess can only trap a small amount of warm air close to the skin, which leaves the human body very vulnerable to heat loss. Without adequate shelter and heat, in cold environments the core temperature can rapidly drop, slowing the rate at which cellular enzymes can function. As the core temperature drops, thermoreceptors relay information to the hypothalamic thermoregulatory control centre, which responds by switching off the production of sweat and reducing blood flow to the skin by initiating vasoconstriction in the dermis.

This minimises heat loss at the skin surface and typically results in the skin taking on a pallid and sometimes bluish appearance (think of how your fingers look when making a snowball).

If this is not sufficient to raise the core temperature towards the set point quickly enough, shivering is initiated where the major skeletal muscle groups contract in spasmodic movements to generate extra heat. If an individual remains in a cold environment for extended periods of time, hormonal adaptations occur such as enhanced release of adrenaline and the thyroid hormones (T3 and T4). This increases the metabolic rate in the longer term (Figure 2.2), generating further internal heat via metabolic thermogenesis. These hormonal adaptations allow the body to adapt to living in a colder environment.

Beneath the dermis of skin is a layer of subcutaneous fat called the hypodermis. This acts as a layer of insulation helping prevent heat loss by holding the heat generated through metabolism within the core of the body. Individuals that are overweight or obese often find it difficult to lose heat because their subcutaneous layers are thicker and this can put them at greater risk of heatstroke, particularly when exercising on hot days. Conversely, older individuals often lose a significant amount of subcutaneous fat as they age and consequently find it harder to retain heat and maintain their core temperature.

Older people also tend to have a lower metabolic rate and a reduced shivering response which places them at increased risk of hypothermia. Hypothermia (remember, hypo means low) is defined as a core temperature of 35°C or lower. At these temperatures cellular enzymatic activity will have slowed significantly and the patient will initially show increased shivering and progressive mental confusion. As hypothermia progresses, the shivering response may be lost, allowing the core temperature to drop further. In severe hypothermia behaviour may become increasingly erratic and illogical. As the core temperature drops below 35°C, the heart rate begins to progressively slow, leading to bradycardia which is defined as a resting heart rate of 60 bpm or less. This leads to reduced tissue and organ perfusion.

Unless treated, severe hypothermia may progressively lead to cardio-respiratory failure and death. However, clinicians have to be extremely careful when pronouncing someone dead as a result of hypothermia since the heart rate and brain activity can be reduced significantly, leading to a torpid state that closely resembles death. Hypothermia is usually treated by moving the patient to a warm environment, ensuring that they are adequately clothed, ideally in multiple layers of dry clothing, and if necessary re-warming the body using heated blankets, water bottles or devices that blow warm air over the body.

Elderly patients are particularly vulnerable to hypothermia in the winter months and this is explored in Grace’s case study.