Haemorrhage is merely another word for bleeding or blood loss. The amount of a haemorrhage may range from the trivial, for example following minor injuries or a small nosebleed, to life-threatening emergencies. Sometimes the haemorrhage and its extent are obvious, but often it may not be detected without careful examination or special tests. A fracture of the femur, for example, can result in very severe hidden bleeding into the thigh. Bleeding may occur slowly and persistently into the gastrointestinal tract from ulcers or tumours and only be detected by biochemical tests on the faeces.
The effects of haemorrhage depend both on the amount and on the rate of the blood loss. Slow haemorrhages may not actually cause a substantial decrease in the volume of blood, as this may be replaced as rapidly as it is lost. However, the lost red blood cells and their pigment, haemoglobin, may not be replaced and often the first signs and symptoms of severe chronic blood loss are those of anaemia: tiredness, pallor, and breathlessness on exertion. People can tolerate quite rapid and relatively large losses of blood with little apparent effect. Healthy people often donate nearly a pint of blood and are unaware of any after-effects. When the volume of blood depleted from the body exceeds about 20%, or 1 litre in an average adult, blood pressure may fall when standing, due to the added effect of blood ‘pooling’ in dependent veins, and the person may faint.
A moderate blood loss results in a decrease in cardiac output (volume of blood pumped per minute by the heart) but no change or even an increase in arterial blood pressure. Because the amount of blood pumped at each heart beat is smaller, the pulse of the arterial blood pressure is diminished, and this provides a smaller stimulus to the baroreceptors. The resulting response is an increase in the degree of constriction both of resistance vessels and of some veins, and also stimulation of the heart. More severe haemorrhage (over about 30% of blood volume or 1.5 litres in an average person) usually leads to a decrease in blood pressure. Baroreceptor stimulation is much reduced and the sympathetic nerves are strongly stimulated. This results in powerful constriction of blood vessels, seen in the skin as pallor. Other effects of the sympathetic nerves are sweating and piloerection (hairs standing on end). The patient is in shock and is pale, sweating, and has a rapid, ‘thready’ pulse.
As well as evoking these rapid reflex responses, haemorrhage also results in increases in the blood concentrations of several hormones, in particular adrenaline and noradrenaline, which augment the effects of sympathetic stimulation; vasopressin (antidiuretic hormone), which causes the kidneys to retain more water (urine volume decreases) ; and angiotensin, which in addition to constricting blood vessels causes secretion of another hormone, aldosterone, which acts on the kidneys to retain both salt and water (see figure).
Severe haemorrhage, if untreated, is a very dangerous condition. The normal compensating responses eventually fail, the sympathetic nerves cease to fire, and activity in the vagus nerve to the heart may increase. This results in fainting. If very low blood pressure and low flow persist they are likely to cause irreversible changes so that even replacement of all the lost blood does not restore blood pressure. The most common cause of death is kidney failure, because the inadequate blood flow can damage the tubules of the kidney. Other problems include septicaemia (infection in the blood) and heart failure. Severe haemorrhage must be treated by measures to stop the bleeding and by transfusion, ideally of blood, but if this is impossible suitable substitute solutions may be used.
Moderate or slow haemorrhages are compensated by the body. The sympathetic stimulation which constricts the arteries and arterioles (resistance vessels) cuts down the flow of blood into the capillaries. This decreases the pressure of blood which would tend to move fluid out through their walls, so that instead the osmotic pressure of the proteins in the blood is unopposed and draws fluid in from the tissues into the circulation. This can replace much of the lost circulating volume within a few hours. The protein constituents are replaced by the liver in days and, if sufficient iron is available, the bone marrow replaces the lost red cells in weeks.
— R. Hainsworth
See also autonomic nervous system; baroreceptors; blood pressure; injury; shock.
