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.
See also autonomic nervous system; baroreceptors; blood pressure; injury; shock.
Hemorrhage (HEM-or-ij) is bleeding or the escape of blood from the blood vessels. The term hemorrhage usually refers to significant bleeding.
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Hemorrhage is any profuse internal or external bleeding from the blood vessels. The most obvious cause of hemorrhage is trauma or injury to a blood vessel. Hemorrhage can also be caused by aneurysms or weak spots in the artery wall that are often present at birth. Over time, the blood vessel walls at the site of an aneurysm tend to become thinner and bulge out like water balloons as blood passes through them, making them more likely to leak and rupture.
Hypertension, or high blood pressure, is often a contributing factor in brain hemorrhage, which can cause a stroke. Other times, vessels simply wear out with age. Uncontrolled diabetes can also weaken blood vessels, especially in the eyes; this is called retinopathy (ret-i-NOP-a-thee). Use of medications that affect blood clotting, including aspirin, can make hemorrhage more likely to occur.
Bleeding disorders can also spark hemorrhages. Among them are hemophilia (he-mo-FIL-e-a), an inherited disorder that prevents the blood from clotting.
Visible blood is the most obvious sign, but sometimes the only way to know a hemorrhage has occurred inside the body is when it causes symptoms or an illness, such as a stroke. In the case of a brain hemorrhage, depending on where the bleeding is occurring, symptoms can include headache; loss of function on one side of the body; vision changes; numbness or weakness; difficulty speaking, swallowing, reading, or writing; balance problems; decreased alertness; vomiting; stiff neck; and confusion, drowsiness, or coma.
People with hemophilia often experience a tingling feeling that alerts them to a hemorrhage.
When bleeding is visible, the causes of most hemorrhages are obvious. Blood tests and spinal fluid tests can show evidence of brain hemorrhage. Computed tomography (CT or CAT) scanning is an important imaging test used to evaluate the brain and other tissues to see if bleeding has occurred.
The first goal in treating a hemorrhage is to stop the bleeding. Hemorrhage caused by trauma or the tearing of blood vessels can be treated by clamping or surgically repairing the tears. Hemorrhage resulting from vessel leakage due to high blood pressure can be treated with medicines to reduce blood pressure, prevent vessel spasm, and reduce pain. Surgery may be needed to reduce the pressure of collected blood in the brain. Blood factors to help the blood clot may be administered to those with bleeding disorders.
A healthy diet, regular exercise, cutting down on excess sodium intake, maintaining a normal weight, and taking prescribed medication properly can often control high blood pressure. Avoiding drug use can also help prevent brain hemorrhage. Cocaine, amphetamines, and alcohol are increasingly associated with brain hemorrhages, particularly in young people. Wearing helmets when bicycling, skateboarding, and rollerblading and always wearing seatbelts in motor vehicles can help prevent serious head injuries. Retinopathy can be prevented or lessened by good control of diabetes, that is, keeping blood sugars at near-normal levels.
hem·or·rhage / ˈhem(ə)rij/ (Brit. haem·or·rhage) • n. an escape of blood from a ruptured blood vessel, esp. when profuse. ∎ a damaging loss of valuable people or resources suffered by an organization, group, or country: a hemorrhage of highly qualified teachers.• v. [intr.] (of a person) suffer a hemorrhage: he had begun hemorrhaging in the night. ∎ [tr.] expend (money) in large amounts in a seemingly uncontrollable manner: the business was hemorrhaging cash.