injury

injury to the body can result from accidents, acts of violence, or surgery. This third category, inflicted by the medical profession itself, is often forgotten, but it does of course cause damage to body tissue — although hopefully the overall result of surgery will be of benefit to the patient.

Injuries can range from damage so minor as to be hardly noticed, to that which is so severe that it causes death or prolonged disability. The size of the problem becomes apparent when it is realized that almost a quarter of all deaths in Europe and the US are caused by injury, particularly to children and young people (six times as many children die from injury than from cancer). In Britain, road traffic accidents are the main cause of death from injury, but in the US gunshot and knife wounds kill more people every two years than the total number of fatalities in the whole of the Vietnam war, and in young black males gunshots at home are the single greatest cause of death.

Understanding the responses to injury (even when the injury is not severe) and developing treatments, is clearly very important. Research indicates that such responses are complicated and variable. They depend on the type and site of injury, the age and health status of the victim, and the time after the injury.

Our bodies can adapt extremely quickly to injury by activating a whole array of responses to limit damage. These occur not only at the specific site of the injury, but also involve most of the general systems in the body, including the heart and circulation, the endocrine, nervous and immune systems, and complex changes often called the ‘defence response’ or ‘acute phase response’. All of these changes are directed towards helping the body to cope with damage, to keep functioning normally, and to start the processes of repair and recovery. However, sometimes, when injuries are severe, it is the body's own defence responses which can actually cause problems and even death, because they are inappropriate or excessive.

Within seconds of injury, responses start to be activated. The first of these — and the one of which we are most aware — is the sensation of pain. Nerve signals from the damaged tissue signal to the brain in fractions of a second a ‘red alert’ to move away from the source of pain and protect the injury. These nerve signals also have other functions. They activate a whole-body response, which has been called the ‘fight or flight’ response, because it involves many of the same changes which occur when we are fearful of danger or need to flee away from threats.

The ‘fight or flight’ response (which is seen even in very primitive organisms) means that the brain automatically activates our sympathetic nervous system to increase heart rate and blood pressure, to direct blood to important regions (the brain, the heart, and in this case the site of injury), and to stimulate release of specific hormones. These hormones tend to stimulate body functions which may be needed to fight off infection, support repair of injuries, and help the heart and other organs cope with the threat to survival. For example, the adrenal gland releases increased amounts of the steroid hormone cortisol (often used in medicines to reduce inflammation); the pancreas releases the hormone glucagon, which mobilizes glucose (needed as a rapid energy supply); and the sympathetic nervous system releases adrenaline and noradrenaline, which not only stimulate the heart and blood vessels, but also start to mobilize body fuels.

Another very important part of the body involved in injury is the immune system, which acts locally at the site of damage and throughout the body. A group of proteins, called cytokines, are produced quickly by immune cells and injured tissues. They, together with other factors released by tissue damage, cause swelling around the injury, and stimulate immune cells in the blood to race into the wound and surrounding tissue. Cytokines also induce the liver to produce a second wave of proteins known as acute phase proteins; the endocrine glands to alter hormone release; the brain to cause changes such as fever and the general symptoms of illness; and many other organs and systems to make various changes. As with most responses to injury, cytokines can help to limit and localize damage and stimulate repair and recovery. However, it is now clear that overproduction of certain cytokines can cause problems, and severe cases may prove fatal, so there is intense research into ways of limiting the production of cytokines.

These general responses are common to many types of injury, but other changes in the body depend on the type or severity of the injury. For example, severe loss of blood (haemorrage) can actually cause a reduction in blood pressure and may result in ‘shock’. Severe pain or injury can also result in shock and loss of consciousness, and toxins released from bacteria in the gut after severe injury can result in a condition similar to severe infection. If this progressive deterioration goes unchecked, it can lead to a fatal condition known as multiple organ failure, where major organs such as the liver, kidney, heart, and lungs are unable to function.

Damage to the brain can be rather different to injury in other parts of the body, for several reasons. Nerve cells (neurons) in the brain, unlike other cells of the body, cannot regrow. Therefore, when they are lost, recovery is limited by the ability of existing brain to compensate, that is, the extent to which functions can be ‘recovered’ in other healthy areas of the brain. Secondly, quite minor damage to the brain can have very major effects on responses in the rest of the body and therefore on survival. For example, damage to very small parts of the lower, most primitive parts of the brain (the brain stem), which control the essential functions of breathing and the heart, can prove fatal, whereas quite severe loss of some other parts of the brain can be survived with only modest disability.

The responses to injury are often divided into acute (early and quite rapid) and chronic (prolonged and sustained). It is usually the early phase which is critical, but of course delayed and prolonged events in the body are also important. The mobilization of fuels necessary for the acute response means that body tissues (fat and protein resources) must be broken down. This, together with loss of appetite or inability to eat after injury, usually means quite significant weight loss. In otherwise healthy adults, this may not be a problem — though even patients who are overweight can have difficulties because protein (mainly muscle) is broken down as well as fat. The implications are much more severe in young, growing children and in elderly patients. Because old people are often underweight and not very mobile, loss of muscle, for example after breaking a leg, can be a real problem.

Finally, the injury itself must recover. This requires repair to the damaged tissue, and possible formation of a scar. Scars are not just a cosmetic problem; they can reduce the normal function of some tissues (particularly the brain) and result in a focus for future problems. Again, research is starting to identify factors — particularly proteins known as growth factors — which can aid repair of injured tissues, and those which lead to scars. Considerable advances in understanding the processes which occur after injury and development of new treatments mean that we can now, with the right medical care, survive some injuries that would once have been fatal.

Physical repair may not of course be the end of the story for the injured person. The prolonged, varied, and complex psychophysiological after-effects of major injury, particularly from assault and violence, known as ‘post-traumatic stress disorder’, have been — and continue to be — a focus of research, especially with respect to the aftermath of war and rape. As more becomes known about the mental and physical processes involved, there is hope for more effective assistance towards recovery.

Nancy J. Rothwell

Bibliography

Rothwell. N. J. and Berkenbosch, E. (ed.) (1994). Brain control of responses to trauma. Cambridge University Press.

See also autonomic nervous system; haemorrhage; immune system; pain; shock; stress; wound healing.