hypoxia

hypoxia means a shortage of oxygen — as compared to anoxia, which means a total lack of it. In common with other mammals, humans have evolved with a system of breathing and blood circulation, which allows intake of oxygen from the air and its transport throughout the body. The tissues need to extract oxygen from the blood constantly at a basic rate for their metabolism, along with the extra needed for work and exercise, and also are accustomed to a certain level of oxygen in their immediate environment. The body can compensate to some extent for a decreased level, but life depends on maintainence of the supply of oxygen. Different organs and tissues can survive lack of oxygen for different lengths of time: the brain is the most rapidly and irrevocably damaged. Because the brain regulates breathing and the circulation — the means by which oxygen is supplied to the whole body, including itself — deprivation of the brain prevents restoration of the supply; a potentially lethal vicious circle.

Hypoxia occurs (i) when there is less than the normal amount of oxygen in the air inhaled; (ii) when breathing is obstructed, is inadequate, or stops; (iii) when oxygen is not transferred normally from the lungs to the blood; (iv) when the blood cannot carry its normal quota of oxygen; (v) when the flow of blood is inadequate, or stops.

The air inhaled

may provide insufficient oxygen either because the atmospheric pressure is low (at high altitude) or when the supply of fresh air is restricted. At high altitude the air is ‘thinner’ in that every molecule of the gas occupies a larger volume. The blood leaves the lungs carrying less oxygen than normal, therefore the tissues are exposed to a lower oxygen level. If this is not too profound, they can still obtain oxygen at the required rate, at least for resting metabolism, because the rate of flow of blood can increase. The tissues are living at a lower level but are still getting a sufficient oxygen supply.

When the supply of fresh air is restricted — with a bag over the head, in a closed cupboard, or in a larger enclosed space crowded with people — oxygen is progressively depleted and exhaled carbon dioxide accumulates. In some circumstances there may be displacement of air by other gases, and the effect of irritant or toxic gases, such as smoke, chlorine, or sulphur dioxide, can complicate the effects of displacement of oxygen.

Disturbance of breathing

Obstruction to breathing can occur either externally (smothering, strangulation, compression of the chest in a crowd disaster) or internally (choking, allergic swelling of the upper airways, asthmatic attacks). In other less drastic ways breathing can become inadequate to keep the oxygen level up to normal, and carbon dioxide down to normal, in the lungs and blood: when breathing becomes mechanically difficult in some types of lung disease; when there is damage to the brain stem or to the upper spinal cord where the nerves arise which activate the muscles of breathing; or when the muscles themselves are weak. Breathing may be depressed by drugs acting on the control centres in the brain, and it may stop entirely in collapse from various causes (concussion, near-drowning, heart attack, electric shock).

If obstruction or cessation of breathing is total, the condition is known as asphyxia: it rapidly causes death by depriving the brain of oxygen. A lesser degree of inadequate breathing is called hypoventilation, characterized by a lowered level of oxygen and a raised level of carbon dioxide in the lungs, blood, and tissues; a person suffering from chronic lung disease, for example, can live for many years in a state of moderate hypoxia. The term suffocation is less precisely defined, but is commonly applied either to obstructed breathing or to lack of fresh air supply.

Oxygen transfer from the lungs to the blood

can be impaired in some types of lung disease because the barrier it has to diffuse across is thickened. So despite breathing as much or more than normal, the blood and tissue oxygen level is below normal, although, again, an adequate supply may be maintained by increased blood flow. There are also conditions (including some congenital heart defects) in which the blood is not routed properly through the lungs, so that some blood bypasses the oxygen supply, with the result that arterial blood is hypoxic.

In all the types of hypoxia described so far, the haemoglobin in the arterial blood is less than fully saturated with oxygen. The redness of blood depends on this saturation. In hypoxia it becomes more blue, and cyanosis is the outward and visible sign of this when blueness tinges the skin.

The oxygen-carrying capacity of the blood

is lowered when red blood cells, and haemoglobin, are in short supply (anaemia): the blood carries less oxygen than normal, although there is sufficient oxygen in the air and in the lungs, and all available haemoglobin is fully saturated. There are also conditions in which the haemoglobin is not all in its normal form. Carbon monoxide poisoning acts by combining with haemoglobin, making it unable to carry oxygen.

Deprivation of blood flow

makes organs and tissues hypoxic: the state of ‘ischaemia’. This can occur either as part of whole-body deprivation in heart failure, or locally where blood vessels are obstructed by arterial disease or by clots, or constricted as in the skin in cold exposure.

Defences against hypoxia

The body has ways to defend itself against hypoxia at each stage of the process of oxygen acquisition: by breathing harder, to get more into the lungs; by crowding more red cells into the blood so that it can carry more in every circulating millilitre; by pumping the blood around at a greater rate; and by widening the blood vessels which supply the vital organs. Most of these adjustments can be made very rapidly.

When oxygen is low — but tolerably so — in inhaled air, and hence in the blood, the arterial chemoreceptors — minute structures in the neck — sense this and, via the brain, cause a reflex increase in breathing. This brings the oxygen concentration in the lungs closer to that of the outside air — it remains low, but not as low as it would be if the breathing did not increase. Stimulation of breathing occurs more dramatically when carbon dioxide is accumulating in the blood whilst oxygen is decreasing, such as in the example of breathing in a confined space.

If hypoxia of a tolerable degree is sustained for weeks, the bone marrow produces extra red blood cells, resulting in polycythaemia. The greater density of red cells brings the oxygen concentration in the blood back towards normal despite their haemoglobin carrying less than it ideally could. The down side is that the thicker blood gives extra work to the pumping heart. This defence mechanism cannot of course operate against anaemia, when the fault itself lies in a deficient production of red blood cells.

The heart compensates for hypoxia by pumping out more blood per minute so that the actual delivery rate of oxygen to the tissues can be kept up despite its lower concentration in the blood.

These automatic attempts at self-preservation operate unless the oxygen lack becomes too profound to sustain brain functions, including that of maintaining breathing itself. At worst, the heart weakens, the blood pressure falls, breathing stops, and cessation of the heartbeat soon follows.

Sheila Jennett

See also altitude; breathing; cyanosis; lungs; oxygen; suffocation.