respiration

respiration is the absorption of oxygen and the output of carbon dioxide that take part in the metabolic processes in the body; it includes the burning of foodstuffs in the tissues, the transport of the gases in the blood and their exchange in the lungs. All but the smallest animals, for example single cells like amoeba, require specialized transport and exchange organs to provide sufficient supply and removal of the large quantities of gases involved. A vigorously exercising athletic human might take up as much as six litres of oxygen per minute, and excrete a similar amount or more of carbon dioxide.

It is usual to divide respiration into external and internal components, linked by the bloodstream that transports the gases but has little metabolism of its own. In mammals, external respiration — the ventilation of the lungs — is achieved by breathing, the mechanical basis of respiration: the terms are sometimes used synonymously. In fishes there is equivalent ‘ventilation’ of the gills with water. At rest an adult human inhales about 6–8 litres of air per minute, of which about two thirds gets to the alveoli and takes part in gas exchange, the rest remaining in the air passages. Fresh air contains 21% oxygen and almost no carbon dioxide. In the lungs, the entry of oxygen into the blood and the release of carbon dioxide results in exhaled gas having about 5% less oxygen and 5% more carbon dioxide. The gases pass in and out of the blood by passive diffusion, due to their relative pressures in the blood and in the alveolar gas, just as a fizzy drink will lose its gas and go flat when exposed to air. At rest an average-sized adult will take up about 250 ml of oxygen each minute, and exhale about 200 ml of carbon dioxide. In vigorous exercise these values can increase over 20-fold in a trained athlete as a result of the increased metabolism in the muscles. The blood in the capillaries leaving the lungs, and therefore in the arteries carrying it round the body, have the same pressures of oxygen and carbon dioxide as those in the alveoli, at least in health. In some lung and heart disease, involving a problem of gas transfer, the arterial blood may have a lower oxygen and a higher carbon dioxide pressure than that in the alveoli, causing hypoxia and hypercapnia. In other types of lung disease these abnormalities in the blood result from inadequate breathing — failure to ventilate the alveoli with sufficient fresh air.

In the blood almost all the oxygen is carried combined with haemoglobin in the red cells, while carbon dioxide is taken up in both plasma and red cells. Once the blood has reached the tissues, internal respiration takes over. Oxygen will diffuse from the capillaries into metabolizing cells, again passively following its pressure gradient. Vigorously contracting muscle cells use up almost all the available oxygen inside them, and strongly pull a fresh supply from the blood by diffusion. The oxygen combines with food materials, mainly sugars and fats, to release heat and energy for contraction of muscle, with water and carbon dioxide as the main waste products. The water enters the general water pool of the body, while the carbon dioxide enters the blood and is carried to the lungs mainly as bicarbonate but also in combination with hemoglobin and plasma proteins. An appreciable amount of carbon dioxide, unlike oxygen, is also free in solution in the plasma. Such aerobic metabolism occurs similarly in the great majority of body cells.

This understanding of the chemical basis of respiration was only developed in the eighteenth century, with the chemical identification of oxygen and carbon dioxide, mainly by Joseph Priestley (1733–1804) and Antoine-Laurent Lavoisier (1743–94). For two thousand years before then there were many speculations about the meaning of breathing, the main idea being that its function was to cool the blood by physically mixing air with it in the arteries and veins. The cooling function seemed proved by the fact that exhaled gas was usually warmer than inhaled air, and the mixing with blood was deduced from the fact that when arteries and veins were cut the blood flowing out was often frothy. (We now know that if large veins are cut they will suck air into the circulation, and it will pass through the heart and lungs and make the haemorrhaging blood bubbly.) In the second century ad Galen wrote ‘respiration is useful to animals for the sake of the heart, which to some extent requires the substance of the air and besides needs very greatly to be cooled because of its burning heat.’ Even primitive men must have known that breathing was necessary for life; according to the Judaeo–Christian tradition, breath created it. ‘And the Lord God formed man of the dust of the ground and breathed into his nostrils the breath of life; and man became a living soul’ (Genesis 2:7). In the seventeenth century much experimental work established that one component of air was essential for life, and that another different fraction was exhaled. These were identified as oxygen and carbon dioxide a century later. There was much speculation about the ‘respiration’ of fetuses. Clearly they could not breathe air, since their lungs were full of liquid. Since the role of the blood circulation in gas exchange was not understood, the respiratory function of the placenta, the ‘external’ respiratory organ of the fetus, was not apparent.

Sophisticated and accurate methods of analysing respiratory gases were developed in the twentieth century, and the mechanisms of external respiration are now well defined. At about the same time development of biochemistry and, later, of molecular biology, led to an understanding of internal respiration. When oxygen combines with carbohydrates every molecule of oxygen creates one molecule of water and one in carbon dioxide. Thus the ratio of the exchange volumes of the two gases, the respiratory quotient, is 1; in practice this would only occur in someone living on a diet of, and metabolizing only carbohydrate — polished rice, for example. Fat and protein contain less oxygen than does carbohydrate, so more oxygen is needed for their consumption than the carbon dioxide that is produced. On an average diet in a developed country the ratio of carbon dioxide excreted to oxygen absorbed is about 0.8. However these values assume that there is enough oxygen for the metabolic needs of all the body — that we are at rest or in a state of entirely aerobic metabolism. ‘Aerobic exercise’ is exercise within this limit. If exercise is severe, not enough oxygen is available for the muscles, which pass an ‘anaerobic threshold’, and energy is provided additionally by the breakdown of carbohydrates without using oxygen and with the formation of lactic acid. This diffuses into the blood, causing a mild acidosis and acting on the bicarbonate to release carbon dioxide which is excreted in the lungs. (Lemon juice, put on self-raising flour, or bicarbonate of soda, will make it fizz as it releases carbon dioxide.) Thus in severe exercise we exhale more carbon dioxide than the oxygen we absorb. After the exercise we retain carbon dioxide in the body and most of the lactic acid is converted back into carbohydrate in the liver. Extra oxygen is needed for this, and the process is referred to as ‘repaying the oxygen debt’.

John Widdicombe

See also breathing; breathing in exercise; carbon dioxide; haemoglobin; lungs; metabolism; oxygen.