Respiratory system

The first breath taken by a baby is accompanied by a loud bout of crying; from then on, breathing is automatic. The importance of breathing is reflected in our language; it occurs in numerous expressions, such as ‘breathtakingly beautiful’ and ‘the breath of life’. Breathing is something we do continuously until the moment we die. Even a person who practises holding his breath can do so for no more than a few minutes; it is not long before he feels an irresistible need to breathe. Breathing is under autonomic control, which means that we breath without thinking about it, taking in exactly the amount of oxygen we require. The distresing symptoms that occur as a result of disorders in which the regulation of breathing goes wrong clearly demonstrate the importance of accurate regulation. Oxygen is indispensable for all living organisms. In the course of evolution, the body’s tissues came to specialize; a need also arose for an organ to look after the oxygen supply, and the result was the lungs. In addition to the supply of oxygen, the human respiratory organs have developed a number of other functions. Smell, for example, is a function of the nose.

Another less obvious function is the use of the respiratory tract for speech. This communicative function can find expression in a great many ways, ranging from conversation to song. The indispensible nature of speech to humans only becomes apparent if removal of the larynx deprives a person of the ability to speak.

Respiration

The word ‘respiration’ conjures up several meanings, including simply the act of breathing in and out. In terms of the body, however, ‘respiration’ has a fairly specific meaning: the exchange of gases between the body and its environment. To this, a biochemist might add the cellular chemical pathways by which oxygen plus energy-containing nutrients, such as glucose, combine to release energy, which is then used to power other chemical reactions in the cell. The oxygen present in the atmosphere is breathed into the lungs, where it is taken up by haemoglobin in the red blood cells and carried to the tissues. Here, food – mostly glucose – is ‘burned’ in the presence of oxygen to release energy, carbon dioxide and water. The carbon dioxide is carried back to the lungs and breathed out, while the water formed is used for other body processes or excreted by the kidneys. So, respiration is, even more specifically, the process of obtaining oxygen and excreting carbon dioxide.

Regulation of respiration

At rest an average of 250 ml of oxygen are required each minute to satisfy the body’s energy needs. The breathing movements are regulated by the respiration centre in the brain. During exercise the oxygen absorption can be increased more than five-fold, by increasing the depth and the frequency of breath.

Gas exchange in the lungs

When we inhale, the air in the tiny air sacs of the lungs – called alveoli – comes into very close contact with the blood. There are only two cell thicknesses separating them. The gases on each side of this thin barrier are always trying to diffuse through it to reach a state of equilibrium, to achieve the same gas concentration on each side. Each gas acts independently, passing from where it is most concentrated to areas where it is less concentrated.

Inhaled air contains about 79 per cent nitrogen, nearly 21 per cent oxygen, 0.04 per cent carbon dioxyde and some water vapour (depending on the weather). This composition undergoes several changes when it reaches the lungs. The inspired air is humidified and mixes with expired air because the lungs do not empty completely after each breath. The concentration of a gas is measured like barometric pressures, in millimetres of mercury and is called gas tension. At sea level, with an atmospheric pressure of 760mm mercury, the tension of oxygen is about 160mm mercury. As a person goes higher above sea level, the oxygen pressure in the environment drops continuously; when that person has reached 5,500 metres it is only about half that it was at sea level. A population living at an altitude of 2,000 metres, e.g. in Denver, USA, is bound to have a slightly lower arterial oxygen pressure at rest than does the sea level population.

The blood arriving in the lungs has oxygen and carbon dioxide dissolved in it, but the two gases are at very different tensions: oxygen tension is lower, and carbon dioxide tension is higher. During its passage through the lungs the blood gases gradually come into equilibrium with the alveolar gases, so that the blood leaving the lungs has its gases at the same tensions as the air in the lungs. This means that carbon dioxide is given off from blood to alveolar air, as the blood takes up oxygen. Expired air contains about 16 per cent oxygen (only 5 per cent less than normal air), 4 per cent carbon dioxide and 6 per cent water.

Diffusion

From each litre of air that is inhaled, about 40 ml are passed to the blood by a process called diffusion. The speed at which oxygen and carbon dioxide pass through the two-cell barrier depends on the thickness of this membrane but to a larger extent on the solubility of the gas – the more soluble it is, the quicker it diffuses through the fluid in the cells. This is not of significance in normal circumstances, but some lung diseases cause scarring and thickening of the walls of the alveoli. This affects oxygen diffusion to a much greater extent than carbon dioxide diffusion, with the resultant alterations in the tensions of these gases in the blood. Measuring blood tensions is therefore a valuable means of assessing lung disease.

Oxygen transport in the blood

Oxygen, on which the human metabolic process entirely depends, is, by comparison with carbon dioxide, relatively insoluble in blood. The body therefore uses an alternative means of oxygen transport -haemoglobin. The red blood cells get their colour from haemoglobin which can combine chemically with oxygen thus enabling it to be transported. Each molecule of haemoglobin combines with four molecules of oxygen, at four different sites. When the first site is occupied by an oxygen molecule this facilitates the combination at the second site, and so on; this means that the fourth molecule of oxygen is the easiest to take up and the most difficult to release. This in turn means that even if the oxygen tension in the lungs is slightly lowered, the blood can still take up its full amount of oxygen. A 20 per cent fall in oxygen tension results in a drop of only 5 per cent in oxygen content of the blood.

These arrangements are very useful physiologically. In normal circumstances only one molecule of oxygen is exchanged in each cycle of blood through the tissues and back to the lungs; so a further three molecules are available for use under exercise conditions. Put another way: in each 100ml of blood there are about 15gm haemoglobin which can carry 19ml of oxygen. The blood returning to the lungs still contains 14ml of oxygen, which means that only 5ml have been used in the tissues. This leaves plenty of reserve capacity.

Carbon dioxide transport in the blood

Unlike oxygen, carbon dioxide is fairly soluble in blood. About 10 per cent of the carbon dioxide is transported in this form. The other 90 per cent is transported either as carbonic acid or attached to haemoglobin.

As carbon dioxide enters the bloodstream from the tissues, an enzyme, carbonic anhydrase, contained in red blood cells causes it to combine with water to form carbonic acid. About 60 per cent of the carbon dioxide is transported in red blood cells in the form of carbonic acid. When a haemoglobin molecule loses one of its oxygen molecules it becomes receptive to carbon dioxide, by which means the remaining 30 per cent of carbon dioxide is transported.

Gas exchange between blood and cells

The whole aim of respiration is to get oxygen right into the cells of the body, where it can be used to help release energy. As the metabolism in the cells uses up oxygen and produces carbon dioxide, oxygen tension falls and carbon dioxide tension rises. The increase in carbon dioxide tension causes this gas to diffuse into the bloodstream, into red blood cells where its conversion into carbonic acid, with the aid of the enzyme carbonic anhydrase, causes increased acidity in these cells.

This increased acidity, together with the reduced oxygen tension in the body cells, causes haemoglobin to release a molecule of oxygen, which diffuses into the body cells.

If there is an increase in cellular activity, as there is in the muscle cells during exercise, the oxygen tension in the cells decreases further and carbon dioxide output correspondingly increases. The resultant increase in acidity together with the lower oxygen tension causes the second, and often the third, oxygen molecules to be released for use by the muscle cell. The breathing rate is increased to enable extra oxygen to be inhaled and the excess carbon dioxide to be carried to the lungs to be exhaled.

Regulation of the acidity of the blood

The acidity of the blood is confined within very narrow limits, because otherwise the metabolic processes in the body will be disrupted. Respiration is one of the mechanisms that regulates this acid/alkaline balance in the blood. On increasing the breathing frequency, more carbon dioxide is expired, with a resultant lowering of the total amount of carbonic acid in the blood.

Hyperventilation as a result of anxiety can impair those regulating mechanisms, because under these circumstances the acidity of the blood becomes too low. This causes complaints such as tingling of the fingers, dizziness or palpitations. Because these symptoms tend to increase anxiety, a vicious circle develops, which is very difficult to break.