CHAPTER VII
RESPIRATION
Life means change. We cannot imagine its continuance without liberation of energy. Arrest of molecular activity is death. There is no possibility of its revival. A watch that has stopped may be started by shaking. On the cessation of molecular activity an animate being becomes inanimate. Dead, it is liable to further chemical changes. Bacteria invade it. They shake down its complex unstable compounds into simple, stable, so-called “inorganic groups”; but the ordered combination with oxygen, which constitutes living, can never recommence. Putrefaction may be prevented by the exclusion of germs. The inanimate mass of organic material may remain unchanged. Its return to life would be a miracle. From time to time a frog is found enclosed in old red sandstone, or some other rock which for countless ages has lain beneath the surface. The cleft through which the frog entered a few hours or days before it was discovered is overlooked. It is supposed to have lived “in a state of suspended animation” for millions of years. The fact that no frogs are to be found among the fossils of the old red sandstone is an objection too casuistical to be seriously entertained. The physiologist’s demand to know what has become of the mountains of solid carbonic acid, water, and urea which the frog must have produced during its unimaginable term of incarceration is regarded as the natural expression of his prejudice—that life cannot continue without molecular change. And he is bound to admit his inability to prove that it cannot. Nevertheless, his experience that, whenever and however he may, by experimental methods, arrest change, he loses the power of causing it to recommence justifies him in his conviction that life is change. Even a living seed is to his mind an organism whose complex constituents are slowly—however slowly—setting free energy by settling down the steps which lead to stability and ultimate, inanimate rest; and the only source of this energy is combination with oxygen. In the case of a seed the oxygen need not come from without. Seeds retain their power of germination after long occlusion in nitrogen or other neutral gases. But all the time some change is occurring, some internal oxidation which resolves their less stable into more stable compounds. Otherwise they would not be alive. A physiologist is willing to believe that this may continue for ten years, fifteen years—for any period that the botanist tells him that he has, under verifiable conditions, observed that it does occur; but when he is told that peas taken from the hand of an Egyptian mummy, or seeds set free by the spades of navvies after a far longer burial, have been found to retain their vitality, his credulity is stretched beyond breaking-point. He cannot imagine a change so slow as to be spread over a geological period, still without exhaustion of all changeable compounds.
The term “respiration” has been extended until it is synonymous with “oxidation.” At one time it was supposed that the combination of oxygen with oxidizable substances occurred in the lungs. The lungs were the hearth of the body, to which the blood brought fuel which burned in the air drawn into them. When it was understood that the actual combination of combustible material with oxygen occurs, not in the lungs, but in the tissues, a somewhat illogical distinction was made between “external respiration”—the combination of oxygen and blood in the lungs—and “internal respiration”—the combination of oxygen and tissue-substances. The terms are not comparable. The taking up of oxygen by the hæmoglobin of blood is a different process to the union of oxygen, after the hæmoglobin has parted with it, with the carbon, hydrogen, and nitrogen of the tissue-substances.
The blood-stream carries both fuel and oxygen to the tissues, but the fuel while in the blood is not in an oxidizable condition. The foods are taken up by the tissues. They enter into combination with their protoplasm. Oxygen also combines with tissue-substances. In proportion as the tissues are active oxidized compounds are split off. They fall into the lymph, whence they are absorbed by the blood. If they are nitrogenous compounds, they are carried to the liver, formed into urea, and passed to the kidneys for elimination. If carbonic acid, it is carried to the lungs for exhalation. The water formed by combination of hydrogen and oxygen may escape from the lungs, the kidneys, or the skin.
Two or three pounds of mixed foods are consumed every day. By the blood they are carried to the tissues, whence an equivalent quantity of waste—that is to say, oxidized—material is removed. About 1½ pounds of oxygen is required to burn the day’s fuel.
The problems of respiration are twofold. In the first place we have to consider the physics and chemistry of the combination of hæmoglobin with oxygen, and of the elimination of carbonic acid from the blood in the lungs; secondly we have to explain the transference of oxygen from hæmoglobin to the tissues, and the reception in the blood of carbonic acid produced by the tissues.
The apparatus by which air is brought into relation with the blood consists of lungs and windpipe. At its upper end, where it joins the portion of the alimentary tract common to deglutition and respiration, the special respiratory tube is protected by the larynx. The nasal chambers belong to the respiratory tract; the gullet, or pharynx, is common to the two functions.
The mucous membrane which lines the nose and windpipe is kept moist in order that it may catch particles of dust drawn in with the air. At the same time the nasal chambers serve to warm the air, and to add moisture to it if it be too dry; for the lining epithelium of the lungs would suffer if dry air came in contact with it. The wall-surface of the nasal chambers is increased by the projection of folded and chambered “turbinate bones.” The importance of warming the air before it is admitted to the lungs is remarkably illustrated in the case of certain sea-birds. The nasal chambers of the frigate-bird, and of some other birds which resemble it, are exceptionally complicated. Since the animal is devoid of any sense of smell, and the air which it breathes must be nearly saturated with moisture, the only function which can be assigned to these convoluted passages is that of warming inspired air.
The larynx will be more minutely described when it is considered as the organ of voice. In connection with respiration, it must be regarded as primarily a valve which closes the entrance to the windpipe during swallowing. It is overhung by a leaf-like appendage—the epiglottis—formed of exceedingly elastic tissue. It was thought until lately that the epiglottis drops over the aperture of the larynx when food is passing down the gullet, and springs up again as soon as the act of deglutition is over; but recent observations have shown that during deglutition the epiglottis is pressed against the back of the tongue, and that the closure of the larynx is effected by its own sphincter muscles. The mucous membrane of the larynx is extremely sensitive to stimulation by anything which would be prejudicial to the tissue of the lungs. When its sensory nerve—the superior laryngeal—is stimulated, the larynx closes. It is the agent in carrying out many reflex actions, in which not the larynx only, but also the muscles of the chest and diaphragm, take part. For example, it immediately stops inspiration if an irritating vapour is present in the air. It stops respiration if any foreign body, such as a crumb of bread or a drop of water, touches the mucous membrane. When the trunk of the nerve is stimulated by an electric current, respiration is inhibited. Further, under suitable stimulation the nerve brings about respiratory movements in which inspiration is gentle and expiration sudden, violent, convulsive. Rib-muscles and diaphragm combine to produce a cough, which ejects the noxious body. Again, its stimulation in a different way probably helps to produce constriction of the smaller bronchi which regulate the amount of air supplied to the air-cells of the lungs; although this constriction may be largely due to a reflex which starts in the air-cells. The epithelium of the air-cells has an immensely rich supply of sensory nerves. In some persons this protective mechanism is very prone to overact its part. A little dust or foul gas in the air leads to such marked contraction of the bronchi that respiration becomes very difficult. Such an exaggerated tendency to reflex action constitutes the neurosis, asthma. In this malady the mechanism is unduly sensitive. Very slight stimulation leads to a maximum discharge of impulses to the muscular tissue of the bronchi.
The trachea has a length of about 4 inches. It extends from the lower edge of the cricoid cartilage, which is easily felt in the neck beneath the thyroid cartilage (Adam’s apple), to the under side of the arch of the aorta, where it divides into the right and left bronchi. The epithelium which lines the trachea and bronchi is ciliated. The cilia propel the secretion which accumulates on its surface upwards towards the larynx. The wall of the windpipe is kept open by rings of cartilage which are incomplete behind, where the trachea and œsophagus are in contact. Rings and plates of cartilage also support the bronchi. The bronchi divide and subdivide until their diameter is reduced to about 0·2 millimetre. Each bronchiole then breaks up into a bunch of very thin-walled, elongated infundibula, club-shaped, and with a diameter about five times that of the bronchiole with which they are connected. They may be three or four times as long as they are broad. The wall of an infundibulum is pitted like a piece of honeycomb into shallow chambers—the air-cells or alveoli.