Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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Blood.

The heart is four-chambered (Fig. 10). Its left ventricle drives the blood round the systemic or greater circulation, the blood returning to the right auricle. The right ventricle drives the blood through the lesser or pulmonary circulation, from which it returns to the left auricle. The walls of all bloodvessels, except capillary tubes, are sufficiently thick to prevent the escape of any of the constituents of blood. To support the pressure of the blood which they contain, the arteries and the larger veins need walls of considerable thickness. The walls of the capillaries allow an interchange between blood and lymph in the manner already described (ssss1).

Blood fresh from the lungs, whether still in the pulmonary veins or in the systemic arteries, is scarlet in colour. Venous blood is darker and purple-red, the depth of its tint varying with the extent to which it has parted with its oxygen. It looks less opaque than arterial blood. With this exception, the physical properties and chemical composition of blood are remarkably constant in all parts of the body. Arterial blood contains more oxygen, venous blood more carbonic acid. Other chemical differences can be recognized, but they are relatively very small. The constancy in the constitution of blood is its most notable character. Bleeding, unless excessive, does not greatly affect it. The number of corpuscles is of course diminished, but even these are replaced with great rapidity. The plasma, after bleeding, soon recovers its proteins and salts. A similar readjustment occurs if normal saline solution (water containing 0·9 per cent. sodic chloride), or even a strong solution of salt, is injected into the blood. Within certain limits it is very difficult to disturb the balance of its constituents. It gets rid of substances added in excess, or replaces substances removed, with remarkable facility. If sugar (glucose) be injected into a vein, it escapes through the capillary walls into the lymph. After a short interval the lymph contains more sugar than the blood. If an excess of protein, whether of a kind foreign to the blood or its own serum-albumin, be injected, it is removed by the kidneys. The blood has various sources from which it can draw out reserves of anything that is lacking, and various ways of getting rid of anything that is in excess. It draws upon the lymph in the tissue-spaces for water. It discharges salts into the lymph. It also takes salts from the lymph. It draws upon the liver for sugar, and probably for proteins also. In a starving animal the blood still contains sugar long after fresh supplies have ceased to reach it from the intestines. The lungs remove its carbonic acid. The kidneys free it from everything which cannot be otherwise removed. It is essential to the well-being of the organism as a whole that a uniform standard of composition should be maintained by the blood.