The circulation of blood is carried out by the heart, which is a hollow muscle that acts like a force pump, and by tubes (blood-vessels) which are respectively classed as arteries, capillaries, and veins. The arteries convey blood from the heart; the veins take blood to the heart; and the capillaries, which are of very minute calibre, form a connection between the arteries and veins. There are two systems in the distribution of blood, namely, the general circulation, and the lung (pulmonary) circulation. In the general circulation, the nutritive matter is carried by the arteries to the capillaries, through the extremely thin walls of which it passes to nourish the various tissues; and the capillaries take up from the tissues waste material, which is brought by the veins to the heart, and which consists chiefly of carbonic acid, water, and broken-up nitrogenous matter. Having completed the round of the general circulation, the blood, which is now loaded with impurities, is carried by the pulmonary arteries to the air-cell capillaries of the lungs, where most of its carbonic acid escapes into the air cells, and is conveyed from them into the outer air by the bronchial tubes, windpipe and air passages of the head. While passing through the capillaries of the air cells, the blood absorbs oxygen from the air that is in these minute cavities, and carries it through the heart to the capillaries of the general circulation, which give it out to a greater or less extent to the tissues, where it is used for purposes of combustion (oxidation). Haemoglobin, which is the colouring matter of the blood, acts as the air-carrier of oxygen to the tissues. On meeting with oxygen (as in the air cells), it forms oxyhaemoglobin, which on being brought into the presence of carbonic acid (as in the capillaries of the tissues) parts with its oxygen. The bright red colour of arterial blood of the general circulation, as when an artery of the external muscles is cut, is due to the oxyhaemoglobin in it. When haemoglobin is uncombined with oxygen, it is of a purple hue, as we may see in ordinary venous blood.

Under usual circumstances of health, the quantity of blood in the body is more or less evenly distributed throughout the system. As this supply of blood is just sufficient for the requirements of the animal, it follows that if there is an unusually large accumulation (congestion) of blood in one part, there will be too little blood (anoemia or bloodlessness) in the remainder of the system; and vice versa. Such local changes can be borne with impunity, if they are restricted within comparatively moderate limits of time and extent. It is, however, impossible to fix the limits which divide congestion and anaemia from health and disease. Cold sets up anaemia by causing contraction of the blood-vessels of the part. Heat and exercise have the opposite effect. As a rule, there is less blood near the surface of the body than in the deeper-lying tissues, owing to the cooling effect of the atmosphere, and to the fact that the work done by the internal organs is more continuous than that of the superficial muscles. Although the terms anaemia and congestion are generally connected with disease, we may correctly apply them to certain healthy conditions, as for instance, the anaemia of the brain during sleep, and the congestion of the blood-vessels of the stomach during the digestion of food in that organ.