Fig. 35—Capillaries a, cells; b, nuclei. (Gray)
Capillaries have one wall, which is the continuation of the inner wall of the artery, thus making the capillary so thin that fluid finds its way easily through it into the surrounding tissues.
Some parts of the body are more vascular than others and some tissues of the body, such as the cornea of the eye, the epidermis, cartilage, the substance of the brain, etc., are entirely destitute of capillaries.
The combined area of all the capillaries of the body is many times greater than the combined area of the trunk vessels. If this were not so, the high pressure on the arterial system would break the thin capillary walls and also the greater area allows the blood to circulate more slowly which gives time for the liberation of oxygen to the tissues and for the absorption of carbon dioxide.
The Venous System.
—The veins, like the arteries, are tubular vessels, their function being to receive the blood from the capillaries and convey it to the auricles of the heart. There are two classes of veins, systemic and pulmonic.
The systemic veins receive the impure or carbonized blood from the capillaries and convey it to the right auricle of the heart.
The pulmonic veins receive the pure oxygenized blood from the lungs and convey it to the left auricle of the heart. The pulmonic veins will be taken up and discussed later under the pulmonary circulation.