Bleeding from arteries and veins
In the arteries the blood is under comparatively high pressure, and if an artery is cut or torn, the blood spurts out in a stream which comes in jets, each jet corresponding to a pulse wave. Because of this high pressure bleeding from an artery is more copious, and therefore more dangerous than bleeding from a vein, and it is more difficult to stem such bleeding. The amount of blood lost depends of course on the size of the artery damaged; a man can bleed to death in a short time from a wound to one of the main arteries.
In the capillaries the blood is under steady and much lower pressure. Consequently it flows through the tissues in a steady stream. Bleeding from the capillaries is of no great consequence. Blood usually oozes slowly from the skin after an abrasion, and can easily be stemmed by pressure.
In the veins the pressure is even lower; in fact, it can even be negative in the large veins of the neck, owing to the suction caused by the act of breathing. Therefore there is the danger of air being sucked into the veins during the act of breathing when someone has a neck wound. When a vein is damaged the blood flows out in a steady stream without jetting, and under comparatively low pressure. Unless the vein is a large one, venous bleeding is readily stemmed by pressure applied to the wound. However, a great deal of blood can be quickly lost from a wound in a large vein.
In ordinary wounds of any size, veins and arteries are both damaged and the bleeding comes from both, although it comes mainly from the arteries.
There is another difference between ateries and veins. The blood in the arteries of the systemic circulation is bright red because it is rich in oxygen. In the veins the blood is purple because it contains almost no oxygen.