HEART AND BLOOD VESSELS 
453 
Figure 402 . — Dia¬ 
gram of a Vein 
Showing the 
Valves. 
In which direc¬ 
tion does the blood 
flow in this dia- 
blood from the heart, and vein is the term applied to the 
vessels which return blood to the heart. There is little 
structural difference between the veins and arteries except 
that the walls of the arteries are thicker, 
and there are no small valves as in the 
veins. As the branches of the arteries be¬ 
come minute, the walls become much thinner, 
thus allowing the food and oxygen to pass 
more easily to the individual cells. These 
minute branches are called capillaries (Latin, 
capillus, hair). From a cluster of capillaries 
a small vein begins which soon connects 
with a slightly larger vein, which leads back 
to the heart through larger and larger veins. 
The blood follows a regular course through 
the body, passing from the left ventricle into 
the aorta , which is the largest artery in the 
body. As soon as the aorta leaves the heart, 
smaller arteries branch from it, and the aorta 
itself also branches until the entire body is 
supplied with blood. The right ventricle gives off 
artery which divides, a branch entering each lung, 
point where an artery leaves a ventricle, there are three half- 
moon-shaped valves which 
prevent the blood from 
flowing back into the heart 
(Figure 402). 
The blood which is car¬ 
ried into the lungs con¬ 
tains a large amount of 
carbon dioxide which gives 
it a dark color. In the lungs the carbon dioxide is given 
off and oxygen taken up, so that when this blood is returned 
to the left auricle, it is of a bright red or “ arterial ” color. 
Every time the heart beats the blood is forced into the 
gram ? 
a short 
At the 
Figure 403 . — Diagram of Capillaries. 
The artery breaks up into minute 
branches, the capillaries, which in turn 
unite to form veins. 
