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The Blood Vessels / The Arteries
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| Arteries Carry Blood Away |
The heart pumps blood out through one main artery called the dorsal aorta. The main artery then divides and branches out into many smaller arteries so that each region of your body has its own system of arteries supplying it with fresh, oxygen-rich blood.
Arteries are tough on the outside and smooth on the inside. An artery actually has three layers: an outer layer of tissue, a muscular middle, and an inner layer of epithelial cells. The muscle in the middle is elastic and very strong. The inner layer is very smooth so that the blood can flow easily with no obstacles in its path.
 Shown on the left is a cross-section through an artiole (a very small artery). Its muscular wall contracts and expands, thus regulating arterial blood pressure. Red blood cells can be seen traveling through it.
Humans and all mammals have what is known as double circulation, meaning that blood passes through the heart twice. On the first pass it is sent to the lungs to collect oxygen and get rid of carbon dioxide. From there it returns to the heart and is pumped at high pressure out into the arterial system. The arteries are elastic in order to cope with the surge of blood. As we age they become stiffer, causing the blood pressure to rise and making the heart work harder.
The muscular wall of the artery helps the heart pump the blood. When the heart beats, the artery expands as it fills with blood. When the heart relaxes, the artery contracts, exerting a force that it strong enough to push the blood along. This rhythm between the heart and the artery results in an efficient circulation system.
You can actually feel your artery expand and contract. Since the artery keeps pace with the heart, we can measure heart rate by counting the contractions of the artery. That's how we take our pulse.
The arteries deliver the oxygen-rich blood to the capillaries where the actual exchange of oxygen and carbon dioxide occurs. The capillaries then deliver the waste-rich blood to the veins for transport back to the lungs and heart.
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