The circulatory system is an amazing series of vessels which are responsible for moving blood to every cell in the body. Blood, in turn, carries vital oxygen and other elements to the cells, and transports waste products to the liver and kidneys for removal. Think of the circulatory system as a massive subway system for your blood. There are two major types of blood vessels - arteries and veins. Although both arteries and veins both carry blood, there are significant differences between them.
Let's take a look at arteries first. Arteries are blood vessels which move blood away from the heart. Arteries are typically large vessels, and are commonly named. The aorta is the largest artery in the body, starting at the heart and holding the entire capacity of the blood flow until it branches off to smaller arteries.
With one exception, all arteries carry oxygenated blood to the various tissues of the body. The one exception is the pulmonary artery. This artery carries blood away from the heart to the lungs. Blood is pumped out of the right ventricle of the heart to the lungs via the pulmonary artery. Once the blood reaches the lungs, it picks up oxygen and is returned to the heart where it will be pumped to the rest of the body.
Arteries must have thick walls. They carry relatively high pressure blood that is pumped out of the left side of the heart. In order for blood to reach every part of the body, the blood must be pumped at high pressure. This means that the arteries must have thick walls and be able to handle a high capacity. Many of the larger arteries have actual muscles in the walls - allowing them to help move the blood along more efficiently.
Most larger arteries have names. The brachial arteries carry blood to the arms. The carotid arteries lead to the brain. Illiac arteries lead to the legs - where they change names and become the femoral arteries. The study of aterial names is quite a large subject in anatomy - leading to many sleepless nights for medical and nursing students.
Now let's take a look at veins. Veins are blood vessels which are responsible for returning blood to the heart. With the exception of the pulmonary veins, all veins carry blood that is low in oxygen content - as that oxygen has been "dropped off" in the tissues and organs of your body. The pulmonary veins carry oxygenated blood as they are returning blood from the lungs to the heart.
Veins carry blood at relatively low pressures compared to arteries. Because of this, they have relatively thin walls. There is no muscle in the walls of veins. Interestingly, some veins in the legs have small one-way valves along their length. This keeps the blood flowing up the legs, and prevents it from pooling in the ankles.
Many veins have names. For example, the superior and inferior vena cava are two of the larger veins that are closest to the heart. Compared to arteries however, veins have much more anatomical variation from person to person, especially in the smaller vessels.
Both veins and arteries are vital for the proper movement of blood throughout your body. They have similar, yet subtly difference functions. Only together can they move oxygen, wastes, and other nutrients around your body.