The Blood System
The heart, veins, arteries and capillaries make up the circulatory system. This is an organ system that transports substances around the body.
The heart is an organ that pumps blood around the body.
The wall of the heart is made from mainly muscle tissue.
Blood enters the atria of the heart (singular atrium).
The atria contract and force blood into the ventricles.
The heart contains valves, whose purpose is to ensure that blood flows in the correct direction.
Blood is transported from the heart to the organs by arteries. It is transported from the organs to the heart by veins.
Arteries have thick walls consisting of muscle and elastic fibres.
Veins have thinner walls. They often contain valves, which is how they prevent the blood from flowing the wrong way.
In the organs, blood flows through very narrow blood vessels called capillaries.
Capillaries have very thin walls -- only one cell thick. This allows substances to pass out of the blood and into the body tissues that need them. The thin walls and narrowness of the capillaries provides a short path for diffusion.
There are actually two separate circulation systems: one going to the lungs and one going to all the other organs in the body.
Stents are devices used in medicine. If a person has a buildup of plaque in their arteries, the arteries can become narrower, restricting the blood flow. A stent can be inserted into the artery and opened up, keeping the artery open.