The heart and blood vessels carry out a transport function. They carry food molecules, water and oxygen to cells and remove waste products such as carbon dioxide. They form the circulatory system.
Instead of just being a single loop the circulation has two interconnected loops, in a sort of figure of eight.
Blood returns from the body to the right atrium. The blood has lost most of the oxygen it carries and is now deoxygenated.
The right ventricle pumps the blood along the pulmonary artery to the lungs where it picks up fresh oxygen. It is now oxygenated.
The oxygenated blood enters the left side of the heart and is pumped out through the aorta to the body.
Once it reaches the capillaries around the body, oxygen diffuses out to the surrounding cells.
The deoxygenated blood is carried back towards the heart in the veins. These join up to form the vena cava which is the largest vein.
One sneaky exam fact is that veins only carry deoxygenated blood except for the pulmonary vein. This is the only one that carries oxygenated blood because it takes blood from the lungs to the left heart ready to get pumped round the body.
This is a special part of the circulation system.
Normally the circulation system takes blood straight back from the capillaries in each organ or tissue.
But the blood from the digestive system carries all sorts of molecules that have been absorbed there. In order to stop the rest of the blood system getting clogged up there is a special detour. It is called the hepatic portal system.
The diagram below shows this system:
Hepatic means to do with the liver. The liver is the factory organ of the body. It deals with all sorts of chemicals, breaking them down and rearranging them.
The hepatic portal vein carries blood to the liver. Then the blood can leave the liver for the heart.