Blood goes round and round the body over and over again in the circulatory system.
The system is made up of the heart, blood vessels and the blood.
The system is there to provide every living cell with supplies of food and oxygen.
The system is also important in removing waste, such as carbon dioxide and nitrogen from the body.
Blood is pumped round the body by a very powerful muscle, the heart.
The heart is divided into two sides, a left and a right.
They are separated by a tough wall called a septum.
One side receives fresh blood which contains oxygen from the lungs and then pumps it to the body.
The other side receives used blood which contains carbon dioxide from the body, which it then pumps to the lungs so the carbon dioxide can be breathed out.
Each side of the heart is divided into chambers: left and right auricles and left and right ventricles.
Fresh blood comes from the lungs through the pulmonary vein and enters the left auricle. When this has filled up with blood it contracts and forces the blood through a valve and into the lower chamber, the left ventricle. As the ventricle fills, the auricle relaxes. Once full, the ventricle contracts and forces blood through another valve and into the aorta. The aorta is the main artery in the body and is a many-branched structure. It branches into arteries that lead to all parts of the body.
Blood is brought back to the heart from all over the body except for the lungs by veins that lead into the right auricle. These veins are known as the superior vena cava and the inferior vena cava. The right auricle contracts and forces blood through a valve and into the right ventricle. From there the impure blood is forced into the pulmonary artery, which takes it to the lungs.
Both sides of the heart work at the same time. When the auricles contract, the ventricles relax and fill with blood. The auricles relax and fill with blood as the ventricles relax. The ventricles contract or beat about 70 times a minute when a healthy adult is resting. Excitement or exercise increase the rate to a hundred or more times a minute.
Arteries take blood away from the heart when the pressure is highest and so need a tough wall.
Veins bring blood back to the heart and have a thinner wall.
Most arteries carry fresh blood, which contains oxygen and is bright red.
Most veins carry used blood back to the heart. This blood is dark red in colour. The exceptions are:
the artery that takes blood from the heart to the lungs contains very little oxygen and is dark red in colour.
the vein that brings blood to the heart from the lungs has blood that is bright red because it contains a lot of oxygen.
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