Cells in mammals require a constant supply of nutrients and oxygen, and a way to remove waste products. Blood is great, as it does all that. Blood needs a way of getting to all cells of the body, a way to… circulate. Without that, blood would just get pulled by gravity towards the centre of the earth. Not a pretty sight I’m afraid.
There are two circulations in the body:
1. The pulmonary circulation takes blood from the heart, pumping it to the lungs in order to oxygenate it.
Key point: the oxygen-rich blood vessels entering an organ are called arteries, while the oxygen-depleted blood vessels leaving an organ are called veins.