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.
2. The systemic circulation takes blood from the heart to everywhere else. Eyes, legs, hand, bum, you name 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.
So a blood vessel entering the liver or kidneys would be an artery. A blood vessel leaving the liver or kidneys would be a…