Every cell in an animal needs a steady supply of some substances and has to get rid of others. Cells need oxygen for aerobic respiration, water for the chemical reactions inside them, and dissolved food molecules such as glucose and amino acids, which provide energy and the building blocks for growth. They also need small amounts of mineral ions, for example calcium for strong bones and iron for making haemoglobin.
Cells also make waste. Carbon dioxide is produced by respiration and has to be removed because it builds up and harms the cell. Urea is made in the liver when excess amino acids are broken down. It is poisonous, so it is carried in the blood plasma to the kidneys and removed in urine.
In a large animal most cells are far from the surface of the body, so the blood carries these substances. Oxygen goes from the lungs to the cells, and carbon dioxide goes from the cells to the lungs to be breathed out. Dissolved food molecules go from the small intestine to every cell, and urea goes from the liver to the kidneys.