Our blood is red because of the substance, hemoglobin, which are contained in our red blood cell [red corpuscles]. The function of these specialized cells is to carry oxygen from our lunges to wherever it is needed in the body. They are formed our bone marrow and the hemoglobin in them, which is made of iron and protein, becomes redder as it picks up oxygen. It loses this color as it deposits oxygen in other cells and picks up carbon dioxide to carry back to the lungs, which is why some of the blood vessels in our hands and arms appear blue through our skin.
As well as carrying oxygen and carbon dioxide, the blood carries nutrients to the cells, so that they can perform their functions, and for cells repair and growth. Waste products are carried back to the liver for removal.
As well as the red cells, our blood also carries white blood cells, which help to fight off disease. All of the cells and nutrients are carried in a pale yellow, thick liquid which is called plasma. The other materials carried in the plasma include a substance called fibrin, which helps the blood to clot when we cut ourselves.