Blood flows everywhere through the human body. The heart pumps blood to all our body cells, supplying them with oxygen and food. Blood also carries proteins and cell fragments called platelets that let blood clot where there has been an injury. Red blood cells carry oxygen to body tissues and remove carbon dioxide. They are red because they contain a protein called hemoglobin that is red in color. Red blood cells are round and thinner in the middle, like a balloon that is partly filled with water. This lets them squeeze through tiny blood vessels without breaking.