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Why is blood red? Because it's mostly made up of red blood cells.
These cells get their color from hemoglobin, a bright red protein that contains iron and is very good at collecting and transporting oxygen. When blood is exposed to air, like when you get a cut or scrape, the hemoglobin absorbs lots of oxygen, making it turn a deep red.
Red blood cells are one of three types of blood cells that all float inside a liquid called plasma, which is mostly water. They look like disks with little indentations in the middle, sort of like Smarties candies or hole-less bagels. But they're very flexible and can change shape when they need to -- which helps them slip into tight places such as tiny capillaries, where oxygen levels are low and carbon dioxide levels are high.
The hemoglobin in red blood cells picks up oxygen when your blood travels through your lungs, and it releases the oxygen to cells as blood circulates through your body. Hemoglobin then absorbs the carbon dioxide from your capillaries and carries it away.
Along with red blood cells, which make up about 40 to 45 percent of your blood, there are white blood cells, also known as leukocytes, that help keep you healthy. Different types of white blood cells destroy parasites, kill bacteria that cause infections, and generally keep your body's immune system working properly.
The third type of blood cells are platelets -- or thrombocytes -- which aid in clotting when you get a cut or other injury. Platelets clump up around a wound to help control bleeding, and they also send a chemical message that helps in the formation of fibrin, a substance that creates a scab so the wound can start to heal.