When you donate blood at a blood drive, what you're donating is whole blood, which is exactly what it sounds like: blood that hasn't had any of its constituent parts removed. That whole blood is made up four primary parts:
What is the composition and volume of whole blood?
Platelets are the cells that help your blood clot when you get a cut or scrape, turning the injury into a scab instead of an ever-oozing open wound. Normally, when you donate blood, an anti-coagulant is added to keep it from clotting.
White blood cells, or leukocytes, fight infections and diseases. Consequently, if you're currently fighting off an illness, like a cold or the flu, your body produces more white blood cells to make you well again. Platelets and white blood cells together constitute less than 1% of whole blood.
Red blood cells, or erythrocytes, carry oxygen to your body's cells and carry away carbon dioxide to be exhaled from the body. Red blood cells make up about 45% of whole blood.
Plasma, which makes up about 55% of whole blood, is the liquid substance that carries the platelets, leukocytes, and erythrocytes — as well as waste, hormones, enzymes, electrolytes, sugars, and other nutrients — throughout the body. Plasma is over 90% water.
It is rare these days for the whole blood that donors give to be transfused untouched into someone else's system. The constituent parts of whole blood are separated, and people are given only those parts that they need in order to avoid complications. Dehydrated patients, for example, are given just the plasma, which gives their body the help they need to get their circulatory system back in working order.
Because whole blood is separated, it's possible that the blood you donate could be used to help more than one person in need.