Red Blood Cells
Red blood cells give blood its colour, which comes from a combination of oxygen and hemoglobin, an iron-containing protein.  The average person has about 25,000,000,000,000 (25 trillion) red blood cells. If they were the size of toonies, a stack of all of your blood cells would be 44,000,000 kilometres high and would almost reach from earth to mars. Red blood cells carry oxygen to the tissues and organs and pick up carbon dioxide and carry it back to the lungs for removal.
Plasma
Plasma on its own is straw-coloured. Ninety per cent of plasma is water, while the other 10 per cent is made up of things like proteins, minerals, vitamins, sugars, fats, and other materials that fight disease and act as chemical messengers. Plasma carries nutrients to all parts of the body and carries waste products out of the body, acting like a cell transportation system. Red blood cells, white blood cells and platelets move around through the plasma.
 
Platelets
Platelets are small, delicate and oddly shaped. When we get a cut or scratch that bleeds, it means that a blood vessel has been cut or broken. Platelets dive into wounds and bind together, getting sticky like glue. They form a seal to help stop bleeding and keep germs and bacteria out of the cut.
White Blood Cells
White blood cells help keep the blood clean and help fight germs and infections. They help us recover from viruses, such as the common cold or chicken pox. Because a donor’s white blood cells may suppress the recipient's immune system by interacting with it, white blood cells are removed from blood donations through a process called leukoreduction.
 
What’s Your Blood Type?
Your blood type determines who you can donate to, or receive blood from. People with type O negative (O-) are considered universal donors, as they can donate red blood cells to all other blood type recipients. That’s why it’s always in high demand — it’s used in emergency situations when there is no time to test a patient’s blood type.
People with type AB positive (AB+) are considered universal recipients for red blood cells as they can receive them from any other blood type donor.
The most common blood type in Canada is O positive (O+), about 39 per cent of the population has it, which is why we have the greatest need for type O blood.
The least common type is AB negative (AB-), the blood type of less than one per cent of the population.