Red blood cells carry oxygen and are used to support major surgery, trauma and childbirth emergencies. They are also used when an illness, such as rheumatoid arthritis or cancer, leads to anaemia, and where red cells break down too soon, such as in sickle cell disease.
Platelets are needed for blood to clot and are used when the bone marrow doesn't work properly - for example during cancer treatment, or when patients can’t produce platelets.
Plasma is used for critically ill patients including burns victims, and patients who have lost large volumes of blood and need to have the full range of proteins replaced.
White blood cells fight infections by destroying bacteria, viruses, and other germs. White blood cells are rarely transfused, but can be used as a temporary measure for people with a low white cell count and severe infection which has not responded to antibiotics.
Of these four components, we usually use three - red cells, platelets and plasma. By giving blood, you can save or improve the lives of up to three different patients in Scotland.
Current blood stock levels across Scotland Sunday 18 April