Blood vessel vessels function to transport blood. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its organs, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the circulatory system Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95–100%) with oxygen. In all veins apart from the pulmonary vein, the saturation of hemoglobin is about 75%. (The values are reversed in the pulmonary circulation.) In
Packed red blood cells used to restore oxygen-carrying capacity in people with anemia due to trauma or other medical problems, and are by far the most common blood component used in transfusion medicine. Historically they were transfused as part of whole blood, but are now typically used separately as RBCs and plasma components. The process of identifying a compatible blood product for transfusion is complicated. Side effects can include allergic reactions including anaphylaxis, red blood cell breakdown, fluid overload, infection, and lung injury. Giving incompatible RBCs to a person can be fatal. With current testing methods in high-income countries the risk of infection is