Summary
Highlights
Unlike many other body parts that can be repaired, synthesized, or replaced, blood remains irreplaceable. Every two seconds, someone in the U.S. needs a blood transfusion, often due to injury, surgery, or cancer treatment. Blood cannot be manufactured or stored for extended periods, making human donation crucial.
Blood is red, sticky, salty, and metallic, making up about 8% of body weight. It's a connective tissue composed of living cells suspended in plasma. Its main functions include transporting oxygen, nutrients, waste, and hormones, regulating body temperature, pH, and fluid volume, and protecting against infection and blood loss.
Donating a pint of blood takes about 20 minutes. Donated blood is tested for diseases and then separated into different components in a centrifuge. This process separates red blood cells (erythrocytes) at the bottom, a thin layer of white blood cells (leukocytes) and platelets in the middle, and yellowish plasma at the top.
Erythrocytes (red blood cells) carry oxygen and carbon dioxide, making up 45% of blood volume. Leukocytes (white blood cells) and platelets protect against toxins and aid clotting, comprising less than 1%. Plasma, which is 55% of blood volume, is 90% water and 10% solutes like electrolytes, proteins (albumin, globulins, fibrinogen), gases, and hormones. Electrolytes help regulate blood chemistry and nerve/muscle function, while plasma proteins balance osmotic pressure and aid immunity and clotting.
Hemostasis is the body's process to prevent excessive blood loss. When a blood vessel is injured, it first constricts, then platelets form a plug by becoming sticky and clumping at the injury site. Fibrin threads then reinforce this plug, forming a mesh that traps blood cells to create a strong clot. This seals the wound, allowing the vessel to heal, after which the clot dissolves. Disorders like hemophilia can impair this process, leading to prolonged bleeding.
Blood typing is crucial for transfusions. Blood types (A, B, AB, O) are determined by agglutinogens (antigens) on red blood cells. The immune system produces antibodies against antigens not present on one's own blood cells. Type AB is the 'universal recipient' as it has both A and B antigens and no antibodies. Type O is the 'universal donor' as it lacks A and B antigens and won't trigger an immune response. Additionally, the Rh factor (positive or negative) also affects compatibility, with Rh-negative individuals needing Rh-negative blood. These two systems combine to create eight major blood types.