The foundations of blood science
An adult carries about five litres of blood, and replaces a few million red cells every second. It is a transport system, a defence force and a repair kit in one fluid — and one of the first tissues that medicine learned to move from one person to another.
Blood clots within minutes of leaving a vessel. Sodium citrate, introduced in 1914, prevented that and made storage possible; Soviet hospitals were storing blood by the early 1930s, and the first hospital blood bank in the United States opened in Chicago in 1937. Today collected blood is separated into red cells, platelets and plasma, each kept under different conditions for days to a year.
Red cells carry surface molecules that differ between people, and plasma carries antibodies against the ones a person lacks. Karl Landsteiner described the ABO groups in 1901, and the Rh factor followed in 1940. Almost fifty blood group systems are now recognised; ABO and Rh matter most for transfusion.
All blood cells descend from stem cells in the bone marrow, demonstrated by Till and McCulloch in 1961. A red cell lasts about 120 days, a platelet only a week to ten days. Transplanting marrow — first achieved between identical twins in the late 1950s, and between siblings who were not twins in 1968 — replaces the whole system.
Platelets stick to a damaged vessel wall within seconds, and a cascade of plasma proteins converts fibrinogen into a mesh of fibrin that holds the plug. Too little clotting is haemophilia; too much is thrombosis.
Fewer than one blood cell in five hundred is white, but these are the mobile part of the immune system: neutrophils that engulf bacteria, lymphocytes that remember infections, and several others. Blood is how they get where they are needed.
Each red cell is packed with haemoglobin, an iron-containing protein that binds oxygen in the lungs and releases it in the tissues. Max Perutz and his colleagues worked out its structure in 1959 and made it public early in 1960. A single changed amino acid in it causes sickle cell disease — the first illness traced to a molecule, by Linus Pauling and colleagues in 1949; Vernon Ingram found the changed amino acid in 1957.
A little over half is plasma: water, salts, and proteins such as albumin, antibodies and clotting factors. The rest is cells — overwhelmingly red cells, with platelets and white cells. William Harvey showed in 1628 that it circulates, pumped by the heart in a closed loop.
Read from the bottom up. What blood is made of explains what it can do; what it can do explains why matching it matters.