Oxygen in human blood is transported mainly by erythrocytes. In combination with hemoglobin, it forms oxygenated (not oxidized, there is no oxidation reaction between iron and oxygen) hemoglobin (oxyhemoglobin). In contrast, about 3 percent of oxygen is transported in dissolved form in plasma. ¹⁾

The Rh system of human blood was discovered by Karl Landsteiner and Alexander S. Wiener. They were experimenting with the blood of rhesus monkeys at the time. The abbreviation “Rh” comes from “Rhesus.” Because of their ease of breeding and anatomical similarity to humans, these monkeys have been and continue to be frequent laboratory animals. ²⁾

Octopuses and squids have blue blood. Instead of red hemoglobin, copper-containing hemocyanin acts as an oxygen transporter (hence the blue color). At low temperatures and low oxygen concentrations in the water, hemocyanin-based transport is more efficient than hemoglobin-based transport. ³⁾

Which blood type is known as a “universal donor”? Group 0 blood can be transfused to anyone if necessary, as there are no anti-A or anti-B antibodies in it. ⁴⁾

AB is the rarest of the AB0 system groups, and Rh- is always less numerous than Rh+. In western Europe, only 1 percent of the population has AB- blood. In the far east, Rh- is very rare, so in Japan, only 0.05 percent of people have AB- blood. ⁵⁾