Rh stands for Rhesus—after the rhesus macaque (Macaca mulatta), or the rhesus monkey (associated with the development of the Rh factor and serum)— and is one of the major blood group systems in humans. The Rh system classifies blood as Rh-positive or Rh-negative, based on the presence or absence of Rh antibodies found in the blood. Similar to the ABO blood type, when it comes to blood transfusions, the Rh often matters: People with Rh-positive blood can have a transfusion of Rh-negative blood without a problem, but people with Rh-negative blood will have a transfusion reaction if they receive Rh-positive blood.

Smaller animals such as hydras do not have a separate circulatory system since their cells are able to efficiently exchange materials (nutrients, gases, and wastes) through diffusion. The cells of these animals are close to the surface and thus can exchange nutrients effectively and efficiently without the need for a circulatory system.