Land-dwelling vertebrates have evolved two separate pumping circuits
Gravity opposes blood flow to elevated parts of the body.
However, the capillaries and alveoli would not be able to withstand the high pressures needed to counteract gravity.
Double circulation provides a vigorous flow of blood to the brain, muscles, etc. because the heart repressurises the blood after it passes through the capillary beds of the gas exchange organs.
- Pulmonary circulation is a lower-pressure circuit to and from the lungs
- Systematic circulation is a higher-pressure circuit to and from the rest of the body
A multichambered heart is required. The pulmonary and systematic circulations are completely separated in the 4-chambered heart of birds and mammals