1.6—Need for hearts and circulation

Diffusion is effective over short distances but becomes too slow when an organism is large or metabolically active. A circulatory system moves substances in bulk between exchange surfaces and tissues.

A pump maintains a pressure gradient and vessels provide a controlled route. The heart sends deoxygenated blood to the lungs and oxygenated blood to body tissues in a double circulation, keeping exchange gradients steep.

An active muscle has a high oxygen demand. Blood flow delivers oxygenated blood close to the cells and removes carbon dioxide, while diffusion completes the final short distance across capillary walls and tissue fluid.

The circulatory system does not replace diffusion: gases and solutes still cross exchange surfaces by diffusion or transport processes. Its role is to shorten effective transport distances and refresh gradients.