Capillaries are the smallest vessels in the circulatory system with walls only one-cell thick. They facilitate the exchange of materials between the blood and surrounding cells, allowing for the delivery of oxygen and nutrients and the removal of waste products.

Veins collect blood from different organs and bring it back to the heart. They have thinner walls compared to arteries because blood in veins is no longer under pressure. Veins have valves that ensure blood flows only in one direction.

Arteries carry blood away from the heart to various organs of the body.

Double circulation refers to the circulation of blood through the heart twice during each cycle. Other vertebrates, such as birds and mammals, exhibit double circulation, with blood passing through the heart once to the lungs or gills for oxygenation and ...

Fish have only two chambers in their hearts, which pump blood directly to the gills for oxygenation before it is circulated to the rest of the body.