Veins have thinner walls compared to arteries as they carry blood back to the heart at lower pressure. Veins have valves to ensure one-directional blood flow, preventing backflow.

Arteries have thick, elastic walls because they carry blood away from the heart under high pressure, helping to withstand and accommodate the force generated by the pumping action of the heart.

Fish have a two-chambered heart, and blood is oxygenated in the gills before directly circulating through the body, resulting in a single passage through the heart per cycle, unlike other vertebrates with double circulation.

Amphibians and many reptiles have three-chambered hearts, allowing some mixing of oxygenated and deoxygenated blood, as their body temperature depends on the external environment.

Birds and mammals have a separated heart to prevent the mixing of oxygenated and deoxygenated blood, ensuring efficient oxygen supply for their high energy needs.