Vertebrate Circulatory Systems

The vertebrate circulatory systems are often being referred to as the cardiovascular system.

Arteries, veins, and capillaries are the three main types of blood vessels.

Arteries: carry blood from the heart to organs throughout the body; branch into arterioles.

Capillaries: microscopic vessels with very thin, porous walls, often form networks called capillary bed; dissolved gases and other chemicals are exchanged by diffusion between the blood and the interstitial fluid across the tissue cells here.

Veins: vessels that carry blood back to the heart; capillaries converge into venules and then into veins.

Single Circulation

In sharks, rays, and bony fishes, blood travels through the body and returns to its starting point in a single circuit (loop), an arrangement called single circulation. 

These animals have a heart that consists of two chambers: an atrium and a ventricle. Blood that leaves the heart passes through two capillary beds before returning to the heart, and when it passes through the gill capillaries (the first capillary bed encountered in the circuit), blood pressure drops substantially, limiting the rate of blood flow in the rest of the animal's body. However, as fish swims, the contraction and relaxation of its muscles help accelerate the pace of its circulation.

Double Circulation: Amphibians

Double circulation provides a vigorous flow of blood to the brain, muscle, and other organs because the heart repressurizes the blood after it passes through the capillary beds of the lungs or skin.

Frogs and other amphibians have a heart with three chambers-two atria and one ventricle. The ventricle, incompletely divided by a valve, diverts most of the oxygen-rich blood from the right atria into the systemic circuit and most of the oxygen-poor blood from the right atrium into the pulmocutaneous circuit. When an amphibian is underwater, it can shut off blood flow to the lungs and circulates blood through its ventricles, enhancing its time of submerging.

Double Circulation: Mammals and Birds

The hearts of mammals and birds have four chambers-two atria and two ventricles. Due to the total separation of its ventricles, birds and mammals cannot vary blood flow to the lungs without varying blood flow throughout the body in parallel.

The four-chambered heart enables a large input of fuel and oxygen and output of waste compare with the circulatory systems found in other animals, as endotherms need much more energy than ectotherms do. This four-chambered heart system arose independently in the distinct ancestors of birds and mammals,  reflecting convergent evolution.

Double Circulation: Reptiles

In the three-chambered heart of turtles, snakes, and lizards, an incomplete septum partially divides the single ventricle into right and left chambers. Their pulmonary and systemic circuits are also connected, which allows blood flow and exchange even when the animal is underwater.

Vertebrate Circulatory Systems 

Reference:

Campbell, et al. (2017). Biology: A Global Approach (11th ed.)