The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. Our bodies actually have two circulatory systems: The pulmonary circulation is a short loop from the heart to the lungs and back again, and the systemic circulation (the system we usually think of as our circulatory system) sends blood from the heart to all the other parts of our bodies and back again. This is an illustration of the circulatory system (World of Anatomy and Physiology 2010).
This here is an illustration of the different vertebrate circulatory systems. Oxygen-rich blood is shown as red and oxygen-poor blood as blue. (a) In fish, the heart has one atrium (A) and one ventricle (V). Blood oxygenated in the gill capillaries flows directly to the capillaries of the systemic circulation without first returning to the heart. (b) In amphibians, the single atrium is divided into two separate chambers. Oxygen-rich blood from the lungs enters one atrium, and oxygen-poor blood enters the other. Oxygen-rich blood is pumped from one ventricle to the body tissues, while oxygen-poor blood is sent from the lungs to the skin, which is a major respiratory organ in amphibians. (c) In birds and mammals, the atrium and ventricle are divided into two separate chambers, forming two hearts. One pumps oxygen-rich blood through the body, and one pumps oxygen-poor blood through the lungs (Gale 2010).
Illustrations of the anterior and interior view of the heart (MUSC Health 2013).