Single and Double Circulation Systems
The circulatory system is a broad term that encompasses the cardiovascular and lymphatic systems. The lymphatic system will be discussed later in this tutorial. The cardiovascular system consists of the heart (cardio) and the vessels required for transport of blood (vascular). The vascular system consists of arteries, veins and capillaries. Vertebrates (animals with backbones like fish, birds, reptiles, etc.), including most mammals, have closed cardiovascular systems. The two main circulation pathways in invertebrates are the single and double circulation pathways.
Single Circulatory Pathways
Single circulatory pathways as shown in the diagram below consist of a double chambered heart with an atrium and ventricle (the heart structure will be described in detail later in this tutorial). Fish possess single circulation pathways. The heart pumps deoxygenated blood to the gills where it gets oxygenated. Oxygenated blood is then supplied to the entire fish body, with deoxygenated blood returned to the heart.
Double Circulatory Systems
Double circulation pathways are found in birds and mammals. Animals with this type of circulatory system have a four-chambered heart.
The right atrium receives deoxygenated from the body and the right ventricle sends it to the lungs to be oxygenated. The left atrium receives oxygenated blood from the lungs and the left ventricle sends it to the rest of the body. Most mammals, including humans, have this type of circulatory system. These circulatory systems are called ‘double’ circulatory systems because they are made up of two circuits, referred to as the pulmonary and systemic circulatory systems.
Humans, birds, and mammals have a four-chambered heart. Fish have a two-chambered heart, one atrium and one ventricle. Amphibians have a three-chambered heart with two atria and one ventricle. The advantage of a four chambered heart is that there is no mixture of the oxygenated and deoxygenated blood.