11 Biology

Circulatory System


There are two types of circulatory pathways in the animals, viz. open or closed.

  • Open Circulatory System: In open circulatory system, blood pumped by the heart passes through large vessels and drained into open spaces or body cavities; called sinuses. This type of circulatory system is present in arthropods and molluscs.
  • Closed Circulatory System: In closed circulatory system blood pumped by the heart is circulated through a closed network of blood vessels. This type of system is present in annelids and chordates. The closed circulatory system is more advantageous because the flow of fluid can be more precisely regulated.

Chambered Heart: A muscular chambered heart is present in all vertebrates. The fishes have a 2-chambered heart; with an atrium and a ventricle. Amphibians and reptiles have a 3-chambered heart; with two atria and one ventricle. But crocodiles have 4-chambered heart. Birds and mammals have 4-chambered heart; with two atria and two ventricles.

Single Circulation: In fishes, the heart pumps out deoxygenated blood to the gills. The oxygenated blood from the gills is supplied to the body parts. Deoxygenated blood is then returned from different body parts to the heart. In single circulation, the blood passes through the heart only once.

Incomplete Double Circulation: In amphibians and reptiles, incomplete double circulation is present. Oxygenated blood is received by the left atrium and deoxygenated blood is received by the right atrium. But, both the bloods get mixed up in the single ventricle which pumps out the mixed blood. In incomplete double circulation, the blood comes to the heart through two different routes, but goes out through a single route.

Complete Double Circulation: Complete double circulation is present in birds and mammals. In this case, the oxygenated blood is received by the left atrium and the deoxygenated blood is received by the right atrium. The oxygenated blood is pumped out through the left ventricle, while the deoxygenated blood is pumped out through the right ventricle. In complete double circulation, there are two separate pathways for oxygenated and deoxygenated bloods.


The human circulatory system is composed of a muscular heart, a network of closed branching blood vessels and blood.


Location and Size: Heart is a mesodermally derived organ. It is situated in the thoracic cavity; in between the two lungs and slightly tilted to the left. It is the size of a clenched fist.

structure of heart

Structure: The heart is protected by a double-walled membrane; called pericardium. The pericardial fluid is filled in this membranous bag.

There are four chambers in the human heart:

  • The two upper chambers are called atria and are relatively smaller.
  • The two lower chambers are called ventricles and are relatively larger.
  • A thin, muscular wall separates the right and the left atria and is called the inter-atrial septum. A thick muscular wall separates the left and the right ventricles and is called the inter-ventricular septum. The atrium and ventricle on the same side are separate by a thick fibrous tissue; called the atrio-ventricular septum. Each of these septa has an opening through which the atrium and the ventricle of the same side are connected.
  • The opening between the right atrium and the right ventricle is guarded by a valve. This valve has three flaps or cusps and hence is called the tricuspid valve. Similarly, a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle.
  • The openings of the right and the left ventricles into the pulmonary artery and the aorta; respectively; are provided with the semi-lunar valves. These valves allow the flow of blood in only one direction and prevent any backward flow.