HUMAN CIRCULATORY SYSTEM CLASS 11

Circulatory Pathways:

• The circulatory patterns are of two types - open or closed. Open circulatory system is present in arthopods and mollusscs.

• Annelids and chordates have a closed circulatory system.

• All vertebrates possess a muscular chambered heart. Fishes have a 2-chambered heart with an artium and a ventricle. Amphibians and the reptiles (except crocodiles, birds and mammals possess a 4-chambered heart with two artia and two ventricles. In fishes the heart pumps out deoxygenated blood (single circulation).

• In amphibians and reptiles, the left artium receives oxygenated blood from the gills/lungs/skin and the right artium gets the deoxygenated blood from other body parta. However, they get mixed up in thw single ventricle which pumps out mixed blood (incomplete double circulation). In birda and mammals, oxygenated and deoxygeneted blood recived by the left and right artia respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up two separate circulatory pathways are present in these organisms, hence, these animals have double circulation. Let us study the human circulatory system.

Human Circulatory System:

• Human circulatory system, also called the blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.

Heart, the mesodermally derived organ is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist. It is protected by a double walled membranous bag, pericardium, enclosing the percardial fluid. Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambera called ventricles. A thin muscular wall called the interatrial septum separates the right and the left artia, whereas a thick-walled, the inter-ventricular septum, seprarates the left and the right ventricles.

The artium and the ventricle of the same side are also separated by a thick fibrous tissue called the artio-ventricular septum. The opening between the right artium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guard the opening between the left artium 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 semilunar valves. The valves in thw heart allows the flow of blood only in one direction from the artia to the ventricles and from the venteicles to the pulmonary artery or aorta. These valves prevent any backward flow. The entire heart us made of cardiac muscles. The walls of ventricles are much thicker than that of the atria. A specialised cardiac musculature called nodal tissue is present in the right upper corner of the right artium called the sino-artial node (SAN). Another mass of this tissue is seen in the lower left corner of the right artium close to the artio-ventricular septum called the atrio-ventricular node (AVN).

• The SAN can generate the maximum number of action potentials, 70-75 min-1 and is responsible for initiating and maintaining the rhyhthmic contractile activity of the heart. Therefore, it is called the pacemaker. Our heart normally beats 70-75 times in a minute (average 72 beats min-1).