HSEB Notes on Physiology of Circulation Class 12

Biology | Zoogy
HSEB Notes on Circulation
Class : 12

Physiology of circulation

Human body is fairly complex and performs wide range of activities. Different organs of body are located far away and may have no connection at all. Our body needs a specialized system of circulation which can connect these organs supplying nutrients, gases, hormones and carrying wastes from these organs. This system of circulation also performs the functions like regulation of things and protection. The system of circulation of some extra cellular fluid is flowing in the body. The structures associated with circulation constitute circulatory system.  Circulatory system must have these basic features

3. A pumping mechanism like heart.

4. A system of tubes through which the circulating fluid can move.

5. Presence of transporting or circulating fluid like blood or lymph.

Structure of heart

A human heart is muscular hollow pumping organ which can maintain continuous flow of blood circulation inside the body. It is roughly triangular in shape. It is about the size of a person’s fist and weighs about 300 grams. It is reddish brown in color. It is situated ventrally in the middle of thoracic cavity in between two lungs. The narrow end of the heart is slightly displaced to the left side. It can be heard towards the left side of the chest.

The heart is enclosed in a double walled membranous sac called pericardium. The inner membrane is attached to the heart. In between two membranes, there is presence of a pericardial fluid. The fluid is shock absorbing so protect heart from any shock and minor injuries. It also allows free movement to the heart.

Anterior broad part is called auricular part and posterior narrow part is called ventricular part. Towards right side of auricular part there is superior and inferior venacavae carrying impure blood from different parts of body except lungs. Towards the left side of auricular part, the pulmonary veins carry pure blood from lungs. The auricular part receives blood. The lower ventricular part sends out blood. The arch of Aorta and Pulmonary trunk carries blood out of the heart.

Internal structure 

The human heart is four chambered. The upper two are right and left atria or auricles. The lower two are right and left ventricles. Auricles are thin walled chambered separated by inter auricular septum. There are presence of muscular ridges called musculi pectinati. Right auricle is having openings for superior and inferior venacavae to receive impure or venous blood. Left auricle is having openings for pulmonary veins to receive pure blood from lungs. The opening of inferior venacava is guarded by valve of Eustachius while the opening coronary sinus is guarded by valve of Thebesius. Right and left auricle open into respective ventricle through an auriculo ventricular apertures. These AV apertures are guarded by valves.

The sinus venosus is completely merged into right auricle. So caval veins directly open into auricles. Truncus arteriosus has split into systemic (aortic) and pulmonary trunk in mammals.

The two ventricles are separated by inter ventricular septum. The septum comes right side from the apex. Ventricles are thick walled than atria. The left ventricle is thicker than right ventricle.

Valves  Bicuspid valve is also known as mitral valve. It is situated between the left auricle and left auricle and left ventricle. It allows unidirectional flow of oxygenated blood from left atrium to left ventricle. It consists of two flaps or cusps. Tricuspid valve is the right AV valve. It consists of three flaps or cusps. It allows impure blood to flow from right atrium to right ventricle.

Both valves are provided with tendons or chords made up of tough strands of connective tissue called chordae tendinae. The chordae tendinae arise from papillary muscle present in the wall of ventricles. Their contractions bring the tightening of chordae tendinae which in turn prevent the valves from turning inside out or from being forced upward during contraction of ventricles.

Semi lunar valves

At the base of pulmonary trunk and aortic arch, there are pulmonary semi lunar and aortic semi lunar valves. Such valve is made up of three flaps attached to inside of arterial wall. These valves allow only unidirectional flow of blood from ventricle to artery and prevent back ward flow.

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