Friday, August 7, 2020

Body Fluids and Circulation - Notes | Class 11 | Part 3: Blood Vascular System - Heart

18. BODY FLUIDS AND CIRCULATION 

3. HEART


It is a mesodermally derived organ located in mediastinum.

It has the size of a clenched fist.

It is protected by double-layered pericardium.

The pericardial space (between pericardial membranes) is filled with pericardial fluid. It reduces the friction between the heart walls and surrounding tissues.

Heart has 4 chambers- two upper atria (auricles) and two lower ventricles.

The walls (cardiac muscles) of the ventricles are much thicker than that of the atria.
The atria are separated by an inter-atrial septum and the ventricles are separated by inter-ventricular septum.

In between atrium and ventricle, there is a thick fibrous atrio-ventricular septum with an opening.

A tricuspid valve (3 muscular flaps or cusps) guards the opening between right atrium & right ventricle. 

A bicuspid (mitral) valve guards the opening between left atrium and left ventricle. These valves allow the flow of blood only in one direction, i.e. from atria to ventricles.

Right ventricle has an opening to pulmonary artery and left ventricle has an opening to aorta. These openings have semi-lunar valves. They prevent backward flow of blood.
 

CONDUCTING SYSTEM OF HEART


It includes nodal tissues, bundles & fibres.

Nodal tissues are specialized cardiac musculature present in heart wall. They are 2 types:
  • Sino-atrial node (SAN) in the right upper corner of the right atrium.
  • Atrio-ventricular node (AVN) in the lower left corner of the right atrium close to the atrio-ventricular septum.
From the AVN, a bundle of fibrous atrio-ventricular bundle (AV bundle) passes through atrio-ventricular septa and divides into right & left branches. Each branch passes through the ventricular walls of its side. In the ventricular wall, it breaks up into minute fibres (Purkinje fibres). These fibres along with the bundles are known as bundle of His.

Nodal tissues generate action potential without any external stimuli, i.e. it is autoexcitable. SAN initiates and maintains contraction of heart by generating action potentials (70-75/min). So, it is called the pacemaker.

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