Structure of the Heart: Location, Layers

The heart is a muscular organ that pumps blood throughout the body, supplying oxygen and nutrients to tissues and removing carbon dioxide and waste products. It is a key component of the circulatory system and is located in the thoracic cavity, between the lungs, slightly tilted to the left. 

1.0Location of the heart

• The heart is present in the thoracic cavity, in space mediastinum (between both the lungs). 
• The heart is slightly tilted to the left. 

2.0Weight of the Heart 

• Adult Male:- 300 gms. 
• Adult Female:- 250 gms.

3.0Coverings and Layers of Heart

The heart is protected by a double walled membranous bag.

Significance of Pericardial Fluid 

• The pericardial fluid acts as a shock absorber.
• Pericardial fluid provides frictionless movements between two pericardial membranes (parietal & visceral). 

4.0Structure of Heart

Chambers of Heart

Our heart has four chambers: two relatively small upper chambers called atria or auricles and two larger lower chambers called ventricles. 

Septa of Heart

• A thin, muscular wall called the interatrial septum separates the right and the left atria, whereas a thick-walled interventricular septum separates the left and the right ventricles. 
• A thick fibrous tissue also separates the atrium and the ventricle of the same side called the atrioventricular septum. 
• However, each septum is provided with an opening through which the two chambers on the same side are connected. 

Vessels and Valves of Heart

• The opening between the right atrium 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 guards the opening between the left atrium and the left ventricle. 
• The semilunar valves provide the openings of the right and left ventricles into the pulmonary artery and the aorta. 

Significance of Valves in Heart 

• The heart's valves allow blood to flow only in one direction from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. 
• These valves prevent any backward flow. 

Significance of Chordae Tendineae and Papillary Muscles 

• Chordae tendineae and papillary muscles prevent the reverse opening of AV valves (Tricuspid and bicuspid) at the time of ventricular systole (contraction of ventricles). 

5.0Conducting System of Heart

Types of heart on the basis of source of Heart Beat

• The entire human heart is made of cardiac muscles. The walls of the ventricles are much thicker than those of the atria because the ventricles have to supply blood to various parts of the body.   
• A specialized cardiac musculature called the nodal tissue is also distributed in the heart. A patch of this tissue is present in the right upper corner of the right atrium called the sino-atrial node (SAN). 
• Another mass of this tissue is seen in the lower left corner of the right atrium, close to the atrioventricular septum, called the atrioventricular node (AVN). 
• Three bundles of muscle fibres connect the AV node and SA node, called internodal pathways.
• A bundle of nodal fibres, the atrioventricular bundle (AV bundle), continues from the AVN, which passes through the atrioventricular septa to emerge on top of the interventricular septum and immediately divides into a right and left bundle. 
• These right and left bundles with AV bundles are known as the Bundle of HIS. 
• These branches give rise to minute fibres called Purkinje fibres throughout the ventricular musculature of the respective sides.
• These fibres and right and left bundle branches are called the Purkinje system. 
• The nodal musculature can generate action potentials without external stimuli; it is auto-excitable. Our heart normally beats 70-75 times a minute (on average 72 beats/minute). 
• SA node (Pacemaker of the heart) (Generate a maximum number of action potentials, i.e., 1 to 72 beats per minute. 
• Atrio-ventricular node (AVN). Present at the lower left corner of the right atrium close to the (atrioventricular septum) (Reserve pacemaker of heart pacesetter of the heart). 
• Correct Pathway : SA node → Inter nodal pathway → AV node → AV bundle → Bundle branches → Purkinje fibers

Number of Beats Per Minute 

• SA Node 70-75
• AV Node 40-60
• Purkinje fibers 20-35