Heart and its structure

• Human heart is a roughly triangular structure and a muscular organ.
• It is situated in the thoracic cavity between the two lungs.
• One third of its part lies on the right side and two third on the left side.
• An average human heart weighs about 300-350 grams which is about 12 cm long and 9 cm broad.
• It is enclosed in a strong double walled membrane called pericardium.
• The pericardium is filled with pericardial fluid that protects the heart from any shock.
• The heart is composed mainly of the cardiac muscle.
• Cardiac muscle is a specialized tissue that contracts automatically, powerfully and without fatigue throughout our lives.

Structure of heart

• The human heart is four chambered structure with two auricles and two ventricles.
• The two upper chambers are called left auricle and right auricle (atria) and the two lower chambers are called left and right ventricles.
• A distinct transverse groove divides the heart into an anterior smaller auricular part and a posterior larger ventricular part.
• This groove is known as the auriculo-ventricular groove or coronary sulcus.
• The right side of the heart is completely separated from the left by septum, so there is no mixing of impure and pure blood in the heart.
• The thickness of the walls in the different heart chambers reflects their function.
• The atria are thin muscled; they pump blood the short distance to the ventricles directly below them.
• The right ventricle is more heavily muscled than either of the atria as it has to force blood a much farther distance to the lungs.
• The left ventricle has the thickest wall as it has to push blood all around the body.
• The blood flows through the heart in one direction only. This is due to the sets of valves which close to prevent back-flow.
• The heart consists of four valves.

A) Auricles

• The auricles are thin walled chambers which are completely separated from each other by a thin vertical inter-auricular septum.
• Each auricle opens into the ventricle of its own side through an auriculo-ventricular aperture.
• Each aperture is guarded by atrio-ventricular valves (AV valves) to prevent blood from returning to the atria when the ventricles contract.
• The right auriculo-ventricular aperture is guarded by tricuspid valve as it consists of three flaps.
• It is also known as right atrio-ventricular valve.
• Similarly, the left atrio-ventricular aperture is guarded by bicuspid valve or mitral valve or left atrio-ventricular valve which consists of two flaps.
• These valves allow blood to flow from the auricle into the ventricle and prevent any back-flow of blood.
• They are attached to walls of the ventricles through long white strands called chordae tendinae.

B) Ventricles

• The ventricles are thick walled chambers which are separated by thick and oblique inter-ventricular septum.
• The left ventricle is larger and its walls are thicker than the right ventricle.
• From the right ventricle originates a pulmonary arch or aorta which carries de-oxygenated blood to the lungs.
• Similarly, from the left ventricle originates a systemic arch or aorta which carries oxygenated blood to various parts of the body.
• The opening of pulmonary arch and systemic arch are guarded by semi-lunar valves.

• Both sets of valves have three semi-lunar (half-moon shaped) flaps.
• These valves allow blood to flow from the ventricle into the aorta and prevent the backward flow of blood into the ventricle.
• Valves are simply strong flaps of tissue and they cannot move on their own.
• As blood begins to flow back, the valve is forced shut and this prevents any further back-flow.

References: 

i) https://training.seer.cancer.gov/anatomy/cardiovascular/heart/structure.html

ii) https://www.news-medical.net/health/Structure-and-Function-of-the-Heart.aspx

Heart and its structure