Heart

1. Definition

The heart is a vital muscular organ responsible for maintaining systemic and pulmonary circulation by acting as a central pump. It comprises four chambers—two atria and two ventricles—which work in a coordinated manner to facilitate the delivery of oxygenated blood to the tissues and the return of deoxygenated blood to the lungs. The heart’s activity is regulated by its intrinsic electrical conduction system, including the sinoatrial node and atrioventricular node, ensuring synchronized contractions and efficient blood flow throughout the body.

2. Anatomy

2.1. Blood supply of the heart

The blood supply of the heart is provided by the coronary arteries (coronary=like a crown) which branch off from the ascending aorta (aortic sinus). The two main arteries are the left coronary artery (LCA) and right coronary artery (RCA), each dividing into smaller branches to supply oxygenated blood to the myocardium. The LCA further divides into the left anterior descending (LAD) and circumflex (LCx) arteries, while the RCA supplies the right side of the heart and parts of the conduction system.

2.2. Dominances

Right coronary dominance:

When the posterior interventricular artery contributes considerably supply of posterior side of the left ventricle.

Left coronary dominance:

When posterior interventricular branch arises from LCA. Here it will supply the entire septum and part of the posterior right of the ventricular wall.

2.3. Venous drainage of the heart

All veins join to form the coronary sinus which is the main vein of the heart. Coronary sinus will drain into the right atrium (sinus venarum cavarum) through the orifice of the coronary sinus.

Great cardiac vein: From apex it ascends in the anterior interventricular sulcus. Joined by oblique/marshal vein.

Middle cardiac vein: Ascends in the posterior interventricular sulcus from the apex. It joins Small cardiac vein.

Small cardiac vein: From the right ventricle running in the coronary sulcus

Other veins: Anterior cardiac veins which drain directly into the RA

Smallest cardiac veins which drain into either of the chambers. This makes some kind of shunt, where mixing of oxygenated and deoxygenated blood happens (though insignificant amount)

2.4. Innervation of the heart

The heart is innervated by sympathetic and parasympathetic fibers from the autonomic branch of the peripheral nervous system. The network of nerves supplying the heart is called the cardiac plexus.

Sympathetic supply comes from the sympathetic trunk and innervates AV node, SA node and all the cardiac muscles.

Parasympathetic branches come from the vagus nerve and only innervate the nodal tissues and not the muscles. Right vagus innervates the SA node while left vagus innervates AV node.

Pericardium is innervated by the phrenic nerve (somatosensory).

3. Histology

The histological constituents of the heart comprise a trio of layers, each with specific structures and functions, including:

1. En­docar­di­um: It is the in­ner­most lay­er lin­ing the cham­bers of the heart. It is made up of sim­ple squa­mous ep­ith­eli­um (en­do­the­li­um) and a thin lay­er of con­nec­tive tis­sue. The en­do­car­di­um in­cludes sub­en­do­the­li­al con­nec­tive tis­sue that con­tains ner­ves, blood ves­sels, and Pur­kin­je fibres for elec­tri­cal con­duc­tion.

2. Myocardium: This is the thick middle layer made of cardiac muscle tissue. The cardiomyocytes in this layer are striated, branched, and connected to each other through intercalated disks with gap junctions and desmosomes for synchronized contraction. The myocardium is the functional muscle layer responsible for the contractile force of the heart.

3. Epicardium: This is the outermost layer, also known as the visceral layer of the serous pericardium. Composed of mesothelial cells and a thin layer of connective tissue, including fat, nerves, and coronary blood vessels, it provides an outer protecting layer for the heart and supports its blood supply.

All these layers together interact in a way that allows the heart to function as a pump: the myocardium would be responsible for the mechanical forces; whereas the endocardium and epicardium would work in the directions of protection and regulation, respectively.

4. Embryology 

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5. Physiology 

Physiology of the heart refers to the function of the heart as a muscular pump in the circulation of blood throughout the body, a process highly integrated with electrical and mechanical events. The major components of the physiology of the heart include:

5.1. Cardiac Cycle

There are mainly two phases to a cardiac cycle:

• Systole: The period of contraction. During ventricular systole, the ventricles contract thereby pumping blood into the aorta and pulmonary artery. This results from the powerful contraction of cardiomyocytes.

• Diastole: It is the relaxation phase. During the ventricular diastole, the chambers of the heart fill with blood. The tricuspid and mitral are the atrioventricular valves that permit blood flow from the atria into the ventricles.

5.2. Cardiac Output (CO)

Cardiac output is the amount of blood that goes through the heart in one minute. It is obtained by the following combination:

• Heart Rate (HR): Amount of heartbeats per minute.
• Stroke Volume (SV): The volume of blood ejected with each pulse.

5.3. Electrical Conduction System

The rhythmic contractions of the heart are governed by its intrinsic electrical conduction system, which includes:

• Sinoatrial Node: The main pacemaker is at the right atrium; it generates the spontaneous action potentials that initiate every heartbeat.

• Atrioventricular Node: This portion delays the electrical impulse to allow for complete atrial contraction before ventricular contraction.

• Bundle of His, Tawara branches and Purkinje Fibers: These conduct impulses rapidly through the ventricles, ensuring a coordinated contraction (direction endocard -> epicard).

5.4. Frank-Starling Mechanism

5.5. Regulation of Heart Rate and Contractility

The heart is ruled by:

• Autonomic Nervous System:
  • • Sympathic stimulation increases heart rate and contractility
    • Parasympathetic stimulation slows down the heartbeat
• Hormonal: epinephrine, thyroid hormones stimulate increases in heart rate and contractility.

5.6. Coronary Circulation

The coronary arteries provide oxygen and nutrition to the myocardium, arising from the root of the aorta. Myocardial blood flow is greater in diastole because contraction during systole compresses the coronary vessels (= retrograde blood flow (= slosh)).