Heart

Heart 

The heart is a conical hollow muscular organ located in the middle mediastinum, enclosed within the pericardium. It is primarily responsible for pumping blood throughout the body to meet metabolic demands.

Anatomical Position and Dimensions

• Positioned posteriorly to the body of the sternum, with approximately one-third on the right and two-thirds on the left of the midline.
• Measures 12 x 8.5 x 6 cm and weighs ~310 g (males) and ~255 g (females).

External Relations

• Anteriorly: Body of the sternum, adjoining costal cartilages, left lung, and pleura (apex).
• Posteriorly: Esophagus, descending thoracic aorta, azygos and hemiazygos veins, thoracic duct.
• Superficially: Bifurcation of the main pulmonary trunk.
• Inferiorly: Diaphragm.
• Laterally: Lungs, pleura.

Layers of the Heart Wall

The heart wall consists of three layers enclosed within the pericardium:

• Epicardium: Outer layer formed by the visceral layer of the serous pericardium.
• Myocardium: Middle muscular layer containing excitable tissue and the conducting system.
• Endocardium: Inner layer consisting of a middle concentric and subendocardial layer.

Structure and Chambers

The heart is divided into right and left halves by septa, each half further divided into two cavities:

• Right Atrium
• Left Atrium
• Right Ventricle
• Left Ventricle

Blood Flow Through the Heart

Blood flows through the heart in a series of steps:

1. Venous blood returns to the right atrium via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus.
2. Right atrium pumps blood through the tricuspid valve into the right ventricle.
3. Right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk for oxygenation in the lungs.
4. Oxygenated blood returns from the lungs to the left atrium via the pulmonary veins.
5. Left atrium pumps blood through the bicuspid (mitral) valve into the left ventricle.
6. Left ventricle pumps blood through the aortic semilunar valve into the ascending aorta to supply the body.

Heart Valves

Maintain unidirectional blood flow and prevent regurgitation:

• Atrioventricular Valves: Tricuspid (right side), Mitral/Bicuspid (left side).
• Semilunar Valves: Aortic (left side), Pulmonary (right side).

Blood Supply

The heart is supplied by two coronary arteries:

• Left Main Coronary Artery: Supplies 80% of blood flow to the heart muscle, branches into the left anterior descending artery and circumflex coronary artery.
• Right Coronary Artery: Supplies the right ventricle, right atrium, and part of the left ventricle's inferior wall.

Venous Drainage and Lymphatics

Venous drainage occurs via coronary veins and the coronary sinus, which opens into the right atrium. Lymphatic drainage mainly targets brachiocephalic and tracheobronchial nodes.

Nerve Supply

The heart is controlled by the autonomic nervous system:

• Sympathetic Nervous System: Increases heart rate.
• Parasympathetic Nervous System: Decreases heart rate.
• Conduction System: Includes the sinoatrial node, atrioventricular node, and special fibers for electrical impulses.

Conclusion

The heart's intricate anatomy and physiological functions ensure its vital role in circulatory dynamics and overall body function.