Heart Anatomy

What is the Heart?

The heart is a powerful muscle that is roughly the size of a closed hand. It continuously pumps blood to all parts of the body via a complex network of blood vessels, supplying oxygen to every cell. It beats constantly during our lives, with a typical rate of 80 beats per minute. By the age of 80, the heart will have beat more than 3 billion times! The heart’s location in the chest is symbolic of its crucial role in emotions such as love, spirituality, courage, and determination. It is a fascinating and remarkable organ, and there is much to discover about its structure and function.

The Pericardium

The heart is located in the middle of the chest in the mediastinum and is surrounded by the pericardium, also known as the pericardial sac. The pericardium is made up of two layers: the fibrous pericardium, which is a tough and inflexible layer that protects the heart from overexpansion, and the serous pericardium, which is made up of two layers: the parietal layer and the visceral layer. The fibrous pericardium, which is the outermost layer, helps to keep the heart in shape, and decreases its compliance. The serous pericardium, which is the innermost layer, is divided into two layers: the parietal and visceral layer.

The inner layer of the serous pericardium is called the parietal layer, which lines the inside of the fibrous pericardium. This inner layer turns inward at several key points, such as the aorta, pulmonary trunk, superior and inferior vena cava, and pulmonary veins, to cover the heart and the large blood vessels. When it does this, it is then referred to as the visceral layer or epicardium. This folding creates a space known as the pericardial space, which contains a small amount of pericardial fluid (20 – 40 mL) that helps the heart move smoothly.

Layers of the Heart Wall

The epicardium is the outermost layer of the heart, also known as the visceral layer of the pericardium. It surrounds the heart and the great vessel roots.

The myocardium is the middle layer of the heart and is the thickest. It is composed of cardiac muscle cells and is responsible for the contraction of the heart.

The innermost layer of the heart is the endocardium which is composed of squamous epithelium, some connective tissue, and a subendocardium. The subendocardium includes the conduction system of the heart. The endocardium covers the heart valves and continues to line all the vessels and form the capillaries.

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Heart Chambers

The heart is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. Blood coming from the systemic circulation enters the right atrium and then passes into the right ventricle, which pumps blood through the pulmonary circulation. 

Blood coming from the pulmonary circulation enters the left atrium and then passes into the left ventricle, which pumps blood through the systemic circulation.

Let’s look at the chambers in detail:

  • The right atrium represents the right border of the heart. It contains the sinus venarum, which allows blood from the systemic circulation to flow into the right atrium. Between the right atrium and the left atrium is the interatrial septum and the fossa ovalis, which is a remnant of the embryologic oval foramen (foramen ovale).
  • The right ventricle forms the majority of the anterior part of the heart and touches the internal surface of the ribcage. The highly muscular interventricular septum separates the two ventricles.
  • The left atrium receives blood from the pulmonary circulation. It forms the base and most posterior region of the heart.
  • The left ventricle represents the apex of the heart and most of the diaphragmatic border.

Great Vessels

Let’s look at the chambers in detail:

  • The right atrium represents the right border of the heart. It contains the sinus venarum, which allows blood from the systemic circulation to flow into the right atrium. Between the right atrium and the left atrium is the interatrial septum and the fossa ovalis, which is a remnant of the embryologic oval foramen (foramen ovale).
  • The right ventricle forms the majority of the anterior part of the heart and touches the internal surface of the ribcage. The highly muscular interventricular septum separates the two ventricles.
  • The left atrium receives blood from the pulmonary circulation. It forms the base and most posterior region of the heart.
  • The left ventricle represents the apex of the heart and most of the diaphragmatic border.

Remember that arteries transport blood away from the heart and veins transport blood toward the heart. In the figure above, areas in pink represent almost fully oxygenated blood, while areas in blue represent blood with a lower oxygen content. Starting at the right atrium, blood from the body is returned to the heart via the superior vena cava (SVC) and the inferior vena cava (IVC). 

The SVC returns blood from all tissues above the diaphragm, and the IVC returns blood from all tissues below the diaphragm. Because this blood is returning from the organs, it has less oxygen. 

This deoxygenated blood enters the right atrium, then passes through the tricuspid valve into the right ventricle, which pumps it into the pulmonary circulation and the lungs.

The main vessel that is responsible for connecting the right ventricle to the pulmonary capillaries is the pulmonary trunk, which divides into the right and left pulmonary arteries. 

Although these are arteries, they contain deoxygenated blood. After the blood is fully reoxygenated in the lungs, it is returned to the left atrium via the pulmonary veins. This highly oxygenated blood then passes to the left ventricle via the mitral valve, where it is ejected into the aorta. The highly oxygenated blood in the aorta is then transported to all the organs in the body.

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