Exercise 21 Gross Anatomy Of The Heart

The heart, a remarkable organ, serves as the engine of our circulatory system, tirelessly pumping life-sustaining blood throughout our bodies. Understanding its intricate anatomy is crucial for anyone in the medical field, and even for individuals simply curious about the wonders of the human body. This exploration will dissect the gross anatomy of the heart, providing a comprehensive overview of its external and internal structures.

External Anatomy: A First Look

The heart, roughly the size of a clenched fist, resides within the mediastinum, the space between the lungs in the thoracic cavity. Its orientation is slightly angled, with its apex (pointed end) directed towards the left hip and its base (broadest part) located beneath the second rib. From an external perspective, several key features are immediately apparent:

Pericardium: This double-layered sac surrounds and protects the heart. The outer layer, the fibrous pericardium, is a tough, connective tissue layer that anchors the heart within the mediastinum and prevents overfilling. The inner layer, the serous pericardium, is a thinner, double-layered membrane. The parietal layer of the serous pericardium is fused to the fibrous pericardium, while the visceral layer (also known as the epicardium) adheres directly to the heart's surface. Between the parietal and visceral layers is the pericardial cavity, filled with a small amount of serous fluid that lubricates the heart and reduces friction during contraction.
Atria: These are the two superior chambers of the heart, the right atrium and the left atrium. They are primarily receiving chambers, collecting blood returning to the heart from the systemic and pulmonary circulations, respectively. Auricles, small, wrinkled, protruding appendages, increase the atrial volume.
Ventricles: These are the two inferior, larger chambers of the heart, the right ventricle and the left ventricle. They are the main pumping chambers, responsible for propelling blood into the pulmonary and systemic circulations. The left ventricle has a significantly thicker wall than the right ventricle, reflecting its greater workload in pumping blood throughout the entire body.
Major Vessels: Several large blood vessels are connected to the heart, facilitating the inflow and outflow of blood. These include:
- Superior Vena Cava: Returns deoxygenated blood from the upper body to the right atrium.
- Inferior Vena Cava: Returns deoxygenated blood from the lower body to the right atrium.
- Pulmonary Trunk: Carries deoxygenated blood from the right ventricle to the lungs for oxygenation. It quickly bifurcates into the right and left pulmonary arteries.
- Pulmonary Veins: Return oxygenated blood from the lungs to the left atrium. Typically, there are four pulmonary veins (two from each lung).
- Aorta: The largest artery in the body, it carries oxygenated blood from the left ventricle to the systemic circulation. The ascending aorta arches to form the aortic arch, from which major arteries branch off to supply the head, neck, and upper limbs. The aorta then descends through the thorax and abdomen as the descending aorta.
Coronary Sulcus (Atrioventricular Groove): This groove encircles the heart, marking the boundary between the atria and ventricles. It contains coronary arteries and veins, which supply the heart muscle itself with blood.
Anterior and Posterior Interventricular Sulci: These grooves on the anterior and posterior surfaces of the heart mark the location of the interventricular septum, the wall that separates the right and left ventricles. They also contain coronary arteries and veins.

Internal Anatomy: Chamber by Chamber

A deeper exploration into the heart's interior reveals a complex network of chambers, valves, and specialized structures that ensure unidirectional blood flow.

Right Atrium

The right atrium receives deoxygenated blood from three sources: the superior vena cava, the inferior vena cava, and the coronary sinus.

Superior Vena Cava: Drains blood from the body regions superior to the diaphragm.
Inferior Vena Cava: Drains blood from the body regions inferior to the diaphragm. A valve, the valve of the inferior vena cava, guards the opening, directing blood flow during fetal development.
Coronary Sinus: Collects blood draining from the myocardium (heart muscle).
Fossa Ovalis: A shallow depression in the interatrial septum (the wall separating the right and left atria). This is a remnant of the foramen ovale, an opening that allowed blood to bypass the fetal lungs before birth. The foramen ovale typically closes shortly after birth, leaving the fossa ovalis.
Pectinate Muscles: Ridges of muscle tissue on the anterior wall of the right atrium.
Tricuspid Valve (Right Atrioventricular Valve): Blood flows from the right atrium into the right ventricle through this valve, which has three cusps (flaps).

Right Ventricle

The right ventricle receives deoxygenated blood from the right atrium and pumps it into the pulmonary trunk.

Trabeculae Carneae: Irregular ridges of muscle on the inner ventricular walls.
Papillary Muscles: Cone-shaped projections of muscle that extend from the ventricular wall.
Chordae Tendineae: Thin, strong connective tissue strings that attach the cusps of the tricuspid valve to the papillary muscles. These strings prevent the valve cusps from everting (flipping backward) into the right atrium during ventricular contraction.
Pulmonary Semilunar Valve: As the right ventricle contracts, it forces blood through this valve into the pulmonary trunk. This valve has three pocket-like cusps. When the ventricle relaxes, blood flows backward towards the ventricle, filling the cusps and sealing the valve to prevent backflow into the right ventricle.
Conus Arteriosus (Infundibulum): The superior part of the right ventricle that leads into the pulmonary trunk.

Left Atrium

The left atrium receives oxygenated blood from the lungs via the four pulmonary veins.

Pulmonary Veins: Four veins (two from each lung) that deliver oxygenated blood to the left atrium.
Pectinate Muscles: Present only in the auricle of the left atrium.
Mitral Valve (Bicuspid Valve, Left Atrioventricular Valve): Blood flows from the left atrium into the left ventricle through this valve, which has two cusps.

Left Ventricle

The left ventricle receives oxygenated blood from the left atrium and pumps it into the aorta, which distributes it throughout the systemic circulation.

Trabeculae Carneae: Similar to the right ventricle, the left ventricle also has irregular ridges of muscle on its inner walls.
Papillary Muscles and Chordae Tendineae: Similar to the right ventricle, these structures attach to the cusps of the mitral valve and prevent eversion during ventricular contraction.
Aortic Semilunar Valve: As the left ventricle contracts, it forces blood through this valve into the aorta. This valve also has three pocket-like cusps that prevent backflow of blood into the left ventricle during ventricular relaxation.
Aorta: The ascending aorta receives blood from the left ventricle.

Heart Wall: Layers of Protection and Power

The heart wall itself is composed of three distinct layers:

Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium. It is a thin, serous membrane containing blood capillaries, lymph capillaries, and nerve fibers.
Myocardium: The middle and thickest layer, composed primarily of cardiac muscle tissue. This layer is responsible for the heart's contractile force. The arrangement of cardiac muscle fibers is complex and allows for efficient and powerful pumping action.
Endocardium: The innermost layer, a thin, glistening sheet of endothelium that lines the heart chambers and covers the heart valves. It is continuous with the endothelium of the blood vessels entering and leaving the heart.

Heart Valves: Guardians of Unidirectional Flow

The heart valves are crucial for ensuring that blood flows in the correct direction through the heart. There are two types of valves: atrioventricular (AV) valves and semilunar (SL) valves.

Atrioventricular (AV) Valves: These valves are located between the atria and ventricles. The tricuspid valve is on the right side of the heart, and the mitral valve is on the left side. During ventricular diastole (relaxation), the AV valves are open, allowing blood to flow from the atria into the ventricles. During ventricular systole (contraction), the AV valves close to prevent backflow of blood into the atria. The chordae tendineae and papillary muscles play a critical role in preventing valve eversion.
Semilunar (SL) Valves: These valves are located at the exit of each ventricle. The pulmonary semilunar valve is located between the right ventricle and the pulmonary trunk, and the aortic semilunar valve is located between the left ventricle and the aorta. During ventricular systole, the SL valves are forced open as blood is ejected from the ventricles. During ventricular diastole, the SL valves close to prevent backflow of blood into the ventricles.

Coronary Circulation: Nourishing the Heart

The heart, like any other organ, requires its own blood supply to function properly. This is provided by the coronary arteries, which arise from the base of the aorta.

Right Coronary Artery (RCA): Supplies blood to the right atrium, right ventricle, and the posterior aspect of the heart. It typically branches into the right marginal artery and the posterior interventricular artery.
Left Coronary Artery (LCA): Supplies blood to the left atrium, left ventricle, and the anterior aspect of the heart. It branches into the left anterior descending artery (LAD) and the circumflex artery. The LAD supplies the anterior ventricular walls and the interventricular septum. The circumflex artery supplies the left atrium and the posterior-lateral aspect of the left ventricle.

The coronary veins collect deoxygenated blood from the myocardium and return it to the right atrium via the coronary sinus. The major coronary veins include the great cardiac vein, the middle cardiac vein, and the small cardiac vein.

Clinical Significance: When the Heart's Anatomy is Compromised

Understanding the gross anatomy of the heart is essential for diagnosing and treating various cardiovascular conditions. Several common clinical conditions are directly related to the heart's anatomical structures:

Myocardial Infarction (Heart Attack): Typically caused by a blockage in one or more of the coronary arteries, leading to ischemia (reduced blood flow) and damage to the myocardium. The location and extent of the infarction depend on which coronary artery is affected.
Valvular Heart Disease: Occurs when one or more of the heart valves are damaged or dysfunctional. This can result in stenosis (narrowing of the valve opening) or regurgitation (backflow of blood through the valve). Valvular heart disease can be caused by congenital defects, rheumatic fever, or other conditions.
Congestive Heart Failure (CHF): A condition in which the heart is unable to pump enough blood to meet the body's needs. CHF can be caused by a variety of factors, including myocardial infarction, valvular heart disease, and hypertension (high blood pressure).
Cardiomyopathy: A disease of the heart muscle that makes it difficult for the heart to pump blood. There are several types of cardiomyopathy, including dilated cardiomyopathy, hypertrophic cardiomyopathy, and restrictive cardiomyopathy.
Pericarditis: Inflammation of the pericardium. This can cause chest pain and other symptoms.
Cardiac Tamponade: Compression of the heart due to fluid accumulation in the pericardial cavity. This can impair the heart's ability to pump blood effectively.
Congenital Heart Defects: Structural abnormalities of the heart that are present at birth. These defects can involve the heart chambers, valves, or major blood vessels. Examples include atrial septal defect (ASD), ventricular septal defect (VSD), and tetralogy of Fallot.

Development of the Heart

Understanding the development of the heart provides valuable insight into the potential for congenital defects. The heart begins to develop very early in gestation, around the third week.

Cardiogenic Area Formation: Mesodermal cells migrate to the anterior part of the embryonic disc and aggregate to form the cardiogenic area.
Heart Tube Formation: The cardiogenic area develops into two endocardial tubes, which fuse to form a single heart tube.
Heart Looping: The heart tube undergoes a complex looping process, forming the characteristic shape of the heart.
Septation: Septa (walls) form to divide the heart tube into the four chambers (two atria and two ventricles). This process involves the growth and fusion of endocardial cushions.
Valve Development: The heart valves develop from the endocardial cushions.
Great Vessel Formation: The aorta and pulmonary trunk develop from the truncus arteriosus.

Disruptions in any of these developmental stages can lead to congenital heart defects.

Conclusion: A Symphony of Structure and Function

The gross anatomy of the heart is a testament to the intricate design of the human body. Its chambers, valves, vessels, and layers work in perfect harmony to ensure the continuous circulation of blood, delivering oxygen and nutrients to every cell in the body. A thorough understanding of the heart's anatomy is crucial for healthcare professionals and anyone seeking a deeper appreciation for the marvels of human physiology. From the protective pericardium to the powerful myocardium and the precisely engineered valves, each component plays a vital role in maintaining this life-sustaining pump.