Exploring the Human Heart

Exploring the Anatomy and Function of the Human Heart

The heart is a muscular organ that is responsible for pumping blood throughout the body. It is located in the chest, slightly to the left of the midline, and is enclosed by a sac called the pericardium. The heart is divided into four chambers, each with a specific function. The chambers are separated by valves, which prevent the backflow of blood and ensure that blood flows in one direction.

The Anatomy of the Human Heart

Location of the Heart

The human heart is roughly the size of a closed fist and weighs between 250 and 350 grams. 

The heart is located in the chest, specifically in the mediastinum, which is the area between the lungs. It is slightly tilted to the left side of the body and is protected by the ribcage. The heart sits on the diaphragm, which is a sheet of muscle that separates the chest cavity from the abdominal cavity. The apex of the heart, which is the pointed end, is directed towards the left hip, while the base, which is the broadest part, is directed towards the right shoulder. The heart's location and orientation are critical for its function as it allows it to receive and pump blood efficiently throughout the body.

Pericardium

The pericardium is a double-layered sac that surrounds and protects the heart. It is composed of two layers, the fibrous pericardium, and the serous pericardium.

The fibrous pericardium is the outermost layer of the pericardium. It is a tough, inelastic, and fibrous sac that surrounds and stabilizes the heart within the chest cavity. The fibrous pericardium also attaches the heart to surrounding structures, such as the sternum, diaphragm, and great blood vessels. This attachment helps to prevent excessive movement of the heart during contraction and relaxation, thereby maintaining its position within the chest cavity.

The serous pericardium is the inner layer of the pericardium and is divided into two layers, the parietal and visceral layers. The parietal layer lines the fibrous pericardium, while the visceral layer, also known as the epicardium, covers the surface of the heart. The space between the parietal and visceral layers is filled with a small amount of fluid known as pericardial fluid, which acts as a lubricant, allowing the heart to beat smoothly without friction.

The pericardium plays a crucial role in protecting the heart from injury and trauma. It acts as a shock absorber, absorbing any external pressure or force that could damage the heart. Additionally, the pericardium acts as a barrier against infection and inflammation that could damage the heart.

Furthermore, the pericardium helps to regulate the pressure and volume within the heart. During times of increased pressure or volume, such as during exercise, the pericardium helps to limit the expansion of the heart, preventing it from overstretching or becoming damaged.

Layers of the Heart Wall

It is composed of three layers: the epicardium, the myocardium, and the endocardium.

The epicardium is the outermost layer of the heart and is composed of a thin layer of connective tissue covered by a layer of epithelial cells. The myocardium is the middle layer and is composed of cardiac muscle cells that are responsible for the heart's pumping action. The endocardium is the innermost layer and lines the inside of the heart chambers and the heart valves.

The Four Chambers of the Heart

The heart is divided into four chambers: the right atrium, right ventricle, left atrium and left ventricle.

How Blood Flows from The Right Atrium to the Left Ventricle?

The right atrium receives deoxygenated blood from the body through two large veins, the superior vena cava and the inferior vena cava. The right atrium then contracts, pushing the blood through the tricuspid valve and into the right ventricle.

The right ventricle pumps the deoxygenated blood to the lungs through the pulmonary artery, where it picks up oxygen and releases carbon dioxide. The pulmonary valve separates the right ventricle from the pulmonary artery and prevents the backflow of blood into the ventricle.

The left atrium receives oxygenated blood from the lungs through the pulmonary veins. The left atrium then contracts, pushing the blood through the mitral valve and into the left ventricle.

The left ventricle pumps oxygenated blood to the rest of the body through the aorta. The aortic valve separates the left ventricle from the aorta and prevents the backflow of blood into the ventricle.

Heart Chambers: Myocardial Thickness and Function

The myocardium is the thick muscular layer of the heart that makes up the bulk of the heart's wall. The thickness of the myocardium varies among the heart chambers and plays a critical role in their function.

The myocardium of the left ventricle is the thickest of all the heart chambers. This is because the left ventricle pumps oxygenated blood to the rest of the body, which requires a lot of force. The thick myocardium of the left ventricle enables it to generate the necessary force to pump blood to the rest of the body. Additionally, the thick myocardium of the left ventricle helps to prevent the heart from overstretching or becoming damaged due to the high pressure required for pumping blood to the systemic circulation.

The right ventricle also has a thick myocardium, although not as thick as that of the left ventricle. The right ventricle pumps deoxygenated blood to the lungs for oxygenation, which requires less force than pumping blood to the systemic circulation. However, the thick myocardium of the right ventricle helps to ensure that blood is effectively pumped to the lungs and prevents the heart from overstretching or becoming damaged.

In contrast, the myocardium of the atria is relatively thin compared to that of the ventricles. The atria are responsible for receiving blood from the body and lungs and pumping it into the ventricles. The thin myocardium of the atria allows them to efficiently pump blood into the ventricles without generating too much force, which could damage the delicate blood vessels in the lungs and body.

Fibrous Skeleton of the Heart

The fibrous skeleton of the heart is a network of dense connective tissue that forms a structural framework within the heart. It consists of four dense fibrous rings that encircle the atrioventricular and semilunar valves and the base of the aorta and pulmonary trunk. These rings serve as attachment sites for the heart's valve leaflets and the myocardium, the heart's muscle tissue.

The fibrous skeleton of the heart plays several critical roles in maintaining the heart's function. First, it provides structural support to the heart and helps to maintain the shape and size of the heart's chambers. The fibrous rings also provide a barrier between the atria and ventricles, preventing electrical impulses from spreading inappropriately between the chambers and ensuring that the heart beats in a coordinated and synchronized manner.

Another important function of the fibrous skeleton is to provide a stable base for the heart's valves. The fibrous rings anchor the valve leaflets in place and prevent them from prolapsing or inverting during the cardiac cycle. This is crucial for maintaining the heart's ability to pump blood effectively and prevent the backward flow of blood.

The Valves of the Heart

The heart valves ensure that blood flows in one direction through the heart. 

There are four valves in the heart: the tricuspid valve, the pulmonary valve, the mitral valve, and the aortic valve.

The tricuspid valve is located between the right atrium and right ventricle and has three leaflets that open and close to allow blood to flow from the atrium to the ventricle.

The pulmonary valve is located between the right ventricle and the pulmonary artery. It has three leaflets that open and close to allow blood to flow from the ventricle to the artery.

The mitral valve, also known as the bicuspid valve, is located between the left atrium and left ventricle and has two leaflets that open and close to allow blood to flow from the atrium to the ventricle.

The aortic valve is located between the left ventricle and the aorta and has three leaflets that open and close to allow blood to flow from the ventricle to the aorta.

Read more: Heart Valves and Murmurs

Major Vessels of the Heart

The heart has two major veins (the superior and inferior vena cava and the pulmonary veins) and two major arteries (the pulmonary artery, and the aorta) that connect to the heart.

Superior and Inferior Vena Cava

The superior and inferior vena cava are the two major veins that return deoxygenated blood from the body to the right atrium of the heart. The superior vena cava carries deoxygenated blood from the upper part of the body, including the head, neck, and arms, while the inferior vena cava carries deoxygenated blood from the lower part of the body, including the legs and abdomen.

The superior vena cava is located above the heart and connects to the right atrium at the upper back of the heart. The inferior vena cava is located below the heart and enters the right atrium at the lower back of the heart.

Pulmonary Artery

The pulmonary artery is the artery that carries deoxygenated blood from the right ventricle of the heart to the lungs. It is the only artery in the body that carries deoxygenated blood, and it is unique in that it carries blood away from the heart and toward the lungs, while other arteries carry oxygenated blood away from the heart and toward the body's tissues.

Once the blood reaches the lungs, it is oxygenated and returns to the heart via the pulmonary veins. The pulmonary artery divides into two branches, the left and right pulmonary arteries, which then further divide into smaller arterioles and eventually into capillaries within the lungs. The capillaries allow for the exchange of carbon dioxide and oxygen, and the oxygenated blood is then carried back to the heart via the pulmonary veins.

Pulmonary Veins

Unlike other veins in the body, the pulmonary veins carry oxygenated blood from the lungs to the heart. There are four pulmonary veins, two from each lung, that connect to the left atrium of the heart. The pulmonary veins are unique in that they are the only veins in the body that carry oxygenated blood.

Aorta

The aorta is the largest artery in the body and carries oxygenated blood from the left ventricle of the heart to the rest of the body. It is shaped like an upside-down tree with numerous branches, which supply blood to every tissue and organ in the body. The aorta begins at the top of the heart and runs down the chest and abdomen before dividing into smaller arteries in the lower part of the body.

The aorta is divided into several sections based on its location in the body. These sections include the ascending aorta, the aortic arch, and the descending aorta. The aortic arch has three major branches that supply blood to the head, neck, and arms.

Blood Supply to the Heart Muscles

The heart muscle, or myocardium, requires a constant supply of oxygen and nutrients to function properly. Coronary circulation is the system of blood vessels that supply the heart muscle with oxygen and nutrients. Coronary circulation is made up of three types of blood vessels: arteries, capillaries, and veins.

Coronary Arteries: 

The coronary arteries are the blood vessels that originate from the aorta and supply blood to the heart muscle. The coronary arteries consist of the left and right coronary arteries, which branch into smaller arterioles and capillaries that supply the myocardium. 

The left coronary artery originates from the left aortic sinus and branches into two major arteries: the left anterior descending artery (LAD) and the left circumflex artery (LCX). The LAD supplies blood to the front and left side of the heart, including the left ventricle, the interventricular septum, and the apex of the heart. The LAD is sometimes referred to as the "widow maker" artery because a blockage in this artery can quickly lead to a heart attack. The LCX supplies blood to the back and left side of the heart, including the left atrium and the left ventricle.

The right coronary artery originates from the right aortic sinus and supplies blood to the right side of the heart, including the right atrium, the right ventricle, and the inferior portion of the left ventricle. The right coronary artery also gives rise to the posterior descending artery (PDA), which supplies blood to the back and bottom of the heart.

Capillary Network: 

The coronary arterioles branch into capillaries, which are the smallest blood vessels in the body. The capillaries form a dense network around each individual heart muscle cell or cardiomyocyte. This network of capillaries is responsible for delivering oxygen and nutrients to the cardiomyocytes and removing waste products produced by the cells. 

Coronary Veins: 

The coronary veins are the blood vessels that drain blood from the heart muscle and return it to the right atrium of the heart. The coronary veins originate from the capillary network and converge into larger vessels, including the great cardiac vein, the middle cardiac vein, and the small cardiac vein. 

These veins ultimately drain into the coronary sinus, a large vein that empties into the right atrium of the heart. The coronary veins also play an important role in maintaining coronary circulation by returning deoxygenated blood and waste products to the heart and lungs for removal.

How Does the Heart Pump Blood: The Conduction System

The human heart's ability to pump blood throughout the body is due to a complex network of specialized cells and fibers known as the conduction system. One vital aspect of the conduction system is the autorhythmic fibers, which are responsible for regulating the heart's rhythm and ensuring proper blood flow.

What are Autorhythmic Fibers?

Autorhythmic fibers, also known as pacemaker cells, are specialized cells located in the sinoatrial (SA) node, which is located in the upper part of the right atrium. These cells are responsible for initiating and coordinating the electrical impulses that cause the heart to contract and pump blood.

Unlike other cells in the heart, autorhythmic fibers have the ability to generate electrical impulses spontaneously, without any external stimulation. This spontaneous depolarization is what makes them the natural pacemakers of the heart.

Conduction Pathway

The electrical impulses generated by the autorhythmic fibers travel along a specialized pathway called the conduction system. The conduction system consists of several structures including the SA node, the atrioventricular (AV) node, the bundle of His, and the Purkinje fibers.

The SA node initiates the electrical impulse, which spreads through the atria and causes them to contract. The impulse then reaches the AV node, located between the atria and the ventricles, where it is delayed briefly to allow the atria to empty their blood into the ventricles before the ventricles contract.

After passing through the AV node, the impulse travels along the bundle of His, a bundle of specialized fibers that run through the septum of the heart, and then branches out into the Purkinje fibers, which spread throughout the ventricles. The electrical impulses cause the ventricles to contract and pump blood out of the heart and into the circulatory system.

Regulation of Heart Rate

The rate at which the autorhythmic fibers generate electrical impulses determines the heart rate. The SA node typically sets the pace of the heartbeat, generating about 60 to 100 impulses per minute. However, other factors such as hormones, neurotransmitters, and the autonomic nervous system can also influence the heart rate.

The sympathetic nervous system, which is activated during times of stress or physical activity, can increase the heart rate by releasing the hormone epinephrine. Conversely, the parasympathetic nervous system, which is activated during times of rest and relaxation, can decrease the heart rate by releasing the neurotransmitter acetylcholine.

Conclusion

The human heart is a remarkable organ that plays a vital role in the circulatory system. Its intricate structure and function allow for the efficient pumping of blood throughout the body, ensuring that every tissue and organ receives the oxygen and nutrients it needs to function properly.