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The human heart is an awe-inspiring organ at the core of our circulatory system. Understanding the detailed structure of the heart is fundamental to comprehending its vital functions and potential malfunctions that lead to cardiovascular diseases. This research article provides an in-depth examination of the heart's structure, so it has numerous pictures to illustrate the complex terms and describe different aspects of the human heart for improved learning.
Gross Anatomy of the Heart:
The human heart, a remarkable muscular organ, is located in the chest cavity (thoracic cavity), slightly left of the center. The sternum is in front of the heart, the spine is behind it, and the lungs are on either side. The heart's size varies among individuals, with the average adult heart roughly the size of a clenched fist weighing about 250-350 grams.
The pericardium is a protective membrane surrounding the heart and consists of two layers. The outer layer, called the fibrous pericardium, is powerful and dense. It maintains the heart in place while preventing overdistension. The inner layer, called the serous pericardium, consists of two layers: the parietal layer, which lines the fibrous pericardium, and the visceral layer, which is closely attached to the heart surface. The pericardial cavity lies between these layers and contains a small amount of fluid that reduces friction during heart contractions.
The heart wall comprises three distinct layers: the epicardium, myocardium, and endocardium. The epicardium is the outermost layer and serves as a protective covering, housing blood vessels and nerves. Beneath the epicardium lies the myocardium, the thick, muscular middle layer responsible for contracting and pumping blood. The myocardium contains cardiac muscle cells (cardiomyocytes) intricately arranged in a cross-linked pattern, ensuring efficient and synchronized contractions. Finally, the endocardium, the innermost layer, lines the heart chambers and valves, providing a smooth surface to facilitate blood flow while preventing clot formation.
The heart's unique shape, resembling an inverted cone, is tailored to function as a powerful pump. The left ventricle, responsible for systemic circulation, is larger and more muscular, as it must propel blood throughout the body. The right ventricle, in contrast, pumps blood to the lungs for oxygenation, requiring less force. This shape optimizes blood flow, allowing for the efficient distribution of oxygenated blood to tissues and deoxygenated blood to the lungs, thus sustaining life and maintaining overall cardiovascular health.
Chambers and Valves:
The heart's internal structure is a marvel of engineering, with four chambers working in harmony to pump blood throughout the body. Pulmonary circulation sends blood to the lungs for oxygenation, while systemic circulation pumps oxygen-rich blood to the body and returns deoxygenated blood to the heart. Red is traditionally used to show oxygenated blood, and blue is used to show blood that is oxygen deficient.
The two atria are at the top of the heart and receive deoxygenated blood from the body and oxygenated blood from the lungs. The right atrium receives deoxygenated blood from the body through the superior and inferior vena cava, while the left atrium receives oxygenated blood from the lungs via the pulmonary veins.
The ventricles are located below the atria in the heart and act as the powerhouse. Deoxygenated blood is pumped to the lungs via the pulmonary artery through the right ventricle. Lungs oxygenate the blood, and carbon dioxide is released. On the other hand, the left ventricle forcefully propels oxygen-rich blood throughout the body through the aorta, which supplies all organs and tissues with vital oxygen and nutrients.
The heart contains valves to ensure unidirectional blood flow. The atrioventricular valves, the mitral valve (bicuspid), and the tricuspid valve separate the atria from the ventricles. They open to allow blood flow from the atria into the ventricles during relaxation (diastole) and close tightly during contraction (systole) to prevent backflow.
The semilunar valves, also called the aortic and pulmonary valves, guard the exits of the two ventricles. When the ventricles contract, these valves open and let the blood flow into the aorta and pulmonary artery. Conversely, when the ventricles relax, the semilunar valves shut to stop the blood from returning to the ventricles.
Histology of the Heart:
The heart wall has three main layers: the outer epicardium, the middle myocardium, and the inner endocardium. The myocardium is the thickest layer consisting of cardiac muscle cells or cardiomyocytes. These highly specialized cells have striations similar to skeletal muscles but possess an automaticity that enables them to contract rhythmically without external stimulation.
Cardiomyocytes interconnect through intercalated discs, which are specialized structures. These discs contain gap junctions that allow for fast transmission of electrical impulses between cells. This intercellular connection ensures the heart muscle contracts in sync, which promotes efficient pumping action.
The connective tissue, specifically collagen and elastin, provides structural support to the heart and forms the fibrous skeleton. The fibrous skeleton surrounds the valves and serves as an anchor for the cardiomyocytes, contributing to the coordination and direction of their contractions during the cardiac cycle. Take a look at this article for more information about the structure and function of muscle cells: https://www.skieslimit.org/post/unveiling-the-intricacies-of-muscle-contraction-a-phenomenon-of-strength-and-control.
The Electrical Conduction System:
The heart's electrical conduction system regulates the rhythmic contractions of its chambers. The process begins with the sinoatrial node (SA node), frequently referred to as the heart's natural pacemaker. The SA node produces electrical impulses that spread throughout the atria, leading to their contraction and blood's movement into the ventricles.
Afterward, the electrical signals move toward the atrioventricular node (AV node), bridging the atria and ventricles. Once there, the electrical signals slow down briefly to allow the ventricles to fill up before they start contracting.
The electrical impulses from the AV node travel down the bundle of His, a specialized conduction pathway that branches into the left and right bundle branches. These branches then reach the Purkinje fibers that widely distribute the electrical impulses throughout the ventricles. As a result, the ventricles contract in a coordinated manner, ensuring efficient ejection of blood into the aorta and pulmonary artery.
Get Body Smart (n.d.). Heart Anatomy. Getbodysmart.com.https://www.getbodysmart.com/heart-anatomy/
Innerbody (n.d.). Heat-Cross-section View. Innerbody.com.https://www.innerbody.com/image/card02.html
[Elara Systems]. (2016, December 13). How the Heart Works Video: Cardiomyocyte [Video]. YouTube. https://www.youtube.com/watch?v=OAWZym8dWcw&ab_channel=ElaraSystems
Johns Hopkins Medicine (n.d.). Anatomy and Function of the Heart's Electrical System. Hopkinsmedicine.org. https://www.hopkinsmedicine.org/health/conditions-and-diseases/anatomy-and-function-of-the-hearts-electrical-system