## Structure of Walls of Heart This section details the anatomical structure of the human heart, focusing on its chambers, wall layers, valves, and the relationship between structure and function. ### Heart Chambers The human heart is a four-chambered organ responsible for pumping blood. - **Atria (singular: atrium):** The two upper chambers that act as receiving rooms for blood entering the heart. - **Right Atrium:** Receives deoxygenated blood from the body via the *superior vena cava*, *inferior vena cava*, and *coronary sinus*. - **Left Atrium:** Receives oxygenated blood from the lungs via the four *pulmonary veins*. - **Ventricles:** The two lower, more muscular chambers that pump blood out of the heart. - **Right Ventricle:** Pumps deoxygenated blood to the lungs via the *pulmonary trunk*. - **Left Ventricle:** Pumps oxygenated blood to the rest of the body via the *aorta*. - **Septa:** Muscular walls that separate the chambers to prevent the mixing of oxygenated and deoxygenated blood. - ***Interatrial Septum:*** Separates the right and left atria. - ***Interventricular Septum:*** Separates the right and left ventricles. ### Layers of the Heart Wall The wall of the heart is composed of three distinct tissue layers: - ***Epicardium:*** The thin, outermost serous membrane that forms the smooth outer surface of the heart. It is also known as the visceral layer of the serous pericardium and contains the coronary blood vessels that supply the heart muscle. - ***Myocardium:*** The thick, middle layer composed of *cardiac muscle cells*. This layer is responsible for the heart's powerful contractions. - ***Endocardium:*** The smooth, innermost layer lining the heart chambers. It consists of simple squamous epithelium over connective tissue. The heart valves are formed from folds of the endocardium. It is continuous with the endothelial lining of the large blood vessels. ### Variation in Wall Thickness The thickness of the myocardium varies between chambers, directly reflecting its workload. | Chamber | Wall Thickness | Reason for Thickness | | :--- | :--- | :--- | | **Atria** | Thin | Only need to pump blood a short distance into the ventricles. | | **Right Ventricle** | Thicker than atria | Pumps blood only to the lungs (*pulmonary circuit*), which are close to the heart. | | **Left Ventricle** | Thickest (3x thicker than right) | Must generate enough force to pump blood throughout the entire body (*systemic circuit*). | ### Heart Valves and Supporting Structures Valves ensure unidirectional blood flow by preventing backflow. - **Atrioventricular (AV) Valves:** Located between the atria and ventricles. - ***Tricuspid Valve:*** Located between the right atrium and right ventricle; has *three cusps* (flaps). - ***Bicuspid (Mitral) Valve:*** Located between the left atrium and left ventricle; has *two cusps*. - **Semilunar (SL) Valves:** Located at the exit of the ventricles. - ***Pulmonary Semilunar Valve:*** Between the right ventricle and the pulmonary trunk. - ***Aortic Semilunar Valve:*** Between the left ventricle and the aorta. - **Supporting Structures for AV Valves:** - ***Papillary Muscles:*** Cone-shaped muscular pillars within the ventricles. - ***Chordae Tendineae:*** Strong, thin connective tissue strings that attach the papillary muscles to the cusps of the AV valves. - *Function:* When the ventricles contract, the papillary muscles also contract, pulling on the chordae tendineae to prevent the AV valves from being forced back into the atria. ### Diagrams of the Heart ### Biological Significance The specialized structure of the heart's chambers, walls, and valves allows for the highly efficient, coordinated, and unidirectional pumping of blood required to maintain separate pulmonary and systemic circulatory systems, vital for oxygen delivery and waste removal in the body.