6.1 Structure and function of the heart
4 min read•august 14, 2024
The heart, a muscular powerhouse, pumps blood through our bodies. It's divided into four chambers: two on top and two below. These work together to move blood through the pulmonary and systemic circulations.
The heart's structure is key to its function. Layers like the , , and protect and power the heart. ensure blood flows in the right direction, while the feed the heart itself.
Heart Anatomy and Function
Cardiac Chambers and Septa
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- The heart is a four-chambered muscular organ located in the thoracic cavity between the lungs, posterior to the sternum, and slightly left of the midline
- The right and left atria are the upper chambers of the heart that receive blood from the venous system (right atrium) and the lungs (left atrium)
- The right and left ventricles are the lower chambers of the heart that pump blood to the lungs (right ventricle) and the (left ventricle)
- The is a thick muscular wall that separates the left and right ventricles, preventing the mixing of oxygenated and deoxygenated blood
- The separates the left and right atria
Coronary Circulation and Great Vessels
- The coronary arteries, arising from the base of the , supply oxygenated blood to the myocardium (heart muscle)
- The great vessels include:
- : returns deoxygenated blood from the upper body to the right atrium
- : returns deoxygenated blood from the lower body to the right atrium
- : carry deoxygenated blood from the right ventricle to the lungs
- : carry oxygenated blood from the lungs to the left atrium
- Aorta: carries oxygenated blood from the left ventricle to the systemic circulation
Layers of the Heart Wall
Epicardium and Pericardium
- The epicardium is the outermost layer of the heart, consisting of a serous membrane that reduces friction during heart contractions and provides protection
- The pericardium is a fibrous sac that surrounds and protects the heart, anchoring it to the surrounding structures and preventing overfilling of the heart
- Parietal pericardium: outer layer of the pericardium, attached to the great vessels and diaphragm
- Visceral pericardium (epicardium): inner layer of the pericardium, directly attached to the heart surface
Myocardium and Endocardium
- The myocardium is the middle layer of the heart, composed of cardiac muscle tissue responsible for the heart's contractile function
- Cardiac muscle cells (cardiomyocytes) are striated, branched, and interconnected by intercalated discs
- Intercalated discs contain gap junctions that allow for rapid electrical impulse propagation and coordinated contraction
- The endocardium is the innermost layer of the heart, lining the chambers and valves, providing a smooth surface for blood flow and preventing blood clotting
- Endothelial cells of the endocardium secrete nitric oxide (NO) and endothelin to regulate vascular tone and cardiac function
- Purkinje fibers, specialized conduction cells, are found in the subendocardial layer and rapidly conduct electrical impulses
Function of Heart Valves
Atrioventricular (AV) Valves
- The atrioventricular (AV) valves, namely the tricuspid (right) and mitral (left) valves, are located between the atria and ventricles
- AV valves prevent backflow from the ventricles to the atria during ventricular contraction (systole)
- The has three cusps, while the has two cusps (bicuspid valve)
- The and anchor the AV valves to the ventricular walls, preventing the valves from inverting during ventricular contraction
Semilunar Valves
- The , namely the pulmonary (right) and aortic (left) valves, are located at the base of the pulmonary artery and aorta, respectively
- Semilunar valves prevent backflow from these vessels into the ventricles during ventricular relaxation (diastole)
- Each semilunar valve has three crescent-shaped cusps that open and close passively in response to pressure changes
- The cusps of the semilunar valves are thinner and more delicate compared to the AV valves, as they do not have chordae tendineae or papillary muscle attachments
Pulmonary vs Systemic Circulation
Pulmonary Circulation
- The is the portion of the cardiovascular system that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium
- The pulmonary circulation is a low-pressure, low-resistance system compared to the systemic circulation
- Pulmonary arteries have thinner walls and larger diameters relative to their systemic counterparts
- Pulmonary capillaries have a larger total cross-sectional area, allowing for efficient gas exchange
- The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs, while the pulmonary veins carry oxygenated blood from the lungs to the left atrium
Systemic Circulation
- The systemic circulation is the portion of the cardiovascular system that carries oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium
- The aorta, the largest artery in the body, carries oxygenated blood from the left ventricle to the systemic circulation
- Ascending aorta: initial portion of the aorta, gives rise to the coronary arteries
- Aortic arch: curved portion of the aorta, gives rise to the brachiocephalic, left common carotid, and left subclavian arteries
- Descending aorta: portion of the aorta that descends through the thorax (thoracic aorta) and abdomen (abdominal aorta)
- The superior and inferior vena cava are the major veins that return deoxygenated blood from the systemic circulation to the right atrium
- Superior vena cava: formed by the union of the left and right brachiocephalic veins, draining the head, neck, and upper extremities
- Inferior vena cava: formed by the union of the left and right common iliac veins, draining the lower extremities, pelvis, and abdomen
Key Terms to Review (39)
Angiogram: An angiogram is a medical imaging technique used to visualize the inside of blood vessels and organs, primarily to check for blockages or abnormalities in the circulatory system. This procedure involves injecting a contrast dye into the bloodstream, which makes the blood vessels visible on X-ray images. Understanding angiograms is crucial for assessing heart health and diagnosing various cardiovascular conditions, as they can reveal issues like coronary artery disease or aneurysms.
Aorta: The aorta is the largest artery in the human body, responsible for transporting oxygen-rich blood from the heart to the rest of the body. It emerges from the left ventricle of the heart and branches into smaller arteries, ensuring that vital organs and tissues receive an adequate supply of blood. This crucial role in circulation makes the aorta an essential component in maintaining overall cardiovascular health.
Aortic Valve: The aortic valve is a crucial component of the heart that controls blood flow from the left ventricle into the aorta, which distributes oxygen-rich blood to the rest of the body. It acts as a one-way gate that opens to allow blood to exit the heart during contraction and closes to prevent backflow when the heart relaxes. Its proper function is essential for maintaining efficient circulation and overall cardiovascular health.
Arrhythmia: Arrhythmia is a condition characterized by irregular heartbeats, which can manifest as a heart that beats too fast, too slow, or with an abnormal rhythm. This abnormality can affect the heart's ability to pump blood effectively, leading to potential health risks. Understanding arrhythmia involves exploring how the structure and function of the heart relate to the electrical signals that control the heartbeat, as well as how these signals influence the cardiac cycle and the associated heart sounds.
Atherosclerosis: Atherosclerosis is a condition characterized by the buildup of plaque within the arterial walls, leading to narrowed and hardened arteries that can restrict blood flow. This process significantly impacts heart function, blood vessel health, and overall circulatory routes, potentially resulting in serious cardiovascular events such as heart attacks or strokes.
Atria: Atria are the two upper chambers of the heart, responsible for receiving blood from the body and lungs. They play a crucial role in the overall function of the heart by acting as reservoirs for blood before it is pumped into the lower chambers, known as ventricles. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs, making them essential in maintaining efficient blood circulation.
Atrioventricular valves: Atrioventricular valves are specialized structures located between the atria and ventricles of the heart, ensuring one-way blood flow from the upper chambers to the lower chambers. These valves, which include the tricuspid and mitral valves, play a crucial role in maintaining efficient circulation by preventing backflow of blood during ventricular contraction, thus supporting the heart's overall function and rhythm.
Blood flow regulation: Blood flow regulation refers to the mechanisms that control the distribution and flow of blood throughout the body, ensuring that tissues receive adequate oxygen and nutrients while removing waste products. This process is vital for maintaining homeostasis, as it adjusts to varying physiological demands during activities like exercise or rest, and is intricately linked to the structure and function of the heart.
Blood pressure: Blood pressure is the force exerted by circulating blood against the walls of blood vessels, primarily arteries, during the cardiac cycle. It is a critical physiological parameter that reflects the health of the cardiovascular system and influences various bodily functions. Blood pressure is expressed in millimeters of mercury (mmHg) and is typically represented as two values: systolic pressure, the pressure during heartbeats, and diastolic pressure, the pressure between beats. Understanding blood pressure helps in assessing heart function, blood flow, and overall vascular health.
Cardiac cycle: The cardiac cycle is the series of events that occur in the heart during one complete heartbeat, including the contraction and relaxation of the heart muscles. This cycle consists of two main phases: systole, where the heart contracts to pump blood out, and diastole, where the heart relaxes and fills with blood. Understanding the cardiac cycle is essential for grasping how the heart functions to maintain blood circulation throughout the body.
Cardiac output: Cardiac output is the volume of blood the heart pumps per minute, reflecting the efficiency of the heart as a pump and the body’s overall ability to deliver oxygen and nutrients to tissues. It is a crucial measurement that depends on heart rate and stroke volume, which are influenced by various factors including heart structure, blood vessel dynamics, and the body's circulatory needs during different activities.
Chordae tendineae: Chordae tendineae are fibrous cords that connect the papillary muscles to the atrioventricular valves in the heart, specifically the mitral and tricuspid valves. These structures play a crucial role in maintaining proper valve function during the cardiac cycle by preventing the valves from inverting when the ventricles contract. They ensure that blood flows in the correct direction through the heart, supporting efficient circulation.
Coronary arteries: Coronary arteries are blood vessels that supply oxygen-rich blood to the heart muscle itself, ensuring its proper function and health. These arteries branch off from the aorta and encircle the heart, providing essential nutrients to the myocardium, which is the thick middle layer of the heart wall. Understanding their structure and function is crucial for recognizing how the heart maintains its vital pumping action and how blockages can lead to serious health issues such as heart attacks.
Echocardiogram: An echocardiogram is a diagnostic test that uses ultrasound waves to create images of the heart's structure and function. This non-invasive procedure provides valuable information about the size, shape, and motion of the heart, as well as how well the heart chambers and valves are working. By assessing these features, an echocardiogram plays a critical role in understanding cardiac health and detecting potential heart conditions.
Electrical conduction: Electrical conduction refers to the process by which electrical impulses are transmitted through specialized cells in the heart, allowing for coordinated contractions and efficient blood flow. This conduction system is crucial for maintaining the rhythmic beating of the heart, ensuring that the chambers contract in a synchronized manner, which is essential for effective circulation.
Endocardium: The endocardium is the innermost layer of tissue that lines the chambers of the heart and covers the heart valves. This thin, smooth layer plays a crucial role in providing a smooth surface for blood flow, minimizing turbulence and helping to maintain efficient circulation within the heart. Additionally, it is involved in regulating the heart's electrical signals and protecting the underlying heart muscle from damage.
Epicardium: The epicardium is the outermost layer of the heart wall, serving as a protective layer that encapsulates the heart. It is composed of connective tissue and serves not only as a barrier to protect the heart but also plays a role in providing support and facilitating movement during heart contractions. The epicardium is essential for maintaining the overall structure and function of the heart, interacting closely with other heart layers.
Heart failure: Heart failure is a medical condition where the heart is unable to pump blood effectively, leading to inadequate blood flow to meet the body's needs. This condition can arise from various causes, including damage to the heart muscle, valve problems, or high blood pressure. Understanding heart failure involves looking at the heart's structure and function, as well as how it interacts with the cardiac cycle and generates sounds that indicate changes in its activity.
Heart rate: Heart rate is the number of times the heart beats in one minute, measured in beats per minute (bpm). It is a critical indicator of cardiovascular health and reflects the overall functioning of the heart, which is responsible for pumping blood throughout the body. The heart rate can change based on various factors, such as physical activity, stress levels, and overall health status, providing insights into the body's physiological responses.
Inferior vena cava: The inferior vena cava is a large vein that carries deoxygenated blood from the lower half of the body back to the heart. It plays a crucial role in the circulatory system by collecting blood from the lower extremities, pelvis, and abdomen and transporting it to the right atrium of the heart, where it enters the pulmonary circulation for oxygenation.
Interatrial septum: The interatrial septum is a thin wall of tissue that separates the left and right atria of the heart. This structure plays a crucial role in maintaining the proper flow of blood through the heart by preventing mixing of oxygen-rich blood in the left atrium with oxygen-poor blood in the right atrium, ensuring efficient circulation throughout the body.
Interventricular septum: The interventricular septum is a thick, muscular wall that separates the left and right ventricles of the heart. This structure is crucial in maintaining the distinct pressures required for the proper functioning of the heart, allowing it to pump oxygenated blood to the body and deoxygenated blood to the lungs without mixing. Its integrity is essential for efficient cardiac function, as it prevents backflow and ensures that each side of the heart can work effectively during contraction.
Mitral valve: The mitral valve is a crucial heart valve located between the left atrium and the left ventricle, responsible for preventing backflow of blood during ventricular contraction. This valve plays a significant role in ensuring efficient blood flow from the lungs to the body, coordinating with other heart structures to maintain proper circulation and pressure within the heart chambers.
Myocardial infarction: Myocardial infarction, commonly known as a heart attack, occurs when blood flow to a part of the heart muscle is blocked for a long enough time that part of the heart muscle is damaged or dies. This condition is primarily caused by the buildup of fatty deposits in the coronary arteries, leading to their narrowing and potential blockage, which disrupts the heart's ability to pump blood effectively.
Myocardium: The myocardium is the muscular middle layer of the heart wall, responsible for the contraction and pumping action of the heart. This layer is made up of cardiac muscle tissue, which is unique because it can contract involuntarily and rhythmically, ensuring that blood is efficiently circulated throughout the body. The strength and efficiency of the myocardium are crucial for maintaining adequate blood flow and overall cardiovascular health.
Papillary Muscles: Papillary muscles are small, cone-shaped muscles located in the ventricles of the heart. They attach to the heart's interior via chordae tendineae, which connect to the atrioventricular (AV) valves, helping to keep the valves closed during ventricular contraction. These muscles play a crucial role in ensuring proper blood flow and preventing backflow into the atria.
Pulmonary arteries: Pulmonary arteries are blood vessels that carry deoxygenated blood from the heart to the lungs for oxygenation. Specifically, they branch off from the right ventricle of the heart and transport blood to the left and right lungs, where carbon dioxide is exchanged for oxygen. This process is crucial for maintaining the body's oxygen levels and overall cardiovascular function.
Pulmonary circulation: Pulmonary circulation is the part of the cardiovascular system responsible for carrying deoxygenated blood from the heart to the lungs and returning oxygenated blood back to the heart. This vital process allows for gas exchange, where carbon dioxide is removed from the blood and oxygen is replenished, ensuring that tissues throughout the body receive the oxygen they need to function effectively.
Pulmonary valve: The pulmonary valve is a semilunar valve located between the right ventricle of the heart and the pulmonary artery. Its main function is to prevent the backflow of blood into the right ventricle after it has been pumped into the pulmonary artery during ventricular contraction. This valve plays a crucial role in the heart's efficiency by ensuring that oxygen-depleted blood moves toward the lungs for oxygenation and then returns to the heart.
Pulmonary veins: Pulmonary veins are blood vessels that carry oxygenated blood from the lungs to the left atrium of the heart. These veins play a crucial role in the circulatory system by ensuring that oxygen-rich blood is delivered to the heart for distribution throughout the body. Each lung has its own set of pulmonary veins, typically four in total, two from each lung, which are essential for maintaining efficient gas exchange and overall cardiovascular health.
Pumping action: Pumping action refers to the mechanical function of the heart that facilitates the circulation of blood throughout the body. This process involves rhythmic contractions and relaxations of the heart muscle, specifically in the ventricles, which propel blood into the arteries and ensure that oxygen-rich blood reaches tissues while returning deoxygenated blood to the lungs for reoxygenation.
Semilunar valves: Semilunar valves are specialized structures located at the exit points of the heart's ventricles, specifically at the aorta and pulmonary artery. They play a crucial role in ensuring unidirectional blood flow, preventing backflow into the ventricles after contraction. These valves are composed of three crescent-shaped flaps, which open to allow blood to exit the heart and close tightly to prevent any regurgitation as the heart relaxes.
Stroke volume: Stroke volume is the amount of blood pumped by the left ventricle of the heart in one contraction. This measurement is crucial for understanding how effectively the heart is functioning, as it directly impacts cardiac output and overall circulatory health, linking to heart structure and function, the dynamics of blood flow through vessels, the phases of the cardiac cycle, and blood pressure regulation mechanisms.
Superior vena cava: The superior vena cava is a large vein that carries deoxygenated blood from the upper half of the body to the right atrium of the heart. This essential vessel plays a critical role in the circulatory system, as it collects blood from the head, neck, arms, and upper torso, ensuring that deoxygenated blood returns to the heart for reoxygenation in the lungs.
Systemic circulation: Systemic circulation is the part of the circulatory system responsible for carrying oxygenated blood away from the heart to the body and returning deoxygenated blood back to the heart. This process ensures that all body tissues receive the necessary oxygen and nutrients while removing carbon dioxide and waste products. The heart acts as a central pump, specifically the left ventricle, which pushes oxygen-rich blood into the aorta, distributing it throughout the body.
Tricuspid valve: The tricuspid valve is one of the four main valves of the heart, located between the right atrium and the right ventricle. It plays a crucial role in regulating blood flow, ensuring that blood moves in the correct direction from the atrium to the ventricle while preventing backflow during ventricular contraction. The structure of the tricuspid valve includes three flaps, or cusps, that open and close with each heartbeat, contributing to the efficient functioning of the heart's pumping mechanism.
Valves: Valves are specialized structures in the heart that regulate blood flow through the heart's chambers and into the major arteries. They ensure that blood flows in a one-way direction, preventing backflow and maintaining efficient circulation. Proper functioning of these valves is crucial for the overall efficiency of the heart and influences the cardiac cycle and heart sounds.
Ventricles: Ventricles are the two lower chambers of the heart responsible for pumping blood out of the heart. They play a critical role in the circulatory system by receiving blood from the atria and then contracting to send it to the lungs or the rest of the body, depending on whether they are the right or left ventricle. Their structure is designed to handle high pressures, allowing for efficient blood circulation.
William Harvey: William Harvey was a 17th-century English physician who is best known for his discovery of the circulation of blood in the human body. His work laid the foundation for modern physiology and changed how we understand the function and structure of the heart, as well as the mechanics behind the cardiac cycle and heart sounds.