Anatomy and Physiology of the Heart
Location and Orientation of the Heart
Mediastinum: The heart is located in the mediastinum, which is the space between the lungs.
Anterior Border: The sternum serves as the anterior protection for the heart.
Posterior Border: The thoracic spine and portions of the esophagus and trachea are located behind the heart.
CPR Context: Cardiopulmonary Resuscitation (CPR) is effective because the heart is positioned between the rigid structures of the sternum and the spine, allowing for compressions that "squish" the heart to move blood.
Attachment: The heart sits directly on the diaphragm and is physically attached to it, a detail that is clearly visible during anatomical dissections.
General Circulation Circuits
Pulmonary Circuit: This division carries blood to the lungs and back to the heart. It is primarily driven by the right side of the heart, specifically the right ventricle.
Systemic Circuit: This division carries blood to the rest of the body and back. It is driven by the left side of the heart (the left ventricle). This circuit includes the blood supply to the heart muscle itself (coronary circulation).
The Pericardium
Definition: The pericardium is a double-walled sac that surrounds and protects the heart.
Layers: * Visceral Pericardium: Also known as the epicardium, this is the deepest layer and is located directly on the surface of the heart muscle. The term "viscera" refers to the internal organs or "guts," signifying its deep placement. * Parietal Pericardium: This is the outer layer. It is often referred to as the fibrous pericardium because it is exceptionally tough, similar to a thinner version of the Dura mater.
Pericardial Cavity: The space between the visceral and parietal layers. It is filled with pericardial fluid.
Functions: * Friction Reduction: Internal friction is damaging to tissues. The fluid allows the heart to move constantly without wearing away its surface against surrounding tissues. * Filling Restriction: The tough pericardium limits how much the heart can expand. While removing it might allow an athlete to fill their heart with more blood initially, over-stretching the heart can lead to damage, ineffective blood expulsion, or an increased risk of blood clots.
Layers of the Heart Wall
Epicardium: The outermost layer, which is synonymous with the visceral pericardium.
Myocardium: The middle, muscular layer composed of cardiac muscle. * Metabolic Flexibility: The myocardium is highly adaptable regarding nutrient sources because it must work continuously without rest. It utilizes carbohydrates, fats, ketones, and even lactic acid.
Endocardium: The innermost layer that lines the heart chambers. It protects the heart muscle from the "torrential" and turbulent blood flow, preventing erosion of the internal tissues.
Heart Morphology and Visualization
Anterior View: Identifiable by a lack of large openings. The major blood vessels coming off the top are visible, but the primary entry points for major veins are not prominent.
Posterior View: Features multiple openings where blood vessels enter the heart chambers.
Transverse Section: In a cross-section of the thoracic cavity, the heart's relationship to the esophagus and the primary bronchi (the split from the trachea) is visible.
Dissection Note: In a living or recently deceased subject, the coronary vessels are often buried under a layer of superficial fat, which surgeons must move through to perform procedures like bypass surgery.
Coronary Circulation
Coronary Arteries: The heart receives the first supply of oxygenated blood leaving the left ventricle through two arteries branching off the base of the aorta. * Right Coronary Artery (RCA): Feeds the right side of the heart. Branches include the marginal artery (runs down the right margin) and the posterior interventricular artery (runs between the ventricles on the back side). * Left Coronary Artery (LCA): Feeds the left side of the heart. Branches include the circumflex artery (circles around the heart) and the anterior interventricular artery (runs down the front between the ventricles).
Cardiac Veins: These collect deoxygenated blood from the heart muscle. * Great Cardiac Vein: Runs with the anterior interventricular artery. * Middle Cardiac Vein: Runs with the posterior interventricular artery. * Small Cardiac Vein: Runs with the right marginal artery.
Coronary Sinus: A pouch or collection spot on the posterior side where the great, middle, and small cardiac veins fuse. It empties directly into the right atrium.
Clinical Definitions and Treatments
Heart Attack vs. Heart Failure: A heart attack is an acute blockage of blood flow leading to cell death. Heart failure is a weakened state where the heart cannot effectively pump blood to perfuse the body’s organs (frequently leading to kidney failure).
Ischemia: A term referring to low blood flow.
Angioplasty (Balloon Treatment): A procedure where a small cell is inflated inside an artery to crush plaque and reestablish flow. It is often temporary.
Coronary Stent: A permanent plastic or metal mesh tube used to keep an artery open. Some are medicated with anti-inflammatories to prevent scar tissue buildup.
The Pathway of Blood Flow
Deoxygenated blood enters the Right Atrium via the Superior Vena Cava, Inferior Vena Cava, and the Coronary Sinus.
Blood passes through the Tricuspid Valve (Right AV Valve) into the Right Ventricle.
Blood is pumped through the Pulmonary Valve (Pulmonary Semilunar Valve) into the Pulmonary Trunk.
The trunk splits into the Right and Left Pulmonary Arteries leading to the lungs.
Oxygenated blood returns via four Pulmonary Veins (two from each lung) into the Left Atrium.
Blood passes through the Mitral Valve (Bicuspid Valve / Left AV Valve) into the Left Ventricle.
Blood is pumped through the Aortic Valve (Aortic Semilunar Valve) into the Aorta for systemic distribution.
Valve Anatomy and Physiology
Purpose: Valves prevent the backflow of blood, which is essential for maintaining systemic oxygen levels.
Atrioventricular (AV) Valves: Located between atria and ventricles. They feature specialized structures: * Chordae Tendineae: Described as having a "Harry Potter swing" to their name, these are "heart strings" that tether the valve flaps. * Papillary Muscles: Muscles in the ventricle walls that anchor the chordae tendineae.
Mechanism: When ventricles contract, they generate significant pressure (). The papillary muscles contract to provide tension on the chordae tendineae, preventing the valve flaps from blowing backwards into the atria, a condition known as valve prolapse.
Semilunar Valves: Located at the base of the pulmonary trunk and aorta. These do not have chordae tendineae because they encounter less force () during heart relaxation (diastole) when blood simply falls back into the "cups" of the valve to close them.
Fetal Heart Anatomy and Bypasses
Rationale: In utero, the lungs are not used for gas exchange (the placenta performs this task); the fetus merely "exercises" the lungs by inhaling and exhaling amniotic fluid.
Foramen Ovale: A hole in the interatrial wall that allows blood to pass directly from the right atrium to the left atrium, bypassing the lungs. After birth, it closes to become the Fossa Ovalis.
Ductus Arteriosus: A short vessel connecting the pulmonary trunk to the aorta, allowing a second lung bypass. After birth, it spasms shut to become the Ligamentum Arteriosum.
Congenital Heart Defects
General Theme: Deoxygenated blood enters the systemic circulation, reducing overall oxygen levels.
Patent Ductus Arteriosus / Patent Foramen Ovale: "Patent" means open; these bypasses fail to close after birth.
Ventricular Septal Defect (VSD): A hole in the wall (septum) between the ventricles.
Tetralogy of Fallot: A complex condition involving four defects: 1. Pulmonary Valve Stenosis: Narrowing of the valve, making it hard to push blood to the lungs. 2. Right Ventricular Hypertrophy: The right side thickens and loses volume because it is working too hard to overcome the stenosis. 3. Ventricular Septal Defect (VSD): A hole between ventricles. 4. Shifted Aorta: The aorta sits over the septal defect instead of the left ventricle, allowing deoxygenated blood from the right side to enter the aorta directly.
Heart Hypertrophy and Pathologies
Concentric Hypertrophy (Bad): Caused by chronic stress such as high blood pressure (hypertension). The heart muscle thickens rapidly without proportional remodeling. This reduces the internal volume of the chambers, leading to decreased blood output and a vicious cycle where the brain signals the heart to work even harder.
Eccentric Hypertrophy (Good): Known as "Athlete’s Heart." Caused by temporary exercise stress. The heart grows stronger and the chamber volume increases. Example: Lance Armstrong had a resting heart rate of due to his heart’s massive stroke volume.
Dilated Cardiomyopathy: The opposite of hypertrophy; the ventricle walls become thin and weak. This leads to a loss of cross-bridges between actin and myosin, resulting in low contractile force and heart failure.
Heart Fibrosis: The replacement of healthy cardiac tissue with stiff, fibrous connective tissue, often due to chronic inflammation (metabolic syndrome). The heart becomes stiff and fails to relax fully, preventing adequate filling.
Heart Murmurs: Caused by stiffened, fibrotic valve flaps that do not seal perfectly, leading to the regurgitation of blood.
Questions & Discussion
Question: Why do we have bypasses for the lungs in a fetus if we still send some blood there?
Response: The lungs are still living tissue. Even if they aren't exchanging air, they need nutrients and oxygen to survive and grow; therefore, you divert some—but not all—blood away from them.
Question: Can heart fibrosis be reversed?
Response: It was long thought to be permanent, but some studies suggested that chronically active individuals show less fibrosis and that exercise might even slightly reduce existing fibrosis, though more validation is needed.