💀Anatomy and Physiology I Unit 19 – The Cardiovascular System – The Heart

Key Structures and Anatomy

• Heart located in the mediastinum, the central compartment of the thoracic cavity between the lungs
• Roughly the size of a closed fist and weighs between 200 to 425 grams (7 to 15 ounces)
• Enclosed in a double-layered sac called the pericardium
  • Superficial fibrous pericardium is a tough, inelastic layer
  • Deep serous pericardium consists of the parietal pericardium and visceral pericardium (epicardium)
• Pericardial cavity between the parietal and visceral layers contains pericardial fluid that lubricates the heart during contractions
• Apex of the heart points inferiorly and to the left, while the base is the posterior aspect and attaches to the great vessels
• Coronary sulcus is a deep groove that runs around the heart and marks the division between the atria and ventricles
• Anterior interventricular sulcus and posterior interventricular sulcus mark the division between the left and right ventricles

Heart Layers and Chambers

• Heart wall composed of three layers: epicardium (external), myocardium (middle), and endocardium (inner)
  • Epicardium is the visceral layer of the serous pericardium
  • Myocardium is the thickest layer and consists of cardiac muscle tissue
  • Endocardium is a thin layer of endothelium and connective tissue that lines the chambers and covers the heart valves
• Four chambers: right atrium, left atrium, right ventricle, and left ventricle
• Atria are the receiving chambers for blood returning to the heart
  • Right atrium receives deoxygenated blood from the systemic circulation via the superior and inferior venae cavae
  • Left atrium receives oxygenated blood from the lungs via the pulmonary veins
• Ventricles are the discharging chambers that pump blood out of the heart
  • Right ventricle pumps deoxygenated blood to the lungs through the pulmonary trunk
  • Left ventricle pumps oxygenated blood to the systemic circulation through the aorta
• Interatrial septum separates the right and left atria, while the interventricular septum separates the right and left ventricles

Blood Flow Through the Heart

• Deoxygenated blood from the body enters the right atrium via the superior and inferior venae cavae
• Blood flows from the right atrium into the right ventricle through the tricuspid valve
• Right ventricle pumps blood through the pulmonary valve into the pulmonary trunk, which divides into the right and left pulmonary arteries to the lungs
• Oxygenated blood returns from the lungs via the pulmonary veins into the left atrium
• Blood flows from the left atrium into the left ventricle through the mitral (bicuspid) valve
• Left ventricle pumps oxygenated blood through the aortic valve into the aorta, which distributes blood to the systemic circulation
• One-way flow through the heart is maintained by the atrioventricular valves (tricuspid and mitral) and semilunar valves (pulmonary and aortic)
  • Atrioventricular valves prevent backflow from the ventricles into the atria during ventricular contraction (systole)
  • Semilunar valves prevent backflow from the pulmonary trunk and aorta into the ventricles during ventricular relaxation (diastole)

Cardiac Conduction System

• Specialized cardiac muscle cells initiate and conduct electrical impulses that stimulate the heart to contract
• Sinoatrial (SA) node, the primary pacemaker, located in the right atrium near the opening of the superior vena cava
  • SA node spontaneously generates electrical impulses at a rate of 60-100 times per minute
• Atrioventricular (AV) node located in the interatrial septum near the tricuspid valve
  • AV node receives impulses from the SA node and delays the signal before sending it to the ventricles
• Bundle of His emerges from the AV node and divides into the right and left bundle branches, which conduct impulses to the respective ventricles
• Purkinje fibers are specialized conducting fibers that rapidly distribute the impulses throughout the ventricular myocardium, causing ventricular contraction
• Conduction system ensures coordinated contraction of the atria followed by the ventricles for efficient pumping of blood

Cardiac Cycle and Heart Sounds

• Cardiac cycle refers to the sequence of events that occur during one complete heartbeat
• Consists of systole (contraction) and diastole (relaxation) of the atria and ventricles
• Atrial systole: atria contract, pumping blood into the ventricles
• Ventricular systole: ventricles contract, pumping blood into the pulmonary trunk and aorta
  • Atrioventricular valves close at the beginning of ventricular systole, producing the first heart sound (S1 or "lub")
  • Semilunar valves close at the end of ventricular systole, producing the second heart sound (S2 or "dub")
• Atrial diastole: atria relax and fill with blood from the venae cavae and pulmonary veins
• Ventricular diastole: ventricles relax and fill with blood from the atria
• Lub-dub sounds are produced by the closing of the heart valves and can be heard using a stethoscope
• Abnormal heart sounds, such as murmurs or extra sounds, may indicate underlying cardiac disorders

Major Blood Vessels

• Aorta is the largest artery in the body and arises from the left ventricle
  • Ascending aorta gives rise to the coronary arteries that supply the heart muscle
  • Aortic arch gives rise to the brachiocephalic trunk, left common carotid artery, and left subclavian artery
  • Descending aorta is divided into the thoracic aorta and abdominal aorta
• Pulmonary trunk arises from the right ventricle and divides into the right and left pulmonary arteries, which carry deoxygenated blood to the lungs
• Superior vena cava returns deoxygenated blood from the upper body to the right atrium
• Inferior vena cava returns deoxygenated blood from the lower body to the right atrium
• Pulmonary veins (four in total) return oxygenated blood from the lungs to the left atrium
• Coronary arteries (right and left) supply oxygenated blood to the heart muscle
  • Right coronary artery supplies the right atrium, right ventricle, and posterior portions of the left ventricle
  • Left coronary artery divides into the left anterior descending and circumflex arteries, supplying the left atrium and left ventricle

Regulation of Heart Function

• Autonomic nervous system plays a crucial role in regulating heart rate and contractility
  • Sympathetic nervous system increases heart rate and contractility through the release of norepinephrine and epinephrine
  • Parasympathetic nervous system decreases heart rate through the release of acetylcholine via the vagus nerve
• Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure and send signals to the cardiovascular center in the medulla oblongata
  • Cardiovascular center adjusts heart rate and vascular tone to maintain stable blood pressure
• Chemoreceptors in the aortic and carotid bodies detect changes in blood oxygen, carbon dioxide, and pH levels
  • Chemoreceptor stimulation can lead to adjustments in heart rate and respiratory rate to maintain homeostasis
• Hormones, such as epinephrine, norepinephrine, and thyroid hormones, can also influence heart rate and contractility
• Cardiac output ($CO$) is the volume of blood pumped by the heart per minute and is calculated as: $CO = HR × SV$
  • $HR$ is heart rate (beats per minute)
  • $SV$ is stroke volume (volume of blood ejected per beat)
• Preload, afterload, and contractility are factors that influence stroke volume and cardiac output

Clinical Applications and Disorders

• Atherosclerosis is the buildup of plaque in the coronary arteries, leading to reduced blood flow to the heart muscle
  • Can cause angina (chest pain) or myocardial infarction (heart attack) if blood flow is severely compromised
• Coronary artery bypass grafting (CABG) is a surgical procedure that redirects blood flow around blocked coronary arteries using grafts from other blood vessels
• Valvular disorders, such as stenosis (narrowing) or regurgitation (leakage), can impair the heart's ability to pump blood efficiently
  • Mitral valve prolapse is a common condition where the mitral valve leaflets bulge into the left atrium during ventricular contraction
• Arrhythmias are abnormalities in the heart's rhythm, which can be caused by disorders of the conduction system or other factors
  • Atrial fibrillation is a common arrhythmia characterized by rapid, irregular contractions of the atria
  • Ventricular tachycardia and ventricular fibrillation are life-threatening arrhythmias that require immediate medical attention
• Pacemakers are implantable devices that generate electrical impulses to regulate heart rate in patients with certain arrhythmias or conduction disorders
• Congestive heart failure occurs when the heart is unable to pump sufficient blood to meet the body's metabolic needs
  • Can be caused by various factors, including coronary artery disease, hypertension, and valvular disorders
• Echocardiography is a non-invasive imaging technique that uses ultrasound to assess heart structure and function
  • Helps diagnose valvular disorders, congenital heart defects, and other cardiac abnormalities