🫀Anatomy and Physiology II Unit 1 – Heart Anatomy and Function

Heart Structure and Location

• Located in the mediastinum, the central compartment of the thoracic cavity between the lungs
• Roughly the size of a closed fist, weighing between 250-350 grams in adults
• Situated behind the sternum and costal cartilages, two-thirds to the left of the midline
• Rests on the diaphragm, the muscular partition separating the thoracic and abdominal cavities
• Enclosed within a fibrous sac called the pericardium, which anchors it to the surrounding structures
• Consists of four chambers: right atrium, right ventricle, left atrium, and left ventricle
• Atria are smaller and have thinner walls compared to the ventricles
• Left ventricle has the thickest walls as it pumps blood to the entire body (systemic circulation)

Layers of the Heart Wall

• Composed of three distinct layers: epicardium, myocardium, and endocardium
• Epicardium is the outermost layer, consisting of mesothelium and connective tissue
  • Protects and lubricates the heart's surface, allowing smooth movement within the pericardial sac
• Myocardium is the middle layer, made up of cardiac muscle tissue
  • Responsible for the heart's contractile function, pumping blood through the chambers
  • Thickest in the left ventricle, followed by the right ventricle, and thinnest in the atria
• Endocardium is the innermost layer, a thin endothelial lining
  • Provides a smooth, non-thrombogenic surface for blood flow
  • Continuous with the endothelial lining of blood vessels entering and leaving the heart
• Coronary blood vessels and nerves are located between the epicardium and myocardium

Chambers and Valves

• Four chambers: right atrium (RA), right ventricle (RV), left atrium (LA), and left ventricle (LV)
• Atria receive blood from the veins and pump it into the ventricles
  • RA receives deoxygenated blood from the superior and inferior vena cava
  • LA receives oxygenated blood from the pulmonary veins
• Ventricles pump blood out of the heart to the lungs (RV) or the body (LV)
• Four valves ensure unidirectional blood flow through the heart
  • Atrioventricular (AV) valves: tricuspid valve (RA to RV) and mitral valve (LA to LV)
  • Semilunar valves: pulmonary valve (RV to pulmonary artery) and aortic valve (LV to aorta)
• AV valves are attached to papillary muscles in the ventricles by chordae tendineae
  • Prevent valve leaflets from inverting into the atria during ventricular contraction
• Semilunar valves have cup-shaped cusps that fill with blood during ventricular relaxation, preventing backflow

Blood Flow Through the Heart

• Deoxygenated blood enters the RA via the superior and inferior vena cava
• Blood flows from the RA to the RV through the tricuspid valve
• RV pumps blood through the pulmonary valve into the pulmonary artery and lungs for oxygenation
• Oxygenated blood returns to the LA via the pulmonary veins
• Blood flows from the LA to the LV through the mitral valve
• LV pumps oxygenated blood through the aortic valve into the aorta and systemic circulation
• Coronary arteries branch off the aorta to supply the heart muscle with oxygenated blood
• Deoxygenated blood from the heart muscle drains into the coronary sinus, which empties into the RA

Cardiac Conduction System

• Specialized cardiac muscle cells and fibers that initiate and coordinate heart contractions
• Consists of the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers
• SA node, the heart's natural pacemaker, spontaneously generates electrical impulses
  • Located in the upper wall of the RA near the opening of the superior vena cava
• Impulses from the SA node spread through the atria, causing atrial contraction
• AV node, located in the interatrial septum, delays the impulse before it enters the ventricles
  • Allows time for atrial contraction to complete before ventricular contraction begins
• Bundle of His, a specialized conduction pathway, transmits the impulse from the AV node to the ventricles
  • Divides into the left and right bundle branches, which run along the interventricular septum
• Purkinje fibers, a network of conduction fibers, rapidly distribute the impulse throughout the ventricles
  • Ensures coordinated and efficient ventricular contraction from apex to base

Heart Sounds and Cardiac Cycle

• Two main heart sounds: S1 (lub) and S2 (dub), produced by the closing of heart valves
• S1 occurs at the beginning of ventricular systole, caused by the closure of AV valves
• S2 occurs at the beginning of ventricular diastole, caused by the closure of semilunar valves
• Cardiac cycle refers to the sequence of events in one complete heartbeat
  • Consists of systole (contraction) and diastole (relaxation) of the atria and ventricles
• Atrial systole: atria contract, pumping blood into the ventricles (AV valves open, semilunar valves closed)
• Ventricular systole: ventricles contract, pumping blood into the pulmonary artery and aorta (AV valves closed, semilunar valves open)
• Atrial and ventricular diastole: all chambers relax and fill with blood (AV valves open, semilunar valves closed)
• Pressure changes in the chambers and blood vessels during the cardiac cycle
  • Ventricular pressure rises above atrial pressure during systole, closing AV valves
  • Ventricular pressure rises above arterial pressure, opening semilunar valves and ejecting blood

Major Blood Vessels

• Aorta: largest artery in the body, carries oxygenated blood from the LV to the systemic circulation
  • Ascending aorta, aortic arch, and descending aorta (thoracic and abdominal portions)
  • Branches supply head, neck, arms (arch), thoracic organs, and abdominal organs
• Pulmonary artery: carries deoxygenated blood from the RV to the lungs
  • Divides into left and right pulmonary arteries, one for each lung
• Pulmonary veins: four veins (two from each lung) that carry oxygenated blood from the lungs to the LA
• Superior vena cava: formed by the union of the left and right brachiocephalic veins
  • Drains deoxygenated blood from the upper body (head, neck, arms, and upper thorax) into the RA
• Inferior vena cava: largest vein in the body, drains deoxygenated blood from the lower body into the RA
• Coronary arteries: branch off the ascending aorta to supply the heart muscle with oxygenated blood
  • Left and right coronary arteries, each with several branches (e.g., left anterior descending, circumflex)
• Coronary sinus: a large vein that drains deoxygenated blood from the heart muscle into the RA

Regulation of Heart Function

• Intrinsic regulation: the heart's ability to adjust its contractility and heart rate without external input
  • Starling's law of the heart: increased venous return stretches the heart muscle, leading to increased contractility
  • Length-tension relationship: optimal overlap of actin and myosin filaments at a certain sarcomere length
• Autonomic nervous system (ANS) modulates heart rate, contractility, and conduction velocity
  • Sympathetic nervous system (SNS) stimulation: increases heart rate, contractility, and conduction velocity
    • Mediated by beta-1 adrenergic receptors in the heart, activated by norepinephrine and epinephrine
  • Parasympathetic nervous system (PNS) stimulation: decreases heart rate and conduction velocity
    • Mediated by muscarinic receptors (M2) in the heart, activated by acetylcholine released from the vagus nerve
• Hormonal regulation: endocrine factors that influence heart function
  • Thyroid hormones (T3 and T4) increase heart rate, contractility, and cardiac output
  • Atrial natriuretic peptide (ANP), released by atrial cells in response to stretching, promotes vasodilation and natriuresis
• Baroreceptor reflex: helps maintain blood pressure homeostasis by modulating heart rate and vascular tone
  • Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure
  • Increased blood pressure stimulates baroreceptors, leading to decreased SNS and increased PNS activity (lowering heart rate and contractility)
• Chemoreceptor reflex: detects changes in blood oxygen, carbon dioxide, and pH levels
  • Peripheral chemoreceptors (carotid and aortic bodies) and central chemoreceptors (medulla oblongata)
  • Hypoxia, hypercapnia, or acidosis stimulate chemoreceptors, leading to increased SNS activity and respiratory rate