Anatomy of the Heart

• The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system, providing oxygen and nutrients to tissues and removing carbon dioxide and other waste products.

Anatomy of the Heart

• The adult human heart is about the size of a fist and weighs approximately 250-350 grams.
• It is located in the thoracic cavity, between the lungs, in the mediastinum.
• The heart is enclosed in a double-walled sac called the pericardium.
• The pericardium consists of two layers: the fibrous pericardium (outer layer) and the serous pericardium (inner layer).
• The serous pericardium is further divided into the parietal layer (lining the fibrous pericardium) and the visceral layer (epicardium) covering the heart surface.
• The space between the parietal and visceral layers contains pericardial fluid, which reduces friction during heartbeats.
• The heart wall is composed of three layers: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).
• The epicardium is the same as the visceral layer of the serous pericardium.
• The myocardium is the thickest layer, consisting of cardiac muscle responsible for the heart's pumping action.
• The endocardium lines the inner chambers of the heart and is continuous with the endothelium of the blood vessels.
• The heart has four chambers: two atria (right and left) and two ventricles (right and left).
• The atria are the receiving chambers, collecting blood from the body (right atrium) and lungs (left atrium).
• The ventricles are the pumping chambers, responsible for propelling blood to the lungs (right ventricle) and the rest of the body (left ventricle).
• The right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus.
• The left atrium receives oxygenated blood from the pulmonary veins.
• The right ventricle pumps deoxygenated blood into the pulmonary artery, which leads to the lungs.
• The left ventricle pumps oxygenated blood into the aorta, which distributes it to the systemic circulation.
• Valves within the heart ensure unidirectional blood flow:
  • The atrioventricular (AV) valves (tricuspid and mitral) prevent backflow from the ventricles into the atria.
    • The tricuspid valve is located between the right atrium and right ventricle.
    • The mitral (bicuspid) valve is located between the left atrium and left ventricle.
  • The semilunar valves (pulmonary and aortic) prevent backflow from the arteries into the ventricles.
    • The pulmonary valve is located between the right ventricle and the pulmonary artery.
    • The aortic valve is located between the left ventricle and the aorta.
• The fibrous skeleton of the heart provides structural support, anchors the heart valves, and electrically isolates the atria from the ventricles.
• The heart's blood supply is provided by the coronary arteries, which arise from the aorta.
• The main coronary arteries are the left main coronary artery (LMCA), the left anterior descending artery (LAD), the left circumflex artery (LCX), and the right coronary artery (RCA).
• The coronary veins collect deoxygenated blood from the heart muscle and return it to the right atrium via the coronary sinus.

Cardiac Muscle Tissue

• Cardiac muscle cells (cardiomyocytes) are short, branched cells with one or two centrally located nuclei.
• Cardiomyocytes are connected by intercalated discs, which contain gap junctions and desmosomes.
• Gap junctions allow for the rapid spread of electrical signals between cells, enabling coordinated contraction.
• Desmosomes provide strong mechanical attachments between cells.
• Cardiac muscle has a large number of mitochondria, reflecting its high energy demands.
• Cardiac muscle contraction is similar to skeletal muscle contraction but with some key differences:
  • Action potentials in cardiac muscle cells have a prolonged plateau phase due to the influx of calcium ions.
  • Cardiac muscle has a longer refractory period, which prevents tetanus and allows the heart to relax and fill with blood between contractions.
• Cardiac muscle relies primarily on aerobic respiration for ATP production.

Cardiac Conduction System

• The cardiac conduction system consists of specialized cardiac muscle cells that initiate and distribute electrical impulses throughout the heart.
• The sinoatrial (SA) node, located in the right atrium, is the heart's primary pacemaker, generating impulses at a rate of 60-100 beats per minute.
• The atrioventricular (AV) node, located in the interatrial septum, delays the impulse slightly, allowing the atria to contract before the ventricles.
• The AV bundle (bundle of His) conducts the impulse from the AV node to the ventricles.
• The right and left bundle branches carry the impulse through the interventricular septum.
• Purkinje fibers distribute the impulse throughout the ventricular myocardium, causing ventricular contraction.
• The heart rate and force of contraction are modulated by the autonomic nervous system:
  • The sympathetic nervous system increases heart rate and contractility.
  • The parasympathetic nervous system (vagus nerve) decreases heart rate.
• The heart is also influenced by hormones (e.g., epinephrine, thyroid hormone) and other factors such as ion concentrations (e.g., potassium, calcium).

Cardiac Cycle

• The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, including atrial and ventricular systole (contraction) and diastole (relaxation).
• Systole is the phase of contraction, during which the heart chambers pump blood.
• Diastole is the phase of relaxation, during which the heart chambers fill with blood.
• The cardiac cycle consists of the following phases:
  • Ventricular filling: During diastole, the atria and ventricles are relaxed, and blood flows passively from the atria into the ventricles through the open AV valves.
  • Atrial systole: The atria contract, pushing additional blood into the ventricles.
  • Isovolumetric contraction: The ventricles begin to contract, increasing pressure and closing the AV valves.
  • Ventricular ejection: The ventricles continue to contract, and when the pressure exceeds that in the pulmonary artery and aorta, the semilunar valves open, and blood is ejected.
  • Isovolumetric relaxation: The ventricles relax, decreasing pressure and causing the semilunar valves to close. The AV valves are also closed.
• Heart sounds are produced by the closing of the heart valves:
  • The first heart sound ("lub") is caused by the closing of the AV valves at the beginning of ventricular systole.
  • The second heart sound ("dub") is caused by the closing of the semilunar valves at the beginning of ventricular diastole.
• Cardiac output (CO) is the amount of blood pumped by each ventricle per minute.
• CO = heart rate (HR) x stroke volume (SV)
• Heart rate is the number of heartbeats per minute.
• Stroke volume is the amount of blood pumped by each ventricle with each beat.
• Factors affecting stroke volume include preload (the degree of stretch of the heart muscle before contraction), contractility (the force of contraction), and afterload (the resistance against which the heart must pump).
• Blood pressure (BP) is the force exerted by the blood against the walls of the blood vessels.
• BP is typically measured in the brachial artery and expressed as systolic pressure (pressure during ventricular contraction) over diastolic pressure (pressure during ventricular relaxation).
• Blood pressure is regulated by several mechanisms, including the autonomic nervous system, hormones, and the kidneys.