Work 2: Cardiovascular system: the heart.

• • The base of the heart is located at the level of the third costal cartilage
  • The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages
  • The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly
• Location of the Heart:

  • The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum
  • The heart is separated from other mediastinal structures by the pericardium or pericardial sac and sits in the pericardial cavity
  • The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages
  • The great veins (superior and inferior venae cavae) and great arteries (aorta and pulmonary trunk) are attached to the superior surface of the heart, called the base
• Heart Chambers:

  • The heart has four hollow chambers: two atria and two ventricles
  • The right and left atria serve as volume reservoirs for blood being sent into the ventricles
  • The interatrial septum divides the atrial chambers
  • The ventricles serve as the pumping chambers of the heart
  • The interventricular septum separates the ventricles
  • The thickness of a chamber wall depends on the amount of high-pressure work the chamber does
  • The walls of the left ventricle are the thickest because it pumps blood against the resistance of the systemic circulation
• Heart Valves and their function:

  • The right atrioventricular valve (tricuspid valve) is between the right atrium and the right ventricle
  • The pulmonary valve (right semilunar valve) emerges from the right ventricle at the base of the pulmonary trunk
  • The mitral valve (bicuspid valve or left atrioventricular valve) is located at the opening between the left atrium and left ventricle
  • The aortic valve (aortic semilunar valve) is at the base of the aorta and prevents backflow from the aorta
• Blood Flow Through the Heart:

  • Valves open to allow forward flow of blood through the heart and prevent backward flow
  • Pressure changes within the heart affect the opening and closing of the valves
  • The amount of blood stretching the chamber and the degree of contraction of the chamber wall determine the pressure
• Structure of Cardiac Muscle:

  • Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres
  • T tubules transmit the impulse from the sarcolemma to the interior of the cell
  • Numerous mitochondria provide energy
  • Cardiac muscle cells have intercalated discs for synchronized contractions
• Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres
• T tubules transmit the impulse from the sarcolemma to the interior of the cell
• Numerous mitochondria are present in cardiac muscle cells for energy
• Intercalated discs are found at the junction of different cardiac muscle cells
• An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions
• Specialized conducting components of the heart include:

  • Sinoatrial node
  • Internodal pathways
  • Atrioventricular node
  • Atrioventricular bundle
  • Right and left bundle branches
  • Purkinje fibers
• The SA node initiates the action potential, which sweeps across the atria
• There is a delay of approximately 100 ms at the atrioventricular node, allowing the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle
• The impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, reaching the right papillary muscle via the moderator band
• Ventricular contraction begins after the impulse spreads to the contractile fibers of the ventricle
• The SA node is the heart's primary pacemaker
• Pacemaker cells in lower areas, such as the junctions tissue and the Purkinje fibers, initiate an impulse only when they don't receive one from above
• The SA node has a firing rate of 60 to 100 beats/minute
• The AV node has a firing rate of 40 to 60 beats/minute
• The Purkinje fibers have a firing rate of 20 to 40 beats/minute
• Four characteristics of cardiac cells that influence generation and transmission of electrical impulses:

  • Automaticity: ability to spontaneously initiate an impulse
  • Excitability: a cell's response to an electrical stimulus
  • Conductivity: ability of a cell to transmit an electrical impulse to another cardiac cell
  • Contractility: ability of a cell to contract after receiving a stimulus
• The prepotential in cardiac conductive cells is due to a slow influx of sodium ions until the threshold is reached, followed by rapid depolarization and repolarization
• Cardiac contractile cells have a long plateau phase due to the influx of calcium ions, allowing for extended refractory period
• An ECG tracing shows the P wave, QRS complex, and T wave, with indicated intervals and segments
• Common ECG abnormalities include:

  • Second-degree or partial block
  • Atrial fibrillation
  • Ventricular tachycardia
  • Ventricular fibrillation
  • Third-degree block
• The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole
• Heart sounds can be recorded using phonocardiograms or auscultograms, with murmurs graded on a scale of 1 to 6
• The heart is regulated by neural and endocrine controls, as well as environmental factors including electrolytes
• Resting cardiac output can be calculated as CO = HR × SV, where CO is the amount of blood pumped by each ventricle in one minute
• Factors influencing stroke volume include preload, contractility, and afterload
• Cardiac activity is regulated by sympathetic cardiac nerves that increase cardiac activity and parasympathetic nerves that slow cardiac activity