The heart : human block 4

• Position, surfaces, and borders of the heart:

  • Apex: 5th intercostal space
  • Base: Posterior aspect
  • Anterior surface: Behind sternum & ribs
  • Superior border: Great vessels enter/leave
  • Inferior border: Lies on diaphragm
  • Right border: Faces right lung
  • Left border: Faces left lung
• Mediastinum:

  • Central compartment of the thoracic cavity
  • Contains: Heart, Great Vessels, Thymus, Oesophagus, Trachea
• Landmarks of the heart:

  • Coronary sulcus marks division between atria & ventricles
  • Anterior interventricular sulcus marks division between ventricles
  • Auricles are atrial appendages that increase capacity
• Pericardium:

  • Fibrous pericardium is tough & inelastic, rests on/attached to diaphragm, open end fused with great vessels
  • Serous pericardium has parietal layer fused to fibrous pericardium, visceral layer continuous/part of epicardium, pericardial cavity contains pericardial fluid
• Layers of the heart:

  • Epicardium (outer) - visceral serous pericardium
  • Myocardium (middle) - cardiac muscle
  • Endocardium (inner) - continuous with endothelium of large vessels of heart
• Chambers and valves:

  • Right atrium (RA): Receives deoxygenated blood from vena cavae and coronary sinus
  • Right ventricle (RV): Pumps deoxygenated blood to lungs (pulmonary circulation)
  • Left atrium (LA): Receives oxygenated blood from lungs via pulmonary veins
  • Left ventricle (LV): Pumps oxygenated blood into aorta (systemic circulation)
  • Atrioventricular (AV) valves: Right AV is tricuspid, Left AV is bicuspid (mitral)
  • Semilunar (SL) valves: 3 semilunar cusps at origin of emerging arteries (pulmonary & aortic)
• Great vessels:

  • Inferior vena cava
  • Superior vena cava
  • Pulmonary trunk
  • Pulmonary arteries
  • Pulmonary veins
  • Ascending aorta
  • Aortic arch
  • Brachiocephalic (branching to Right Subclavian & Right Common Carotid)
  • Left Common Carotid
  • Left Subclavian
• Systemic and pulmonary circulations
• Coronary circulation:

  • Aorta
  • Right coronary
  • Posterior interventricular
  • Right marginal
  • Left coronary
  • Circumflex
  • Left marginal
  • Diagonal
  • Anterior interventricular
  • Coronary sinus
• Blood components:

  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (thrombocytes)
  • Plasma
• Blood functions:

  • Transportation of nutrients & waste
  • Protection through the immune system
  • Regulation of hormones, proteins, etc
  • Haematopoiesis: blood production
  • Haemorrhaging: blood loss
• Red blood cells (erythrocytes):

  • Produced in bone marrow
  • Erythropoietin - kidney
  • No nucleus
  • Haemoglobin protein
  • Haematocrit (40-45%)
  • Anaemia (low iron)
  • Polycythaemia (high viscosity)
• White blood cells (leukocytes):

  • Protection through phagocytosis
  • Granulocytes (neutrophils, eosinophils & basophils)
  • Agranulocytes (lymphocytes & monocytes)
  • Abnormal levels: Leukopenia (low WBC count), Leukocytosis (high WBC count)
• Platelets (thrombocytes):

  • Cell fragments that control blood loss
  • Normal platelet count (150,000 – 450,000 platelets per μl)
  • Thrombocytosis (>450,000)
  • Thrombocytopenia (<150,000)
• Plasma:

  • Straw-coloured liquid in which blood cells are suspended
  • Composed of water (92%), proteins (major protein = albumin), glucose, electrolytes
• Summary:

  • Position, surfaces, and borders of the heart
  • Landmarks of the heart: Coronary sulcus, auricles, ant/post interventricular sulcus
  • Pericardium and layers of the heart
  • Chambers and valves: RA, RV, LA, LV, AV valves & SL valves
  • Great vessels: SVC, IVC, PT, PA, PV & AA
  • Systemic and pulmonary circulations
  • Coronary circulation (LCA & RCA)
  • Blood functions, blood production & loss
  • 4 main components: red cells, white cells, platelets, and plasma
• Two vascular beds arranged in series:

  • Pulmonary circulation: Removes CO2 and absorbs O2
  • Systemic circulation: Delivers O2 and removes CO2
• Tissues within the systemic circulation are arranged in parallel
• Excitation in systemic circulation is initiated in the sinoatrial node
• Excitation travels across the atria by cell-to-cell conduction through channels called gap junctions
• Conduction is delayed in the atrioventricular node
• Excitation continues through a specialized fast conduction system composed of Purkinje cells, resulting in rapid excitation of the ventricles (completed by tissue conduction via gap junctions)
• Regional action potential differences are essential for the coordinated excitation of the heart
• Blood flow in the heart is determined by pressure difference across valves
• Cardiac output is the volume of blood ejected from the heart per minute
• Cardiac output can be calculated as:

  • Cardiac Output = Heart Rate × Stroke Volume
• Heart rate is determined by pacemaker electrical activity in the sinoatrial node
• Stroke volume is determined by cardiac mechanical factors: Preload, Afterload, and Contractility
• Autonomic nervous system is the primary controller of heart rate
• Sympathetic pathway:

  • Response to stimulation begins slowly with the release of norepinephrine
  • Downstream effects are mediated by a relatively slow second messenger system involving the production of cAMP
  • Response decays gradually
• Parasympathetic pathway:

  • Rapid response to stimulation with the release of acetylcholine
  • Downstream effects are mediated by specialized acetylcholine-regulated K+ channels
  • Rapid decay due to the presence of cholinesterase in SA and AV nodes
• Heart rate is subject to various reflex responses:

  • Baroreceptor Reflex: Sudden change in arterial blood pressure causes an inverse change in heart rate
  • Bainbridge Reflex: Distension of the right atrium by increased venous return alone causes an increase in heart rate
• Stroke volume is determined by:

  • Preload (ventricular filling = end-diastolic volume)
  • Afterload (resistance to blood flow = total peripheral resistance)
  • Contractility (force and speed of ventricular contraction at a given preload and afterload)
• Stretch increases myosin-actin interactions, by increasing sensitivity to Ca2+ and reducing inter-filament spacing
• Stroke volume regulation:

  • Regulated by sympathetic and parasympathetic activity
  • Sympathetic activity increases stroke volume by increasing magnitude and rate of force generation
• Electrical excitation causes myocardial contraction, resulting in pressure generation in the atria and ventricles
• Pressure differences between chambers determine valve status and blood flow
• Changes in preload, afterload, or contractility alter cardiac mechanical function
• The Wiggers diagram and pressure-volume loop represent cardiac hemodynamics and are essential for understanding cardiac function