Part 2 - Heart

• Mediastinum


  Middle compartment of the thoracic cavity

  
• Mediastinum


  • Superior extent: superior thoracic aperture
  • Inferior extent: diaphragm
  • Anterior extent: sternum
  • Posterior extent: thoracic vertebrae
  • Lateral extent: lungs and pleura

  
• Contents of superior mediastinum (anterior to posterior)

  • Thymus gland (remnant)
  • Veins (brachiocephalic vv. and SVC)
  • Arteries (pulmonary trunk and branches, arch of aorta and branches)
  • Trachea (vagus nerve just posterior)
  • Oesophagus
  • Nerves (cardiac plexus and left laryngeal nerve)
  • Thoracic duct and lymphatics

  
• Contents of anterior subdivision of inferior mediastinum


  • Remnant of thymus gland
• Contents of posterior subdivision of inferior mediastinum


  • Oesophagus
  • Vagus nerve
  • Thoracic duct
  • Azygos vein
  • Descending aorta
  • Sympathetic trunks (right and left)
  • Lymph nodes

  
• Middle subdivision of inferior mediastinum


  • Space enclosed by the pericardium
  • Between the mediastinal pleurae
  • Lies in the inferior mediastinum
  • Largest division of the inferior mediastinum
  • Pericardium and heart
  • Great vessels (ascending aorta, SVC, pulmonary trunk)
  • Cardiac plexus
  • Phrenic nerve and pericardiophrenic vessels

  
• Structures on right side of mediastinum


  • Right vagus nerve
  • Azygos vein and its arch
  • SVC
  • Phrenic n. and pericardiophrenic arteries
  • IVC
  • Trachea
  • Sympathetic trunk
  • Oesophagus
  • Pericardium
  • Root of lung

  
• Structures on left side of mediastinum


  • Left subclavian artery
  • Left vagus nerve and left recurrent laryngeal nerve
  • Greater splanchnic nerve
  • Phrenic n. and pericardiophrenic arteries
  • Thoracic duct
  • Aortic arch and thoracic aorta
  • Sympathetic trunk
  • Oesophagus
  • Pericardium
  • Root of lung

  
• Thoracic duct


  • Origin: chyle cistern in the abdomen
  • Course: ascends through aortic hiatus in the diaphragm at T12, ascends in the posterior mediastinum between the oesophagus and azygos vein, at T4 it crosses to the left, ascends to superior mediastinum, arches laterally and then inferiorly, anterior to subclavian artery, enters at the union of left internal jugular and subclavian veins

  
• Trachea


  • Blood supply: inferior thyroid a., bronchial aa.
  • Venous drainage: inferior thyroid venous plexus
  • Innervation: vagus nn., recurrent laryngeal nn., sympathetic trunks
• Phrenic nerves


  • Origin: descends over scalenus anterior to enter thorax
  • Accompanied by pericardiophrenic vessels
  • Passes anterior to lung roots between mediastinal pleura and pericardium
  • Sensory fibers supply pleurae, pericardium and diaphragm
  • Motor fibers supply diaphragm

  
• Right vagus nerve


  • Gives rise to right recurrent laryngeal nerve
  • Runs parallel to right side of trachea
  • Passes posterior to right brachiocephalic vein, SVC and root of right lung

  
• Left vagus nerve


  • Loops around arch of aorta
  • Gives rise to left recurrent laryngeal nerve
• Thoracic oesophagus


  • In superior mediastinum: posteriorly T1-T4, on left - aortic arch, thoracic duct, left rec. laryngeal n., left common carotid a., on right - right rec. laryngeal n. and right vagus n.
  • In inferior mediastinum: posteriorly T5-T10, thoracic duct, branches of hemiazygos vein, anteriorly - pericardium, on left - thoracic aorta, on right - azygos vein
  • Constricted by: aortic arch on left, left main bronchus anteriorly, oesophageal hiatus
  • Innervation: oesophageal plexus, vagus nn., sympathetic trunk

  
• Major diaphragm openings


  • Caval hiatus (T8) - IVC, terminal branches of right phrenic n.
  • Oesophageal hiatus (T10) - oesophagus, right and left vagus nn.
  • Aortic hiatus (T12) - descending aorta, thoracic duct
• Fibrous pericardium


  • Continuous superiorly with tunica adventitia of great vessels
  • Attached anteriorly to posterior surface of sternum by sternopericardial ligaments
  • Bound posteriorly by loose connective tissue to structures in posterior mediastinum
  • Continues inferiorly with central tendon of diaphragm

  
• Parietal layer of serous pericardium


  • Fused with internal surface of fibrous pericardium
• Visceral layer of serous pericardium


  • Reflected onto heart where it forms epicardium
  • Becomes continuous with parietal layer of serous pericardium where SVC, IVC and pulmonary veins enter the heart, and where aorta and pulmonary trunk leave the heart

  
• Pericardial cavity


  • Separates the 2 serous layers and has a thin film of fluid
• Layers of the heart wall


  • Epicardium - thin mesothelium formed by visceral serous pericardium
  • Myocardium - thick, helical muscle layer
  • Endocardium - thin inner layer that also covers valves

  
• Transverse pericardial sinus


  • Located between the aorta and pulmonary trunk anteriorly and the left atrium and SVC posteriorly
  • Connects the 2 sides of the pericardial cavity

  
• Oblique pericardial sinus


  • Located between the pulmonary veins and IVC posteriorly and the left atrium anteriorly
  • A blind ending sinus

  
• Pericardial arterial supply


  • Internal thoracic aa.
  • Pericardiophrenic aa.
  • Musculophrenic aa.

  
• Pericardial venous drainage


  • Internal thoracic vv.

  
• Pericardial innervation


  • Phrenic nn. (C3-C5) - sensory
  • Vagus nn. - parasympathetic
  • Sympathetic trunks - vasomotor fibers
• Pericarditis


  Inflammation usually causing chest pain and referred pain to C3-C5 dermatomes (top of the shoulder)
• Position of the heart


  • Lies in the middle mediastinum
  • Behind the body of the sternum and the 2nd to 6th costal cartilages
  • 1 third lies to the right of the midline and 2 thirds lies to the left
  • Lies opposite to T5-T8 vertebrae in supine position and T6-T9 vertebrae in erect position

  
• Surfaces of the heart


  • Apex - inferolateral part of the left ventricle
  • Base (posterior surface) - mainly formed by left atrium and slightly by right atrium, faces posteriorly towards T6-T9, extends superiorly to bifurcation of pulmonary trunk and inferiorly to coronary sinus, receives pulmonary veins on right and left sides of left atrium and receives SVC and IVC at superior and inferior ends of right atrium
  • Sternocostal (anterior) - mainly formed by right ventricle, and slight by right atrium and left ventricle
  • Diaphragmatic (inferior) - formed mainly by the left ventricle and partly by the right ventricle

  
• Borders of the heart


  • Superior border - formed by right and left atria and auricles
  • Right border - right atrium, between superior and inferior vena cava
  • Left border - formed by left ventricle and a little by the left auricle
  • Inferior border - formed mainly by the right ventricle and slightly by the left ventricle

  
• Surface markings of heart


  • Superior border: 3rd right costal cartilage to 2nd intercostal space on the left of sternum
  • Inferior border: Sternal end of 6th right costal cartilage to midclavicular line in 5th left IC space
  • Right border: Extends from 3rd right costal cartilage to near 6th right costal cartilage
  • Left border: 2nd left IC space to midclavicular line in 5th left IC space
• Sulci of the heart


  • Atrioventricular (coronary) sulcus - on the right and left sides
  • Anterior interventricular sulcus
  • Posterior interventricular sulcus

  
• Right atrium


  • Right auricle - continuous muscular pouch
  • 3 openings: SVC, IVC, coronary sinus
  • Internal structures: smooth, thin-walled posterior part (sinus venarum) where all 3 veins open, rough, muscular anterior portion composed of pectinate muscles, right AV orifice, sulcus terminalis, crista terminalis, fossa ovalis, SA node is between the SVC and crista terminalis

  
• Left atrium


  • Openings for 4 pulmonary veins in its smooth posterior wall
  • Smaller auricle containing pectinate muscle

  
• Right ventricle


  • Tapers superiorly into the conus arteriosus which leads to the pulmonary trunk
  • Interior surface has irregular muscular elevations - trabeculae carneae
  • Supraventricular crest - thick muscular ridge separating the ridged muscular wall from the smooth wall of conus arteriosus
  • Tricuspid valve - guards right AV orifice, has posterior, septal and anterior cusps

  
• Right atrium


  • Rough, muscular anterior portion composed of pectinate muscles
  • Right AV orifice where blood leaves the right atrium
  • Sulcus terminalis- shallow vertical groove that externally separates smooth and rough parts of the wall
  • Crista terminalis- internally separates smooth and rough parts of the wall
  • Fossa ovalis- thumb-shaped depression on the interatrial septum
  • Remnant of the foramen ovale and its valve in the foetus
  • Border- limbus ovalis
  • SA node is between the SVC and crista terminalis

  
• Left atrium


  • Openings for 4 pulmonary veins in its smooth posterior wall
  • Smaller auricle containing pectinate muscle

  
• Right ventricle


  • Tapers superiorly into the conus arteriosus which leads to the pulmonary trunk
  • Interior surface has irregular muscular elevations- trabeculae carneae
  • Supraventricular crest- thick muscular ridge separating the ridged muscular wall from the smooth wall of conus arteriosus
  • Tricuspid valve- guards right AV orifice
  • Has posterior, septal and anterior cusps
  • Chordae tendineae arise from the papillary muscles and attach to the valve cusps
  • Papillary muscles- prevent prolapse of tricuspid valve
  • Anterior- largest, arising from anterior wall and attaching to anterior and posterior cusps
  • Posterior- consists of several parts, arising from the inferior wall of right ventricle and attaches to posterior and septal cusps
  • Septal- arise from IV septum and attaches to anterior and septal cusps
  • Septomarginal trabecula (moderator band)- traverses the right ventricle from the inferior part of the IV septum to the base of the anterior papillary muscle

  
• Left ventricle


  • Walls are 2-3 times thicker than right ventricle
  • Trabeculae carneae are finer and more numerous
  • Conical cavity longer than right ventricle
  • 2 papillary muscles- larger than in right ventricle
  • Anterior and posterior
  • Mitral valve guards left AV orifice
  • Aortic vestibule- smooth supero-anterior outflow region leading to the aortic valve

  
• Atrioventricular valve function


  1. Blood returns to the heart, fills the atria and puts pressure on against the AV valves, forcing them open

  2. As ventricles fill, valve flaps hang limply into ventricles

  3. Ventricles contract, forcing blood against AV valve cusps and causing them to close

  4. Papillary muscles contract and chordae tendineae tighten, preventing cusps from everting into atria

  
• Semilunar valve function


  1. As ventricles relax and intraventricular pressure falls, blood flows back from the arteries

  2. The cusps are filled with blood since they are cup-shaped causing them to close