Chapter 3 – The Thorax (Part 4 – Pages 97-117) Mediastinum and Heart
1. Mediastinum Definition
The mediastinum is a central compartment of the thoracic cavity surrounded by loose connective tissue.
It separates the lungs and contains vital structures including the heart, major blood vessels, trachea, esophagus, and more.
2. Boundaries of the Mediastinum
The mediastinum extends from the thoracic inlet (superiorly) to the diaphragm (inferiorly).
Laterally, it is bounded by the mediastinal pleura (part of the pleura covering the lungs).
Anteriorly, it is bordered by the sternum, and posteriorly by the thoracic vertebrae.
3. Contents of the Mediastinum
Heart and great vessels (aorta, superior and inferior vena cavae, pulmonary arteries and veins).
Trachea and main bronchi.
Esophagus.
Thymus gland.
Nerves (e.g., phrenic and vagus nerves).
Lymphatic structures.
4. Structures that Pass Through the Mediastinum
Blood vessels (such as the aorta and inferior vena cava).
Nerves (including the vagus and phrenic nerves).
The esophagus and trachea.
5. Four Subregions of the Mediastinum
Superior Mediastinum: Contains vessels such as the aortic arch and brachiocephalic vein.
Anterior Mediastinum: Located between the sternum and pericardium.
Middle Mediastinum: Contains the heart and surrounding pericardium.
Posterior Mediastinum: Located behind the heart, containing structures such as the esophagus and descending aorta.
6. Anterior Mediastinum Boundaries (Fig. 3.53)
Bounded anteriorly by the sternum and posteriorly by the pericardium.
7. Contents of the Anterior Mediastinum
Thymus gland (in children and adolescents).
Connective tissue, fat, and lymph nodes.
8. Contents of the Middle Mediastinum
Heart enclosed within the pericardium.
Major blood vessels attached to the heart.
9. Pericardium Overview (Fig. 3.54)
a) Definition
The pericardium is a double-walled sac containing the heart and the roots of the greater vessels.
b) Fibrous Pericardium Description
The fibrous pericardium is the tough outer layer made of dense connective tissue, providing protection and anchoring the heart within the thorax.
c) Serous Pericardium Description
The serous pericardium is a thinner, more delicate layer, consisting of two parts:
Parietal Layer: Lines the fibrous pericardium.
Visceral Layer (Epicardium): Covers the heart's surface.
d) Pericardial Cavity
The cavity between the parietal and visceral layers of the serous pericardium, containing pericardial fluid, which reduces friction during heartbeats.
10. Attachments of the Fibrous Pericardium
Attached to surrounding structures such as the diaphragm, sternum, and the great vessels of the heart, anchoring the heart in place.
11. Important Nerve in the Fibrous Pericardium (Fig. 3.55)
The phrenic nerve travels within the fibrous pericardium, important for diaphragm contraction and referred pain to the shoulder due to its origin in the cervical spine.
12. Arteries Supplying the Pericardium (Fig. 3.55)
Pericardiacophrenic artery and branches from the musculophrenic artery and aorta.
13. Nerves Innervating the Pericardium
Phrenic nerve (sensory innervation).
Vagus nerve also has some influence.
Referred pain may be experienced in the shoulder due to innervation origins.
14. Pericarditis Definition
Inflammation of the pericardium, causing chest pain and other symptoms, may interfere with heart function.
15. Shape of the Heart and Orientation (Fig. 3.57)
The heart is cone-shaped, with the apex pointing down and to the left (towards the left hip), and the base facing upward and backward towards the right shoulder.
16. Surfaces of the Heart (Fig. 3.57)
Base Surface: Superior region where great vessels attach.
Other Surfaces:
Anterior (sternocostal) surface: Faces the sternum.
Diaphragmatic surface: Faces the diaphragm below.
Right surface: Faces the right lung.
Left surface: Faces the left lung.
17. Functions of the Two Pumps of the Heart (Fig. 3.63)
a) Right Side Pump
Responsible for pulmonary circulation: pumps deoxygenated blood to the lungs.
b) Left Side Pump
Responsible for systemic circulation: pumps oxygenated blood to the rest of the body.
18. Two Chambers of Each Pump (Figs. 3.63 and 3.59)
Each pump consists of:
Right Pump: Right atrium and right ventricle.
Left Pump: Left atrium and left ventricle.
19. Differences in Wall Thickness of Atria and Ventricles
Atria have thinner walls as they only need to deliver blood to the ventricles.
Ventricles have thicker walls to generate the force needed to pump blood out of the heart and into the systemic and pulmonary circuits.
20. Thicker Left Ventricular Wall
The left ventricular wall is thicker due to the higher pressure needed to pump blood throughout the systemic circulation compared to the right ventricle which serves the lower pressure pulmonary circulation.
21. Three Septa Separating the Heart Chambers
Interatrial Septum: Divides the right and left atria.
Interventricular Septum: Divides the right and left ventricles.
Atrioventricular Septum: Separates the atria from the ventricles.
22. Features of the Base of the Heart (Fig. 3.58)
The base is the upper part of the heart where the chambers are attached to major vessels, typically visible include both atria, especially the left atrium, and openings for pulmonary veins.
23. Orientation of the Apex Relative to the Base (Fig. 3.57)
The apex is oriented inferiorly and anteriorly, located at the left fifth intercostal space, while the base is directed superiorly and posteriorly.
24. Relative Location of the Heart in the Thorax (Fig. 3.65)
The heart is typically located slightly to the left of the median sagittal plane, behind the sternum, but generally within the thoracic cavity's midline space.
25. Anterior Surface Features (Fig. 3.59)
The anterior (sternocostal) surface presents the right atrium and part of the right ventricle, reflecting the structural adaptation for the heart's position in the thoracic cavity.
26. Diaphragmatic Surface Features (Fig. 3.60)
The diaphragmatic surface contains the left and right ventricles with significant grooves for the coronary arteries, aiding the structural integrity during contraction.
27. Sulci on the Heart's Outer Surface (Fig. 3.62)
Coronary Sulcus: Located between atria and ventricles; houses coronary vessels.
Anterior Interventricular Sulcus: Separates the left and right ventricles anteriorly.
Posterior Interventricular Sulcus: Separates the left and right ventricles posteriorly.
28. Veins Delivering Blood to the Right Atrium (Fig. 3.66)
Superior Vena Cava: Drains blood from the upper body.
Inferior Vena Cava: Drains blood from the lower body.
Coronary Sinus: Drains blood from the heart muscle itself.
29. Opening and Valve Between Right Atrium and Right Ventricle
Tricuspid Valve: Opens during diastole to allow blood flow from the right atrium to the right ventricle; closes during ventricular contraction (systole).
30. Internal Areas of the Right Atrium
Smooth Area: Located near the opening of the vena cava.
Rough Area: Contains pectinate muscles, aiding in contraction force.
31. Transitions Between Areas in Right Atrium
a) External Groove (Fig. 3.58)
The external ridge marking the transition where the smooth wall meets the rough pectinate area.
b) Internal Ridge (Fig. 3.66)
The internal feature, known as the crista terminals, separates the two areas internally.
32. Other Features of the Right Atrium (Fig. 3.66)
a) Right Auricle
A small, muscular pouch that increases atrial volume.
b) Musculi Pectinate (Pectinate Muscles)
Muscles that increase the force of contraction; found within the rough area of the atrium.
c) Opening of the Coronary Sinus
Located posteriorly, receives deoxygenated blood from the heart muscle.
d) Interatrial Septum
Divides the right and left atrium.
e) Fossa Ovalis
Remnant of the foramen ovale that existed in the fetus, allowing blood shunting from right to left atrium.
33. Features of the Right Ventricle (Fig. 3.67)
a) Conus Arteriosus (Infundibulum)
Smooth-walled region leading into the pulmonary trunk; regulates blood flow.
b) Trabeculae Carneae
Irregular muscular columns that provide structural reinforcement and assist in contraction.
c) Papillary Muscles
Muscular projections that anchor the chordae tendineae, controlling valve closure.
d) Chordae Tendineae
Tendinous cords connecting papillary muscles to the tricuspid valve, preventing eversion.
e) Tricuspid Valve
Composed of three cusps, prevents backflow into the atrium during ventricular contraction.
34. Mechanisms Preventing Blood Backflow
Tricuspid Valve prevents blood from flowing backward into the right atrium during contraction.
The chordae tendineae and papillary muscles prevent the valve from inverting (eversion) into the atrium.
35. Pulmonary Valve Function
Prevents backflow from the pulmonary trunk into the right ventricle, with cupping structures known as pulmonary sinuses that assist in valve function by ensuring closure during diastole.
36. Veins Carrying Oxygenated Blood Into Left Atrium (Figs. 3.60 and 3.70)
Pulmonary Veins: Four veins (two from each lung) that deliver oxygen-rich blood to the left atrium.
37. Divisions of the Interior of the Left Atrium (Fig. 3.70)
Rough Surface: Houses pectinate muscles.
Smooth Surface: Comprises the majority of the atrial wall.
38. Valve of the Foramen Ovale (Fig. 3.70)
Also known as the septum primum, it acts as a flap valve in the fetal heart, allowing blood to bypass the non-functioning fetal lungs.
39. Features of the Left Ventricle (Fig. 3.71)
a) Entry Point for Blood from Left Atrium
Blood enters through the mitral (bicuspid) valve.
b) Shape and Size of Left Ventricle
The left ventricle is thicker and more muscular than the right due to higher pressure demands of systemic circulation.
c) Exit Point
Blood exits into the aorta through the aortic valve during ventricular contraction.
d) Trabeculae Carneae
Similar to the right ventricle, present for added support and contraction efficiency.
e) Papillary Muscles and Chordae Tendineae
Also present to prevent valve inversion during contraction.
f) Two Parts of the Interventricular Septum
Contains muscular (main) and fibrous (top) components for stability.
g) Mitral Valve
Composed of two cusps: regulates blood flow between left atrium and ventricle.
40. Backflow Prevention from Aorta into Left Ventricle (Fig. 3.72)
Aortic Valve: prevents backflow into the left ventricle; is a three-cusp valve compared to the two-cusp mitral valve, providing better structural reinforcement against high pressures.
41. Potential Problems in Cardiac Valve Diseases
Stenosis: Narrowing of the valve opening, reducing blood flow.
Regurgitation: Valves fail to close properly, allowing backward flow of blood, leading to inefficient heart function.
42. Common Congenital Heart Defects
Examples include Atrial Septal Defect (ASD), Ventricular Septal Defect (VSD), and Tetralogy of Fallot. These defects may affect normal heart function and require intervention.