Anatomy of the Heart

• The mediastinum refers to a group of structures located in the middle mass between the lungs, bound together by connective tissue attached to the central compartment of the thorax.
• It is divided into superior and inferior compartments.
• The inferior mediastinum is further divided into anterior, middle, and posterior portions.
• The heart is located within the middle portion of the inferior mediastinum.
• The remaining boundaries of the mediastinum are the vertebrae posteriorly and the sternum anteriorly.

Components within the Mediastinum

• Superior mediastinum lies above T4/T5
• Inferior mediastinum lies below T4/T5
• Anterior mediastinum
• Middle mediastinum
• Posterior mediastinum

Structure and Layers of the Heart

• The base is a roughly quadrilateral region where the great vessels (e.g., aorta/pulmonary trunk) enter/exit.
• The apex is the most inferior point where the ventricles taper down and meet, oriented downward, anteriorly, and to the left.
• Contraction occurs in a wave-like manner, first atria, then ventricles, terminating at the apex.
• The pericardium (pericardial sac) is composed of two fused layers: fibrous and parietal serous pericardium.
• The epicardium (visceral serous pericardium), myocardium (muscle), and endocardium (endothelial layer) make up the layers of the heart.

Pericardial Sinuses

• The serous pericardium extends as separate sleeves over the superior vena cava on the right side and the aorta and pulmonary trunk on the left, creating the transverse pericardial sinus.
• The sleeve of serous pericardium that encloses the superior vena cava also encloses the openings of the inferior vena cava and pulmonary veins, creating the oblique pericardial sinus.

Heart Chambers and Grooves

• The heart has four chambers: 2 atria and 2 ventricles.
• The atria are separated from the ventricles by an atrioventricular groove or the coronary sulcus in which the coronary sinus sits.
• The ventricles are separated by two additional grooves on the heart's external surface called the posterior and anterior interventricular grooves.
• The coronary sulcus is a depression surrounding the heart at the atrioventricular junction.

Right Atrium

• The right atrium receives deoxygenated blood from the superior and inferior vena cava and the coronary sinus.
• The internal structure includes a rough muscular wall containing musculi pectinati (muscular fibers raised in parallel ridges).
• Externally, the right auricle is an ear-like muscular pouch attached to the right atrium.
• The opening of the coronary sinus is a feature of the internal wall.

Prenatal Circulation

• Blood from the umbilical vein enters the right atrium and bypasses the right ventricle.
• Blood then goes through the foramen ovale to the left atrium.
• Blood that does enter the right ventricle is shunted from the pulmonary trunk to the aorta via the ductus arteriosus.

Postnatal Circulation

• Systemic and pulmonary blood flow separates with closing of fetal shunts; the foramen ovale becomes the fossa ovalis.
• The ductus arteriosus closes and is renamed the ligamentum arteriosum.

Embryological Remnants

• The foramen ovale is the fetal shunt that redirects blood from the right to the left atrium so blood from the right ventricle does not reach under-developed lungs. Once closed the foramen ovale becomes the fossa ovalis.
• The ductus arteriosus is the fetal shunt that redirects blood from the pulmonary trunk to the aorta. The shunt allows blood instead of going through under-developed lungs. Once closed the ductus arteriosus becomes the ligamentum arteriosum.

Right Ventricle

• The right ventricle receives blood from the right atrium during atrial contraction while the ventricles are relaxed.
• The walls features muscular ridges called the trabeculae carnae
• The AV valve in the right ventricle is called the Right AV Valve
• It then pumps blood to the lungs (via the pulmonary trunk) for oxygenation.
• Chordae tendinae attach to conical-shaped papillary muscles, which project upward from the floor of the ventricle.

Left Atrium

• It composes the posterior and superior portion of the heart.
• Newly oxygenated blood enters via the 4 pulmonary veins.
• A finger-like extension of muscle protruding from the left atrium is called the left auricle.
• The walls are generally smooth, except for pectinate muscles in the left auricle.

Left Ventricle

• Most apex of the heat is formed by the left ventricle
• It has muscular walls that 2x as thick as those found in the right ventricle, which allows it to more forcefully propel blood throughout systemic circulation.
• Chordae tendinae attach to the cusps of the bicuspid (mitral) valve and to the conical papillary muscles.
• Trabeculae carnae cover most of the internal wall.
• The left ventricular wall 2x as thick as the AV valve

Characteristics specific to the Atria

• Muscular pouches, the auricles attached to chamber walls anteriorly.
• Pectinate muscles

Characteristics specific to Ventricles

• Papillary muscles, which pull valves closes.
• Chordae tendinae (string-like cords attached to cups of valves)
• Trabeculae carnae (interwoven muscular network) which can be found lining the walls

Heart Valves

• Atrioventricular: separate atria from ventricles
• Bicuspid (mitral): left atrium/ventricle, two cusps
• Tricuspid: right atrium/ventricle, three cusps
• Semilunar:
  • At the base of the pulmonary trunk and the aorta
  • Pulmonary valves when exiting the right ventricle,
  • Aortic valves where it exits the left ventricle.
• Atrioventricular Valves Open:
  • When the atria contract pushing the AV valve open
  • Blood passes through atria into correspoding vetricle
  • Atrial pressure will then decrease and the AV valves close
• Atrioventricular Valves Closed:
  • Once valves are closed ventricles contract and the valves must remain closed while blood is pushed out into the pulmonary arteries or aorta
  • To do this the papillary muscles contract and pull on chordae tendinae which pulls on the cusps

Anatomy of AV Valves

• Atrioventricular valves on both the left and the right, lie at the interface between the atria and ventricles. They only allow one-way passage of blood.
• The closure of valves is the responsibility of the papillary muscles

Semilunar valves

• Semilunar valves are located at the base of the pulmonary trunk as well as the origin of the aorta.
• The valves regulate blood during the changes in the pressure from the contraction of the heart.
• Semilunar valves prevent blood from flowing backwards once the ventricles relax.
• Semilunar valves do not have attached chordae tendinae.

Categories for Blood Circulation

• Pulmonary circulation:
  • Right ventricle to lungs for oxygenation
  • Oxygenated blood returns to left atrium
• Systemic circulation:
  • Oxygenated blood from left ventricle to the aorta, supplying body tissues
• Coronary circulation:
  • Right/left coronary arteries arise from the aorta
  • Supplies oxygenated blood to the heart itself
• Hepatic portal circulation:
  • Collects venous blood from the gastrointestinal tract
  • Transports it to the liver before returning to the heart

Coronary Arteries

• The heart muscle's blood supply via coronary arteries.

• During diastole (ventricular relaxation), blood pools near the closed aortic valves for the right and left coronary arteries.

• Terminal Branches of each coronary artery include the following:

  • Right coronary artery (RCA):
    • posterior interventricular artery
    • marginal artery
  • Left coronary artery (LCA):
    • circumflex artery
    • left anterior descending artery

Anastomoses between the Coronary Arteries

• Anastomoses between the coronary arteries:
• RCA and the circumflex branch of the LCA form an anastomosis
• The anterior and posterior interventricular arteries also do

Conduction System of the Heart

• Contraction of the cardiac muscle is modulated by this system
• Specialized myocardial cells make up the system which create and conduct impulses inside the heart
• The Sinoatrial (SA) Node:
  • Located subepicardially
  • This is the pacemaker because it is impulse generating
  • Overrides impulses from other regions of the heart due to doing so more rapidly
• The Atrioventricular (AV) Node
  • Located subendocardially and serve as an electorcal connection between the atria and ventricles
  • It is a waypoint where the impulse generated is briefly delayed
  • This is to provide the atria time to completely contract prior to the contraction of a ventricle

Autonomic Regulation

• The SA Node receives both Sympathetic (from spinal nerves T1-T5) and Parasympathetic (from Vagus nerve CN X) which makes the node extremely sensitive
• To this end Sympathetic Stimulation increases SA node activity and Parasympathetic stimulation decreases SA node activity.

Impulse Pathway

• SA node initiates impulse
• Impulse conducted to walls of atria
• Atria contract
• Impulses enter the AV node
• Impulse travels to the bundle and bundle branches
• Papillary Muscles contract
• Ventricles Contract