the heart

Mediastinum

Location of the heart:
- Right atrium and right ventricle positioned anteriorly
- Left atrium and left ventricle positioned posteriorly
- Apex of the heart is directional inferior, base is superior
Shape of the heart is compared to an upside-down pyramid

Two layers surround the heart:
- Fibrous pericardium: outer layer, made of fibrous tissue
- Serous pericardium: inner layer, divided into two parts:
- Parietal layer: lines the fibrous pericardium
- Visceral layer (epicardium): adheres to the heart surface
Contains pericardial space filled with pericardial fluid
The heart is enclosed in a "bag within a bag" structure, facilitating movement and reducing friction

Right atrium: receives deoxygenated blood from the body
- Formed by the absorption of the sinus venosus during embryonic development
Right ventricle: pumps blood to the pulmonary trunk, walls are thinner than left ventricle
Left atrium: receives oxygenated blood from the lungs, predominantly smooth
Left ventricle: pumps oxygenated blood to the body, walls are thicker than right ventricle due to higher pressure requirement

Blood returns to the heart via:
- Superior vena cava: returns blood from above the diaphragm
- Inferior vena cava: returns blood from below the diaphragm
- Coronary sinus: returns blood from the heart muscle itself
Blood flows through:
- Right atrium → tricuspid valve (right atrioventricular valve) → right ventricle → pulmonary valve (semilunar) → pulmonary artery → lungs (for oxygenation)
- Lungs → four pulmonary veins → left atrium → left atrioventricular valve (mitral valve) → left ventricle → aortic valve (semilunar) → aorta
Significant chambers and valves:
- Right atrioventricular valve (tricuspid): three cusps (septal, anterior, posterior)
- Left atrioventricular valve (bicuspid or mitral): two cusps
- Semilunar valves: consist of aortic and pulmonary semilunar valves which prevent backflow

Fossa ovalis: derived from the foramen ovale, closes as pressure dynamics change after birth
Ductus arteriosus: becomes the ligamentum arteriosum after birth, connecting the pulmonary trunk to the aorta, bypassing the non-functioning lungs
Ductus venosus: becomes the ligamentum venosum, connecting the umbilical vein to the inferior vena cava, thus bypassing the liver

SA Node:
- Pacemaker of the heart located near the entrance of the superior vena cava
Pathway of impulse conduction:
- SA Node → AV Node → Bundle of His → Right and Left Bundle Branches → Purkinje fibers
- Bachmann's bundle: coordinates contraction between right and left atria

Atrioventricular valves prevent backflow during ventricular contraction
Semilunar valves open to allow blood flow during ventricular contraction and close when the ventricles relax, producing heart sounds ("lub" for closing AV valves, "dub" for closing semilunar valves)