Structure of the Heart and the Path of Blood

The heart is located within the thoracic cavity and rests on the diaphragm.
It is bordered laterally by the lungs.
The heart is relatively small, conical in shape, and approximately the size of a person's clenched fist.
It typically weighs between 250 to 350 grams.
The heart is situated to the left of the body midline, posterior to the sternum, in the middle mediastinum.
The heart is rotated, with the right side (comprising the right atrium and right ventricle) positioned anteriorly relative to the left side (comprising the left atrium and left ventricle), which is located more posteriorly.

Outer Portion:
- Known as the fibrous pericardium.
- Composed of tough, dense connective tissue.
- Attached to both the sternum and the diaphragm.
Inner Portion:
- Known as the serous pericardium, it is a thin, double-layered serous membrane.
- Contains two layers:
- Parietal Layer: The layer that lines the fibrous pericardium.
- Visceral Layer: Also known as the epicardium, which adheres to the heart tissue.

Epicardium:
- The outermost layer of the heart wall.
- Also recognized as the visceral layer of the serous pericardium.
- Composed of simple squamous epithelium with an underlying layer of fat.
Myocardium:
- The middle layer of the heart wall, primarily made up of cardiac muscle tissue.
- It is the thickest layer among the three heart layers.
Endocardium:
- The innermost layer lining the internal surfaces of the heart and external surfaces of heart valves.
- Composed of thin endothelium overlying areolar connective tissue.

The heart contains four hollow chambers: two smaller atria and two larger ventricles.

Structure: Thin-walled chambers located superiorly in the heart.
Function:
- Atria receive blood returning to the heart from both the systemic and pulmonary circuits:
- The right atrium receives blood from the systemic circuit.
- The left atrium receives blood from the pulmonary circuit.
- Blood entering an atrium is passed to the adjacent ventricle.

Structure: Inferior chambers of the heart.
Function:
- Each ventricle ejects blood into major arteries.
- The pulmonary trunk carries blood from the right ventricle into the pulmonary circuit.
- The aorta conducts blood from the left ventricle into the systemic circuit.

Chordae Tendineae:
- Also referred to as the heart strings.
- These structures attach to the tricuspid and bicuspid valves, functioning to prevent valvular prolapse.

Functionality: Receives venous blood from the systemic circuit and the heart muscle itself.
Major Vessels:
- Superior Vena Cava (SVC): Drains blood from the head, upper limbs, and upper trunk.
- Inferior Vena Cava (IVC): Drains blood from the lower limbs and trunk.
- Coronary Sinus: Drains blood from the heart wall.
The interatrial septum forms a wall separating the right and left atria.

Also known as the tricuspid valve.
It separates the right atrium from the right ventricle.
Structure: Composed of three triangular flaps.
Blood flows from the right atrium through this valve into the right ventricle and is forced closed when the right ventricle contracts, preventing backflow.

Functionality: Receives deoxygenated blood from the right atrium.
Separation: The interventricular septum forms a wall between the right and left ventricles.
Papillary Muscles:
- Located on the internal wall surface, these cone-shaped muscular projections anchor the chordae tendineae to prevent their eversion during contraction.

The pulmonary semilunar valve marks the exit of the right ventricle into the pulmonary trunk.
The pulmonary trunk divides into right and left pulmonary arteries, which carry deoxygenated blood to the lungs for gas exchange.

Located within the walls of both ventricles, just before the connection to the pulmonary trunk and aorta.
Composed of three thin, pocket-like cusps.
- As blood is pumped into the arterial trunks, it pushes against the cusps, forcing the valves open.
- When contraction ceases, blood is prevented from flowing back into the ventricles by the cusps inflating and blocking backflow.

Once gas exchange occurs in the lungs, oxygenated blood travels through the pulmonary veins to the left atrium.
It has a smooth posterior wall with openings for approximately four pulmonary veins: two from the left lung and two from the right lung.

Also known as the bicuspid valve or mitral valve.
It separates the left atrium from the left ventricle.
Similar to the right AV valve, it has chordae tendineae.
Oxygenated blood flows from the left atrium into the left ventricle, and the valve is forced closed during left ventricle contraction to prevent backflow.

The largest of all four heart chambers
- Features a wall that is typically three times thicker than the wall of the right ventricle.
Functionality: It generates sufficient pressure to force oxygenated blood from the lungs into the aorta and throughout the systemic circuit.
The right ventricle only pumps blood to the nearby lungs.
At the superior end of the left ventricle, the aortic semilunar valve marks the entrance into the aorta.

Arteries:
- Function: Carry blood away from the heart.
- Notably, the pulmonary arteries are the only arteries carrying deoxygenated blood (arising from the pulmonary trunk, branching to the right and left lungs).
Veins:
- Function: Carry blood toward the heart.
- The pulmonary veins are unique in that they are the only veins carrying oxygenated blood (two from the right lung and two from the left lung).

Pathway for the Pulmonary Circuit:
- Blood enters through the right atrium → right atrioventricular/tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries and returns via pulmonary veins to the left atrium.
Pathway for the Systemic Circuit:
- Blood enters through the left atrium → left atrioventricular/bicuspid/mitral valve → left ventricle → aortic valve → aorta → circulates through the body and returns to the right atrium via the superior and inferior vena cavae.

Arteries:
- Begin with the aorta, branching into the right and left coronary arteries.
- Various arteries supply other tissues.
Veins:
- Drain tissues via capillaries, leading to venules and ultimately the inferior/superior vena cavae which return blood to the right atrium.

Coronary Arteries:
- Referred to as left and right coronary arteries, they emerge from the base of the aorta near the top of the heart.
Cardiac Veins:
- These veins drain into the coronary sinus, which opens into the right atrium near the inferior vena cava.