The Heart: Structure and Location

Overview of Heart Anatomy

The heart is a muscular organ located in the thoracic cavity, slightly left of the midline, and protected by the sternum and rib cage. It functions as the central pump of the cardiovascular system, circulating blood throughout the body.

• Chambers: The heart consists of four chambers: two atria (right and left) and two ventricles (right and left).

• Valves: Four main valves ensure unidirectional blood flow: the atrioventricular (AV) valves (tricuspid and mitral/bicuspid) and the semilunar (SL) valves (pulmonary and aortic).

• Major Vessels: The aorta, pulmonary trunk, and vena cavae are major vessels connected to the heart.

• Internal Structures: Structures such as the septum, chordae tendineae, and papillary muscles support valve function and chamber separation.

Example: The left ventricle pumps oxygenated blood into the aorta for systemic circulation.

Mechanical Events: The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle encompasses all events associated with blood flow through the heart during one complete heartbeat. It is divided into systole (contraction) and diastole (relaxation), driven by pressure gradients that ensure unidirectional blood flow.

• Ventricular Filling (Passive): Both atria and ventricles are relaxed; AV valves are open, SL valves are closed. Approximately 80% of blood flows passively into the ventricles.

• Atrial Systole: Atria contract (corresponding to the P wave on ECG), ventricles remain relaxed. AV valves stay open, SL valves closed. The remaining 20% of blood is actively pumped into the ventricles.

• Isovolumetric Contraction: Atria relax, ventricles contract (QRS complex). All valves are closed, ventricular pressure rises, closing AV valves and producing the first heart sound ("lub").

• Ventricular Ejection: Ventricles contract, AV valves closed, SL valves open. Ventricular pressure exceeds arterial pressure, forcing SL valves open and ejecting blood into the arteries.

• Isovolumetric Relaxation: Both atria and ventricles are relaxed (T wave). All valves are closed. Backflow in the aorta and pulmonary trunk closes SL valves, producing the second heart sound ("dub").

Example: The cardiac cycle repeats with each heartbeat, maintaining continuous blood circulation.

Key Terms and Definitions

• Systole: The phase of the cardiac cycle when the heart muscle contracts and pumps blood.

• Diastole: The phase when the heart muscle relaxes and allows chambers to fill with blood.

• Pressure Gradient: The difference in pressure that drives blood flow from areas of higher to lower pressure.

• AV Valves: Valves between atria and ventricles (tricuspid on the right, mitral/bicuspid on the left).

• SL Valves: Valves at the base of the large arteries leaving the heart (pulmonary and aortic).

Phases of the Cardiac Cycle: Summary Table

Additional info:

• The cardiac cycle is essential for maintaining effective circulation and tissue perfusion.

• Heart sounds ("lub-dub") are produced by the closing of heart valves during the cardiac cycle.