The Heart

Structure and Anatomy of the Heart

The heart is a muscular organ roughly the size of a clenched fist, situated in the mediastinum of the thoracic cavity. It consists of four distinct chambers: two thin-walled atria that receive blood, and two thick-walled ventricles that pump blood out. A muscular wall called the septum divides the left and right sides, preventing the mixing of oxygenated and deoxygenated blood. Blood flow is directed in one direction by four primary valves:

• Tricuspid valve: Located between the right atrium and right ventricle.
• Mitral (bicuspid) valve: Located between the left atrium and left ventricle.
• Aortic valve: Situated between the left ventricle and the aorta.
• Pulmonary valve: Situated between the right ventricle and the pulmonary artery.

The heart wall itself comprises three layers: the outer epicardium, the muscular myocardium responsible for contraction, and the inner lining called the endocardium.

Functions of the Heart

The primary role of the heart is to act as a dual pump, driving blood through the pulmonary and systemic circuits. The right side of the heart manages pulmonary circulation, collecting deoxygenated blood from the body via the superior and inferior vena cavae and pushing it to the lungs to release carbon dioxide and pick up oxygen. The left side manages systemic circulation, receiving freshly oxygenated blood from the lungs and distributing it to all systemic organs and tissues.

Beyond circulation, the heart also functions as an endocrine organ. Specialized muscle cells in the atria release Atrial Natriuretic Peptide (ANP) when stretched by high blood volume, signaling the kidneys to excrete more sodium and water to lower blood pressure.

Conducting System of the Heart

The heart possesses the unique property of autorhythmicity, meaning it can generate its own electrical impulses without nervous system stimulation. This electrical signal follows a specific pathway through the heart muscle:

1. Sinoatrial (SA) node: Known as the natural pacemaker, located in the right atrial wall, initiating each heartbeat.
2. Atrioventricular (AV) node: Delays the impulse slightly to allow the atria to empty fully into the ventricles.
3. Bundle of His (AV bundle): Transmits the signal from the AV node down into the interventricular septum.
4. Bundle branches: Split into left and right pathways, carrying the impulse toward the apex of the heart.
5. Purkinje fibers: Spread the electrical charge rapidly through the ventricular myocardium, causing a synchronized contraction.

Cardiac Activities and the Cardiac Cycle

The cardiac cycle describes all the mechanical and electrical events that occur during one single heartbeat. It is divided into two main phases: systole (contraction and emptying) and diastole (relaxation and filling).

• Atrial systole: The atria contract, pushing the remaining blood through the open AV valves into the ventricles.
• Ventricular systole: The ventricles contract, causing the AV valves to snap shut (creating the first heart sound, 'lub') and forcing the semilunar valves open to eject blood into the aorta and pulmonary trunk.
• Ventricular diastole: The ventricles relax, the semilunar valves snap shut (creating the second heart sound, 'dub'), and the heart fills passively with blood from the atria.

Electrocardiography (ECG)

An electrocardiogram (ECG or EKG) is a graphical recording of the electrical activity generated by the heart muscle during the cardiac cycle. Electrodes placed on the skin detect these tiny electrical currents and display them as wave deflections on paper or a monitor. A standard normal ECG waveform consists of specific features:

• P wave: Represents atrial depolarization, the electrical signal spreading through the atria before they contract.
• QRS complex: Represents ventricular depolarization, the larger electrical wave that triggers ventricular contraction.
• T wave: Represents ventricular repolarization, the recovery phase as the ventricles return to their resting electrical state.