Anatomy and Physiology of the Human Heart

Anatomy and Function of the Human Heart

The heart is located in the anterior mediastinum to the left of the midline of the body and weighs between 250 and 300 grams. It forms a cone shape, with its apex (lower corner) pointing downward, to the left, and forward. It has three faces: anterior, inferior, and posterior.

The Three Layers of the Heart

• External (Pericardium): Consists of two layers (leaves) with serous pericardial fluid between them that reduces friction.
• Middle (Myocardium): The heart muscle itself, composed of striated muscle in involuntary bundles.
• Internal (Endocardium): The inner lining of the chambers and heart valves.

The Heart Conduction System

The electrical conduction system of the heart consists of:

• Sinoatrial (SA) node (the natural pacemaker)
• Atrioventricular (AV) node
• Bundle of His
• Right and left branches of the Bundle of His
• Purkinje fibers

Understanding General Diastole

Cardiac diastole is the period of time when the heart relaxes after contraction in preparation for filling with circulating blood. Ventricular diastole is when the ventricles relax, and atrial diastole is when the atria are relaxed. Together, they are known as general diastole, lasting approximately half of the cardiac cycle length, which is about 0.4 seconds.

During ventricular diastole, ventricular pressure falls below the peak reached during systole. When left ventricular pressure falls below the pressure of the left atrium, the mitral valve opens, and the left ventricle fills with blood that had been accumulating in the left atrium. About 70% of the filling of the ventricles occurs spontaneously. Likewise, when right ventricular pressure falls below that of the right atrium, the tricuspid valve opens, and the right ventricle fills with blood that was accumulating in the right atrium.

The Mechanism of Atrial Systole

Atrial systole is the contraction of the myocardium (muscle) of the left and right cardiac atria. Normally, both atria contract simultaneously. The term systole is equivalent to muscle contraction, while electrical systole is the electrical activity that stimulates the myocardium of the heart chambers to contract. This is immediately followed by mechanical systole, which is the actual mechanical contraction of the heart. As the atria contract, blood pressure within them increases, forcing blood out into the ventricles. Atrial systole lasts about 0.1 seconds.

The Mechanism of Ventricular Systole

Ventricular systole is the contraction of the muscles of the left and right ventricles, which follows atrial systole. Ventricular systole lasts about 0.3 seconds.

Calculating Cardiac Output

Cardiac output (CO) is the volume of blood being ejected per minute from the left ventricle through the aorta (or from the right ventricle through the pulmonary artery).

It is equivalent to stroke volume (SV) — the amount of blood ejected by the ventricle with each contraction — multiplied by the heart rate (HR):

CO (mL/min) = SV (mL/beat) × HR (beats/min)

For example, in an adult male:

CO = 70 mL/beat × 75 beats/min = 5.25 L/min

Factors Regulating Stroke Volume

Three main factors regulate stroke volume:

• Preload: The degree of stretching of the heart muscle fibers before contraction.
• Contractility: The force of contraction of each ventricular myocardial fiber.
• Afterload: The pressure that must be exceeded so that blood is successfully expelled from the ventricles.