Which of the following best describes the stroke volume (SV) of the heart?

A. Distension: Cardiac muscle tissue is capable of stretching to accommodate varying volumes of blood, a property shared with smooth muscle (in blood vessels and hollow organs) and skeletal muscle to some extent. Distension alone does not distinguish cardiac muscle from other muscle types.

B. Relaxation: Relaxation is a fundamental property of all muscle types, including skeletal, smooth, and cardiac muscle. After contraction, all these muscles return to their resting length to allow subsequent filling or preparation for the next contraction.

C. Contracting: Contraction is a universal property of all muscle types. Skeletal, smooth, and cardiac muscles all contract via actin–myosin cross-bridge cycling in response to appropriate stimuli.

D. Autorhythmicity: Cardiac muscle is unique in its ability to generate spontaneous action potentials without external nervous stimulation. Specialized pacemaker cells within the sinoatrial node initiate rhythmic depolarization, allowing the heart to maintain a coordinated and continuous heartbeat. This intrinsic automaticity is distinct from skeletal and smooth muscle, which require neural or hormonal stimuli to contract.

Correct answer: True

The pericardium is a fibroserous, double-walled sac located in the mediastinum that encloses the heart and the roots of the great vessels. It consists of an outer fibrous layer that provides structural support and limits excessive cardiac movement, and an inner serous layer, which is divided into parietal and visceral layers separated by a small amount of pericardial fluid. This fluid acts as a lubricant, reducing friction as the heart contracts and relaxes. The pericardium’s anatomical position and structure protect the heart from mechanical injury, infection, and sudden overexpansion, while maintaining optimal conditions for cardiac function.