With extreme stimulation from the SA node, the AV node can transmit impulses at rates higher than 220 per minute without affecting heart function.

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.

A. Both ventricles: Stroke volume (SV) is the volume of blood ejected by a ventricle during a single contraction, measured from end-diastolic volume minus end-systolic volume. The left ventricle pumps blood into systemic circulation, while the right ventricle pumps blood into the pulmonary circulation, making SV a measure relevant to both ventricles.

B. Left ventricle: The left ventricle pumps oxygenated blood into the aorta for systemic circulation. While left ventricular SV is often emphasized clinically due to systemic perfusion, stroke volume is conceptually defined for both ventricles, not exclusively the left.

C. Right ventricle: The right ventricle ejects deoxygenated blood into the pulmonary trunk for gas exchange in the lungs. Like the left ventricle, it contributes to the stroke volume, so SV is not limited solely to the right ventricle.

D. Both atria: The atria function as filling chambers that contribute to ventricular preload by delivering blood through the atrioventricular valves. While atrial contraction assists ventricular filling, stroke volume specifically refers to blood ejected from the ventricles, not the atria.