Which of the following terms identifies the anatomical region found between the lungs that extends from the sternum to the vertebral column and from the first rib to the diaphragm?

Correct Answer: D

Rationale: The mediastinum is the anatomical region between the lungs, stretching from the sternum to the vertebral column and first rib to diaphragm, containing the heart, trachea, esophagus, and major vessels. The epicardium is the heart's outer layer, not a region. The abdominal cavity lies below the diaphragm, unrelated. The pericardium encases the heart within the mediastinum, not the broader space. This central compartment's role in housing vital structures makes it a critical anatomical landmark, key in thoracic surgery and understanding mediastinal pathology like tumors or infections.

Correct Answer: B

Rationale: Cardiac muscle fibers depolarize longer than skeletal muscle due to a prolonged plateau phase in their action potential, driven by calcium influx through L-type channels, lasting ~200-300 ms versus skeletal's ~2-5 ms. This extended depolarization ensures sustained contraction, preventing tetanus and allowing complete ventricular ejection per beat. Skeletal muscle's brief depolarization suits rapid, repeated motions. 'Same' ignores this distinction; cardiac's unique refractory period matches its continuous duty. This longer phase, key to heart rhythm, is critical in ECG interpretation and antiarrhythmic drug effects targeting calcium channels.

Correct Answer: C

Rationale: Cardiac output (CO) is the blood volume ejected from either ventricle left into the aorta (systemic), right into the pulmonary trunk (lungs) per minute, ~5 L total. 'Left, aorta' or 'right, pulmonary' alone is half; 'right, aorta' is wrong. This dual definition reflects parallel circuits, key in measuring heart efficiency, critical in failure where CO drops, a broad metric in physiology.

Correct Answer: C

Rationale: The myocardium, the heart wall's thickest layer, is cardiac muscle driving contraction thickest in the left ventricle (~1-1.5 cm) for systemic pressure. Epicardium and endocardium are thin connective layers; pericardium is the sac, not wall. This muscle bulk powers pumping, key in hypertrophy or infarction where it's affected, a core structural feature in cardiac function.

Correct Answer: D

Rationale: Alveoli (D) are not in the conducting zone, per the key they're respiratory zone structures for gas exchange. The nose (A) filters/moistens air (e.g., 90% humidity). The trachea (B) and bronchi (C) conduct air (16 generations of branching), with cartilage/mucus clearing debris. The conducting zone (nose to terminal bronchioles) warms, humidifies, and cleans air (e.g., 37°C), per physiology. Alveoli (300 million) perform external respiration (600 mL/min O₂), not conduction. This excludes D its role in diffusion (0.2 μm barrier) contrasts with A's filtration, B's patency, or C's airflow, marking it as the exception.