The exchange of gases between the external environment and the lungs ______.

Correct Answer: B

Rationale: External respiration is the exchange of gases oxygen in, carbon dioxide out between the external environment and the lungs, occurring in the alveoli where air meets blood. 'Respiration' broadly includes this plus cellular processes, too vague here. Cellular respiration is intracellular, using oxygen to produce energy, not lung-based. 'None' is wrong external respiration fits precisely. This process involves air entering via inhalation, diffusing across alveolar walls into capillaries, and CO2 exiting, driven by partial pressure gradients. It's the lungs' primary role, distinct from internal or cellular phases, a critical distinction in respiratory physiology for gas transport understanding.

Correct Answer: A

Rationale: Heart valves are made of dense connective tissue, primarily collagen, providing strength and flexibility to withstand pressure changes, with a core of fibrous tissue layered by endothelium. Areolar connective tissue is loose, not robust enough. Hyaline cartilage is rigid, found in the trachea, not valves. Cardiac muscle forms the heart wall, not valves. This dense tissue, avascular and resilient, ensures durability cusps flex without stretching key in valve function, relevant in pathology like calcification where flexibility diminishes, impacting flow.

Correct Answer: D

Rationale: The cardiac action potential follows: SA node (pacemaker, initiates), AV node (delays signal), bundle of His (transmits to ventricles), Purkinje fibers (spreads to ventricular myocardium), driving normal contraction. Other sequences disrupt this flow e.g., starting with Purkinje or misordering AV node delays atrial-ventricular timing. This precise path, from SA node atop the right atrium to Purkinje's ventricular reach, ensures atria contract before ventricles, optimizing filling and ejection, a fundamental rhythm in cardiac physiology, disrupted in arrhythmias like bundle branch block.

Correct Answer: C

Rationale: Increased stroke volume (SV) blood ejected per beat results from increased preload (more venous return stretches ventricles, per Frank-Starling), decreased afterload (lower arterial resistance eases ejection), and increased contractility (stronger myosin-actin interaction, often catecholamine-driven). Increased afterload resists ejection, reducing SV. Decreased preload or contractility lowers filling or force, cutting SV. This trio optimizes SV by enhancing filling, easing outflow, and boosting power, key in exercise or heart failure management where these factors shift, a core concept in cardiac output dynamics.

Correct Answer: B

Rationale: The epicardium is another name for the visceral pericardium, the serous layer adhering to the heart's surface, distinct from the parietal pericardium and fibrous outer sac. Endocardium lines chambers, myocardium is muscle, fibrous pericardium is external. This synonym reflects its pericardial role, key in friction reduction, critical in epicardial pathology like inflammation, a precise layer nomenclature.