Which of these statements is true about internal respiration?

Correct Answer: B

Rationale: Internal respiration is the exchange of gases oxygen from blood to tissues, carbon dioxide from tissues to blood occurring at the cellular level via diffusion across capillaries. ATP production is cellular respiration's outcome, not the exchange itself. Alveolar-blood exchange is external respiration, not internal. Breathing from atmosphere to alveoli is ventilation, not respiration's tissue phase. This process sustains metabolism, delivering oxygen for energy and removing CO2 waste, distinct from lung-based external respiration, a vital concept in understanding systemic oxygen transport and tissue oxygenation in physiology.

Correct Answer: C

Rationale: The pulmonary semilunar valve prevents blood from flowing back into the right ventricle, closing after systole as pulmonary artery pressure exceeds ventricular pressure, ensuring one-way flow to the lungs. The tricuspid valve stops backflow into the right atrium, not ventricle. The bicuspid (mitral) serves the left side. The aortic semilunar valve guards the left ventricle. This valve's role in pulmonary circulation is crucial, maintaining forward flow, key in right heart dynamics and conditions like pulmonary regurgitation affecting lung perfusion.

Correct Answer: D

Rationale: The second heart sound ('dupp') occurs when semilunar valves (aortic and pulmonary) close, marking the end of ventricular systole as ventricular pressure drops below arterial pressure, halting ejection. Valvular stenosis is a condition, not an event. Semilunar valves opening starts systole, tied to the first sound ('lub') with AV valve closure. AV closing precedes systole, not the second sound. This 'dupp,' sharper and higher-pitched, reflects elastic recoil of closed valves, a key auscultatory marker in the cardiac cycle, critical for timing and diagnosing valve issues like regurgitation.

Correct Answer: D

Rationale: The left ventricle has the thickest myocardium, up to 1-1.5 cm, pumping against high systemic pressure (~120 mmHg) to the aorta, requiring robust muscle. The right ventricle (~0.3-0.5 cm) handles lower pulmonary pressure (~25 mmHg). Atria, thin-walled (~0.1-0.2 cm), only fill ventricles. This thickness gradient reflects workload left ventricle's force sustains body-wide circulation, key in hypertrophy or failure where it thickens or weakens, a core structural adaptation.

Correct Answer: A

Rationale: The aortic valve, a semilunar valve, isn't atrioventricular (AV) it guards the aorta from the left ventricle, not atrium-to-ventricle. Mitral (bicuspid) and tricuspid are AV valves, linking atria to ventricles. This distinction semilunar versus cuspid defines flow roles, key in valve disease like aortic stenosis, a clear anatomical category.