In Aves, the exchange of gases occurs within the __________.

Correct Answer: A

Rationale: In birds (Aves), gas exchange occurs in the lungs, not air sacs. Their unique system features rigid lungs with parabronchi, where oxygen and carbon dioxide diffuse into blood continuously, unlike human tidal breathing. Air sacs act as bellows, moving air unidirectionally through the lungs, but lack capillaries for exchange they store and pump air. 'Air sacs and lungs' overstates sacs' role; 'none' is wrong lungs are key. This efficient design supports high metabolism for flight, a distinct adaptation in avian respiration, critical for understanding comparative physiology and bird-specific respiratory conditions.

Correct Answer: C

Rationale: Gap junctions electrically connect cardiac muscle fibers, allowing rapid ion flow between cells via connexin channels, synchronizing contractions across the myocardium for a unified heartbeat. Desmosomes anchor fibers mechanically, not electrically. Tight junctions seal cells, rare in heart tissue. Interneurons are neural, not muscular. These gap junctions, within intercalated discs, enable the heart's autorhythmic, coordinated action, a key feature distinguishing cardiac from skeletal muscle, essential in physiology and arrhythmias where connectivity falters, disrupting rhythm.

Correct Answer: C

Rationale: Increased plasma potassium (hyperkalemia) decreases heart rate by depolarizing cardiac cells, slowing pacemaker activity and conduction severe cases stop the heart. Norepinephrine (sympathetic) and thyroid hormone increase rate by boosting metabolism and SA node firing. Increased calcium enhances contractility, not slowing rate in excess, it speeds it. Potassium's membrane effect, shifting resting potential, disrupts rhythm, a clinical concern in renal failure or drug effects, contrasting with stimulatory factors, key in ECG changes and arrhythmia management.

Correct Answer: C

Rationale: During heart transplants, vagus nerves (parasympathetic) are severed, losing their slowing effect on the SA node, raising resting heart rate to ~100 bpm its intrinsic pace. Glossopharyngeal nerves sense pressure, not rate directly. Cardiac accelerator (sympathetic) nerves speed rate, intact or regrown later. Phrenic nerves drive diaphragm, not heart. This denervation removes 'brake,' key in post-transplant care, explaining tachycardia and need for rate control.

Correct Answer: A

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.