The tiny air sacs present in human lungs are called _______.

Correct Answer: A

Rationale: Alveoli are the tiny air sacs in the lungs, numbering about 500 million, where gas exchange occurs oxygen enters the blood, and carbon dioxide exits via diffusion across their thin walls. The bronchus (singular) and bronchioles are airways leading to alveoli, not sacs themselves bronchi branch from the trachea, and bronchioles are smaller terminal passages. 'All' is incorrect; only alveoli fit the description. Their vast surface area (about 100 m²) and capillary network make them the lungs' functional units, essential for oxygenation, a key focus in respiratory anatomy and diseases like emphysema, where alveoli degrade.

Correct Answer: B

Rationale: The anterior interventricular sulcus, a groove on the heart's front surface, marks the boundary between the right and left ventricles, housing the anterior interventricular artery. The coronary sulcus encircles the heart, separating atria from ventricles. The posterior interventricular sulcus, on the back, also divides ventricles but is less prominent anteriorly. The coronary sinus is a vein, not a groove. This sulcus's visibility and vascular role make it a key landmark, guiding surgical and imaging approaches, essential in cardiac anatomy for locating ventricular divisions.

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.