The heart contracts because of an electrical impulse. Where in the heart does this impulse start?

A. Sinoatrial node:The sinoatrial (SA) node, located in the right atrium near the superior vena cava, is the natural pacemaker of the heart. It generates electrical impulses that initiate each heartbeat and set the rhythm for the entire heart.

B. Atrioventricular node:The atrioventricular (AV) node delays the electrical impulse slightly to allow ventricular filling before transmitting it to the ventricles. While critical for coordinated contraction, it does not start the impulse.

C. Right ventricle:The right ventricle contracts in response to impulses conducted through the AV node and bundle branches. It is not the origin of the heartbeat.

D. Left atrium:The left atrium receives impulses propagated from the SA node via the atrial conduction pathways. It does not generate the initial electrical signal.

A. Segmental bronchi:Each lung contains multiple segmental (tertiary) bronchi-10 in the right lung and 8–10 in the left lung. They serve individual bronchopulmonary segments, so their total exceeds five.

B. Choanae:The choanae are the paired posterior nasal apertures connecting the nasal cavity to the pharynx. There are only two, not five, and they are unrelated to the lungs.

C. Laryngeal cartilages:The larynx has several cartilages (e.g., thyroid, cricoid, arytenoid), but they number more than five and are part of the upper airway, not the lungs.

D. Lobes:The lungs are divided into five lobes total: three in the right lung (superior, middle, inferior) and two in the left lung (superior, inferior). Lobes are anatomical subdivisions important for lung function and clinical localization.

E. Tracheal cartilages:The trachea contains 16–20 C-shaped cartilaginous rings, which are far more than five. These rings support the trachea but do not describe lung anatomy.