The central idea is the axial skeleton, the main axis of the skeleton that includes the skull, vertebral column, and rib cage. This part provides central support for the body and protects the brain and spinal cord, which matches exactly what’s described. The appendicular skeleton covers the limbs and their girdles, not the central axis. The skeletal system refers to all bones and related structures as a whole, not just this central portion. The term central skeleton isn’t a standard anatomical label, so axial skeleton is the precise term.

Metatarsals are the five long bones in the foot that run between the tarsal bones in the hind/midfoot and the phalanges in the toes. They form the main framework of the midfoot and transmit weight from the ankle (tarsals) forward to the toes through the metatarsophalangeal joints. This arrangement enables things like the arch support and the push-off motion during walking or running. The other structures mentioned aren’t the long bones that connect tarsals to phalanges: tarsals are the ankle/heel bones, the fibula is a leg bone, and the patella is the kneecap.

Melatonin regulates the sleep-wake cycle and is produced by the pineal gland. The pineal gland synthesizes melatonin from serotonin, with enzymes like AANAT and HIOMT catalyzing the steps. Melatonin levels rise in darkness, signaling to the brain that it’s time to sleep, and are suppressed by light, helping you wake up. This ties directly to the daily rhythm we rely on to feel sleepy at night and alert during the day, which is why melatonin is the hormone linked to sleep-wake regulation from the pineal gland. Cortisol follows a different pattern, helping wake you up in the morning but not produced by the pineal gland. Adrenaline is part of the fight-or-flight response and isn’t the sleep-regulating hormone produced by the pineal gland. Serotonin is a neurotransmitter involved in mood and is a precursor to melatonin, but the actual sleep-regulating hormone produced by the pineal gland is melatonin.

The atrioventricular valves separate the atria from the ventricles. There are two of them: the tricuspid valve between the right atrium and right ventricle, and the mitral valve between the left atrium and left ventricle. They open to let blood flow from the atria into the ventricles, and they close when the ventricles contract to prevent backflow into the atria. This distinction is different from the semilunar valves, which separate the ventricles from the great arteries.