Body Systems

Passages that mention body systems rarely quiz a single organ in isolation. They hand you a chain: air into the lungs, oxygen into the blood, blood pumped by the heart, oxygen delivered to a working muscle, or food broken down step by step until nutrients reach the bloodstream. The task is to read a diagram or a short setup and follow the handoff from one stage to the next, noticing what each stage adds or removes.

Two mechanisms carry almost all of this content. First, diffusion: oxygen crosses from the alveolus into capillary blood because the alveolar air holds more oxygen pressure than the blood arriving there, and the same drop-from-high-to-low rule moves carbon dioxide back out, costing no energy. Second, pumping: the heart's output, called cardiac output, is heart rate multiplied by stroke volume, the volume pushed out per beat. Digestion follows a sequence rather than a single rule: mouth starts mechanical and enzyme breakdown, stomach adds acid and more enzymes, small intestine finishes digestion and absorbs nutrients into the blood, large intestine reclaims water.

These pieces connect because the body treats oxygen delivery as a product, not a single number: delivery equals cardiac output times how much oxygen the blood actually carries. Raise the altitude, and the blood carries less oxygen per liter, so the heart must pump more liters per minute to keep delivery steady, explaining why heart rate climbs at altitude or during exercise even though the lungs and muscles have not changed their own demands. A question may give you a table of these linked numbers and ask you to spot which one must change to keep the others balanced.