Trace the path of blood through the heart's double circuit, describe gas exchange at the alveoli, and outline the digestive pathway from mouth to large intestine.
The heart has four chambers: two upper atria receive incoming blood, and two lower, more muscular ventricles pump blood back out. Blood flow forms a double loop: the pulmonary circuit carries oxygen-poor blood from the right side of the heart to the lungs, where it picks up oxygen and releases carbon dioxide, and back to the heart's left side; the systemic circuit then carries that oxygen-rich blood out to the rest of the body and back again, oxygen-poor, to the heart's right side. Three types of blood vessels serve distinct roles: arteries carry blood away from the heart under high pressure and have thick, muscular walls; veins carry blood back toward the heart at low pressure and rely on one-way valves to keep blood moving in the right direction against gravity; and capillaries, only a single cell thick, are the actual site of exchange with surrounding tissue. Blood itself carries red blood cells, which use the protein hemoglobin, a callback to Unit 2's quaternary protein structure, to bind and carry oxygen, white blood cells, which defend against pathogens and preview next week's immune system, and platelets, which trigger clotting at a wound.
The respiratory system moves air along a single branching pathway: nose or mouth, trachea, bronchi, then progressively smaller bronchioles, ending in tiny grape-like clusters called alveoli, each wrapped in a dense network of capillaries. Alveoli are built around the same surface-area-maximizing principle introduced back in Unit 3: an enormous number of tiny sacs provides a vastly larger total surface area for gas exchange than a single large sac ever could. Breathing itself is driven mechanically by the diaphragm, a sheet of muscle beneath the lungs that contracts and flattens to pull air in, then relaxes to help push air back out. The digestive system works like a disassembly line, matching directly back to Unit 2's macromolecules and enzymes: mechanical and chemical digestion both begin in the mouth, where the enzyme amylase starts breaking down starch; food travels down the esophagus to the stomach, which churns it mechanically while the enzyme pepsin begins digesting protein in a strongly acidic environment; the small intestine is where most chemical digestion finishes and where nutrients are absorbed, through finger-like projections called villi that, again, maximize surface area; and the large intestine reabsorbs water, hosts large communities of beneficial bacteria, and compacts remaining waste.
Mastery looks like: They can trace blood through the heart's double circuit, name the three blood vessel types and their roles, and outline the digestive pathway with the enzyme active at each major stage.
Common stumbles: Reversing which heart side serves the pulmonary versus the systemic circuit; forgetting that chemical digestion begins in the mouth, not the stomach.