Eleven systems, one student — the survey of the body they live in.
Suggested pace: 3 weeks. Mastery looks like:
The human body is the capstone of the survey: roughly 37 trillion cells organized into tissues, tissues into organs, organs into cooperating systems. No single system survives alone — the lungs are useless without blood to carry their oxygen, the muscles useless without nerves to command them. The body is, in the truest sense, a civilization of cells, and its governing principle is homeostasis: the continuous maintenance of stable internal conditions — temperature near 98.6°F, balanced blood sugar, steady pH — against a changing outside world. Shivering, sweating, and thirst are homeostasis made visible.
The circulatory system is the courier service: the four-chambered heart drives blood through arteries, capillaries, and veins, delivering oxygen and nutrients and hauling away wastes. The right side of the heart sends oxygen-poor blood to the lungs; the left side sends freshly oxygenated blood to the body. The respiratory system supplies the cargo: air travels down the trachea into branching bronchi and ends in the alveoli — hundreds of millions of microscopic air sacs wrapped in capillaries, where oxygen and carbon dioxide trade places. Every concept from the cellular-energy unit lands here: those alveoli exist to feed mitochondria.
The digestive system disassembles food into molecules cells can use, beginning in the mouth, where chewing and the enzyme salivary amylase start breaking down starch. The stomach's acid bath continues the work; the small intestine, lined with absorbing villi and aided by the liver and the pancreas, completes digestion and absorbs the nutrients; the large intestine reclaims water. The excretory system handles quality control: the kidneys filter the entire blood supply many times a day, removing wastes and balancing water and salts, producing urine. The pancreas appears twice in this story — it also releases the hormone insulin, which regulates blood sugar, a flagship example of the endocrine system's chemical messengers at work.
The skeletal system is more than a frame: bones support the body, protect organs, and manufacture blood cells in their marrow. The muscular system moves the frame — skeletal muscle under voluntary command, smooth muscle running the internal organs, cardiac muscle beating without rest. Command and control belong to the nervous system, whose basic working unit is the neuron: brain and spinal cord process, peripheral nerves carry signals at speeds your student can actually test with a dropped-ruler reaction-time experiment at the kitchen table.
Patrolling everything is the immune system: skin and mucous membranes as outer walls, and white blood cells as the standing army that recognizes and destroys invaders — with vaccination training that army on a wanted poster before the criminal ever arrives. Close the unit by reconnecting the systems your student has now toured: a single jog upstairs recruits muscles, accelerates heart and lungs, triggers sweat for cooling, and adjusts blood chemistry — homeostasis conducted live, by the civilization of cells they have carried all along.
Every correct answer climbs one rung. Climb forever. Badges at 10, 25, 50, and 100 lifetime rungs.
GENO — a robot you can actually TALK to — has studied this entire unit and is available day or night, in 32 languages, at no cost. Ask him to re-explain any idea on this page, quiz you out loud, or go deeper than the lesson goes.