Lifespan
The human body is not a fixed object — it is a process that unfolds over time. The same person is biologically very different as an embryo, a teenager, and an 80-year-old. This root follows that arc.
Each stage has its own characteristic biology: what is growing, what is maturing, what is declining, and which systems are under the most change.
Stages
Section titled “Stages”- Prenatal development — from a single fertilized cell to a newborn.
- Infancy and childhood — rapid growth and the build-out of body and brain.
- Adolescence and puberty — sexual maturation and the transition to an adult body.
- Adulthood — the long plateau of maturity and peak function.
- Aging and senescence — the gradual decline of function in later life.
Why it matters
Section titled “Why it matters”Almost every fact in anatomy and physiology has a “when” attached to it. Bone is still being laid down in a child and slowly lost in an elder; the immune system is naive at birth and weathered in old age. Seeing the body across its lifespan keeps any single snapshot in context.