Physiology
The study of bodily function reveals integrated systems of extraordinary sophistication—homeostatic mechanisms that maintain life through feedback loops, coordination, and apparent purposiveness.
Integrated Systems
The body's systems work in concert, each depending on and supporting the others. This integration cannot arise piecemeal—the systems must function together from the start.
The cardiovascular system is useless without the respiratory system; the nervous system requires the endocrine system for sustained responses. This mutual dependence suggests coordinated design rather than gradual accumulation.
- Cardiovascular-Respiratory Integration: Heart, lungs, and blood vessels form an integrated gas exchange system. The heart adjusts output to oxygen demand; breathing rate responds to CO₂ levels. Neither works alone.
- Neuroendocrine Coordination: The nervous system provides rapid responses; the endocrine system sustains them. The hypothalamus bridges both, integrating stress response, metabolism, and reproduction.
- Immune-Nervous Crosstalk: The immune system communicates with the brain through cytokines; the brain modulates immunity through the vagus nerve. Mind and body are integrated.
- Gut-Brain Axis: The enteric nervous system contains 500 million neurons. Gut bacteria influence mood, cognition, and behavior. The body is more integrated than reductionism supposed.
Development: From One Cell to Trillions
The body's ability to develop from a single fertilized cell into a complex organism with trillions of cells, hundreds of cell types, and precise spatial organization is perhaps biology's greatest mystery.
Development is not merely growth but orchestrated differentiation—cells becoming heart, brain, bone, and blood in the right places at the right times. This requires information beyond the genome: positional information, morphogen gradients, and temporal coordination.
- Embryonic Development: From one cell to 37 trillion, differentiated into 200+ cell types in precise three-dimensional arrangement. The information required exceeds what DNA alone can specify.
- Morphogenesis: Tissues fold, migrate, and shape themselves into organs. The heart loops, the brain folds, the gut rotates—all following a developmental program of staggering complexity.
- Wound Healing: A coordinated cascade of inflammation, proliferation, and remodeling restores tissue integrity. The body knows what it should be and works to restore it.
- Regeneration: Salamanders regenerate entire limbs; humans regenerate liver. These capacities reveal latent programs for restoration—the body as self-repairing system.