Physiology and Behavior
Homeostasis
Physiology is about maintaining internal stability. Negative feedback reverses a deviation (thermoregulation, blood glucose via insulin and glucagon, osmoregulation via ADH and the kidneys). Positive feedback amplifies a change (childbirth, blood clotting) and is rarer. Trap: choosing positive feedback for routine regulation.
Systems
Nervous: neurons generate action potentials by Na+ influx (depolarization) and K+ efflux (repolarization); neurotransmitters cross synapses. Endocrine: hormones travel in blood; insulin lowers glucose, glucagon raises it; thyroid and adrenal hormones regulate metabolism and stress. Circulatory and respiratory: gas exchange by diffusion across thin, moist surfaces; hemoglobin carries O2. Digestive: enzymes break down macromolecules; villi increase absorption. Excretory: kidneys filter blood and reabsorb water and glucose. Immune: innate versus adaptive (antibodies, memory cells).
Plants: transpiration pulls water up xylem; stomata balance gas exchange and water loss; phloem carries sugars; auxins direct growth.
Behavior
Innate behaviors are genetically programmed (reflexes, fixed action patterns, taxis/kinesis); learned behaviors include habituation, imprinting, classical and operant conditioning, and insight. Social behavior includes communication, territoriality, dominance hierarchies, and kin selection (Hamilton's rule: help when rB > C).
Worked example
Body temperature rises → sensors detect the deviation → the hypothalamus triggers sweating and vasodilation → temperature falls back toward the set point. Rule of thumb: if the response opposes the initial change, it is negative feedback.