Physiology: Gradients, Feedback, and Quantitative Regulation
Physiology is the study of how gradients are created and spent. Nearly every system moves something-ions, water, blood, gas-down an electrochemical or pressure gradient, and regulation adjusts the size of that gradient or the resistance in its path. Learn negative feedback as the default: a sensor detects deviation, an integrating center compares it to a set point, and an effector reverses the change. Positive feedback is rare and explosive, as in uterine contractions and the clotting cascade.
Core quantitative ideas: Fick's principle for diffusion (rate proportional to area and gradient, inversely to thickness); osmosis and tonicity; the Nernst/Goldman relationship for resting membrane potential; the cardiac output equation; Starling forces across capillaries; renal filtration, reabsorption, and secretion; and acid-base handling via the bicarbonate buffer and Henderson–Hasselbalch equation. Hormonal axes-hypothalamus, pituitary, target gland-usually close the loop with negative feedback.
Traps: confusing osmolarity (total solute particles per volume) with tonicity (effective osmoles that move water); assuming all filtration is reabsorption; forgetting that parasympathetic activity slows the heart while sympathetic activity increases rate and contractility; and mixing up affinity with efficacy at receptors.
Worked example: cardiac output = heart rate × stroke volume. At 70 beats/min and 70 mL/beat, CO = 4,900 mL/min, about 4.9 L/min. If heart rate doubles while stroke volume is unchanged, CO doubles; if stroke volume falls by half, CO falls by half.
Rule of thumb: flow equals gradient divided by resistance. When a question asks what changes a rate, ask which of those three variables moved.