Human Physiology: Systems, Homeostasis and Regulation
Loops, signals, effectors
Physiology items usually present a disturbance - water loss, cold, high blood glucose - and ask you to trace the feedback loop to the effector and predict the change in urine, blood or body temperature. Answer in the order receptor, control centre, effector, response.
Core rules
- Feedback: negative feedback reverses the deviation; positive feedback amplifies it (oxytocin in labour, clotting).
- Neuron: resting potential about −70 mV is maintained by the Na⁺/K⁺ ATPase; a stimulus opens Na⁺ channels, and reaching threshold triggers an all-or-none action potential; K⁺ efflux repolarises; myelinated axons conduct saltatorily; at a synapse, Ca²⁺ entry releases neurotransmitter.
- Endocrine: peptide hormones bind surface receptors and act through second messengers; steroid hormones enter the cell; insulin from beta cells lowers glucose by promoting uptake and glycogenesis; glucagon raises it; ADH inserts aquaporins in the collecting duct; aldosterone increases Na⁺ reabsorption; thyroxine raises metabolic rate.
- Kidney: filtration at the glomerulus, reabsorption in the proximal tubule (glucose, most water), countercurrent multiplication in the loop of Henle, fine adjustment in the collecting duct.
Worked example
Dehydration: plasma osmolarity rises, hypothalamic osmoreceptors detect it, ADH is released from the posterior pituitary, more aquaporins are inserted in the collecting duct, water reabsorption increases, and a small volume of concentrated urine is produced; thirst is stimulated. Reasoning: the loop ends by restoring osmolarity, which then reduces ADH secretion.
Marks lost
Saying ADH increases urine volume; claiming impulse amplitude grows with stimulus strength; swapping insulin and glucagon targets; stating that aldosterone reabsorbs water rather than driving Na⁺ reabsorption that water follows.