Homeostasis — A/L Biology

What must I remember about Homeostasis?

• Homeostasis is the physiological maintenance of a dynamic, steady internal environment (extracellular fluid: interstitial fluid and plasma) within narrow physiological limits despite external environmental fluctuations.
• Negative Feedback System components: Sensor/Receptor detects deviation from set point -> Control Center (integrator, often in hypothalamus) compares signal to set point -> Effector executes corrective responses to counteract the initial deviation.
• Thermoregulation in humans: Controlled by preoptic area of Hypothalamus; when body temperature rises, heat-loss mechanisms activate (cutaneous vasodilation, eccrine sweating); when temperature falls, heat-conservation and heat-production mechanisms activate (cutaneous vasoconstriction, shivering thermogenesis, non-shivering thermogenesis via brown fat).
• Osmoregulation: Osmoreceptors in hypothalamus detect increased plasma osmolarity, stimulating thirst and triggering posterior pituitary release of ADH (Vasopressin); ADH inserts aquaporin-2 water channels into collecting duct apical membranes, maximizing water reabsorption and producing concentrated urine.
• Blood Glucose Regulation: Normal fasting blood glucose is 70-100 mg/dL (3.9-5.6 mmol/L); deviations trigger endocrine pancreas to secrete Insulin (lowering glucose via glycogen synthesis in liver/muscle and GLUT4 uptake in adipose) or Glucagon (raising glucose via glycogenolysis and gluconeogenesis).

Test yourself (1 minute)

1. Negative vs Positive Feedback

   Negative feedback counteracts disturbances to restore equilibrium; Positive feedback amplifies deviations until a climactic event occurs (e.g., oxytocin during labor contractions, LH surge in ovulation, platelet plug formation).

2. Countercurrent multiplier in kidney

   Loop of Henle establishes hyperosmotic medullary gradient (up to 1200 mOsm/L), creating the osmotic driving force required for ADH-mediated water reabsorption in collecting ducts.

3. Ectothermy vs Endothermy

   Endotherms (mammals, birds) generate internal metabolic heat to maintain constant core body temperature; ectotherms rely on behavioral adaptations to absorb environmental heat.

Practise Homeostasis questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

What should I study first?

What comes next?

Next in the syllabus

Where does Homeostasis fit in the syllabus?

Year 2 · Term 1