Biology: Homeostasis and Response

Method

1. State the general negative feedback model before answering any homeostasis question: a receptor detects a deviation from the set point (norm value), a coordinator (often the hypothalamus) processes this information, and an effector (a muscle or gland) brings the level back towards the set point, at which point the corrective response is itself switched off.
2. For blood glucose questions, link a rise in blood glucose concentration to insulin secretion by β cells of the islets of Langerhans, which binds to receptors on liver and muscle cells to increase glucose uptake and stimulate glycogenesis (glucose to glycogen); link a fall in blood glucose concentration to glucagon secretion by α cells, which stimulates glycogenolysis (glycogen to glucose) and gluconeogenesis (glucose from non-carbohydrate sources such as amino acids) in the liver.For blood glucose questions, link a rise in blood glucose concentration to insulin secretion by beta cells of the islets of Langerhans, which binds to receptors on liver and muscle cells to increase glucose uptake and stimulate glycogenesis (glucose to glycogen); link a fall in blood glucose concentration to glucagon secretion by alpha cells, which stimulates glycogenolysis (glycogen to glucose) and gluconeogenesis (glucose from non-carbohydrate sources such as amino acids) in the liver.
3. Distinguish Type 1 diabetes (the immune system destroys insulin-producing β cells, so little or no insulin is produced, treated by insulin injection) from Type 2 diabetes (body cells lose sensitivity to insulin, so glucose uptake falls despite normal or high insulin levels, associated with obesity and often managed initially through diet).Distinguish Type 1 diabetes (the immune system destroys insulin-producing beta cells, so little or no insulin is produced, treated by insulin injection) from Type 2 diabetes (body cells lose sensitivity to insulin, so glucose uptake falls despite normal or high insulin levels, associated with obesity and often managed initially through diet).
4. For heart rate control, trace the pathway from receptor to response: chemoreceptors (detecting blood pH/CO2 concentration) and baroreceptors (detecting blood pressure) send impulses to the medulla oblongata, which increases heart rate via the sympathetic nervous system and the sinoatrial node (SAN), or decreases it via the parasympathetic (vagus) nerve; remember the SAN, not the nervous system, actually initiates each heartbeat, since cardiac muscle is myogenic.
5. For kidney and osmoregulation questions, follow the pathway of a molecule through the nephron: ultrafiltration at the glomerulus and Bowman's capsule forces small molecules (water, ions, glucose, urea) out of the blood under pressure, selective reabsorption at the proximal convoluted tubule reclaims glucose and amino acids by active transport, and the loop of Henle creates a concentration (water potential) gradient in the medulla that allows water to be reabsorbed osmotically from the collecting duct.
6. For ADH questions, explain the pathway as a negative feedback loop: a fall in blood water potential is detected by osmoreceptors in the hypothalamus, which triggers the posterior pituitary gland to release more ADH into the blood; ADH makes the collecting duct walls more permeable to water (by inserting aquaporins), so more water is reabsorbed and urine becomes more concentrated (lower volume, higher solute concentration).
7. For ECG or rate calculations, treat the trace like any periodic graph: measure the time between two identical, repeated features (such as consecutive R waves) to find the period, then use rate = 60 / period (in seconds) to convert to beats per minute.

Practice questions

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