Biology courses teach the nervous, endocrine and excretory systems one at a time, but your body never meets a problem one system at a time. A runner on a hot day is fighting rising core temperature, falling blood volume, shrinking glycogen stores and soaring oxygen demand all at once, and the fixes interfere with each other: sweating cools the body but drains the plasma that keeps blood pressure up. Understanding homeostasis means following a disturbance across systems and seeing which loop wins when two compete. This lesson works through three cases, then shows how to draw an integrated feedback flowchart and design an investigation that measures a feedback response.
Case 1: a marathon on a hot day
| Stage | Disturbance | Detected by | Responses (system) |
|---|---|---|---|
| Start | Muscles need far more O₂ and glucose | Motor cortex signals; chemoreceptors sense rising CO₂ | Sympathetic nerves raise heart rate and stroke volume (nervous, circulatory); faster, deeper breathing (respiratory); adrenaline and glucagon release glucose from liver glycogen (endocrine) |
| 20–40 min | Core temperature rises from muscle heat | Thermoreceptors and the hypothalamus | Skin arterioles dilate and sweat glands secrete, up to 1–2 L of sweat per hour (integumentary, circulatory) |
| 1–2 h | Plasma volume falls; blood becomes more concentrated and blood pressure drops | Hypothalamic osmoreceptors; baroreceptors; kidney cells | Thirst and ADH (water saved); renin–angiotensin–aldosterone (Na⁺ and water saved); urine becomes scant and dark (excretory) |
| Late race | Glycogen depleted; blood volume still falling | Glucose sensors in the pancreas; baroreceptors | Glucagon and cortisol keep glucose up; heart rate drifts upward to maintain output with less blood volume |
The loops compete for the same blood. Skin needs blood flow to shed heat, working muscles need it for oxygen, and the brain and heart must keep their supply. As sweat loss lowers plasma volume, the body protects blood pressure first: skin blood flow and sweating are reduced, so heat is no longer removed fast enough and core temperature climbs. Above about 40 °C the hypothalamus itself begins to fail, the person becomes confused and may stop sweating: heat stroke, a medical emergency. A loss of about 2% of body mass as sweat already impairs performance. The opposite error is dangerous too. A slow runner who drinks far more plain water than they sweat dilutes blood sodium (hyponatremia); water moves into cells by osmosis, and swelling brain cells cause confusion and seizures. Sports drinks containing Na⁺ and drinking to thirst reduce both risks.
Case 2: diabetic ketoacidosis
In type 1 diabetes the immune system has destroyed the pancreas's beta cells, so without injected insulin, cells cannot take up glucose. Diabetic ketoacidosis (DKA) develops when insulin is missing for hours to days, often during an illness or a missed dose. Endocrine: with no insulin to oppose it, glucagon drives the liver to release ever more glucose, so blood glucose soars (often above 14 mmol/L, compared with a normal 4 to 7), while cells starve and break down fat for fuel. The liver turns fatty acids into ketone bodies, which are acids, so blood pH falls. Excretory: glucose in the filtrate exceeds what the nephron's carriers can reabsorb, so it stays in the urine and holds water there by osmosis. The person urinates large volumes, loses Na⁺ and K⁺, becomes dehydrated and very thirsty. Respiratory: chemoreceptors in the medulla and carotid arteries detect the falling pH and trigger deep, rapid breathing, which blows off CO₂. By Le Chatelier's principle, the bicarbonate buffer below shifts left, using up H⁺. The kidneys also excrete extra H⁺ and make new HCO₃⁻. The breath may smell fruity from acetone, a ketone. Treatment restores all three: IV fluids, insulin and potassium.
Case 3: fight or flight
A car swerves toward you. Within a second, the hypothalamus fires sympathetic nerves: heart rate and force rise, bronchioles dilate, pupils widen, digestion pauses and blood is redirected from gut to skeletal muscle. Within seconds the adrenal medulla releases adrenaline into the blood, reinforcing and prolonging every one of those effects and making the liver break down glycogen. If the stress continues for minutes to hours, the hypothalamus releases CRH, the pituitary ACTH and the adrenal cortex cortisol, which keeps glucose high by making it from proteins. The three tiers show why the body uses both systems: nerves are fast and targeted, hormones are slower but reach every tissue and last longer. Afterward the parasympathetic system slows the heart and cortisol feeds back to shut off CRH and ACTH.
Building feedback flowcharts and designing investigations
- Start with the stimulus as a change from the set point ("plasma solute concentration rises"), not a vague event ("running").
- Name the specific receptor and control centre (osmoreceptors in the hypothalamus), not just "the brain".
- Write the messenger on each arrow: a nerve or a named hormone, with the gland that releases it.
- Name the effector and its response (collecting-duct cells insert aquaporins; more water reabsorbed).
- Close the loop with a feedback arrow marked with a minus sign back to the stimulus. For an integrated case, draw each loop and join them where they share a variable, such as blood volume.
| Investigation | Independent variable | Dependent variable | Controls and safety |
|---|---|---|---|
| Heart-rate recovery | Intensity of a 2-minute step test (step height or rate) | Heart rate at rest, at the end of exercise, and every 30 s for 5 min | Same participant, posture and time of day; screen for heart or breathing conditions; anyone may stop at any time |
| Glucose tolerance curve | Time after a standard glucose drink (or compare a simulated diabetic and non-diabetic data set) | Blood glucose (mmol/L) at 0, 30, 60, 90 and 120 min | In school, use published or simulated data, not blood sampling; a clinical test is done fasting with a 75 g glucose drink |