Homeostasis & Feedback Loops

Homeostasis comes from the Greek words "homoios" (similar) and "stasis" (standing still). It describes the body's remarkable ability to keep its internal environment stable — even when the outside world is constantly changing. Think of it like a thermostat in a house. The thermostat is set to 20°C. When the room gets too cold, the heating switches on. When it gets too warm, the heating switches off. The room temperature stays near 20°C regardless of the weather outside. Your body does this for dozens of variables simultaneously: - Body temperature — maintained at ~37°C - Blood glucose — maintained at ~4–6 mmol/L - Blood pH — maintained at exactly 7.35–7.45 - Blood pressure — maintained at roughly 120/80 mmHg at rest - Blood sodium — maintained at 135–145 mmol/L Each variable has a set point — the normal target value. When the actual value drifts away, the body detects it and acts to bring it back. This detection-and-correction process is called a feedback loop.

Positive feedback amplifies a change rather than opposing it. It is used when the body needs to drive a process quickly to completion. Blood clotting: Vessel damage → clotting cascade begins → clotting factors recruit more clotting factors → rapid clot formation. Stops when the vessel is sealed and the stimulus (exposed vessel wall) is gone. Childbirth: Baby's head presses on cervix → oxytocin released → stronger contractions → more cervical pressure → more oxytocin. Escalates until birth, then stops. Nerve impulse: Stimulus opens sodium channels → Na⁺ rushes in → cell interior becomes more positive → MORE sodium channels open → explosive electrical signal generated. The key difference from negative feedback: Positive feedback is not self-correcting — it escalates until something stops it (the trigger disappears or a threshold is reached). It is useful for one-time events that need to happen rapidly.

Blood glucose must stay within 4–6 mmol/L. Two opposing hormones manage this: Insulin (from pancreatic beta cells): - Released when blood glucose RISES after eating - Tells cells to take up glucose; tells liver to store it as glycogen - Blood glucose falls back to normal Glucagon (from pancreatic alpha cells): - Released when blood glucose FALLS during fasting - Tells liver to break down glycogen and release glucose - Blood glucose rises back to normal What goes wrong in diabetes: - Type 1 — immune system destroys beta cells → no insulin → blood glucose rises unchecked - Type 2 — cells resist insulin → pancreas compensates, then fails → chronic high blood glucose Persistently high blood glucose damages blood vessels and nerves over time — causing blindness, kidney failure, and nerve damage.

About 60% of the body is water. The concentration of salts in that water must stay in a narrow range. Two hormones manage this: ADH (Antidiuretic Hormone): - Made by hypothalamus, released by pituitary - Released when blood is too concentrated (dehydrated) - Tells kidneys to reabsorb water → concentrated dark urine - Also triggers thirst Aldosterone: - Made by adrenal glands - Released when blood pressure or sodium falls - Tells kidneys to reabsorb sodium → water follows → blood volume expands → pressure rises Clinical failures: - SIADH — too much ADH → excess water retained → dangerously low sodium - Diabetes insipidus — too little ADH → massive dilute urine → severe dehydration - Conn's syndrome — excess aldosterone from adrenal tumour → high blood pressure + low potassium