explain the principles of homeostasis in terms of internal and external stimuli, receptors, coordination systems (nervous system and endocrine system), effectors (muscles and glands) and negative feedback

Homeostasis in Mammals – Cambridge A‑Level Biology 9700 (Topic 14)

1. What is Homeostasis?

Definition – Homeostasis is the collection of processes that keep the internal environment of an organism within narrow limits (the set‑point) despite continual changes in the external environment.

It is essential because enzymes, cells and whole organ systems work optimally only within a restricted range of temperature, pH, osmolarity, glucose concentration, etc.

2. Key Quantitative Concepts

Set‑point – the ideal value for a physiological variable (e.g. 37.0 °C core temperature, 5.5 mmol L⁻¹ blood glucose).

Range – the normal physiological window around the set‑point (e.g. 36.5–37.5 °C for temperature).

Deviation – the difference between the measured value and the set‑point (positive or negative).

Feedback gain – the proportion of the deviation corrected in one feedback cycle. A high gain restores the set‑point quickly but may cause overshoot.

3. Stimuli

Any change that threatens internal balance is a stimulus. Stimuli are divided into:

  • Internal stimuli – rise in blood glucose, fall in arterial O₂, fall in blood pH, increase in blood pressure, change in plasma osmolarity.

  • External stimuli – ambient temperature, light intensity, availability of food or water, mechanical injury, humidity.

4. Receptors (Sensors)

Receptors detect a stimulus and convert it into a nerve impulse (electrical) or a chemical signal (hormone). The main transduction mechanisms are ion‑channel opening or G‑protein‑coupled receptor (GPCR) pathways.

Receptor typeLocationStimulus detected
ThermoreceptorsSkin, hypothalamic pre‑optic areaTemperature change
Chemoreceptors (carotid & aortic bodies)Carotid sinus, aortic archPaO₂, PaCO₂, blood pH
BaroreceptorsCarotid sinus, aortic archArterial pressure (stretch)
OsmoreceptorsHypothalamic supra‑optic nucleusPlasma osmolarity
Proprioceptors (muscle spindles, Golgi tendon organs)Muscle & tendonMuscle length & tension
Mechanoreceptors (bladder, skin)Urinary bladder wall, skinStretch or pressure
Pancreatic β‑cells (glucose sensors)Islets of LangerhansBlood glucose (via GLUT2 uptake)

5. Coordination Systems

5.1 Nervous System – fast, short‑term control

  • Stimulus → afferent neuron → specialised control centre (e.g. hypothalamic thermoregulatory centre).

  • Control centre integrates the input and sends rapid efferent signals via somatic or autonomic motor neurons.

  • Typical rapid responses: shivering, vasoconstriction/dilation, changes in heart rate and ventilation.

5.2 Endocrine System – slower, long‑term control

  • Control centres (hypothalamus, anterior/posterior pituitary) release hormones into the bloodstream.

  • Hormones travel to distant target cells, bind specific receptors and modify gene expression or enzyme activity.

  • Typical longer‑term responses: alteration of basal metabolic rate, regulation of water balance, chronic glucose control.

5.3 Integration of Nervous & Endocrine Pathways

Most homeostatic responses involve both systems working together.

  • Hypothalamic‑Pituitary‑Adrenal (HPA) axis – stress (external stimulus) → CRH (hypothalamus) → ACTH (anterior pituitary) → cortisol (adrenal cortex). Cortisol raises blood glucose and suppresses immune activity; high cortisol feeds back negatively on hypothalamus and pituitary.

  • Hypothalamic‑Pituitary‑Thyroid (HPT) axis – cold exposure → TRH (hypothalamus) → TSH (anterior pituitary) → T₃/T₄ (thyroid) → increased basal metabolic rate and heat production.

  • Renin‑Angiotensin‑Aldosterone System (RAAS) – fall in arterial pressure → renin release (kidney) → angiotensin II formation → vasoconstriction + aldosterone secretion (adrenal cortex) → Na⁺ and water re‑absorption → blood‑volume and pressure restored.

6. Effectors

Effectors are organs, tissues or cells that carry out the corrective action.

  • Muscles – skeletal muscle shivering (heat), smooth muscle in arterioles (vasoconstriction/dilation), skeletal muscle contraction (proprioceptive feedback).

  • Glands – sweat glands (evaporative cooling), pancreatic islets (insulin, glucagon), adrenal medulla (adrenaline), thyroid (thyroid hormones).

  • Kidneys – adjust water re‑absorption (ADH) and Na⁺ re‑absorption (aldosterone) to control blood volume.

  • Blood vessels – change diameter to regulate heat loss, blood pressure and tissue perfusion.

7. Feedback Mechanisms

7.1 Negative Feedback (the dominant mechanism)

The response reduces the original deviation and the system stabilises.

  1. Stimulus moves a variable away from its set‑point.

  2. Receptor detects the deviation.

  3. Signal is transmitted to a control centre.

  4. The centre activates an appropriate effector.

  5. The effector’s action opposes the original change.

  6. The variable returns toward the set‑point; reduced receptor firing terminates the response.

7.2 Positive Feedback – when it is useful

Positive feedback amplifies a change and is employed only when a rapid, self‑reinforcing response is advantageous.

  • Oxytocin during labour – uterine stretch → oxytocin release → stronger contractions → more stretch.

  • Platelet aggregation – damaged vessel exposes collagen → platelets release ADP → recruitment of additional platelets.

Both loops are terminated by a separate mechanism (delivery of the baby, clot formation) that restores homeostasis.

8. Hormones Central to Homeostasis (summary)

HormonePrimary homeostatic roleMain target organ(s)
InsulinLowers blood glucoseMuscle, adipose tissue, liver
GlucagonRaises blood glucoseLiver
Adrenaline (epinephrine)Rapid increase in heart rate, bronchodilation, glycogenolysisHeart, lungs, liver, muscle
ADH (vasopressin)Conserves waterKidney collecting ducts
AldosteroneIncreases Na⁺ re‑absorption (and K⁺ secretion)Distal nephron
Thyroid hormones (T₃, T₄)Raise basal metabolic rate → heat productionAll cells (increase mitochondrial activity)
CortisolMaintains blood glucose during prolonged stress; anti‑inflammatoryLiver, immune cells
Renin, Angiotensin II, Aldosterone (RAAS)Restore blood volume and pressureKidney, blood vessels, adrenal cortex

9. Example: Regulation of Body Temperature

  1. Stimulus – ambient temperature falls below the hypothalamic set‑point (~37 °C).

  2. Receptors – cutaneous thermoreceptors and hypothalamic thermosensitive neurons increase firing as temperature drops.

  3. Control centre – pre‑optic area of the hypothalamus integrates the input.

  4. Integration of pathways

    • Nervous: Sympathetic outflow → skeletal‑muscle shivering and vasoconstriction of cutaneous arterioles (rapid heat production & reduced loss).

    • Endocrine: HPT axis → TRH → TSH → T₃/T₄ → increased basal metabolic rate (long‑term heat production).

  5. Effectors – shivering muscles, constricted skin vessels, thyroid hormones.

  6. Negative feedback – as core temperature approaches 37 °C, thermoreceptor firing declines, sympathetic and hormonal output fall, and the system stabilises.

10. Practical Links (Paper 5 skills)

  • Measure core temperature (rectal/tympanic) and plot temperature vs. time to illustrate a negative‑feedback curve.

  • Monitor blood glucose with a glucometer; calculate deviation from the set‑point and evaluate insulin/glucagon responses.

  • Use ELISA or radio‑immunoassay to quantify hormones (e.g., ADH, cortisol) in stress or dehydration experiments.

  • Design an experiment to test the effect of ambient temperature on shivering intensity – formulate hypothesis, control variables, collect quantitative data.

11. Summary Table of Homeostatic Components

ComponentFunctionTypical mammalian example
StimulusDisturbs the internal environment.Cold ambient temperature, high blood glucose, low arterial O₂, low plasma osmolarity.
Receptor (sensor)Detects the stimulus and generates an electrical or chemical signal.Thermoreceptors, carotid body chemoreceptors, osmoreceptors, baroreceptors, pancreatic β‑cells.
Control centreIntegrates signals and decides on a response.Hypothalamic nuclei, anterior pituitary, kidney juxtaglomerular cells (renin release).
EffectorExecutes the corrective action.Shivering skeletal muscle, sweat glands, renal tubules, arterioles, adrenal cortex.
Feedback typeTerminates the response once the set‑point is restored.Negative feedback (temperature, glucose, blood pressure); positive feedback (oxytocin in labour).

12. Suggested Diagram (to be drawn by students)

Flow diagram of a negative‑feedback loop:

Stimulus → Receptor → Control centre → Effector → Response → Restored set‑point (arrow back to Receptor)