Physiology Foundations: Clinical Reasoning with Homeostasis—From Symptom to Loop Failure

Clinical reasoning with homeostasis: from symptom to loop failure

Clinical problems often look like a list of symptoms (fever, dizziness, sweating, low blood pressure). A homeostasis-based approach turns that list into a structured question: which regulated variable is off, what compensations should be happening, and where in the control system could the failure be (sensor, integrator, effector, or signal pathway). This chapter gives a reusable method and then applies it to common beginner-friendly scenarios.

What you are trying to identify

• Regulated variable: the quantity the body tries to keep within a range (e.g., core temperature, plasma glucose, arterial pressure/effective circulating volume).
• Direction of change: too high or too low relative to the usual range for that person.
• Expected compensation: predictable responses that should push the variable back toward its range (e.g., sweating to lower temperature; tachycardia/vasoconstriction to support blood pressure; counterregulatory hormones to raise glucose).
• Failure point: the most likely broken link—sensor (detection), integrator (decision/set-point control), effector (organs doing the work), or signal pathway (hormone/nerve delivery or receptor response).

(1) Reusable diagnostic flowchart

Use this flowchart whenever you see a symptom that could reflect a control-system disturbance.

START: Patient has symptom(s) suggesting dysregulation (e.g., hot, shaky, dizzy, thirsty).  |  Step 1 — Pick the regulated variable most directly linked to the symptom.  |  Step 2 — Measure/estimate it (vitals, point-of-care tests, exam clues).  |  Step 3 — Decide direction: HIGH or LOW?  |  Step 4 — Predict expected compensation for that direction.  |  Step 5 — Compare with what you observe:      A) Compensation present and appropriate?         - YES → likely primary disturbance is upstream (load/driver), loop intact.         - NO  → suspect failure within loop.      B) Compensation present but mismatched (wrong direction)?         - suspect integrator/set-point problem or misleading sensor input.  |  Step 6 — Localize failure point:      - Sensor failure: variable abnormal but detection absent/inaccurate.      - Integrator failure: wrong target/command; responses inappropriate for actual variable.      - Signal pathway failure: command generated but not transmitted/received.      - Effector failure: command arrives but organ cannot execute response.  |  Step 7 — Build a cause-and-effect chain and test it with one more data point. END

Quick mapping: what “appropriate compensation” looks like

In real patients, you will not see every compensation. The key is whether the pattern you do see makes sense for the measured direction of the variable.

(2) Worked cases with explicit cause-and-effect chains

Case A: Fever vs. hyperthermia (same symptom, different loop logic)

Presentation 1: A patient with chills, feeling cold, and shivering. Temperature is 39.0°C. Skin feels cool; patient wants blankets.

• Step 1–3 (variable and direction): Core temperature is high.
• Step 4 (expected compensation if temperature is high): The body should try to lose heat (sweating, vasodilation, seeking cool environment).
• Step 5 (compare to observed): Observed responses are heat-gaining (chills, shivering, seeking warmth). That is the wrong direction for a truly “too hot” state.
• Step 6 (localize failure point): Wrong-direction responses point to an integrator/set-point issue: the system is behaving as if the body is “too cold” relative to a raised target.

Cause-and-effect chain (fever):

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• Pyrogenic signals (e.g., from infection/inflammation) → integrator raises the target temperature → current temperature is now “below target” → shivering/vasoconstriction/behavioral warming → temperature rises to the new target → later, when the target returns toward normal, sweating/vasodilation may occur.

Presentation 2: A patient after exertion in a hot environment: hot, flushed, confused. Temperature is 40.5°C. Skin is hot; sweating may be absent or present early then stop.

• Step 1–3: Core temperature is high.
• Step 4: Expected compensation: heat loss.
• Step 5: If you see strong heat-loss efforts (profuse sweating, vasodilation) but temperature remains high, compensation is appropriate but insufficient.
• Step 6: This suggests the loop is intact but overwhelmed by heat load, or an effector failure (e.g., inability to sweat effectively, impaired heat dissipation) or signal pathway issue (e.g., impaired autonomic output).

Cause-and-effect chain (hyperthermia conceptually):

• Excess heat production/heat gain + limited heat loss capacity → temperature rises above normal → integrator commands heat loss → effectors attempt sweating/vasodilation → if environment/hydration/physiology prevents adequate heat loss, temperature continues to rise despite appropriate commands.

Clinical reasoning takeaway: The same measured variable (high temperature) can show either mismatched compensation (points to integrator/set-point change) or matched but failing compensation (points to effector limitation or overwhelming load).

Case B: Hypoglycemia from insulin excess (appropriate compensation that can’t win)

Presentation: A person with diabetes accidentally takes too much rapid-acting insulin. They develop sweating, tremor, palpitations, hunger, confusion. Fingerstick glucose: 45 mg/dL (2.5 mmol/L).

• Step 1–3: Regulated variable: plasma glucose is low.
• Step 4: Expected compensation: signals that raise glucose availability (autonomic symptoms prompting eating; hormonal counterregulation increasing glucose output and limiting uptake).
• Step 5: Observed: sweating/tremor/palpitations/hunger are appropriate for low glucose (they are part of the body’s attempt to correct it).
• Interpretation: The loop is trying to compensate, but the driver (excess insulin effect) is pushing glucose down faster than compensation can restore it.
• Step 6 (failure point): Not primarily a sensor/integrator failure; it is an external effector-like force overriding the system (pharmacologic insulin acting as a powerful “effector” promoting glucose uptake/storage).

Cause-and-effect chain:

• Excess insulin effect → increased glucose uptake/storage + reduced hepatic glucose output → plasma glucose falls → brain and peripheral sensors detect low glucose → autonomic activation produces warning symptoms and drives food-seeking → counterregulatory hormones attempt to raise glucose → if insulin effect remains high and intake is delayed, glucose stays low → neuroglycopenic symptoms worsen.

How to test your chain with one more data point: Ask: “Are warning symptoms present?” If yes, detection and signaling are likely intact. If a patient has low glucose without warning symptoms, consider a sensor/signaling problem (e.g., impaired autonomic response) rather than simply insulin excess.

Case C: Dehydration and blood pressure compensation (appropriate compensation indicates volume problem)

Presentation: Several days of vomiting/diarrhea and poor intake. Patient feels dizzy on standing. Exam: dry mucous membranes, decreased skin turgor. Vitals: HR 110, BP 95/60 lying; standing BP drops further with symptoms.

• Step 1–3: Regulated variable to focus on: effective circulating volume/arterial pressure is functionally low (especially on standing).
• Step 4: Expected compensation: increased heart rate, vasoconstriction, thirst, reduced urine output.
• Step 5: Observed: tachycardia and orthostatic symptoms suggest the body is attempting compensation but cannot maintain pressure with position change.
• Step 6: Most likely problem is not the integrator; it is a loss of volume (the “load” on the system). The effectors may be working, but there is insufficient fluid to support pressure.

Cause-and-effect chain:

• Fluid loss → reduced venous return → reduced stroke volume → arterial pressure drops (especially upright) → sensors detect reduced pressure/stretch → integrator increases sympathetic output and fluid-conserving signals → heart rate rises and vessels constrict → partial support of pressure but may be insufficient until volume is restored.

Variant to localize a different failure point: If a dehydrated patient has low blood pressure without tachycardia, ask whether the expected compensation is absent. That pattern can suggest a signal pathway issue (e.g., impaired autonomic output) or an effector issue (e.g., heart unable to increase rate/contractility, or medications blocking responses).

(3) Practice vignettes: identify variable, compensation, and likely failing component

For each vignette, answer three questions: (1) regulated variable, (2) is compensation appropriate, (3) most likely failure point (sensor, integrator, signal pathway, effector, or overwhelming load).

Vignette 1

A child has a temperature of 39.2°C and is wrapped in blankets, shivering, and says they feel cold. Hands and feet are cool.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Vignette 2

A marathon runner collapses on a hot day. Temperature is 40.3°C. Skin is hot and flushed. They are confused. Early reports say they were sweating heavily, then sweating decreased.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Vignette 3

A person with diabetes is found shaky and sweaty. Glucose is 48 mg/dL. They report taking their usual insulin dose but skipped lunch.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Vignette 4

A person with diabetes has glucose of 45 mg/dL but reports no palpitations, tremor, or sweating. They only notice they are confused and tired.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Vignette 5

An older adult has dizziness when standing. Lying BP 100/65, HR 105. Standing BP 80/55 with worse symptoms; HR rises to 120. Mucous membranes are dry.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Vignette 6

A patient has low blood pressure (85/55) and feels lightheaded. Heart rate is 55. They recently started a medication that limits heart rate increases. They have no signs of dehydration.

• Regulated variable: __________
• Compensation appropriate? __________
• Most likely altered component: __________

Answer key with cause-and-effect hints (use after attempting)

Step-by-step template you can reuse on new problems

• 1) Name the variable in one phrase: “This looks like a problem of ____.”
• 2) Anchor with one measurement: a vital sign, point-of-care test, or exam proxy.
• 3) Predict 2–3 compensations you would expect if the variable is high vs. low.
• 4) Check the pattern: Are the compensations present, absent, or reversed?
• 5) Localize: reversed → integrator/sensor mismatch; absent → sensor/signal/effector failure; present but insufficient → overwhelming load or effector limitation.
• 6) Write a short chain using arrows (A → B → C) and identify one additional observation that would support or refute it.