PHYSYRA

See the body respond, second by second.

Welcome to Physyra. Choose a scenario and press play to watch the body respond step by step. Click any part of the body to learn its anatomy. Use Explore to change the body yourself, then test what you know with Challenge and Cases.

What the body is dealing with

Time and steps

About Physyra

What it is, how it works, where the numbers come from, and how to use it in class.

For students and teachers

Physyra turns physiology into something you can watch. Each scenario runs a connected model of the heart, blood vessels, lungs, nerves and temperature control, so you can see not only what changes but the order in which it changes and why.

Ideas it helps teach

  • Homeostasis and negative feedback, shown as a live control loop for each scenario.
  • How organ systems depend on each other: one change spreads to the rest of the body.
  • Why timing matters: fast nerve reflexes, slower chemical and hormonal responses, and very slow fluid and heat changes.
  • How bodies differ: compare a typical adult with a trained athlete or an older adult.

Ways to use it in class

  • Share a link that opens a specific scenario (use Copy link), or send students straight to Explore, Challenge or Cases.
  • Ask students to predict before pressing play, then explain any difference.
  • Export the simulated data as a CSV file for graphing and lab-style write-ups.

Learning goals by scenario

Stand upVenous pooling, the baroreflex, why pulse pressure narrows, and why older adults are prone to dizziness on standing.
Blood lossCompensated shock, the Frank–Starling mechanism, and why a normal blood pressure can hide serious loss.
Hold your breathCO₂ as the main driver of breathing, the oxygen–hemoglobin curve, and the diving reflex.
Acute stressThe sympathetic–adrenal response and the parasympathetic rebound.
FeverThe hypothalamic set point, skin vasoconstriction, shivering, and sweating when the fever breaks.
Cold exposureHeat conservation and production, and how hypothermia begins.
Heat & dehydrationSweating, plasma volume, and cardiovascular drift.
ExerciseCentral command, cardiac output, muscle blood flow and heat production.

How the model works

Physyra uses a small set of linked equations rather than a full-body simulation, so each relationship can be seen and explained.

  • Mean arterial pressure is cardiac output multiplied by vascular resistance. Cardiac output is heart rate times stroke volume.
  • Stroke volume follows venous return (the Frank–Starling mechanism) and contractility, and shrinks when blood pools, volume drops, or the heart beats very fast.
  • A baroreflex compares pressure with a set point and adjusts sympathetic drive after a short delay, which sets heart rate and vessel tone.
  • Arterial CO₂ is CO₂ production divided by ventilation. Lung oxygen falls during a breath-hold, and saturation follows an S-shaped hemoglobin curve.
  • Core temperature is heat produced minus heat lost, with skin blood flow, sweating and shivering driven by the gap from the hypothalamic set point.
  • A typical adult at rest starts at about 65 beats per minute, a 75 mL stroke volume, a 93 mmHg mean pressure and 5 litres of blood.

Limits

The numbers are illustrative and were tuned to match textbook ranges, not measured from real people. The model leaves out hormones, kidneys, individual differences and disease. It is a teaching tool, not a diagnostic tool, and it should be reviewed by a qualified instructor before being used for assessment.

Sources

Concepts and typical values are based on standard references. Please check editions and page numbers against your own copies.

  1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier.
  2. Boron WF, Boulpaep EL. Medical Physiology. Elsevier.
  3. Rowell LB. Human Cardiovascular Control. Oxford University Press, 1993.
  4. Rowell LB. Human cardiovascular adjustments to exercise and thermal stress. Physiological Reviews, 1974;54(1):75–159.
  5. Parkes MJ. Breath-holding and its breakpoint. Experimental Physiology, 2006;91(1):1–15.
  6. Boulant JA. Role of the preoptic-anterior hypothalamus in thermoregulation and fever. Clinical Infectious Diseases, 2000;31(Suppl 5):S157–S161.
  7. American College of Surgeons. Advanced Trauma Life Support (hemorrhage classification).
  8. Tortora GJ, Derrickson B. Principles of Anatomy and Physiology. Wiley.
  9. Drake RL, Vogl AW, Mitchell AWM. Gray’s Anatomy for Students. Elsevier.

Physyra uses simplified models of physiology to show how systems influence each other. The numbers are illustrative, not clinical, and nothing here is medical advice, diagnosis or treatment guidance. Real bodies vary a great deal.