# Flight Physiology Basics

> The atmosphere, respiration, and gas laws relevant to aviation.

- **Subject:** 040 — Human Performance Limitations
- **Section:** 040-02 — Basics of Aviation Physiology and Health Maintenance
- **Topic code:** 040-02-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/human-performance-limitations/basics-of-aviation-physiology-and-health-maintenance/basics-of-flight-physiology/

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## The Atmosphere
Standard Atmosphere (ISA) assumes:
*   **Pressure**: 1013.25 hPa at MSL, decreasing with altitude.
*   **Temperature**: +15°C at MSL, decreasing by 2°C/1000 ft (1.98°C) up to the Tropopause (36,090 ft).
*   **Composition**: 78% Nitrogen, 21% Oxygen, 1% other gases. (Constant up to ~70,000 ft).

## Gas Laws
*   **Boyle's Law ($P_1 V_1 = P_2 V_2$)**: Pressure and volume are inversely proportional.
    *   Relevance: Trapped gases expand on ascent (ears, sinuses, teeth, stomach, intestine).
*   **Dalton's Law ($P_{Total} = P_1 + P_2 + ...$)**: Total pressure is the sum of partial pressures.
    *   Relevance: As altitude increases, total pressure drops, so the partial pressure of oxygen ($ppO_2$) drops, leading to Hypoxia.
*   **Henry's Law**: The amount of gas dissolved in a liquid is proportional to its partial pressure.
    *   Relevance: Decompression Sickness (DCS). Nitrogen bubbles form in blood when pressure drops rapidly (Scuba diving + Flying).

## Hypoxia

![Students in hypobaric chamber simulating hypoxia](https://upload.wikimedia.org/wikipedia/commons/a/a7/US_Navy_090603-N-6651N-001_Students_sit_in_the_hypobaric_chamber_at_the_Aviation_Survival_Training_Center_to_simulate_the_effects_of_high_altitude_on_the_body._The_students_determine_what_their_individual_hypoxia_symptoms_are.jpg)

A state of oxygen deficiency in the body.
1.  **Hypoxic Hypoxia**: Insufficient oxygen in the air (Altitude).
2.  **Anemic Hypoxia**: Blood cannot carry oxygen (CO Poisoning, Smoking reduces hemoglobin efficiency).
3.  **Stagnant Hypoxia**: Blood not moving (G-forces, Heart failure, Cold).
4.  **Histotoxic Hypoxia**: Cells cannot use oxygen (Alcohol, Cyanide).

**Symptoms**: Euphoria, cyanosis (blue lips/fingernails), impaired judgment, tunnel vision.
**Time of Useful Consciousness (TUC)**: Decreases rapidly with altitude (e.g., FL300: 1-2 mins; FL400: 15-20 secs).

## Hyperventilation
Over-breathing usually caused by stress/anxiety.
*   **Effect**: Flushes too much CO2 from the blood (Hypocapnia). Blood becomes alkaline.
*   **Symptoms**: Dizziness, tingling in hands/feet, visual tunneling (similar to hypoxia).
*   **Action**: Slow breathing rate.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.