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Respiratory Physiology Overview

Jun 12, 2025

Overview

This lecture provides an overview of respiratory physiology, focusing on the primary functions of the respiratory system, major gas laws, mechanics of breathing, and clinical relevance.

Purpose of the Respiratory System

  • The respiratory system brings oxygen into the body and removes carbon dioxide.
  • Oxygen is inhaled, transferred to the bloodstream, delivered to tissues, and supports ATP production.
  • Carbon dioxide, a byproduct of cellular respiration, moves from tissues to blood, then to lungs and is exhaled.

Atmospheric Pressure and Its Significance

  • Atmospheric pressure at sea level is 760 mmHg.
  • Gas pressure is the force that gases exert on the walls of a container.
  • Higher altitudes have lower atmospheric pressure due to fewer air molecules.

Gas Laws in Respiration

  • Dalton’s Law: The total pressure of a gas mixture equals the sum of individual partial pressures.
  • Nitrogen is 78% (≈593 mmHg), oxygen 21% (≈160 mmHg), and carbon dioxide 0.03% (≈0.3 mmHg) of atmospheric gases.
  • Each gas moves down its own pressure gradient (from high to low partial pressure).

Mechanics of Breathing (Boyle’s Law)

  • Boyle’s Law: Pressure and volume of a gas are inversely related.
  • Increasing thoracic volume (by contracting diaphragm/external intercostals) decreases lung pressure, allowing air in (inspiration).
  • Decreasing thoracic volume (relaxing muscles) increases pressure, pushing air out (expiration).
  • Pleural cavity creates lower pressure, keeping lungs adhered and following chest movements.
  • Accessory muscles (e.g., abdominal, internal intercostals) are used for deep or forceful breathing.

Gas Exchange and Henry’s Law

  • Henry’s Law: Gas dissolves in liquid proportional to its partial pressure gradient.
  • Oxygen partial pressure drops from 160 mmHg (air) to 104 mmHg (alveoli) to 40 mmHg (tissues).
  • CO₂ partial pressure rises from 0.3 mmHg (air) to 40 mmHg (alveoli) to 45 mmHg (tissues).
  • Gases diffuse down their pressure gradients: O₂ from alveoli to blood to tissues; CO₂ from tissues to blood to alveoli.

Structural and Clinical Aspects

  • Alveoli are lined with water, causing surface tension that could collapse them.
  • Type II pneumocytes secrete surfactant to reduce surface tension and prevent alveolar collapse.
  • In emphysema, loss of elastic tissue causes lungs to remain inflated and airways to collapse during forceful exhalation.

Phases of Respiration

  • Ventilation: Moving air into and out of lungs.
  • External Respiration: Gas exchange between alveoli and blood.
  • Gas Transport: Moving gases through blood.
  • Internal Respiration: Gas exchange between blood and tissues.

Key Terms & Definitions

  • Partial Pressure — Pressure exerted by an individual gas in a mixture.
  • Ventilation — Movement of air in and out of the lungs.
  • External Respiration — Gas exchange between alveoli and capillaries.
  • Internal Respiration — Gas exchange between blood and tissue cells.
  • Surfactant — Substance that reduces surface tension in alveoli, preventing collapse.
  • Emphysema — Disease causing destruction of elastic tissue in lungs.

Action Items / Next Steps

  • Review gas laws (Dalton’s, Boyle’s, Henry’s).
  • Memorize key partial pressures of O₂ and CO₂ at different body sites.
  • Understand the differences between ventilation and respiration.
  • Study the role of surfactant and its clinical significance.

Full transcript