The Evolution of Lungs

Introduction

• Focus on an ancient fishy ancestor
• Lived around 380 million years ago
• Key milestone: Breathing air

Early Life and Diffusion

• Early life evolved in water
• Simple forms like bacteria used diffusion for oxygen
  • Diffusion: Movement from high concentration to low concentration
• Larger organisms needed more oxygen and thus, more efficient methods

Development of Circulatory and Respiratory Systems

• Need for circulatory and respiratory systems for larger organisms
• Respiratory surfaces moved to the inside
• Development of gills

Transition to Lungs

• Earth’s environment changed (warmer, shallower seas with lower oxygen)
• Evolution of lungs in lobe-finned fish
  • New interface for gas exchange
  • Led to the diversification of land-dwelling vertebrates

Human and Animal Lungs

• Modern vertebrates evolved lungs
  • Protective ribs
  • Stiff trachea
  • Strong diaphragm (in mammals)
  • Form the respiratory system
• Respiratory and circulatory systems work together
• Cellular respiration enabled by bulk flow and simple diffusion

The Mechanism of Breathing

• Imagining life without lungs
  • Difficulty in oxygen diffusion across large distances

Principles of Diffusion and Bulk Flow

• Short distance & fewer obstacles key to efficient diffusion
• Oxygen molecule’s path to a neuron (example of poor diffusion)
• Bulk flow: Quick transportation of large quantities
  • Draws in 100 quintillion oxygen molecules in a breath
  • Filters down to alveoli in the lungs for close diffusion to blood

Anatomy of the Respiratory System

Conducting Zone

• Nose: Supported by bones, filters air, warms, and moistens
• Sinuses: Aid in filtering, warming, and moistening air
• Shared pathway with esophagus (epiglottis controls the pathway)
• Trachea: Rigid with rings, prevents collapse from negative pressure
• Bronchi: Split from trachea into two main tubes

Respiratory Zone

• Bronchioles narrow down into alveolar ducts and sacs
• Alveoli: Site of gas exchange, lined with wet membranes
  • Oxygen dissolves in membrane, diffuses into blood
  • CO2 diffuses out to be exhaled
• 700 million alveoli in lungs, providing 75 m² surface area

Summary

• Respiration relies on diffusion and bulk flow
• Anatomy and mechanisms evolved over 400 million years

Credits

• Thank you to Patreon patrons and contributors to Crash Course