Breathing and Exchange of Gases

🔬 How We Breathe: The Basics

Our respiratory system has two main parts:

  • 🎯 Conducting part: Carries air from nostrils to tiny airways (bronchioles). It warms, cleans, and humidifies air.
  • 🔄 Exchange part: Alveoli (tiny air sacs) where oxygen (O_2) and carbon dioxide (CO_2) swap between air and blood.

🛡️ The Lung Chamber

Lungs sit in an air-tight thoracic chamber made of:

  • Backbone (dorsal)
  • Breastbone (ventral)
  • Ribs (sides)
  • Dome-shaped diaphragm (floor)

When this chamber expands/shrinks, lungs follow! We can’t directly change lung size—it depends on the thoracic cavity.

💨 Steps of Respiration

  1. Breathing: Inhaling air → Exhaling CO_2-rich air.
  2. Gas diffusion: O_2/CO_2 move across alveoli walls.
  3. Gas transport: Blood carries gases.
  4. Tissue exchange: Gases swap between blood and body cells.
  5. Cellular respiration: Cells use O_2 to make energy, releasing CO_2.

📉 Mechanism of Breathing

Breathing = Inspiration (inhale) + Expiration (exhale). Air moves due to pressure differences:

  • ✅ Inspiration: Lungs pressure < atmospheric pressure → Air rushes in.
  • ❌ Expiration: Lungs pressure > atmospheric pressure → Air pushed out.

🎭 How Muscles Help

During inspiration:

  • Diaphragm contracts → flattens ↓
  • External intercostal muscles lift ribs up ↗️
  • Result: Thoracic volume ↑ → Lung volume ↑ → Pressure ↓ → Air enters!

During expiration:

  • Diaphragm relaxes → arches up ↑
  • Ribs/sternum drop ↘️
  • Result: Thoracic volume ↓ → Lung volume ↓ → Pressure ↑ → Air exits!

Fun fact: We breathe 12-16 times/minute! 😮

📏 Lung Volumes & Capacities

Key volumes (average values):

  • Tidal Volume (TV): Normal breath air → 500 mL
  • Inspiratory Reserve Volume (IRV): Extra air after deep inhale → 2500-3000 mL
  • Expiratory Reserve Volume (ERV): Extra air after forced exhale → 1000-1100 mL
  • Residual Volume (RV): Air left after forced exhale → 1100-1200 mL

Important capacities:

  • Vital Capacity (VC): Max air exhaled after deep inhale = ERV + TV + IRV
  • Total Lung Capacity (TLC): Total air in lungs after deep inhale = RV + ERV + TV + IRV

🌬️ Gas Exchange

Gases move by simple diffusion (high → low concentration) in:

  1. Alveoli (air ↔ blood)
  2. Tissues (blood ↔ cells)

Partial pressure (pO_2, pCO_2) drives this! Check the pressures:

GasAtmosphereAlveoliDeoxygenated BloodOxygenated BloodTissues
O_2 (mm Hg)159104409540
CO_2 (mm Hg)0.340454045

Note: O_2 flows alveoli → blood → tissues. CO_2 flows tissues → blood → alveoli! 🔄

🚨 Important Concepts for NEET

  1. Muscle roles in breathing: Diaphragm + intercostals control pressure gradients.
  2. Respiratory volumes/capacities: Memorize TV, IRV, ERV, RV, VC, TLC definitions & values.
  3. Gas exchange principles: Partial pressure gradients (use the table!) and diffusion factors.
  4. Pressure changes: How lung volume affects pressure during inhale/exhale.

Keep practicing—you’re doing great! 💪