Mechanism of Breathing
Breathing is the movement of air into and out of lungs. It has two phases:
- Inspiration (Inhalation): air enters lungs
- Expiration (Exhalation): air leaves lungs
Core Principle: Pressure Gradient Air always moves from high pressure → low pressure.
- Air enters lungs when intrapulmonary pressure (inside lungs) becomes less than atmospheric pressure.
- Air goes out when intrapulmonary pressure becomes more than atmospheric pressure.
Pressures Involved
1. Atmospheric pressure (Patm): pressure of outside air
2. Intrapulmonary pressure (Ppul): pressure inside alveoli/lungs
3. Intrapleural pressure (Pip): pressure in pleural cavity (between pleura)
Key points:
- Pip is always negative (lower than Patm) → helps keep lungs expanded.
- Changes in thoracic volume change lung volume, which changes Ppul.
Muscles Involved
1. Diaphragm (Main Muscle)
- Dome-shaped at rest
- Contracts → becomes flat → thoracic volume increases
2. Intercostal Muscles (Between ribs)
- External intercostals: help in inspiration (lift ribs up and out)
- Internal intercostals: help in forced expiration (pull ribs down and in)
Key point: Quiet breathing mainly uses diaphragm + external intercostals.
Inspiration (Active Process)
Inspiration is usually active because muscles contract.
Sequence (Step-by-step):
- Diaphragm contracts → flattens
- External intercostals contract → ribs + sternum move upward & outward
- Thoracic cavity volume increases (both antero-posterior and dorso-ventral dimensions)
- Lungs expand → pulmonary volume increases
- Intrapulmonary pressure decreases (Ppul < Patm)
- Air rushes into lungs until pressures equalize
Volume ↑ → Pressure ↓ → Air IN
Expiration (Usually Passive)
Normal expiration is usually passive due to elastic recoil.
Sequence (Step-by-step):
- Diaphragm relaxes → becomes dome-shaped again
- External intercostals relax → ribs move downward & inward
- Thoracic volume decreases
- Lungs recoil → pulmonary volume decreases
- Intrapulmonary pressure increases (Ppul > Patm)
- Air moves out of lungs
Volume ↓ → Pressure ↑ → Air OUT
Forced Breathing (During exercise, coughing, blowing)
Forced Inspiration
- Extra help from accessory muscles (neck/shoulder region) to lift ribs more
Forced Expiration (Active)
- Internal intercostals contract
- Abdominal muscles contract (push diaphragm upward)
- Thoracic volume decreases more → more air pushed out
Key point: Quiet expiration is passive, but forced expiration is active.
Breathing Rate and Spirometer
- Normal breathing rate: 12–16 breaths/minute
- Spirometer: measures lung volumes/capacities and helps assess pulmonary function.
Lung Volumes and Capacities
Volumes
- Tidal Volume (TV): ~500 mL (air in a normal breath)
- Inspiratory Reserve Volume (IRV): extra air inhaled forcibly after normal inspiration
- Expiratory Reserve Volume (ERV): extra air exhaled forcibly after normal expiration
- Residual Volume (RV): air left after forceful expiration
Why RV is important: prevents lung collapse and allows continuous gas exchange.
Capacities
- IC = TV + IRV
- EC = TV + ERV
- VC = IRV + TV + ERV (maximum air exhaled after maximum inhalation)
- TLC = VC + RV
Key point: Capacities are combinations of volumes.
Mechanism of Breathing
Golden Rule: Air moves High pressure → Low pressure
Inspiration:
- Diaphragm contracts (flat) + External intercostals contract (ribs up & out)
- Thoracic volume ↑ → Intrapulmonary pressure ↓ → Air IN
Expiration (quiet):
- Muscles relax + elastic recoil
- Thoracic volume ↓ → Intrapulmonary pressure ↑ → Air OUT
Forced Expiration: Internal intercostals + abdominal muscles
Intrapleural pressure: always negative (keeps lungs expanded)
TV: ~500 mL RV: prevents lung collapse
Capacities:
- VC = IRV + TV + ERV
- TLC = VC + RV
💡 Questions and Answers
Q1. What creates the pressure gradient for inspiration?
A1: When I inhale, my chest expands, so lung volume increases and the pressure inside lungs (intrapulmonary pressure) becomes less than atmospheric pressure. Then air automatically enters.
Key points:
- Volume ↑
- Ppul ↓ (below Patm)
- Air enters due to pressure difference
Q2. Describe the shape of the diaphragm during inspiration and expiration.
A2: During inhalation the diaphragm contracts and becomes flat. During exhalation it relaxes and becomes dome-shaped again.
Key points:
- Inspiration: flat diaphragm
- Expiration: dome-shaped diaphragm
Q3. Which muscles are involved in normal quiet breathing?
A3: In normal breathing, mainly the diaphragm and external intercostal muscles work.
Key points:
- Diaphragm + External intercostals
- Quiet expiration mostly passive
Q4. How does the thoracic volume change during inspiration?**
A4: Thoracic volume increases in two ways: the diaphragm increases it by moving down and the ribs increase it by moving up and out.
Key points:
- Diaphragm → increases volume (downward movement)
- Ribs/sternum → up & out by external intercostals
Q5. What is the role of a spirometer?
A5: A spirometer helps measure how much air I inhale/exhale and calculates lung volumes and capacities, so doctors can check lung function.
Key points:
- Measures lung volumes/capacities
- Used for pulmonary function assessment
Q6. Why does air enter lungs during inspiration?
A6: Because the pressure inside lungs becomes lower than outside, so air flows inside to equalize the pressure.
Key points:
- Ppul < Patm
- Air flows high → low pressure
Q7. Why is expiration considered passive in normal breathing?
A7: In normal exhalation, muscles relax and lungs automatically come back due to elastic recoil, pushing air out without extra effort.
Key points:
- Muscle relaxation
- Elastic recoil causes air out
Q8. Why is residual volume important?
A8: Residual volume is important because it keeps lungs from collapsing and helps gas exchange continue even between breaths.
Key points:
- Prevents lung collapse
- Maintains continuous gas exchange