Respiration is a vital biological process that enables organisms to release energy from food. It involves two major steps:
- Breathing (Ventilation): Intake of oxygen (O₂) and release of carbon dioxide (CO₂)
- Cellular Respiration: Oxidation of food to release energy
This chapter focuses on the mechanism of breathing, gas exchange, transport of gases, and regulation of respiration in humans.
What is Breathing?
Breathing is the physical process of inhalation and exhalation.
- Inhalation (Inspiration): Intake of oxygen-rich air
- Exhalation (Expiration): Removal of carbon dioxide-rich air
It is different from respiration, which is a chemical process.
Respiratory Organs in Humans
The human respiratory system includes:
- Nostrils
- Nasal cavity
- Pharynx
- Larynx
- Trachea
- Bronchi and bronchioles
- Lungs
Key Components
1. Nostrils & Nasal Cavity
- Filter dust using hairs and mucus
- Warm and moisten air
2. Pharynx
- Common passage for food and air
3. Larynx (Voice Box)
- Produces sound
- Prevents food entry using epiglottis
4. Trachea (Windpipe)
- Supported by C-shaped cartilage rings
- Divides into bronchi
5. Bronchi and Bronchioles
- Conduct air into lungs
6. Lungs
- Pair of spongy organs
- Right lung: 3 lobes
- Left lung: 2 lobes
Alveoli: Site of Gas Exchange
- Tiny air sacs (≈300 million in humans)
- Provide large surface area (~70 m²)
- Surrounded by capillaries
Features of alveoli:
- Thin walls (one cell thick)
- Moist surface
- Rich blood supply
Mechanism of Breathing
Breathing involves pressure changes in the thoracic cavity.
Inhalation (Inspiration)
- Diaphragm contracts → becomes flat
- External intercostal muscles contract
- Thoracic cavity volume increases
- Pressure decreases
- Air enters lungs
Exhalation (Expiration)
- Diaphragm relaxes → dome-shaped
- Intercostal muscles relax
- Thoracic volume decreases
- Pressure increases
- Air is expelled
Respiratory Volumes and Capacities
Respiratory Volumes
- Tidal Volume (TV): ~500 mL (normal breathing)
- Inspiratory Reserve Volume (IRV): Extra inhaled air (~2500–3000 mL)
- Expiratory Reserve Volume (ERV): Extra exhaled air (~1000–1100 mL)
- Residual Volume (RV): Air remaining in lungs (~1100–1200 mL)
Respiratory Capacities
- Vital Capacity (VC): TV + IRV + ERV
- Total Lung Capacity (TLC): VC + RV
Exchange of Gases
Gas exchange occurs by diffusion.
Exchange in Alveoli
- Oxygen diffuses from alveoli → blood
- Carbon dioxide diffuses from blood → alveoli
Based on partial pressure gradients:
| Gas | Alveoli | Blood |
|---|---|---|
| O₂ | High | Low |
| CO₂ | Low | High |
Exchange in Tissues
- O₂ moves from blood → tissues
- CO₂ moves from tissues → blood
Transport of Gases
Transport of Oxygen
- 97% transported by hemoglobin as oxyhemoglobin
- 3% dissolved in plasma
Transport of Carbon Dioxide
- 70% as bicarbonate ions (HCO₃⁻)
- 20–25% as carbaminohemoglobin
- 5–7% dissolved in plasma
Oxygen Dissociation Curve
- Sigmoid-shaped curve
- Shows relation between PO₂ and hemoglobin saturation
Factors affecting curve:
- pH
- Temperature
- CO₂ concentration
Bohr Effect:
Increase in CO₂ shifts curve right → more O₂ released
Regulation of Respiration
Respiration is controlled by:
Respiratory Centers (Brain)
- Located in medulla oblongata and pons
- Regulate breathing rate
Chemical Regulation
- CO₂ is the main regulator
- Increase in CO₂ → increases breathing rate
Disorders of Respiratory System
1. Asthma
- Inflammation of bronchi
- Causes breathing difficulty
2. Emphysema
- Damage to alveoli
- Reduces gas exchange
3. Bronchitis
- Inflammation of bronchi
- Excess mucus production
4. Pneumonia
- Infection of lungs
- Fluid-filled alveoli
5. Occupational Respiratory Disorders
- Caused by dust exposure (e.g., silica, asbestos)
Important Differences
Breathing vs Respiration
| Feature | Breathing | Respiration |
|---|---|---|
| Type | Physical | Chemical |
| Site | Lungs | Cells |
| Energy | Not released | Energy released |
Key NEET & CBSE Points
- Functional unit: Alveoli
- Surface area of lungs: ~70 m²
- Tidal volume: 500 mL
- Main respiratory pigment: Hemoglobin
- CO₂ transport: Mainly as bicarbonate ions
- Respiratory center: Medulla oblongata
Quick Revision Notes
- Breathing = mechanical process
- Gas exchange = diffusion
- O₂ transport = hemoglobin
- CO₂ transport = bicarbonate
- Regulation = CO₂ levels
Frequently Asked Questions (FAQs)
Q1. What is the main site of gas exchange?
Alveoli in the lungs.
Q2. Why is hemoglobin important?
It transports oxygen efficiently in blood.
Q3. What is tidal volume?
Volume of air inhaled/exhaled during normal breathing (~500 mL).
Q4. What regulates breathing?
Mainly carbon dioxide levels in blood.
Conclusion
The process of breathing and gas exchange is essential for survival as it ensures a constant supply of oxygen and removal of carbon dioxide. Understanding this system helps in mastering key physiological concepts required for CBSE exams and NEET.