Unit 8
Human Biology
Chapter 8.2
Respiratory SystemClass 12 Biology – Respiratory System Notes PDF
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Introduction
The human respiratory tract includes the nose, pharynx, larynx, trachea, bronchi, bronchioles and lungs. Gas exchange occurs mainly across the thin walls of alveoli.
1. Human Respiratory Tract
Fig. 1 – Human Respiratory System
| Part | Function |
|---|---|
| Nasal cavity | Filters, warms and moistens inspired air. |
| Pharynx | Common passage associated with respiratory and digestive pathways. |
| Larynx | Voice production and passage of air. |
| Trachea | Conducts air; supported by cartilaginous rings. |
| Bronchi / bronchioles | Distribute air through the lungs. |
| Alveoli | Main site of gas exchange. |
2. Alveoli and Respiratory Surface
Alveoli are tiny air sacs surrounded by dense capillary networks. Their walls and capillary walls are very thin, allowing rapid diffusion of oxygen and carbon dioxide.
Fig. 2 – Alveolus and Capillary
- Large total surface area.
- Very thin diffusion barrier.
- Moist respiratory surface.
- Rich blood supply.
- Continuous ventilation maintains diffusion gradients.
3. Mechanism of Breathing
Inspiration
- Diaphragm contracts and flattens.
- External intercostal muscles raise the ribs.
- Thoracic volume increases.
- Pressure inside lungs falls below atmospheric pressure.
- Air enters the lungs.
Expiration
- Diaphragm relaxes and becomes dome-shaped.
- Ribs move downward and inward during quiet expiration.
- Thoracic volume decreases.
- Pressure inside lungs rises.
- Air leaves the lungs.
Fig. 3 – Inspiration vs Expiration
4. Exchange of Respiratory Gases
Gas exchange occurs by diffusion along partial-pressure gradients. Oxygen moves from alveolar air into pulmonary blood, while carbon dioxide moves from blood into the alveoli.
5. Transport of Oxygen and Carbon Dioxide
Oxygen
Most oxygen is transported bound reversibly to haemoglobin in red blood cells as oxyhaemoglobin; a small amount remains dissolved in plasma.
Carbon Dioxide
Carbon dioxide is transported in several forms, especially as bicarbonate ions, with smaller amounts bound to haemoglobin or dissolved in plasma.
Fig. 4 – Gas Transport Between Lungs and Tissues
6. Respiratory Volumes and Capacities
| Term | Meaning |
|---|---|
| Tidal volume | Air inspired or expired during a normal quiet breath. |
| Inspiratory reserve volume | Additional air that can be inspired after normal inspiration. |
| Expiratory reserve volume | Additional air that can be expired after normal expiration. |
| Residual volume | Air remaining in lungs after maximal expiration. |
| Vital capacity | Maximum amount exhaled after a maximal inspiration. |
Fig. 5 – Simplified Spirogram
7. Quick Revision & Exam Points
- Draw and label the human respiratory system.
- Why are alveoli efficient gas-exchange surfaces?
- Explain inspiration and expiration.
- Explain exchange of O₂ and CO₂ in lungs.
- How is oxygen transported in blood?
- How is carbon dioxide transported?
- Define tidal volume and vital capacity.
- Alveoli = main gas-exchange surface.
- Inspiration increases thoracic volume.
- Expiration decreases thoracic volume.
- O₂ mostly travels with haemoglobin.
- CO₂ mostly travels as bicarbonate.
- Diffusion follows partial-pressure gradients.
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