The Respiratory System and the Mechanism of Breathing: Quick Revision

A brief note on The Respiratory System and the Mechanism of Breathing. Read it before a test. Try each question at the end before you open its answer.

Respiratory organs

• Sponges, coelenterates and flatworms: simple diffusion over the body surface.
• Earthworms: moist cuticle. Insects: tracheal tubes.
• Aquatic arthropods, molluscs and fishes: gills.
• Amphibians: lungs and moist skin (cutaneous). Reptiles, birds and mammals: lungs.

The human respiratory system

• Path: external nostrils, nasal chamber, pharynx, larynx, trachea, primary bronchi, secondary and tertiary bronchi, bronchioles, terminal bronchioles, alveoli.
• The larynx is the sound box. The epiglottis covers the glottis during swallowing.
• The trachea divides at the fifth thoracic vertebra. Incomplete cartilaginous rings hold the airway open.
• Conducting part: nostrils to terminal bronchioles. Exchange part: alveoli and their ducts.
• Two lungs, each with a double pleura and pleural fluid. The thoracic chamber is air-tight, with the diaphragm below.

The mechanism of breathing

• Inspiration: the diaphragm and the external intercostal muscles contract. Chest volume rises, lung pressure falls below atmospheric, air flows in.
• Expiration: both relax. Chest volume falls, lung pressure rises slightly above atmospheric, air flows out.
• Additional abdominal muscles strengthen the breathing.
• Rate: 12 to 16 breaths a minute. Air moved: about 6000 to 8000 mL a minute.

Volumes and capacities

• TV about 500 mL. IRV 2500 to 3000 mL. ERV 1000 to 1100 mL. RV 1100 to 1200 mL.
• IC = TV + IRV. EC = TV + ERV. FRC = ERV + RV.
• VC = TV + IRV + ERV. TLC = VC + RV.

A worked example

A person has TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL and RV = 1200 mL. Find the vital capacity.

1. VC = TV + IRV + ERV.
2. VC = 500 + 3000 + 1100 = 4600 mL.

Practice with new numbers

A program to try

The program turns four volumes into the five capacities.

Mistakes to avoid

• Saying that gas exchange takes place in the trachea. It takes place in the alveoli.
• Saying that the diaphragm relaxes in inspiration. It contracts.
• Putting the residual volume into the vital capacity. It is not breathed out.
• Mixing up EC and ERV. EC = TV + ERV.

Check yourself

1. At which vertebra does the trachea divide?

2. What happens to the lung pressure in inspiration?

3. Which volumes add up to the functional residual capacity?

4. Write the vital capacity as a sum.

Sources

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter Pulmonary Ventilation.