Respiratory Organs

Respiratory organs are the structures through which animals take in ${\mathrm{O}}_2$ and give out $\mathrm{C}{\mathrm{O}}_2$. This page covers why cells need a steady supply of oxygen, how respiratory organs differ across animal groups, and the human respiratory system from the external nostrils to the alveoli, with the pleura, the conducting and exchange parts and the thoracic chamber. It follows the NCERT Class 11 chapter Breathing and Exchange of Gases. NEET often asks which animals use which respiratory organs, the order of the air passages and the role of each part.

1. Why Organisms Need Breathing

• Organisms use oxygen (${\mathrm{O}}_2$) to break down simple molecules such as glucose, amino acids and fatty acids indirectly.
• This breakdown releases the energy needed for various activities.
• ★ Exam imp Carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$), which is harmful, is released during these catabolic reactions.
• So ${\mathrm{O}}_2$ must be supplied continuously to the cells, and the $\mathrm{C}{\mathrm{O}}_2$ they produce must be removed.
• The rhythmic rise and fall of the chest that we can feel is due to breathing.

2. Respiratory Organs in Different Animals

• The mechanism of breathing varies among animal groups.
• It depends mainly on the animal's habitat and its level of organisation.
• ★ Exam imp Gills: special vascularised structures used for gas exchange in water; this is branchial respiration.
• ★ Exam imp Lungs: vascularised bags used by terrestrial forms; this is pulmonary respiration.
• Cutaneous respiration: gas exchange through the moist skin, as in frogs.

Examples to Remember

3. Human Respiratory System

3.1 Air passages: nostrils to trachea

1. A pair of external nostrils opens out above the upper lip.
2. The nostrils lead through the nasal passage into the nasal chamber.
3. The nasal chamber opens into the pharynx. Part of the pharynx is the common passage for food and air.
4. The pharynx opens through the larynx region into the trachea.

• ★ Exam imp Larynx: a cartilaginous box that helps in sound production; hence it is called the sound box.
• Glottis: the opening of the larynx.
• Epiglottis: a thin, elastic, cartilaginous flap that can cover the glottis.

3.2 Trachea, bronchi, bronchioles and alveoli

• Trachea: a straight tube that extends up to the mid-thoracic cavity.
• ★ Exam imp The trachea divides at the level of the 5th thoracic vertebra into a right and a left primary bronchus.
• Each bronchus divides repeatedly into secondary and tertiary bronchi, then bronchioles.
• The branching ends in very thin terminal bronchioles.
• Incomplete cartilaginous rings support the trachea, the primary, secondary and tertiary bronchi, and the initial bronchioles.
• Each terminal bronchiole gives rise to many alveoli.
• Alveoli: very thin, irregular-walled, vascularised, bag-like structures.
• Lungs: the branching network of bronchi, bronchioles and alveoli.

3.3 Lungs and pleura

• We have two lungs.
• ★ Exam imp Each lung is covered by a double-layered pleura, with pleural fluid between the two layers.
• The pleural fluid reduces friction on the lung surface.

3.4 Conducting part and exchange part

3.5 The thoracic chamber

• The lungs lie in the thoracic chamber, which is anatomically air-tight.

• ★ Exam imp Any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity.
• This arrangement is essential for breathing, because we cannot alter the pulmonary volume directly.

4. Steps of Respiration

Respiration in humans involves five steps, in this order:

1. Breathing (pulmonary ventilation): atmospheric air is drawn in and $\mathrm{C}{\mathrm{O}}_2$-rich alveolar air is released out.
2. Diffusion of gases (${\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$) across the alveolar membrane.
3. Transport of gases by the blood.
4. Diffusion of ${\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ between blood and tissues.
5. Utilisation of ${\mathrm{O}}_2$ by the cells for catabolic reactions, with release of $\mathrm{C}{\mathrm{O}}_2$; this is cellular respiration.

5. Exam Essentials

Pairs to Match

Numbers to Remember

• 2 external nostrils; 2 primary bronchi (right and left); 2 lungs.
• Trachea divides at the level of the 5th thoracic vertebra.
• Pleura is double-layered: outer and inner pleural membranes.
• Thoracic chamber has 4 boundaries; respiration has 5 steps.
• The conducting part has 4 functions: transport, cleaning, humidifying and warming air.

6. Quick Revision

• Cells use ${\mathrm{O}}_2$ to break down glucose, amino acids and fatty acids for energy; harmful $\mathrm{C}{\mathrm{O}}_2$ is released.
• Breathing (respiration): exchange of atmospheric ${\mathrm{O}}_2$ with $\mathrm{C}{\mathrm{O}}_2$ produced by cells.
• Breathing mechanisms vary with habitat and level of organisation.
• Body surface: sponges, coelenterates, flatworms. Moist cuticle: earthworm. Tracheal tubes: insects.
• Gills (branchial): aquatic arthropods, molluscs, fishes. Lungs (pulmonary): terrestrial forms; frogs also use skin.
• Air path: nostrils, nasal passage, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.
• Larynx is the sound box; the epiglottis covers the glottis during swallowing.
• Trachea divides at the 5th thoracic vertebra; incomplete cartilaginous rings support the airway up to the initial bronchioles.
• Alveoli are thin, irregular-walled, vascularised sacs; bronchi, bronchioles and alveoli make up the lungs.
• Double-layered pleura with pleural fluid reduces friction; outer layer on thoracic lining, inner on lung surface.
• Conducting part (nostrils to terminal bronchioles) transports, cleans, humidifies and warms air.
• Exchange part (alveoli and ducts) is the only part of the respiratory system where gases diffuse.
• Air-tight thorax: vertebral column, sternum, ribs and diaphragm; thoracic volume change alters lung volume.
• Five steps: breathing, alveolar diffusion, transport, tissue diffusion, cellular use of ${\mathrm{O}}_2$.

7. Solved Examples

8. Practice Questions

Common Mistakes to Avoid