3.2.2 Lower Respiratory Tract

The lower respiratory tract includes the larynx, trachea, bronchi, and lungs.

Larynx

• The larynx is an enlargement in the airway at the top of the trachea and below the pharynx.
• The larynx is composed primarily of muscles and cartilages.
• One of the cartilages is the epiglottis.
• This structure usually stands upright and allows air to enter the larynx.
• During swallowing, the larynx is raised and the epiglottis is pressed downward.
• As a result, the epiglottis partially covers the opening into the larynx and helps to prevent foods and liquids from entering the air passages.
• The opening of the larynx is called glottis.
• It is also lined with a mucous membrane.
• Inside the larynx, there are two vocal cords which are responsible for vocalization.

• Passageway for air.
• Prevents food and liquids from entering the air passages.
• Produces sound (vocalization).

Trachea

• The trachea or windpipe is a membranous tube.
• It consists of dense regular tissue and smooth muscle.
• It is reinforced with 15–20 C-shaped pieces of cartilage.

• Passageway for air to the bronchi.
• The C-shaped cartilage prevents the trachea from collapsing.

Bronchi and Bronchioles

• The trachea divides to form two smaller tubes called primary bronchi.
• The primary bronchi divide into secondary bronchi within each lung.
• There are two secondary bronchi in the left lung and three in the right lung.
• The secondary bronchi, in turn, give rise to tertiary bronchi.
• The bronchi continue to branch, finally giving rise to bronchioles which are less than 1 mm in diameter.
• The bronchioles also subdivide several times to become even smaller terminal bronchioles.
• In the secondary bronchi, the C-shaped cartilages are replaced with cartilage plates.
• The bronchioles and their terminal branches have no cartilage structures.

• Distribute air throughout the lungs.
• Bronchioles regulate airflow.

Alveolar Ducts and Alveoli

• The terminal bronchioles divide to form alveolar ducts.
• These alveolar ducts end at tiny air-filled chambers called alveoli.
• There are over 700 million alveoli present in the lungs.
• The wall of each alveolus is only 0.1 μm thick.
• On its outside is a dense network of blood capillaries.
• Lining each alveolus is moist squamous epithelium.
• This consists of very thin, flattened cells, reducing the distance over which diffusion must occur.
• Collagen and elastin proteins are also present in their walls which allow the alveoli to expand and recoil easily during breathing.

• Site of gas exchange between air and blood.
• Large surface area for diffusion.
• Thin walls for efficient diffusion.

External Structure of Lungs

• The lungs are the principal organs of respiration.
• Each lung is conical in shape.
• Its base rests on the diaphragm.
• Its apex extends to a point just above the clavicle.
• The right and left lungs are separated medially by the heart and mediastinum.
• The left lung has two lobes: superior lobe and inferior lobe.
• The left lung shares space with the heart.
• The right lung has three lobes.
• The hilum is a triangular-shaped depression of both the lungs where the blood vessels and airways pass into the lungs.
• The lungs are spongy due to the presence of alveoli.

• Main organs of respiration.
• House the alveoli where gas exchange occurs.

Summary Table of Respiratory System Structures

Intermediate Level

Upper vs Lower Respiratory Tract

Structure and Function of Respiratory Organs

Lobes of the Lungs

Alveoli

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Respiratory system exchanges gases between body fluid and environment.
• 2. Upper respiratory tract: nostrils, nasal cavity, pharynx.
• 3. Lower respiratory tract: larynx, trachea, bronchi, lungs.
• 4. Nose filters, warms, and moistens air.
• 5. Pharynx is a passageway for air and food.
• 6. Larynx is the voice box; prevents food entry.
• 7. Trachea is reinforced with C-shaped cartilage.
• 8. Bronchi divide into secondary and tertiary bronchi.
• 9. Bronchioles have no cartilage.
• 10. Alveoli are the site of gas exchange.
• 11. There are over 700 million alveoli.
• 12. Alveolar wall is 0.1 μm thick.
• 13. Right lung has three lobes; left lung has two.
• 14. Surfactant reduces surface tension.
• 15. Lungs are spongy due to alveoli.

Remember

• - Upper respiratory tract = nostrils, nasal cavity, pharynx
• - Lower respiratory tract = larynx, trachea, bronchi, lungs
• - Nose = filters, warms, moistens air
• - Pharynx = passageway for air and food
• - Larynx = voice box; prevents food entry
• - Trachea = windpipe; C-shaped cartilage
• - Bronchi = branches of trachea
• - **Bronchioles** = no cartilage
• - **Alveoli** = gas exchange
• - **700 million** = alveoli
• - 0.1 μm = alveolar wall thickness
• - Right lung = three lobes
• - Left lung = two lobes
• - Surfactant = reduces surface tension

Common Mistakes

• 1. Confusing upper and lower respiratory tracts — Upper: nostrils, nasal cavity, pharynx; Lower: larynx, trachea, bronchi, lungs.
• 2. Thinking the pharynx is only for air — It is also for food.
• 3. Forgetting the role of the epiglottis — It prevents food from entering the trachea.
• 4. Mixing up the number of lobes — Right lung has three; left lung has two.
• 5. Thinking bronchioles have cartilage — Bronchioles have no cartilage.
• 6. Confusing the trachea and esophagus — Trachea is for air; esophagus is for food.
• 7. Forgetting the role of surfactant — It reduces surface tension.
• 8. Mixing up the alveolar wall thickness — It is 0.1 μm.
• 9. Believing gas exchange occurs in the bronchi — Gas exchange occurs in the alveoli.
• 10. Thinking the nose only warms air — It also filters and moistens air.

Quick Revision

The respiratory system is responsible for the exchange of gases between body fluid and the outer environment. It is divided into the upper respiratory tract (nostrils, nasal cavity, pharynx) and the lower respiratory tract (larynx, trachea, bronchi, lungs). The nose filters, warms, and moistens air. The pharynx is a passageway for air and food. The larynx is the voice box and prevents food from entering the air passages. The trachea is reinforced with C-shaped cartilage. The bronchi divide into secondary and tertiary bronchi. The bronchioles have no cartilage. The alveoli are the site of gas exchange; there are over 700 million alveoli, each with a wall only 0.1 μm thick. The right lung has three lobes; the left lung has two. Surfactant reduces surface tension and prevents alveoli from closing.

60-Second Revision

• Upper respiratory tract = nostrils, nasal cavity, pharynx
• Lower respiratory tract = larynx, trachea, bronchi, lungs
• Nose = filters, warms, moistens air
• Pharynx = passageway for air and food
• Larynx = voice box; prevents food entry
• Trachea = windpipe; C-shaped cartilage
• Bronchi = branches of trachea
• Bronchioles = no cartilage
• Alveoli = gas exchange
• 700 million = alveoli
• 0.1 μm = alveolar wall thickness
• Right lung = three lobes
• Left lung = two lobes
• Warning: Bronchioles have no cartilage; gas exchange occurs in alveoli

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• 3. Respiratory System of Man

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• 3.3 MECHANISM OF VENTILATION (BREATHING)

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