3.2 Respiratory System of Man
The respiratory system is the biological system responsible for the exchange of gases between the body's fluids (like blood) and the external environment. It is divided into two main regions: the Upper Respiratory Tract and the Lower Respiratory Tract.
3.2.1 Upper Respiratory Tract
The upper respiratory tract conditions the air by filtering, warming, and moistening it before it reaches the lungs. It includes the nose, nasal cavity, and pharynx.
Nose
- Structure: The external part is composed of bone, cartilage, and fatty tissue. The external openings are nostrils.
- Nasal Cavities: Two internal hollow spaces separated by the nasal septum. The anterior parts near the nostrils are called vestibules.
- Lining: Lined with a ciliated mucous membrane.
- Defense and Conditioning Functions:
- Filtering: Nose hairs in the vestibules, along with mucus and cilia, trap and filter foreign particles like dust and microorganisms.
- Moistening: The mucus membrane moistens the inhaled air.
- Warming: A network of blood capillaries beneath the membrane warms the air to approximately 30°C.
- Cleaning: Cilia (tiny hair-like structures) move rhythmically to sweep trapped particles towards the pharynx to be swallowed or expelled.
Pharynx (Throat)
- Structure: A cone-shaped passageway that connects the nasal and oral cavities to the larynx and esophagus.
- Dual Function: Serves as a passageway for both air (respiratory system) and food (digestive system).
- Sections: Conventionally divided into three parts:
- Nasopharynx (upper part, behind the nose)
- Oropharynx (middle part, behind the mouth)
- Laryngopharynx (lower part, leading to the larynx/esophagus)
3.2.2 Lower Respiratory Tract
The lower respiratory tract is responsible for conducting air to the lungs and for the actual exchange of gases. It includes the larynx, trachea, bronchi, and lungs.
Larynx (Voice Box)
- Location: An enlargement in the airway found at the top of the trachea and below the pharynx.
- Structure: Composed of muscles and cartilages.
- Epiglottis: A flap of cartilage that stands upright to allow air into the larynx. During swallowing, it presses down to cover the larynx opening, preventing food and liquids from entering the airway.
- Glottis: The opening into the larynx.
- Vocal Cords: Two cords located inside the larynx that vibrate to produce sound (vocalization).
Trachea (Windpipe)
- Structure: A membranous tube made of dense tissue and smooth muscle.
- Support: Reinforced by 15-20 C-shaped rings of cartilage that prevent it from collapsing.
Bronchi and Bronchioles
- Branching Pathway: The trachea divides into smaller and smaller tubes in a tree-like structure.
- Primary Bronchi: The trachea splits into two primary bronchi, one for each lung.
- Secondary Bronchi: Primary bronchi divide into secondary bronchi (3 in the right lung, 2 in the left).
- Tertiary Bronchi: Secondary bronchi branch into tertiary bronchi.
- Bronchioles: Further branching leads to bronchioles (less than 1 mm in diameter).
- Terminal Bronchioles: The final, smallest subdivisions of the bronchioles.
- Structural Change: The C-shaped cartilage rings of the trachea are replaced by cartilage plates in the bronchi, and there is no cartilage in the bronchioles.
Alveolar Ducts and Alveoli
- Alveolar Ducts: Formed from the division of terminal bronchioles.
- Alveoli: Tiny, air-filled sacs at the end of alveolar ducts. They are the primary sites of gas exchange.
- Quantity: Over 700 million alveoli in the lungs, creating a massive surface area for gas exchange.
- Wall Structure: Extremely thin (0.1 µm), consisting of moist squamous epithelium (thin, flattened cells) to minimize diffusion distance.
- Capillary Network: Each alveolus is surrounded by a dense network of blood capillaries.
- Elasticity: Walls contain collagen and elastin proteins, allowing them to expand and recoil during breathing.
- Surfactant: A lipoprotein film that lines the alveoli. It lowers surface tension to prevent the alveoli from collapsing, speeds up O₂ and CO₂ transport, and helps kill bacteria.
External Structure of Lungs
- Function: The principal organs of respiration.
- Shape and Location: Conical organs resting on the diaphragm, separated by the heart and mediastinum.
- Lobes: The lungs are divided into lobes.
| Left Lung | Right Lung |
|---|---|
| 2 Lobes (superior, inferior) | 3 Lobes |
| Smaller to accommodate the heart | Larger |
- Hilum: A triangular depression on the medial surface of each lung where blood vessels and airways (bronchi) enter and exit.
- Texture: Spongy due to the presence of millions of alveoli.
Possible Questions and Answers
Q: What are the three main functions of the upper respiratory tract? A: To filter, warm, and moisten inhaled air before it reaches the lungs.
Q: What is the role of the epiglottis? A: The epiglottis is a flap of cartilage that covers the opening of the larynx (glottis) during swallowing to prevent food and liquid from entering the trachea and lungs.
Q: How is the structure of the alveoli adapted for efficient gas exchange? A: Alveoli have extremely thin walls (one cell thick), a large collective surface area (over 700 million), a moist lining, and a dense network of surrounding capillaries. This minimizes the distance for gases to diffuse between the air and the blood.
Q: Why does the left lung have fewer lobes than the right lung? A: The left lung has only two lobes because it shares space in the thoracic cavity with the heart, which is tilted to the left. The right lung has three lobes.