13.1. Discuss the functions of main part of respiratory system (Copy)

1. The Conducting Zone (Upper Respiratory Tract)

The respiratory system is functionally divided into two zones. The Conducting Zone includes all the passageways that provide a rigid conduit for air to reach the gas exchange sites. These structures also cleanse, humidify, and warm the incoming air. Because no actual gas exchange occurs here, it is referred to as anatomical dead space (typically $approx 150text{ mL}$).

• Nasal Cavity: The primary entryway for air. It contains coarse hairs (vibrissae) to filter large particles. The nasal conchae create air turbulence, allowing a rich capillary bed to warm the air, while mucous secretions humidify it and trap fine dust and bacteria.
• Pharynx (Throat): A muscular funnel that serves as a common passageway for both food and air. It is divided into the nasopharynx, oropharynx, and laryngopharynx.
• Larynx (Voice Box): A complex cartilaginous structure. Its primary biological function is to provide an open airway and to act as a switching mechanism to route air and food into the proper channels. The epiglottis, an elastic cartilage flap, acts as a lid to cover the laryngeal inlet during swallowing, preventing food from entering the trachea.

2. The Conducting Zone (Lower Respiratory Tract)

Air passes from the larynx into the lower respiratory tract, navigating a heavily branched network of tubes often called the respiratory tree.

• Trachea (Windpipe): A flexible tube ($approx 10text{-}12text{ cm}$ long) that descends into the mediastinum. It is structurally supported by $16text{-}20$ C-shaped rings of hyaline cartilage. These rings prevent the trachea from collapsing under negative pressure during inhalation. The open, non-cartilaginous part of the “C” faces posteriorly, allowing the esophagus to expand into the trachea when swallowing large boluses of food.
• The Mucociliary Escalator: The inner lining of the trachea is composed of pseudostratified ciliated columnar epithelium with abundant goblet cells. Goblet cells secrete sticky mucus to trap debris, while the cilia rhythmically beat upward, moving the contaminated mucus away from the lungs toward the throat to be swallowed or spat out.
• Bronchial Tree: The trachea bifurcates into the Right and Left Primary Bronchi. These divide into secondary (lobar) bronchi, and then tertiary (segmental) bronchi. As the tubes get smaller (eventually becoming bronchioles), the cartilage rings are gradually replaced by irregular plates, and the amount of smooth muscle in the walls increases significantly, allowing for bronchoconstriction and bronchodilation.

3. The Respiratory Zone (Alveoli)

The conducting zone ends at the terminal bronchioles. The Respiratory Zone is defined by the presence of thin-walled air sacs called alveoli, which are the actual sites of gas exchange (diffusion of $O_2$ and $CO_2$).

Human lungs contain roughly 300 million alveoli, providing a massive surface area (approximately $70text{ m}^2$, or the size of a tennis court) for optimal gas diffusion. The alveolar walls are composed of two highly specific cell types:

• Type I Pneumocytes: Extremely thin, simple squamous epithelial cells that form the vast majority ($95%$) of the alveolar wall. Their thinness allows for instantaneous gas diffusion into the adjacent pulmonary capillaries.
• Type II Pneumocytes: Cuboidal cells scattered among the Type I cells. They secrete a critical detergent-like substance called surfactant (a complex of phospholipids and proteins). Surfactant coats the inner alveolar surface, drastically lowering the surface tension of water, preventing the alveoli from collapsing inward on themselves during exhalation.

4. The Lungs and Pleura

The lungs are paired, spongy organs occupying the entire thoracic cavity except for the mediastinum. Due to the presence of the heart on the left side, the lungs are asymmetrical:

• Right Lung: Larger, heavier, and divided into three lobes (superior, middle, inferior).
• Left Lung: Smaller, divided into two lobes (superior, inferior), and features a concave impression called the cardiac notch to accommodate the heart’s apex.

Each lung is enclosed in a double-layered serous membrane called the Pleura. The parietal pleura lines the thoracic wall and diaphragm, while the visceral pleura covers the external lung surface. The ultra-thin slit between them (the pleural cavity) is filled with lubricating pleural fluid, which allows the lungs to glide frictionlessly during breathing and creates the vital negative surface tension that keeps the lungs expanded.

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