Chapter 3: Respiratory System

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Chapter Overview & SLOs

What is the respiratory system? The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.

How does gas exchange take place in the alveoli? Gas exchange occurs through diffusion, where oxygen ($O_2$) moves from high concentration in the alveoli to the blood (low concentration), and carbon dioxide ($CO_2$) moves from high concentration in the blood to the alveoli (low concentration) to be exhaled.

What is the mechanism of inhalation (inspiration)? Inhalation is an active process:

• External intercostal muscles contract, elevating the ribs and sternum outward and upward.
• The diaphragm contracts and flattens (moves downward).
• Thoracic cavity volume increases.
• Intrapulmonary pressure drops below atmospheric pressure.
• Air rushes into the lungs to equalize pressure.

What is the mechanism of exhalation (expiration)? Exhalation is largely a passive process:

• External intercostal muscles relax, ribs and sternum move downward and inward.
• The diaphragm relaxes and returns to its dome shape (moves upward).
• Thoracic cavity volume decreases.
• Intrapulmonary pressure increases above atmospheric pressure.
• Air is forced out of the lungs.

What is the difference between inspired and expired air?

| Component | Inspired Air | Expired Air | |-----------|--------------|-------------| | Oxygen ($O_2$) | ~21% | ~16% | | Carbon Dioxide ($CO_2$) | ~0.04% | ~4% | | Nitrogen ($N_2$) | ~79% | ~79% | | Water vapor | Variable | Saturated | | Temperature | Variable | ~37°C (body temperature) |

What is the effect of exercise on breathing rate? During exercise, muscles produce more $CO_2$, increasing blood $CO_2$ levels. The medulla oblongata (respiratory center in the brain) detects this increase and sends signals to increase breathing rate and depth. Normal resting rate is 15-20 breaths per minute; during intense exercise, it can rise to 30-40 breaths per minute.

What are the functions of the nasal cavity?

• Filtration: Hairs and mucus trap dust, pollen, and pathogens.
• Warming: Rich blood supply warms incoming air to body temperature.
• Humidification: Mucus adds moisture to prevent drying of lung tissue.
• Olfaction: Contains olfactory receptors for smell.

What is the role of the pharynx? The pharynx (throat) serves as a common passageway for both air (from nasal cavity) and food (from mouth). The epiglottis (a flap of cartilage) closes over the larynx during swallowing to direct food into the esophagus and prevent choking (aspiration).

What are common respiratory disorders?

• Emphysema: Condition where alveolar walls break down, reducing total surface area for gas exchange. Primary cause is long-term smoking. Symptoms include severe breathlessness, chronic cough, and fatigue.
• Bronchitis: Inflammation of the bronchi (airways). Acute bronchitis is often caused by viral infections; chronic bronchitis is associated with smoking. Symptoms include persistent cough with mucus, wheezing, and chest discomfort.

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we describe the mechanism of inhalation? The external intercostal muscles contract to elevate the ribs, and the diaphragm contracts (flattening and moving downward), which increases thoracic volume, drops internal pressure below atmospheric levels, and forces air into the lungs. Exhalation is passive: diaphragm relaxes to dome shape, intercostal muscles relax, thoracic volume decreases, pressure increases, air is expelled.
• How do we differentiate between inspired and expired air? Inspired air contains approximately 21% Oxygen and 0.04% Carbon Dioxide, whereas expired air contains 16% Oxygen and 4% Carbon Dioxide due to internal gas exchange (oxygen diffuses into blood, carbon dioxide diffuses out of blood).
• How do we identify the symptoms and causes of Emphysema? Emphysema involves the breakdown of alveolar walls, reducing the total surface area for gas exchange; it is primarily caused by long-term smoking and results in severe breathlessness, chronic cough, and fatigue.
• How do we explain the role of the Pharynx? The Pharynx serves as a common passageway for both air and food, utilizing the epiglottis to direct air into the larynx and prevent choking during swallowing.

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