2.2 The Respiratory System & Gas Exchange

2.2 The Respiratory System & Gas Exchange

The primary objective of the respiratory system is to facilitate gas exchange—supplying oxygen ($O_2$) to body tissues for cellular respiration and removing carbon dioxide ($CO_2$), a metabolic waste product. On the TEAS 7 exam, candidates must demonstrate a thorough understanding of respiratory structures, the mechanics of breathing, alveolar gas transport, and neural-chemical regulatory pathways.

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Anatomy of the Respiratory Tract

The respiratory system is structurally divided into upper and lower tracts, establishing a continuous conduit for air movement and gas transfer.

Upper Tract: Nasal Cavity -> Pharynx -> Larynx
Lower Tract: Trachea -> Mainstem Bronchi -> Bronchioles -> Terminal Bronchioles -> Respiratory Bronchioles -> Alveolar Ducts -> Alveoli

1. Upper Respiratory Tract

• Nasal Cavity: Lined with pseudostratified ciliated columnar mucosa containing goblet cells that produce mucus. The nasal cavity warms, humidifies, and filters incoming air. Cilia sweep trapped debris toward the pharynx. The superior region contains olfactory receptors for smell.
• Pharynx (Throat): Muscular tube serving as a shared passageway for air and food. Subdivided into:
  • Nasopharynx: Posterior to the nasal cavity; air passage only. Houses the pharyngeal tonsils (adenoids) and auditory (Eustachian) tube openings.
  • Oropharynx: Posterior to the oral cavity; passage for both food and air.
  • Laryngopharynx: Inferior portion leading to the larynx (for air) and esophagus (for food).
• Larynx (Voice Box): Cartilaginous structure that routes air and food and produces sound. It contains the epiglottis, a leaf-shaped elastic cartilage flap that folds downward over the glottis (opening to the larynx) during swallowing to prevent food and fluid aspiration into the lower airway.

2. Lower Respiratory Tract

• Trachea (Windpipe): Rigid tube supported by 16 to 20 C-shaped rings of hyaline cartilage that keep the airway open during pressure changes. Lined with a mucociliary escalator.
• Bronchial Tree: The trachea divides at the carina into right and left primary (mainstem) bronchi.
  • The right mainstem bronchus is shorter, wider, and more vertically aligned than the left, making it the most common destination for aspirated foreign bodies.
  • Primary bronchi branch into secondary (lobar) bronchi (3 in the right lung, 2 in the left lung, corresponding to lung lobes), then tertiary (segmental) bronchi, and smaller bronchioles.
  • As airways shrink, cartilage decreases while smooth muscle increases, allowing bronchodilation (sympathetic control) and bronchoconstriction (parasympathetic control).
• Alveoli: Microscopic, thin-walled air sacs (~300 million per lung) surrounded by dense pulmonary capillary beds. Alveoli are the functional units where gas exchange occurs.

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Mechanics of Ventilation (Breathing)

Pulmonary ventilation relies on changes in thoracic volume that generate pressure gradients between the atmosphere and the lungs, adhering to Boyle’s Law ($P \propto 1/V$). Air always flows spontaneously from areas of higher pressure to lower pressure.

• Inhalation (Active Process): Contraction of the primary inspiratory muscles (diaphragm and external intercostals) expands the thoracic cavity, pulling the lungs outward. As volume increases, intrapulmonary pressure drops below atmospheric pressure ($760\text{ mmHg}$), drawing air inward.
• Exhalation (Passive Process): During quiet breathing, exhalation requires no active muscle contraction. Relaxation of inspiratory muscles and passive elastic recoil of lung tissue decrease thoracic volume, raising intrapulmonary pressure above atmospheric pressure and pushing air out. Forced exhalation recruits internal intercostals and abdominal muscles.

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Gas Exchange Across the Respiratory Membrane

External respiration occurs across the respiratory membrane—an ultra-thin (~$0.5,\mu\text{m}$) barrier consisting of:

1. Type I alveolar cells (squamous epithelium)
2. Capillary endothelial cells
3. Fused basement membranes between them

Gas exchange proceeds exclusively via simple diffusion driven by differences in partial pressure ($P$):

• Oxygen Exchange: Deoxygenated venous blood arriving at pulmonary capillaries has a low partial pressure of oxygen ($PO_2 \approx 40\text{ mmHg}$), while alveolar air has a high $PO_2 \approx 104\text{ mmHg}$. Oxygen diffuses down its pressure gradient into blood capillaries, binding to iron in hemoglobin inside red blood cells to form oxyhemoglobin ($HbO_2$).
• Carbon Dioxide Exchange: Deoxygenated blood arriving at pulmonary capillaries has a high partial pressure of carbon dioxide ($PCO_2 \approx 45\text{ mmHg}$), whereas alveolar air has $PCO_2 \approx 40\text{ mmHg}$. Carbon dioxide diffuses out of the capillary blood into the alveoli to be exhaled.

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Pulmonary Surfactant

The inner surface of each alveolus is lined with a thin film of water. Water molecules exhibit high cohesion, creating surface tension that threatens to collapse the delicate alveolus.

To counteract this, Type II alveolar cells secrete pulmonary surfactant—a complex mixture of phospholipids and proteins. Surfactant disrupts hydrogen bonding between water molecules, significantly reducing alveolar surface tension. This prevents alveolar collapse (atelectasis) at the end of exhalation and decreases the muscular effort required to inflate the lungs.

Clinical Correlation: Premature infants born before ~34 weeks gestation often lack adequate surfactant secretion, resulting in Infant Respiratory Distress Syndrome (IRDS), characterized by alveolar collapse and severe dyspnea.

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Neural and Chemical Control of Breathing

Ventilation is controlled automatically by neural centers located in the brainstem:

• Medulla Oblongata: Houses the primary respiratory rhythmicity centers. The Dorsal Respiratory Group (DRG) sets the automatic pace of quiet breathing.
• Pons: Houses pontine centers (apneustic and pneumotaxic centers) that smooth transitions between inhalation and exhalation.

Chemical Regulation (The Primary Respiratory Driver)

Under normal conditions, breathing rate and depth are regulated primarily by arterial carbon dioxide ($CO_2$) levels, not oxygen levels.

1. Carbon dioxide easily crosses the blood-brain barrier into cerebrospinal fluid (CSF), reacting with water to form carbonic acid, which dissociates into hydrogen ions: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^--$
2. Central Chemoreceptors in the medulla oblongata detect the increased $\text{H}^+$ concentration (drop in CSF pH) caused by hypercapnia (elevated arterial $PCO_2$).
3. In response, the medulla sends signals to increase the rate and depth of respiration (hyperventilation), blowing off excess $CO_2$ and returning blood/CSF pH to baseline ($7.35\text{--}7.45$).
4. Peripheral Chemoreceptors in the carotid bodies and aortic arch monitor arterial blood $PCO_2$, $\text{H}^+$ levels, and severe drops in arterial $PO_2$ ($<60\text{ mmHg}$).