Respiratory System - Anatomy

• Small diffusion distance: ≈0.5 μm (about 1/15​ the diameter of a red blood cell) between air and blood.

• Large surface area: ≈100 m² (roughly 1/2​ to 2/3​ of a tennis court), achieved by branching into ≈300 million alveoli.

What two structural features are essential for efficient gas exchange in the lungs, and what are their specific measurements?

• External respiration: Lungs — O2​ is absorbed from the atmosphere into pulmonary capillary blood; CO2​ is excreted.

• Internal (tissue) respiration: Systemic capillaries ↔ tissues — Gas exchange between blood and tissue fluid/cells.

• Cellular respiration: Mitochondria — O2​ is used to break down glucose into ATP, producing CO2​.

What are the three types of respiration, where do they occur, and what happens in each?

Pulmonary ventilation is the bulk movement of air in and out of the lungs (breathing). It is the mechanical process of moving air, whereas respiration refers to the actual gas exchange and metabolic utilization processes.

What is pulmonary ventilation, and how does it differ from respiration?

Ventilatory pump (air) → External respiration (lungs) → Right heart → Left heart → Systemic capillaries → Internal respiration (tissues) → Cellular respiration → CO2​ returns via veins → Right heart → Lungs.

Trace the complete pathway/circuit of air and gas transport starting from the ventilatory pump.

• Upper respiratory tract: High up (e.g., cold/flu).

• Lower respiratory tract: Deeper down (e.g., pneumonia). Lower tract infections are more severe because they occur closer to the blood supply

How do the structural divisions of the respiratory tract differ, and why are lower tract infections more severe?

• Structures: Nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → terminal bronchioles.

• Function: Warms, humidifies, and cleans incoming air. Performs no gas exchange.

What structures make up the conducting zone, and what are their primary functions?

• Structures: Respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.

• Function: Gas exchange occurs across these structures.

What structures make up the respiratory zone, and what is its primary function?

• Conditions: Air must be warm, moist (100% saturated with H2​O), and clean ("wet, warm, sticky").

• Key Features:

  • Nasal hairs: First filter for large particles.

  • Conchae (turbinates): Create turbulence to slow air, mix it, and throw particles onto mucus.

  • Respiratory epithelium: Contains goblet cells and mucous glands to trap debris.

  • Rich vascular bed: Directly underlies epithelium to warm air.

What three conditions must be met for air to be proper "conditioned" before reaching the alveoli, and what nasal features accomplish this?

• Three bony shelves (inferior, middle, superior) on the lateral nasal wall covered in respiratory epithelium.

• They increase surface area and create air turbulence, which slows/mixes air and throws particles onto the sticky mucus layer.

What are the conchae (turbinates) and how does their structure assist in air conditioning?

• Mucus sits on top of cilia tips (not soaking them).

• Cilia beat 10–20×/second in a coordinated wave to move mucus toward the pharynx to be swallowed (stomach acid kills microbes).

• Moves mucus downward from the nose to the pharynx, and upward from the trachea/bronchi to the pharynx.

What is the mucociliary escalator, and how does it function as a primary defense mechanism?

• Mechanism: Paralyzes and eventually destroys cilia, causing mucus to stagnate and trap bacteria.

• Consequence: Higher risk of lung infections and development of a chronic "smoker's cough," as coughing becomes the only remaining mechanism to clear mucus.

How does cigarette smoking impair the mucociliary escalator, and what is the resulting clinical consequence?

• Nasopharynx: Air only.

• Oropharynx: Air + food.

• Laryngopharynx: Air + food.

What are the three regions of the pharynx, and what passes through each?

• Structural: Part of the GI tract.

• Functional: Acts as a shared pathway for both air and food.

What is the main structural role vs. functional classification of the pharynx?

• Epiglottis: Flexible flap that is passively pressed down by the food bolus to seal off the trachea.

• Soft palate: Closes off the nasopharynx to prevent food/liquid from entering the nasal cavity.

• Glottis: Closes to block entry to the larynx.

• Oesophagus: Forced open posteriorly so the food bolus passes safely into it.

What four mechanisms protect the airway during swallowing, and how do they function?

• Dimensions: ≈12 cm long, thickness of a thumb.

• Structure: Supported anteriorly by incomplete "C"-shaped cartilage rings with free ends joined posteriorly by the smooth trachealis muscle to keep the airway permanently open.

What are the basic dimensions of the trachea and what structural feature prevents its collapse?

The oesophagus sits immediately posterior in the groove of the trachealis muscle. The soft, cartilage-free posterior wall allows large food boluses to pass down the oesophagus without pressing against rigid cartilage.

What structure lies directly posterior to the trachea, and how does tracheal anatomy accommodate it?

Pseudostratified ciliated columnar epithelium. The cilia drive the mucociliary escalator upward toward the pharynx to clear trapped debris and mucus.

What type of epithelium lines the trachea, and what is its main function?

• Bronchus: Ciliated epithelium + goblet cells → smooth muscle → mucous glands → cartilage (irregular plates) → adjacent alveoli.

• Bronchiole: Ciliated epithelium (shorter, no goblet cells) + Club cells → prominent smooth muscle → no cartilage → adjacent alveoli.

What is the structural wall composition of a bronchus versus a bronchiole from the lumen outward?

• Cartilage: Present in bronchi (irregular plates); absent in bronchioles.

• Glands/Cells: Bronchi have mucous glands and goblet cells; bronchioles lack these and instead have Club cells (secrete watery, antimicrobial fluid).

• Epithelium: Taller (columnar) in bronchi; shorter (columnar → cuboidal) in bronchioles.

How do bronchi and bronchioles differ in cartilage, mucous glands, and epithelium?

• Bronchus: Smooth muscle has a minor effect because rigid cartilage dominates; main job is to keep a wide-open conduit (like arteries).

• Bronchiole: Prominent smooth muscle controls airway diameter/tone because no cartilage restrains it; main job is to control airflow and resistance (like arterioles).

How do smooth muscle roles and primary functions differ between bronchi and bronchioles?

• Asthma: Bronchiolar smooth-muscle bronchoconstriction (+ inflammation) narrows thousands of bronchioles, restricting airflow.

• Salbutamol: A β2​-adrenoceptor agonist that relaxes smooth muscle, causing bronchodilation to restore airflow. Reversibility of symptoms after use supports an asthma diagnosis.

What is the mechanism of asthma in the bronchioles, and how does Salbutamol reverse it?

• Terminal bronchiole: Last part of the conducting zone — no gas exchange occurs; structurally resembles a small bronchiole.

• Respiratory bronchiole: Transition structure to the respiratory zone — features a few alveoli along its walls, allowing initial gas exchange to occur.

• Alveolar ducts & sacs: Terminal structures of the respiratory zone — made almost entirely of alveoli, dedicated fully to gas exchange.

How do terminal bronchioles, respiratory bronchioles, and alveolar ducts/sacs differ in location and function across the respiratory zones?

• Extremely thin, flat cells (squamous) that make up most of the alveolar surface area.

• Function: Main site where gas exchange occurs.

What are Type I pneumocytes, and what is their primary function?

• Cuboidal cells that secrete surfactant at the air-liquid interface.

• Function: Reduces surface tension to prevent alveolar collapse on expiration and lower the work of breathing.

• Clinical: Premature infants lacking surfactant develop Respiratory Distress Syndrome (RDS).

What are Type II pneumocytes, and what is the role of surfactant?

Mobile immune cells that wander the alveolar spaces and inter-alveolar septa to engulf debris, particles, and pathogens.

What are alveolar macrophages ("dust cells"), and what is their function?

1. Alveolar air space

2. Squamous pneumocyte (Type I) cytoplasm

3. Fused basement membrane (pneumocyte + capillary endothelium)

4. Capillary endothelium

5. Blood plasma

6. Red blood cell

• Total Distance: ≈0.5 μm

What are the layers of the diffusion (blood–air) barrier, in order from air to blood, and what is its total distance?

The basement membrane of the Type I pneumocyte and the capillary endothelium are fused together, which is unusual compared to other tissues and minimizes the diffusion distance for gas exchange.

What structural adaptation of the basement membrane in the diffusion barrier minimizes diffusion distance?

Fibrosis increases connective tissue, which increases barrier thickness. This impairs gas exchange and makes the alveoli harder to inflate.

How does pulmonary fibrosis affect the blood–air diffusion barrier and impair lung function?

• Epithelial height: Transitions from tall (pseudostratified columnar) down to short (squamous).

• Cartilage: Progresses from complete/incomplete rings → irregular plates → completely absent beyond the smallest bronchi.

How do epithelial height and cartilage change along the airway from the trachea to the alveoli?

• Mucous glands / Goblet cells: Present in large airways → absent in bronchioles and the respiratory zone.

• Smooth muscle: Relatively less in large airways → relatively more in small airways (controls diameter) → absent in the true respiratory zone.

How do mucous glands/goblet cells and smooth muscle change along the airway from the trachea to the alveoli?

The primary role shifts from defence and conditioning in the upper airways to gas exchange in the lower respiratory structures.

What is the overarching functional shift as you transition down the respiratory tract?

• Primary bronchi (2): Lead to the right and left lungs.

• Secondary (lobar) bronchi: Lead to the lobes — 2 on the left (space for the heart), 3 on the right.

• Tertiary (segmental) bronchi: Lead to bronchopulmonary segments — typically 8 on the left, 10 on the right.

How do primary, secondary (lobar), and tertiary (segmental) bronchi divide the lungs structurally?

• Definition: A structural subdivision of a lung lobe wrapped in its own connective tissue with an independent air and blood supply.

• Clinical Significance: Disease in one segment does not collapse the whole lung, and surgeons can resect a segment (e.g., with a localized tumor) without compromising neighbouring segments.

What is a bronchopulmonary segment, and why are these segments clinically important?

• Visceral pleura: Inner layer that directly covers and adheres to the lung surface.

• Parietal pleura: Outer layer that lines the inner chest wall (ribs) and upper surface of the diaphragm.

• They are continuous with each other at the hilum (root of the lung).

What are the visceral and parietal pleurae, and where do they become continuous?

1. Friction reduction: Allows the pleural layers to slide smoothly against each other during breathing.

2. Surface tension ("gluing"): Holds the visceral and parietal layers together so the lung is forced to expand and follow chest wall/diaphragm movements.

What two functions does pleural fluid serve in the pleural space?

• The entry of air breaks the surface-tension fluid seal between the visceral and parietal pleurae.

• Consequence: The lung is no longer pulled along by chest wall movements and collapses due to its elastic recoil.

What is a pneumothorax, and what mechanism leads to lung collapse?

• Diaphragm: Flattens to pull the central tendon down; responsible for ≈75% of air movement at rest.

• External intercostal muscles: Lift ribs up and out ("bucket-handle" motion); responsible for ≈25% of air movement at rest.

Which muscles are responsible for quiet inspiration, and what percentage of air movement does each provide?

• Quiet expiration: Passive process relying entirely on elastic recoil of the lungs and chest wall as muscles relax.

• Forced breathing/exercise: Active process recruiting internal intercostals (pull ribs down and in) and accessory muscles (neck, abdominal) to force air out faster and handle larger volumes.

Why is quiet expiration passive, and how does this change during forced breathing or exercise?

Muscle contraction → ↑ thoracic volume → pleura drags lungs → ↑ lung volume → intrapulmonary pressure drops below atmospheric pressure → air flows in down its pressure gradient.

What is the pressure and volume mechanism linking muscle contraction to airflow during inspiration?