describe the structure of the human gas exchange system, limited to: lungs, trachea, bronchi, bronchioles, alveoli, capillary network

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – The Gas Exchange System

The Human Gas Exchange System

This set of notes outlines the key structures involved in the human respiratory pathway that enable the exchange of oxygen and carbon dioxide between the external environment and the bloodstream.

Overview of the Pathway

Air enters the body through the nose or mouth, passes down the trachea, divides into the bronchi, further branches into bronchioles, and finally reaches the alveolar sacs where gas exchange occurs across a dense capillary network.

Key Structures

  • Lungs – Paired organs containing the bronchial tree and alveolar tissue.

  • Trachea – Rigid tube supported by C‑shaped cartilage rings; conducts air to the bronchi.

  • Bronchi – Two primary tubes (right and left) that enter each lung and subdivide into secondary and tertiary bronchi.

  • Bronchioles – Small, smooth‑muscle‑lined airways (< 1 mm diameter) that lack cartilage and terminate in alveolar ducts.

  • Alveoli – Microscopic sac‑like structures (≈ 200 µm diameter) providing a large surface area for diffusion.

  • Capillary Network – Dense mesh of pulmonary capillaries surrounding each alveolus, enabling rapid gas exchange.

Structural Details

StructureMain FeaturesRole in Gas Exchange
LungsEncased in pleura; divided into lobes (right: 3, left: 2); contain the entire bronchial tree.House the alveolar–capillary interface where O₂ enters blood and CO₂ is removed.
Trachea≈ 10–12 cm long; C‑shaped hyaline cartilage rings; lined with pseudostratified ciliated columnar epithelium.Provides a protected, open airway; cilia move mucus and trapped particles upward (mucociliary clearance).
BronchiBranch into secondary (lobar) and tertiary (segmental) bronchi; cartilage plates become irregular; mucous glands present.Distribute inhaled air to each lung lobe; maintain airway patency.
BronchiolesAbsence of cartilage; abundant smooth muscle; terminal bronchioles end in alveolar ducts.Regulate airflow resistance via smooth‑muscle contraction (bronchoconstriction) or relaxation (bronchodilation).
AlveoliThin (≈ 0.2 µm) type I epithelial cells; surfactant‑producing type II cells; elastic fibers.Provide a massive surface area (~ 70 m²) and minimal diffusion distance for O₂ and CO₂.
Capillary NetworkExtremely thin walls (single endothelial cell layer); extensive anastomoses; blood flow matches ventilation.Facilitates rapid diffusion of gases; blood‑air barrier thickness ≈ 0.5 µm.

Principles of Gas Diffusion

The rate of diffusion of a gas across the alveolar–capillary membrane is described by Fick’s law:

\$J = -D \frac{\Delta C}{\Delta x}\$

where J is the flux (amount per unit area per unit time), D is the diffusion coefficient, ΔC is the concentration gradient, and Δx is the thickness of the membrane.

Key factors that increase diffusion rate include:

  1. Large surface area (provided by millions of alveoli).

  2. Thin diffusion barrier (type I cells + capillary endothelium).

  3. Steep concentration gradient (high O₂ in alveoli, low O₂ in blood).

  4. High diffusion coefficient (O₂ and CO₂ are small, non‑polar molecules).

Integration of Structure and Function

The hierarchical branching from the trachea to the bronchioles ensures that air reaches every alveolus efficiently. The elastic recoil of lung tissue and the surface‑active properties of pulmonary surfactant keep alveoli open, preventing collapse (atelectasis) and maintaining optimal diffusion distance.

Suggested diagram: A cross‑sectional illustration of the respiratory tract highlighting the trachea, bronchi, bronchioles, alveolar sacs, and surrounding capillary network.

Summary Points

  • The respiratory system is a series of progressively smaller tubes ending in alveolar sacs.

  • Each structural level contributes to maintaining an open airway, regulating airflow, and maximizing surface area for diffusion.

  • The alveolar–capillary membrane is the critical site where O₂ enters the blood and CO₂ is expelled, governed by diffusion principles.