F1i / 23A04: Explain the structural features of the alveolus that facilitate its function?

19B04: Exam Report

Explain the structural features of the alveolus that facilitate its function?

14% of candidates passed this question.

The question asked for an explanation of the structural features of the alveolus that facilitate its function. Candidates who scored well integrated the specific anatomical and structural elements of the alveolus with the multiple functions of the alveolus, including the relevant explanation regarding mechanisms.

No marks were given for simply listing the structural features of an alveolus without an explanation on how this facilitates function, nor for listing functional requirements of an alveolus without explaining whether or how they are met by its structure. For the same reason, this was one question where simply listing equations with no discussion as to how these relate to the structure and function of the alveoli garnered no marks.

Equations were not required for full marks but may be an efficient way to represent physical relationships that are hard to write in a few words. Common omissions included a description and the role of
collagen and elastin fibres, capillary structure, the filtration function of the membrane, lymphatic drainage, recruitment and distensibility and metabolic functions.

Candidates are encouraged to practice model answer templates for these integrative questions in the months leading up to the exam.

F1i / 23A04: Explain the structural features of the alveolus that facilitate its function?


Blind-ending sac at approx airway generation 23, part of the lung where gas exchange occurs

  • Contain the majority of the lung volume (~3000mL)
  • Contains alveolar deadspace (30ml/kg)
  • Gas flow is slow due to exponential increase in cross sectional area with each airway generation
  • Diffusion is the predominant mechanism of gas movement


The structure of the alveolus is optimised for gas exchange

Anatomical features of the alveolus


  • Spherical shape maximises SA to vol ratio


  • Extremely thin (0.2-0.3um) allowing optimisation of gas exchange as per Fick’s equation (see below)
  • Fragile, alveoli can be damaged by increased capillary pressure
  • Contains fibrin, collagen and elastin
    • Elastic tissue allows alveoli to inflate and deflate without causing shear stress
    • Contributes to alveolar interdependence
      • When small alveoli begin to collapse, surrounding tissues oppose distortion and pull alveoli open
    • Contain pores of Kohn
      • Allows equilibration between alveoli with differing time constants

Cell types

  • Type 1 pneumocytes
    • Line alveolar walls & provide structure
    • Make up ~90% of alveolar SA
  • Type 2 pneumocytes
    • Specialised secretory cells
    • Produce surfactant
  • Alveolar macrophages
    • Alveoli have no cilia
    • Inhaled particles are phagocytosed by alveolar macrophages in septa and lung interstitium


  • 90% phospholipid
    • 40% DPPC (dipalmitoyl phosphatidyl choline)
      • Main active component
    • Other phospholipids
    • Neutral lipids
    • 10% protein
  • Stored in lamella bodies in the cytoplasm of type II pneumocytes
  • Released in response to increased lung volume (inflation), increased RR or endocrine stimulation
  • Arrangement with hydrophilic head in the aqueous phase and hydrophobic tail in the airspace of the alveolus
  • T ½ appro 15-30hrs
  • -ve feedback to type 2 pneumocytes for regulation
  • Reduces surface tension as per La Place’s Law
    • P = \( \large \frac{\text{2T}}{\text{R}}  \)
    • P = distending pressure (dyn/cm2)
    • T = surface tension of liquid (dynes/cm)
    • R = radius (cm)


  • Maintains stability of alveoli
    • As the alveoli decrease in size, the surfactant molecules are pushed together and exert a greater surface tension lowering effect
  • Increases compliance
    • Smaller radius = increased pressure to reinflate
  • Reduces work of breathing
    • Less energy required to reinflate alveoli
  • Role in lung elastic recoil
    • At higher volumes, surface tension rises – prevents overdistension
  • Reduces transudate
    • Surface tension causes the pressure in t3h alveolar lining fluid to be less than alveolar pressure
  • Immune function
    • Antioxidant rich
    • Role in macrophage and neutrophil activity

Alveolar-capillary barrier

3 layers

  • Type 1 pneumocytes
  • Extracellular matrix
    • Provides support/structural integrity
  • Pulmonary capillary endothelium
    • Thin
    • Contains collagen and elastin allowing capacitance

Gas exchange

  • The alveolar capillary barrier is extremely thin, ~0.3um
  • Total alveoli SA of lung is 50-100um
  • These are optimal conditions for gas exchange as given by Fick’s law of diffusion

Fick's Law of Diffusion

  • Diffusion = \( \text{∝} \large \frac{\text{A}}{\text{T}}  \) x D (P1 – P2)

Graham's Law

  • \( \text{Diffusivity =} \large \frac{\text{Solubility}}{\text{√MW}}  \)

Other functions


  • Two-way filtration barrier
    • Prevents particulate matter and pathogens entering areas of gas exchange
      • Immune functions of surfactant
      • Lymph drainage
      • Alveolar macrophages
        • ability to trigger immune response
      • physical barrier for anything larger than 2.5um
    • prevents larger emboli entering lungs/systemic arterial circulation
      • blood clots/tumour cells/other emboli


  • Alveoli synthesize surfactant, and the alveolar capillaries synthesize NO, and heparins
  • Synthesis and storage, and release of proinflammatory mediators
    • Histamine, eicosanoids, endothelin, platelet aggregating factor and adenosine


  • Entire CO passes through lungs
    • Well suited to metabolic processes
  • Inactivation of NA, 5HT, prostaglandins, bradykinin and Ach
    • Adrenaline, ADH + ATII pass though unaltered
  • Conversion of ATI -> ATII by ACE
  • ACE also one of 3 enzymes that metabolizes bradykinin
  • Metabolism/first pass of a number of drugs
    • e.g. fentanyl. Lignocaine and NA


  • Blood reservoir
  • O2 reservoir

Acid-base balance

  • The alveolar-capillary membrane is thin with a huge surface area
    • optimised for the offloading of CO2 to maintain acid/base and is a major buffering system



  • Pulmonary capillaries form a dense network over one or more alveoli to maximise gas exchange
  • 7-10um thick
    • This is just large enough for an erythrocyte to pass through\
    • Blood flows in thin sheets optimising transfer of respiratory gases
  • Thin-walled
    • Allows gas exchange to take place under optimal conditions
  • Distensible
    • Pulmonary circulation receives entirety of CO
    • Low pressure, low resistance system
    • Allows blood to flow slowly and optimise transfer of gases across membrane
    • Allows lung circulation to act as a blood reservoir


  • Small lymph vessels commence at the junction between alveolar and extra alveolar space
  • The lymphatic system surrounds the bronchi and pulmonary vessels
  • Drains towards the hilum where it passes through several lymph nodes
  • Important in immune function
    • Removes debris and antigens and transports them to lymph nodes for antigen presentation
  • Provides oncotic pressure to avoid pulmonary oedema

Author: Erin Maylin