Respiratory System
Divisions of the Respiratory System
 Ventilation
o Transporting O2 from the external environment to the alveolar surface
o Removing CO2 from the alveolar surface to the external environment
o Occurs in the conducting pathways
 Nasal passages, nasopharynx, larynx, trachea, bronchi, bronchioles
 Gas exchange
o O2 diffusion from alveolar gas to arterial blood
o CO2 diffusion from arterial blood to alveolar gas
o Occurs in the respiratory portions
 Alveoli of the lungs (lung parenchyma)
Structure of the Conducting System
- Bronchi branch into bronchioles, which then branch into terminal bronchioles. Terminal bronchioles branch
into respiratory bronchioles, the most distal conducting airways. Alveoli branch directly from the walls of
respiratory bronchioles as alveolar ducts, then alveoli
- Large airways (bronchi, bronchioles)
 Airway wall is thick and composed of various tissue types
 Epithelium (composed of various cell types interspersed between epithelial cells)
o Ciliated pseudostratified columnar epithelial cells
 Cilia stem from apical surfaces
 Cilia contain microtubules arranged in a 9+2 pattern
 Microtubule sliding permits cilia to move in a coordinated way
 Cilia move mucus and particles from airways to the pharyngeal region
o Brush cells
 Non-ciliated
 Have apical microvilli
o Goblet cells
 Non-ciliated
 Secrete pink-staining mucous
 Mucous secretion is a highly-regulated function
- Secretions can change in character (become
more or less watery)
o Serous cells
 Produce more watery secretion
o Clara cells
 Non-ciliated
 Located in terminal airways
 Regulate electrolyte and water transport
Cartilage and mucous
 Metabolize exogenous compounds carried to the lungs
glands
of conducting airway
by air flow or circulation
epithelium
o Kulchitsky cells
 Neuroendocrine cells
 Composed of prominent granules that contain regulatory cell mediators
- Biogenic amines (e.g. serotonin)
- Peptide mediators
 Also located in mucous glands and ducts
o Basal cells
 Cartilage
 Connective tissue
 Smooth muscle
o Muscle contraction has two effects on conducting airways
 Reduces diameter of the airways
 Markedly reduces airflow
 Airway walls too thick for gas exchange to occur
- Small (distal) airways (respiratory bronchioles)
 Airway wall is thinner
o Epithelium is not as tall
o Epithelium is more cuboidal
o CT layer is thinner
o No cartilage or mucous glands
 Airway walls too thick for gas exchange to occur
Ciliated pseudostratified columnar
- Deposition of inhaled particles in the respiratory tract
epithelium of the conducting airways
 Large particles (>5um) collide with upper airway
walls
and are removed from airstream
 Small particles (<0.1um) settle out of airstream in
smaller
airways and are trapped in mucus
 Mucociliary escalator
o Operates throughout the ciliated airways down to the level of terminal bronchioles
o Removes particles once they are deposited in the conducting airways
 Mucous and particles are carried up toward the pharynx by the coordinated beating
action of airway epithelial cilia (beating of cilia is always “up and out”)
 Mucous and particles in the pharynx are then swallowed and removed via the GI tract
 Bacteria can impair the function of the mucociliary escalator
- Bacteria can firmly attach to respiratory cilia and impair their beating
Structure of Gas Exchange System
- Alveoli
 Requires a large surface area (SA) for successful gas exchange and diffusion
o Pulmonary capillary vessels wrap around alveoli to allow for maximal SA
o Pulmonary veins carry oxygenated blood back to the heart
 Epithelium
o At the junction between terminal airways and alveolar ducts, ciliated epithelium disappears
 Type II epithelial cells
- Cuboidal
- Synthesizes surfactant and releases it onto the alveolar surface
 Type I epithelial cells
- Squamous
- Very thin and have few organelles to facilitate gas diffusion
 Capillary endothelial cell
- Squamous
- Very thin except in the nuclear region
- RBCs must deform to pass through narrow capillary space
- Can metabolize circulating mediators and limit their activity
o Gas exchange is possible because of the very thin basal lamina (without CT) between the
alveolar type I epithelial cells and capillary endothelial cells
 Air-blood barrier
Blood vessel and septa of alveolar tissue
Type I epithelial cell, alveolar space, capillary
endothelial cell, and RBC of air-blood barrier

Alveolar macrophages
o Engulf and remove particulate material that reaches the alveolar surface
 Inhaled particles, bacteria, fungal spores
o Engulfed particles are carried away to the pharyngeal region by the mucociliary escalator
o Macrophages can also carry particles through alveolar epithelium to pulmonary lymphatics
o Electron microscopy illustrates unique features of alveolar macrophages
 Irregular pseudopodia extending along the alveolar surface
 Engulfed electron-dense particles within the macrophage
o Species with pulmonary intravascular macrophages (PIMs)
 Ruminants, horses, pigs, cats
o Species without PIMs
 Primates, dogs, chickens, rodents
 Pores of Kohn
o Provide collateral ventilation at the alveolar level
 Highly-evolved defense system
o External environment is brought into close contact with circulation system and there is a large
SA exposed to potentially harmful substances. Thus, protective mechanisms are vital
Respiratory Lymphatics
- Bronchus associated lymphoid tissue (BALT)
 Consists of nodules of lymphoid cells in the airway walls
 Commonly located at airway bifurcations, where particle are deposited
 Functions as a prominent defense mechanism of the bronchociliary system
o Protects from inhaled infectious agents
 There are specialized, non-ciliated epithelial cells that lie over BALT
o These epithelial cells transport antigens from the airway to the underlying lymphoid cells
 Chronic BALT infection (SLIDE 32)
o Leads to chronic immune stimulation, which results in the continuous division of lymphocytes
and the enlargement of BALT nodules (BALT hyperplasia)
BALT nodule (pale green) beneath airway epithelium
-
BALT hyperplasia
Pulmonary lymphatic drainage
 The path of drainage is from pulmonary capillaries to the interstitium to thin-walled lymphatic vessels
 Prevents edema (fluid) from accumulating in the interstitium and interfering with gas diffusion
o Without lymphatic drainage, pulmonary edema develops
Avian lungs
- Ventilation (which occurs during both inspiration and expiration) is accomplished by downward movement of
the keel, which decreases pressure in the thoraco-abdominal cavity and pulls air into the compliant air sacs. Air
flow is then directed through the rigid parabronchi (do not move during the respiratory cycle), where gas
exchange occurs
 Gas exchange via a cross current relationship between air flow and blood flow