Page 273 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Respiratory system (apparatus respiratorius) 255

alveolar surface. Aside from numerous micropinocytotic tion and pressure equilibration between adjacent alveoli.
VetBooks.ir vesicles, signifying their role in trans-epithelial transport, At their free ends, the septa become expanded; smooth
these cells have few organelles. Type I alveolar epithelial muscle cells occurring at these locations act as sphincters.
cells are closely associated with the thin underlying basal
lamina. They form part of the blood–air barrier. Blood–air barrier
Type II alveolar epithelial cells (cellulae magnae or Gas exchange occurs across the blood–air barrier. The pro-
granulares) are relatively large, wedge-shaped or spheroid cess is facilitated by the close apposition of the alveolar
cells (10–12 μm) that occur in small clusters (Figure 11.27). epithelium, and its thin basal lamina, with the wall of the
They are rich in organelles, including abundant character- alveolar capillaries.
istic, apically located multivesicular and lamellar bodies The exchange of gases between the alveoli and erythro-
(cytosomes, 0.2–1.0 μm). The content of these osmiophilic cytes occurs across the following interfaces (Figure 11.27):
secretory granules is high in phospholipids (saturated leci-
thins), glycosaminoglycans and acid phosphatases. The · surfactant coating the surface of the alveolar epithelium,
secretory product is exocytosed to the free alveolar sur- · cytoplasm of the flattened alveolar epithelial cells,
face, where it disperses widely forming a surface-active · basal lamina of the alveolar epithelium,
phospholipid film (pulmonary surfactant). Surfactant · basal lamina of the capillary wall,
has a detergent effect, reducing the surface tension of the · cytoplasm of the capillary endothelium,
alveolar wall by a factor of 5–10. Type II alveolar cells can · blood plasma and the
divide and transform into Type I cells. · plasmalemma of erythrocytes.
Impurities, such as aspirated particles and fluids, are
removed from the alveolar surface by pulmonary alveolar The adjacent basal laminae are often fused. Interstitial con-
macrophages (components of the mononuclear phagocyte nective tissue is sometimes found between these.
system). These cells migrate from the interstitial connective The rate of gas exchange is determined by:
tissue and attach to the internal alveolar wall (Figure 11.27). Any change in the diameter of the blood vessels, cellular
Alveolar macrophages are expelled via the conducting air- surface area or membrane permeability has the potential
ways, together with mucus and desquamated epithelial cells. to impact on respiratory function.

Species variation Bronchial system and gas exchange in birds
Calf, sheep, goat, cat and pig: In the lungs of these The pulmonary bronchi of birds consist of:
species, clusters of lymphocytes and lymphoid nodules
are found in the walls of the airways and in association · the diameter of the capillary lumen,
with blood vessels. This tissue is referred to as BALT · the rate of blood flow in the capillaries,
(bronchus-associated lymphoid tissue). · partial pressures and
· the thickness of the cell membranes and interstitial
The alveolar septum (septum inter-alveolare) constitutes connective tissue.
the common wall of two adjacent alveoli (Figures 11.24 and
11.27). A distinction is made between septa in the alveolar ducts The primary bronchi are also referred to as first-order
and septa between neighbouring alveoli. The inter-alveolar bronchi. They penetrate the horizontal septum and pass
septa increase the surface area of the gas exchange interface. through the lung to its caudal margin, where they open
Both alveolar surfaces of inter-alveolar septa are lined into the abdominal air sacs.
with Type I and Type II alveolar epithelial cells resting The walls of the primary bronchi contain incomplete
on a thin basal lamina. A connective tissue interstitium rings of cartilage that are absent from all subsequent
is formed by a delicate network of collagen (types I and bronchial divisions. In the primary bronchi, the lumen is
III), reticular and elastic fibres and fibrils. The spaces in surrounded by respiratory epithelium, underlaid by elastic
this meshwork are occupied by fibroblasts, fibrocytes, and collagen fibres, seromucous glands and lymphoid tis-
granulocytes, lymphocytes and mast cells. Phagocytic mac- sue. The smooth muscle of the primary bronchi is mostly
rophages, originating from migrating blood monocytes, circular. An adventitia of loose connective tissue is present.
are frequently observed. Inter-alveolar septa house dense Secondary bronchi, or second-order bronchi, extend
capillary networks that are closely apposed to the alveolar from the primary bronchi. According to the direction in
epithelium (Figure 11.27). Also present are contractile cells which they pass, they are grouped into:
that support the function of basket-like networks of elastic
fibres in compressing the lumen of the alveoli during exha- · two primary bronchi (bronchi primarii),
lation. These cells are innervated by non-myelinated nerve · secondary bronchi (bronchi secundarii),
fibres. Pores in the inter-alveolar septa facilitate air circula- · parabronchi (parabronchi) and
· air capillaries (pneumocapillares).

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