From Embryology

Original file(617 × 800 pixels, file size: 125 KB, MIME type: image/jpeg)

Lung Anatomy - Secondary Lobule

Part of a secondary lobule from the depth of a human lung, showing parts of several primary lobules.

Camera drawing of one 50 μm section. X 20 diameters. (Miller.) See also Labeled version

  1. bronchiole
  2. respiratory bronchiole
  3. alveolar duct
  4. atria
  5. alveolar sac
  6. alveolus or air cell
  • m - smooth muscle
  • a - branch pulmonary artery
  • v - branch pulmonary vein
  • s - septum between secondary lobules
Links: Primary Lung Lobule | Gray 975 | Secondary Lung Lobule | Gray 974 | Respiratory System Development

Lung Structure

The lungs are composed of an external serous coat, a subserous areolar tissue and the pulmonary substance or parenchyma.

The serous coat is the pulmonary pleura it is thin, transparent, and invests the entire organ as far as the root.

The subserous areolar tissue contains a large proportion of elastic fibers; it invests the entire surface of the lung, and extends inward between the lobules.

The parenchyma is composed of secondary lobules which, although closely connected together by an interlobular areolar tissue, are quite distinct from one another, and may be teased asunder without much difficulty in the fetus. The secondary lobules vary in size; those on the surface are large, of pyramidal form, the base turned toward the surface; those in the interior smaller, and of various forms. Each secondary lobule is composed of several primary lobules, the anatomical units of the lung. The primary lobule consists of an alveolar duct, the air spaces connected with it and their bloodvessels, lymphatics and nerves.

The intrapulmonary bronchi divide and subdivide throughout the entire organ, the smallest subdivisions constituting the lobular bronchioles. The larger divisions consist of: (1) an outer coat of fibrous tissue in which are found at intervals irregular plates of hyaline cartilage, most developed at the points of division; (2) internal to the fibrous coat, a layer of circularly disposed smooth muscle fibers, the bronchial muscle; and (3) most internally, the mucous membrane, lined by columnar ciliated epithelium resting on a basement membrane. The corium of the mucous membrane contains numerous elastic fibers running longitudinally, and a certain amount of lymphoid tissue; it also contains the ducts of mucous glands, the acini of which lie in the fibrous coat. The lobular bronchioles differ from the larger tubes in containing no cartilage and in the fact that the ciliated epithelial cells are cubical in shape. The lobular bronchioles are about 0.2 mm. in diameter.

Each bronchiole divides into two or more respiratory bronchioles, with scattered alveoli, and each of these again divides into several alveolar ducts, with a greater number of alveoli connected with them. Each alveolar duct is connected with a variable number of irregularly spherical spaces, which also possess alveoli, the atria. With each atrium a variable number (2–5) of alveolar sacs are connected which bear on all parts of their circumference alveoli or air sacs. (Miller.)

The alveoli are lined by a delicate layer of simple squamous epithelium, the cells of which are united at their edges by cement substance. Between the squames are here and there smaller, polygonal, nucleated cells. Outside the epithelial lining is a little delicate connective tissue containing numerous elastic fibers and a close net-work of blood capillaries, and forming a common wall to adjacent alveoli (Fig. 975).

(Text modified from Gray's Anatomy)

Gray's Images: Development | Lymphatic | Neural | Vision | Hearing | Somatosensory | Integumentary | Respiratory | Gastrointestinal | Urogenital | Endocrine | Surface Anatomy | iBook | Historic Disclaimer
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
iBook - Gray's Embryology  
Grays Anatomy Embryology cover.jpg


Gray H. Anatomy of the human body. (1918) Philadelphia: Lea & Febiger.

Cite this page: Hill, M.A. (2024, June 25) Embryology Gray0974.jpg. Retrieved from

What Links Here?
© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G

File history

Yi efo/eka'e gwa ebo wo le nyangagi wuncin ye kamina wunga tinya nan

current16:36, 29 February 2012Thumbnail for version as of 16:36, 29 February 2012617 × 800 (125 KB)Z8600021 (talk | contribs)
20:40, 24 August 2009Thumbnail for version as of 20:40, 24 August 2009462 × 600 (58 KB)S8600021 (talk | contribs)Part of a secondary lobule from the depth of a human lung, showing parts of several primary lobules. Camera drawing of one 50 μ section. X 20 diameters. (Miller.) # bronchiole # respiratory bronchiole # alveolar duct # atria # alveolar sac # alveolus o