3.1 Lung Anatomy and Function
3.1.1 The Normal Lung
3.1.2 The Smoker’s Lung
Section 3 describes the normal lung anatomy, benign lung conditions including infections, vascular abnormalities and inflammatory disease as well as benign lung tumors. These benign conditions may appear in lung screening imaging as solitary pulmonary nodules (SPN), as multiple nodules or as lung masses.
Finally an account will be given of the benign causes of solitary pulmonary nodules (SPN) Indeed, solitary pulmonary nodules are a regular and important finding in screening and lung CT imaging.
3.1 Lung Anatomy and Function
So, in order to understand the way that lung abnormalities are classified and described in diagnostic reports, particularly in pathology reports, some understanding of the components of the lung tissue is helpful.
3.1.1 Normal Lung Anatomy
Firstly, inhaled air is transmitted down the trachea to the main bronchi and then to the bronchioles. In addition, cartilage surrounds the Bronchi and helps to retain the shape and keep them open. The bronchi contain mucinous glands within their wall that contain ‘goblet cells.’
Indeed, larger bronchioles are lined by ciliated columnar epithelium or cuboidal epithelium (smaller bronchioles leading to alveoli). The epithelium contains ciliated columnar cells in larger bronchioles, or non-ciliated in smaller bronchioles.
Bronchioles have a diameter of 1 mm or less and do not have goblet cells, but there are cells called Clara cells that are secretory. Clara cells secrete one of the components of surfactant. A rim of smooth muscle surrounds the bronchioles but they do not have cartilage.
Terminal bronchioles are the smallest of the conducting airways. The terminal bronchioles lead to the respiratory bronchioles. Again, the respiratory bronchioles lead to the alveoli where oxygen exchange to the blood occurs via the alveolar capillaries.
Figure 3.1 Diagram of the Normal
What are the Alveoli
Thin tissue sections of lung examined using a microscope give an appearance of a fine lace-like structure. However, most of the normal lung consists of air spaces lined by thin-walled alveoli.
A single layer of epithelial cells make up the alveoli. Endothelial cells line alveolar capillaries. Between the alveoli, there is a thin layer of connective tissue.
There are two main types of alveolar epithelial cells:
- Type I pneumocytes: large flattened cells – (95% of the total alveolar area) which present a very thin diffusion barrier for air exchange.
- Secondly, we have Type II pneumocytes (5% of the total alveolar area and 60% of the total number of cells). Type II pneumocytes secrete ‘surfactant’ that decreases the surface tension between the thin alveolar walls and stops alveoli collapsing on breathing out.
Figure 3.2 Normal glandular bronchial epithelium.
The glandular cells of the normal bronchi and bronchioles
have surface cilia and contain mucin within their cytoplasm.
This high power photomicrograph is stained for mucin
(purple / pink). PAS (x 60)
Alveolar macrophages are important cells of the immune system that ingest bacteria and foreign particles, and they arise from circulating cells (monocytes) in the blood.
The respiratory portion of the lung consists of respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.
Figure 3.3 Diagram showing the main
constituents of the lung alveolus.
The alveolar-capillary barrier varies from 0.2 to 2.5 µm in thickness.
A very thin basement membrane separates the capillary endothelial cells (in red) and the alveolar epithelial cells. The basement membrane allows
oxygen and carbon dioxide to diffuse. Alveolar capillaries branch
from the pulmonary arterioles.
3.1.2 The Smoker’s Lung Anatomy
Cigarette smoking causes a number of changes in the lung tissue, with the most obvious being the accumulation of fine particles of black carbon. Indeed, although carbon settles within the lung tissue, most of it concentrates in the lymph nodes in the lungs.
Smoking also destroys the cilia that line the bronchi and bronchioles that bring air into the lung. In normal lungs, cilia provide an important defense. The cilia move mucus that contains trapped foreign particles and can be coughed up. In smokers, these particles and carcinogens cannot be removed from the lung, increasing the risk of lung cancer and infection.
Chronic smoking also causes a change to the ciliated glandular epithelium of the bronchi. As a response to injury, the normal glandular epithelium is replaced by squamous epithelium in a process known as ‘squamous metaplasia.’ Squamous metaplasia explains why squamous carcinoma of the bronchus is so common in smokers.
Smoking increases the risk of emphysema, a loss of the elasticity of the lung, causing destruction of alveolar walls and loss of the ability for oxygen to diffuse to the blood.
Figure 3.4 Comparison of the Normal Lung (N)
and the Smoker’s Lung (S).
Photomicrographs show that the normal lung (N) alveolar spaces are clear.
A small bronchus (B) has cartilage (C) in its wall. An arteriole (A) is
included. The smoker’s lung (S) contains deposits of black carbon pigment.
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