An Overview of Human Cells for Light Microscopists
Part II - Human Skin & Human Lungs
A 3D modelling article
by Mol Smith 2010
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The Human Lung
People tend to think of the lungs in a very simplistic way, imagining them to be like a pair of balloons inflating and deflating. In fact, the lungs are far more sophisticated. They are yet another marvel of Nature's sophisticated engineering and, along with the heart, form a critical major organ of the body. Most Microscopists look at the lung in a series of sections through the lobes or Bronchi with an optical microscope. Even at relatively low powers, it is fairly easy to determine diseased cells from healthy ones.

The greatest enemy of human lungs is cigarette smoke, not least because it contains carcinogenic substances which increase the risk of cancers. Many people are also genetically susceptible to other compounds present in the smoke. These compounds will cause a cessation of lung elasticity over time, resulting in ever decreasing lung capacity and functionality, and ultimately causing a slow and miserable death. Although the lungs have mechanisms for clearing dust and particles from their interior, it is not possible for them to unlock and clear carbon particles from the cells. All damage caused by smoke is therefore irreversible! Any article about the human lungs would be less than beneficial if it did not also cover some of the critical diseases, especially where they are preventable. So, I will discuss both healthy and diseased lungs here. If you smoke (like me) you may prefer to negate the awful truth about the consequences, but since I already suffer from COPD and my mother recently died of Lung Cancer, I can only say to any reader that the sooner you give up the better. If you are so addicted (and I know what that is like) consider switching to the
new electronic cigarette and you can continue with your habit with a massively reduced risk of lung damage (almost zero risk).

Almost the whole of your chest region is allocated as the home of your lungs and heart. You can see in my 3D model below (if you have 3D red/cyan glasses, wear them now for true 3D), how the lungs and heart are prominently set above the stomach and intestines.


The Lungs are in two halves with the Right side (1st person perspective) larger than the left side - which (the latter) is smaller to make room for the heart next to it. You can see the various major processes and the Lung's close association to the heart in the 3D model below, where I have made the lungs semi-transparent to reveal the Bronchiole, Alveoli, and other processes inside.

You may wish to consider the model against the x-ray below.

An outline of each of the main processes in the lung and also associated with both the lung and the heart are below.


This is the basic windpipe with one opening arriving within the mouth and the other dividing off into the left and right lungs forming the Bronchi.

Bronchus / Bronchi
The major two branches coming from the lower Trachea into the lungs (one into each lung). The right bronchus divides into three Bronchi, which extend separately into the three lobes of the right lung. The left bronchus divides into two bronchi which penetrate the two lobes of the left lung.
Pulmonary Artery
Pulmonary Vein

Bronchiole / Bronchi
(see Bronchus above)

The bronchioles (bronchi) divide repeatedly into many alveolar tubes lined with cuboidal epithelium. These terminate in hollow lobed air sacs called Alveoli.
There are over 700 million alveoli present in human lungs, representing a surface area of approx. 85 square metres. The wall of each alveoli is just 0.0001 mm thick, and on the outside is a dense network of blood capillaries which have originated from the pulmonary artery and ultimately will rejoin with the pulmonary vein. Oxygen diffuses across the thin membrane represented by the alveolar epithelium and capillary endothelium to pass into the blood plasma. It then combines with hemoglobin in the red blood cells to form Oxyhemoglobin, whilst carbon dioxide diffuses ion the reverse direction into the alveolar cavity. Note: the alveolar capillaries diameter is smaller than the diameter of the red corpuscles. This means
blood pressure squeezes the cell through the capillary, resulting in a slower progress and allowing more time for the gas exchange to take place over a wider area of the blood cell.

This is not shown on the 3D model. The pleura consist of two membranes separated for most of their area by a visceral fluid (a bit like a super fine film) in contact with both surfaces, which allows the two membranes to slide smoothly over each other. The two layers meet only at the hilum of the lung. (
hilum : point at which the lung is connected to the trachea by its bronchus). Any pain a person may feel from their lungs, is probably originating from the Pleura as the lung interior does not contain pain receptors! (Inflammation of the pleura or excessive fluid within the two membranes will cause pain! It is the surface tension between the inner membrane and the outer membrane which hold the lungs open and prevent them from collapsing like deflated balloons.

This is not shown on the 3D model but is part represented by the broken blue line. The action of the diaphragm is responsible for the movement of the lungs (inhalation and exhalation). The diaphragm is an area of muscle attached to the chest cavity. When they pull on the cavity, the chest area expands (the ribs flex to allow this) and the action of the pleural membranes (and the fluid sticking them together) pulls the lungs into expansion. When the Diaphragm relaxes the chest wall contracts allowing the lungs to return to normal - exhalation / expiration occurs.

An interesting point to remember is that not all lungs are equal, in the fact that the lungs of birds, reptiles, amphibians, and Invertebrates all differ from mammalian lungs (ours) and each other. More
info here wiki.

The Optical Microscope can be used to recognise many diseases in sections of lungs of tissue. Before we move on to look at this aspect through an optical microscopist's perspective, you can see an interesting 3D (red/cyan) video of the lungs, and their position in the human frame in the movie below.

The lungs are almost completely transparent so you can see their location
with respect to the heart and other organs more clearly.

Okay, now let's take a look at
lung disease...



Comments to the author
Mol Smith are welcomed.

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Published in May 2010 Micscape Magazine.
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