PAG 1.3

Question 1

1. Using a calibrated eyepiece graticule the mean diameter of an alveolus was calculated to be 170 µm.

Do you think this sounds too small, too large or about right? What is the reasoning behind your answer?

Answer 1

-naturally occurring variability;
-the nature of sampling (an answer further supported by reference to the standard deviation of the mean they have calculated, or even simply the range within their sample
-possible differences between their sample and the way it was prepared and the sample used to derive the 170 µm figure including:
different species,
different age,
healthy or diseased tissue,
different fixation causing different degree of shrinking, inflated or deflated at the point of fixation.

-‘sanity checking’ the 170 µm figure by a combination of knowledge and observation.
For example
knowing that erythrocytes are approximately 7 µm in diameter allows us to get a visual impression of scale either by finding an erythrocyte for direct comparison or by realising that the diameter of the lumen of the capillaries will be about the same;
bearing in mind that alveolar diameter will change significantly during the breathing cycle adds to the uncertainty of any estimate;
knowing that the diffusion distance from air to blood is very short (of the order of just 1 µm) is another way of getting a sanity check on the scale of what is being

Question 2

2. Why is it difficult to see blood capillaries?

Answer 2

The capillaries have a small diameter and very thin walls (the endothelial cells are flattened (and hence unstained)) meaning there is very little material that will take up stain. The lumen is likely to be empty and is itself very narrow. The capillaries therefore are small, low contrast and unobtrusive in these specimens.

Question 3

3. Why are bronchioles so numerous? Why are capillaries so numerous?

Answer 3

3. Why are bronchioles so numerous? Why are capillaries so numerous? Numerous bronchioles are necessary to achieve effective ventilation – allowing air to get to and from all the alveoli. Numerous capillaries are necessary to achieve effective perfusion – allowing blood to flow adjacent to every one of the alveoli. Matching ventilation and perfusion makes an efficient gas exchange system.

Question 4

Why are lung arterioles relatively thin-walled?

Answer 4

4. Why are lung arterioles relatively thin-walled? The blood pressure on the arterial side of the pulmonary circulation is lower than on the arterial side of the systemic circulation. Therefore thinner arteriole walls are adequate to withstand and maintain this pressure. In addition there is no need in pulmonary arterioles’ walls for smooth muscle to allow vasoconstriction since all areas of both lungs will always remain fully perfused.

Question 5

Explain how the features of lung structure seen on these slides account for efficient gas exchange in the lungs.

Answer 5

. Concepts to include: thin walls of alveoli and capillaries (short diffusion distance); large number of alveoli and capillaries (large total surface area) (matching ventilation and perfusion); large numbers of arterioles and bronchioles (maintaining a steep concentration gradient through effective perfusion and

Question 6

equipment

Answer 6

• Light microscope with x10 and x40 objective lenses and an eyepiece graticule
• Stage micrometer
• Pre-prepared slides of sections of mammalian lung tissue

Question 7

procedure.

Answer 7

In lung slides under the microscope you should be able to see the ends of the smallest bronchioles opening into alveolar ducts. These ducts then open into numerous alveolar sacs which then open into the smallest structures, the alveoli.

Owing to the presence of numerous alveolar sacs and alveoli, lung tissue is spongy in appearance. The walls of the alveoli are very thin. There are also numerous tiny blood capillaries, which are very difficult to observe unless they have been injected with coloured dye.

A number of tubular structures cut in a variety of planes are also seen in a thin section. You will see branches of the pulmonary blood vessels. It is hard to tell which are arterioles and which are venules because the arteriole walls are much thinner than arteriole walls in the rest of the body, so that there is little difference between the two types of blood vessels in lung tissue. These blood vessels can be recognised because they contain large numbers of red blood cells (erythrocytes).

You will also see large numbers of bronchioles. These have thin walls with two layers of cells, and no red blood cells inside. If you look carefully you may be able to see bronchioles connected to alveolar sacs.

Question 8

aim

Answer 8

To study the microscopic structure of the mammalian lung and relate structure to function.
1. Using the information above identify blood vessels, bronchioles, alveolar sacs and alveoli in your slide.

2. Use the stage micrometer to calibrate the eyepiece graticule with the x10 objective.
3. Use the x10 objective to draw a low power plan showing the appearance of: a) a bronchiole. b) an alveolar sac and its alveoli. In both cases add labels, annotations and a scale bar.
4. Use the stage micrometer to calibrate the eyepiece graticule with the x40 objective.
5. Use the x40 objective to make a high power drawing to show the cellular detail of three alveoli. Add labels, annotations and a scale bar to your drawing.
6. Calculate the diameter of a typical alveolus from your slide.