Do the lungs just consist of a large tree of alveoli, covered by the pleura, like this picture:
Or are they found "inside" the sac that is the lungs? If it's the later, what fills the space in-between the alveoli and forms the walls of the lung?
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2 Answers
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Alveolar tree is like a bunch of grapes. If you put a bunch of grapes into a plastic bag and put this bag into another bag, you can imagine how the lungs are covered.
Alveoli form the surface of the lungs. There is a membrane called visceral (or pulmonary) pleura that covers the alveoli and this membrane continues into another sheet called parietal (or thoracic) pleura that covers the inner side of the thoracic wall. The space between the membranes is called pleural space.
Here you can see how visceral pleura continues into parietal pleura:
Picture 1. Pleural space (source: sdstate.edu)
Picture 2. from page 1117 of "Cunningham's Text-book of anatomy" (1914) (source: Flickr)
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$\begingroup$ Oh, so aside from the pleura, there is nothing else covering/containing the alveoli? It's just alveoli that are then immediately covered by the visceral pleura? $\endgroup$– DahenCommented Dec 19, 2019 at 19:27
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1$\begingroup$ @Dahen Yes, alveoli are immediately covered by the visceral pleura. The image you pasted may mislead you into thinking that alveoli are very individual and scattered, but they are actually packed together and form a smooth surface similar to the liver (see the real lungs without pleura). $\endgroup$– JanCommented Dec 20, 2019 at 9:24
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1$\begingroup$ One nitpick. I usually encourage students not to think of a pleural sac as a bag containing a lung, but rather a bag containing the pleural fluid, which is smashed in between the lung on one side and the chest wall on the other. The part of the bag that touches the chest wall is parietal. The part that touches the lung parenchyma is visceral. Using the bag of grapes analogy, you'd put water in the bag, then press the bag up against the grapes without ever putting the grapes inside the bag. $\endgroup$– De NovoCommented Dec 20, 2019 at 21:40
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$\begingroup$ That does make it easier to visualize, thanks! $\endgroup$– DahenCommented Dec 21, 2019 at 9:53
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I agree with Jan's answer generally, but thought I'd clarify a few points.
What fills the space between the alveoli in the lungs?
As Jan says, alveoli are packed together. In most cases, the thing next to any particular alveolus is another alveolus. In this case, the space between them is the alveolar wall, which (generally) consists of a pneumocyte and capillary (endothelial cell) with a shared basement membrane. You can see this in the following slide from Yale.
Alveolar sacs can abut other elements of lung parenchyma, including bronchioles, smooth muscle, nerves, and pleura. In this figure from Ross Histology, Ch. 19, you can see an alveolar sac adjacent to visceral pleura.
The relationship between lung parenchyma and pleural sac
I would encourage you to not think of the visceral and parietal pleura as two different bags, both of which contain the lung parenchyma (bag in a bag analogy). There is only one bag (per lung), and the only thing inside of it is the pleural fluid. The alveoli and associated parenchymal tissues are on the outside of the pleural sac just as much as the chest wall. They are just on opposite sides of the sac. This ubiquitous figure illustrates the relationship. The fist represents a lung, the ballon represents the pleura.
