The lungs are made of a soft, elastic, spongy tissue (it is pink in colour - the image above is grey in order to help you visualize the branches, which are in different colours). It is easy to imagine their structure much like the inverse of a tree. Air enters the lungs via the "trunk", the trachea. The trachea branches in two, to form the bronchi . Each bronchus continues to branch out (each branch is a different colour in the image above) until, at the end of each bronchiole, we reach a cluster of alveoli (like grapes on the bough). Alveoli are small sacs where the gas-exchange of CO2 and O2 takes place. There is an intricate web of blood vessels which wrap around each sac, allowing the transfer of gases to take place through the extremely thin walls of the alveoli. The total surface area of the alveoli is very, very large - roughly the size of a tennis court.
The lungs are not equal in size. The right lung is shorter, because the liver sits high, tucked under the ribcase, but it is broader than the left. The left lung is smaller because of the space taken up by the heart (see diaphragm for an image of this). Each lung is separated into lobes branching off the main bronchus; the right lung has three lobes, while the left has only two lobes. As the bronchi branch out, the total area of the two new branches is larger than its parent bronchus, making it extremely easy for the air to rush into the lungs.
Each lung is encased in a sack of tissue called the pleura. The lung is "stuck" to this sack by a small amount of liquid, which creates surface tension. It creates a very smooth surface so that as the lungs expand and contract they can ride over the heart and surrounding elements. Since each lung is in a separate pleural sac, if the chest wall gets punctured, only one lung will collapse. The lungs are "stuck" to the inside of the thorax, also by surface tension. As the ribcase and diaphragm move, the lungs are stretched, drawing air into the lung, or the lungs are compressed, pushing the air out.
There are two types of breaths:
the unconscious breath, for instance when you're sleeping;
the breath you choose to "take", when you regulate your breathing
There are a number of useful terms related to the volume of the lungs at various stages of the respiratory cycle, and related to the measurement of the air that can be inhaled/exhaled and the air that cannot be exhaled.
The quantity of air used in a regular inhalation and exhalation cycle
- Men: rest 750 cc, light work 1670 cc, heavy work 2030 cc
- Women: rest 285 - 393 cc
Average Tidal Volume at Rest: 500 cc
approx. 1500 cc to 2500 cc (males).
approx. 1000 - 1500 cc (males).
approx. 150 cc in airways.
Inspiratory & Expiratory Reserve Volume:
the maximum that can be inhaled or exhaled beyond normal tidal volume
the quantity of air that remains in the lungs and airways, even after a maximum exhalation -
N.B.: We cannot speak on residual air.
Dead Air, CO2, and Yawning:
the last air to leave the alveoli, and the first to be drawn back in is CO2 laden. When breathing very shallowly, the accumulation of CO2 makes us yawn -
Back to Respiration