The lung and gas exchange
We can also gauge the growth of lungs and airways from the efficiency with which it fulfills its task of supplying the body with sufficient oxygen, and of removing carbon dioxide. The supply and removal of gas must meet the requirements of the gas exchanging body mass, hence be appropriate for any body weight.
All gas passes the alveolar surface. Obviously an opportunity to study lung and airway tissue of healthy, growing human individuals is very rarely available. It has been established in other mammals (see illustration) that a pronounced increase in alveolar surface area after birth is followed by a less pronounced increase. In man there is evidence that the increase in lung volume and lung surface area until age 6-8 year is primarily due to an increase in the number of alveoli from about 17 million to about 300 million; in this phase the number of airways still increases. This period of multiplication is followed by one in which the alveoli primarily increase in size.
As regards gas exchange there are
a few trends. The partial pressure of carbon dioxide in blood
(blue) is high at birth, drops dramatically in the first few
hours and subsequently for practical purposes remains constant
during to old age. The lung, therefore, is completely up to
its task of removing gas even when body mass increases from
about 3-3.5 kg in a newborn to 80 kg or more in an adult.
The partial pressure of oxygen (yellow) changes appreciably: low immediately after delivery, an approximately exponential rise until adulthood, and then a slow decline. Do keep in mind that due to the oxygen binding capacity of fetal and adult hemoglobin the quantity of oxygen carried per unit of blood volume varies much less than you might glean from from the trend in the oxygen tension.
- Lung growth is at first characterized by an increase in the number of alveoli, an subsequently by an increase in the dimensions of alveoli.
- The capacity to remove carbon dioxide is closely tuned to body mass.
- The partial pressure of oxygen varies during growth and aging.
The latter phenomenon can be understood if we consider the elastic properties of lung and thorax during growth and aging.