Respiration can be divided into three different consecutive
processes: ventilation, external or pulmonary respiration and internal or
tissular respiration.
 |
| Respiratory System |
Ventilation is the process related to the movement of the
air, from the environment to the interior of our lungs and from the interior of
our lungs to the environment. So, ventilation can also be split into two
different processes, inhalation when the air flows into our lungs, and
exhalation when the air flows from our lungs.
Our lungs have no inner muscles, the movement of the air
depends on the muscular movements that involve the rib cage and the diaphragm.
The lungs are surrounded by two thin tissular layers called pleurae. The outer
one, called parietal pleura, covers the inner surface of the rig cage. The
inner one, called visceral pleura, covers the surface of the lungs. Between
these two pleura we can find a fluid called pleural liquid. Pleural liquid
maintains both pleurae bounded.
When the rib cage moves, the parietal pleura moves too, and
it leaves to the movement of visceral pleura, that is firmly stuck due to the
pleural liquid. Visceral pleura moves lungs.
How does inhalation take place? It is quite simple. External
intercostal muscles contract, and this movement makes the rib cage ascend. This
makes the rib cage increase its volume. At the same time, the diaphragm
contracts. Both muscular contractions cause the lungs to increase their volume,
because they are bound to the rib cage and the diaphragm. So the pressure
inside the lungs falls, and becomes lower than air pressure. Gases flow from
places with high pressure to places with low pressure, so the air enters the
lungs.
Exhalation is a passive process. Lung are made up of many
contractile fibers, they are very elastic structures. So, when rib external
intercostal muscles and diaphragm relax, the elastic properties of lungs make
them return to their original size. Lungs decrease in size, so air pressure
inside them rises and becomes higher than air pressure. And the air exits the
lungs, because fluids flow from high pressure places to low pressure places.
Sometimes we need to increase the speed of inhalation and
exhalation. when this happens the elastic properties of the lung are not
sufficient to maintain the ventilatory rate. We must expel air more quickly.
This process is called forced exhalation, and is carried out by internal
intercostal muscles and abdominal muscles. When these muscles contract, the rib
cage returns to its original position very quickly, making the lungs shrink in
few seconds.
This happens, for instance, when we are doing physical
exercise and we need to breathe quickly to provide oxygen to our muscles, or
when we want to shout, or whistle or even sing.
 |
| Inhalation and exhalation |
When we talk about external respiration we refer to the
process of gases exchange between lungs and blood. It takes place in pulmonar
alveoli, tiny spherical structures of our lungs, covered by a very thin
epithelial layer and surrounded by capilar vessels.
The air from our environment enters into the alveoli (thanks
to inhalation). This air is rich in oxygen, but has a low quantity of carbon
dioxide. The blood that has reached the capilar vessels of the alveoli comes
from the body, and has a low quantity of oxygen, and a high quantity of carbon
dioxide.
Chemical substances tend to move from high concentration
places to low concentration places (this is called osmosis), so oxygen moves
from the air to the blood, whereas carbon dioxide moves from the blood to the
air. This process is called pulmonary or external respiration.
Due to this, when the blood exits the lungs it has a high
concentration of oxygen and a low concentration of carbon dioxide.
To improve the transport, oxygen travels in the blood linked
to a protein called hemoglobin. This protein can be found inside the red cells
(erythrocytes). This is the reason why erythrocytes are very important to our
body: they are the cells that transport oxygen from the lungs to the tissues.
 |
| Hemoglobin and hero group by Openstax College |
Carbon dioxide travels in our blood transformed in carbonic
acid. This is why when we consume a big amount of oxygen, because we are doing
hard physical exercise, for instance, the pH of our blood decreases: the
accumulation of carbon dioxide increase the amount of carbonic acid in the
blood and acids tend to reduce the pH of solutions.
 |
| Respiratory System and Exchange of Gases, by NLHBI |
When the blood arrives at the tissues the situation is just
the opposite as in the lungs: the erythrocytes are charged with oxygen, however
the cells of the tissues have consumed the oxygen to obtain energy (during the
cell respiration), so there are a low quantity of oxygen in there. On the other
hand, cell respiration has produce a high amounts of carbon dioxide, so the
tissues have a high quantity of this product.
This is the reason why oxygen moves from the blood (that
comes from the lungs with high amounts of oxygen) to the tissues, and at the
same time carbon dioxide moves from the tissues to the blood. This exchange is
called tissular or internal respiration.
Deoxygenated blood, full of carbon dioxide, must return to
the lungs to exchange gases again, and the cycle has been completed.
