6.4 - Gas Exchange

Uploaded by luizgmello on 05.11.2012

Hey everybody and welcome to topic 6.4, Gas Exchange!
In Topic 6.1, Digestion, we discussed how the body absorbs and assimilates nutrients.
Our diet consists of different sources of proteins, lipids, nucleic acids and carbohydrates,
as well as nutrients and vitamins. Carbohydrates play a particularly crucial role, as enzymes
will break large molecules such as starch into smaller molecules like maltose and, eventually,
glucose. You remember that glucose is an essential molecule in the production of energy in the
form of ATP in the mitochondria of eukaryote cells, and that includes humans.
Another essential component for the production of ATP is oxygen. You remember that oxygen
allows pyruvate to enter the Krebs cycle and increase the yield of ATP production in a
process known as aerobic cellular respiration. This oxygen arrives at cells thanks to the
combined effort of the transport and respiratory system.
If everything I just said sounds like Greek to you (and you’re not Greek), it’s a
good idea to review Topic 3. Chemistry of Life is a complex, abstract topic and it never
hurts to go review it once in a while. -
There are three terms that need to be clearly distinguished. They are ventilation, gas exchange
and cell respiration. Ventilation is a muscular action that results
in the movement of air into and out of the lungs. This is a physical process that requires
energy, since it requires muscles to contract. Some of the muscles involved in ventilation
are the diaphragm, separating the torax from the abdomen, and the intercostal muscles on
the inside and outside of the ribs. Gas exchange is the exchange of O2 and CO2.
In the respiratory system, it occurs around a structure called the alveoli. There, the
blood releases CO2 into the lungs to be exhaled, and red blood cells attract the recently inhaled
oxygen molecules. This process is also physical, as it just involves the movement of molecules
without any chemical transformation. As they move across membranes in favor of a concentration
gradient, there’s no energy needed. Lastly, cell respiration, which was described
earlier. Anaerobic cellular respiration, which doesn’t depend on oxygen, begins in the
cytoplasm to produce small amounts of ATP and pyruvate as a by-product. If oxygen is
present, aerobic cellular respiration will take place, with pyruvate entering the mitochondria
and the Krebs cycle. Both types of cellular respiration are now chemical processes, because
they chemically transform glucose into ATP and pyruvate and later into carbon dioxide,
water and more ATP. -
The need for ventilation emerged as organisms became larger and more complex. Amphibians,
such as frogs and toads, can obtain oxygen and release carbon dioxide through the surface
of their skin. Hence, there’s no need for lungs or a ventilation system. This is an
evolutionary barrier for the occupation of drier areas, where it would be difficult to
maintain such a moist surface, a requirement for gas exchange to take place. That’s when
reptiles come in! Their thick scales prevent water loss and help maintain homeostasis,
but gas exchange now needs to occur internally, in the lungs. So ventilation ensures that
there is a constant supply of air to the lungs. You may be wondering why the inside of the
lungs isn’t filled with water so that oxygen could diffuse through it, rather than needing
to spend energy to move air in and out of the lungs. The answer is that diffusion through
water is too slow for large, complex animals with a high oxygen demand. Fish use gills
to breath obtain oxygen from the water they swim through: now think of the largest fish
you know and compare its size to a whale (which has lungs and breathes in air). The ability
to absorb oxygen efficiently is a big factor in limiting the size of non-mammals in aquatic
environments. -
I mentioned earlier the alveoli as the structure in the respiratory system where gas exchange
takes place. This is what it looks like. An alveolus looks like a raspberry, with small
alveolar sacs surrounded by capillaries – the very location of gas exchange! Each alveolus
connects to a bronchiole, which in turn connect to the bronchi and then to the trachea and
your mouth and nose. As usual, structure relates to function, so you can probably deduce that
the shape of the alveolar sacs increase surface area, that they are made up of a single layer
of cells (reducing the diffusion distance) and are very moist, like the skin of a frog,
to facilitate the diffusion of gases, which must be dissolved for diffusion to take place.
The constant blood supply from the capillaries ensures that there are always red blood cells
available to attract oxygen from the inside of the alveoli. This is so important that
the heart pumps blood to the lungs directly, collects the blood back, and then pumps it
to the rest of the body. -
This simple diagram contains the structures you need to be able to draw from scratch.
You must be able to draw this with an inset containing a magnified view of an alveolus
as in the previous slide. -
Before we look at this next slide, do the following. Take both your hands and put them
on the bottom of your ribcage. Take a deep breath. Hold it for a second. And exhale. Now, if you're
careful, as you're doing this, you'll be able to tell all of the mechanisms of ventilation
that are taking place. Here we go.
This last slide contains the mechanisms of ventilation, in other words, the series of
events that lead to inspiration and expiration. If you remember inspiration, everything is
expiration is just the opposite. For instance, in inspiration, the external intercostals
muscles contract, so in expiration, they must relax. Outlining the steps in ventilation
is a popular question for this topic, so make sure you know it.
And this concludes topic 6.4. Make sure you’re taking thorough notes and that you have successfully
attempted the drawing of the respiratory system, with the inset, without referring to your
notes. Keep at it and I’ll see you guys in class!