Shadowing Practice: Respiratory System 5, Podcast on breathing - Learn English Speaking with Video

Now ventilation describes the process whereby air is sucked into the lungs and blown out of the lungs.

Ventilation is the physical transfer of air in and out of the lungs.

This is the process we would normally simply refer to as breathing.

And it's one of the fundamental things that we need to understand, this mechanism of ventilation.

because if we understand the mechanism of ventilation we're in a position to do something about it

when that mechanism goes wrong and you will come across many situations in your clinical life where there's problems of ventilation.

So you might come across patients with chest injuries of various types such as a pneumothorax

or a haemothorax or a pulmonary contusion and you'll certainly come across people with airways diseases such as chronic bronchitis,

emphysema, chronic asthma, acute asthma, pneumonia, various other pulmonary infections.

So this is something you're going to come across almost on a daily basis

and of course it fits in quite nicely with our priorities in clinical care which are airway, breathing and circulation.

So the patent airway

and the ability to ventilate the lungs is always right at the top of our priority of clinical interventions.

Now first of all we need to think a little bit about the anatomy.

So we have the chest wall, now there's 12 pairs of ribs in the chest wall,

12 ribs on either side connected by the sternum at the front

and the ribs are connected to the sternum via the costal cartilage and in between the ribs we have the intercostal muscles.

So costal means to do with ribs So the intercostal muscles are the muscles between the ribs.

And at the bottom of the thoracic cavity, separating the thoracic cavity from the abdominal cavity below,

we have the diaphragm which is a domed sheet of muscle.

Now the first thing we need to do is to breathe in the process of inspiration.

So if you put your hands on your front of your chest

and take a deep breath in I think you can probably see that your hands move up and out.

So to facilitate the process of inspiration the external intercostal muscles contract

and that has the effect of pulling the rib cage

that is the chest wall or the thoracic wall up

and out and at the same time if you put your hand on your tummy and you take a breath in

if you take a deep breath in with your hand on your tummy i think you can probably see

that your hand moves up the way so during inspiration the chest wall moves up and out and the abdomen moves outwards.

Now the reason that the abdomen is moving outwards is that the diaphragm is moving down.

So to facilitate inspiration the diaphragm flattens and in doing

so moves down and that's going to press on the contents of the abdominal cavity

and that's what makes your tummy go out when you take a breath in.

So we have a situation where the ribs

and intercostal muscles have pulled the chest wall up and out and the diaphragm is flattened.

Now these two measures are going to increase the volume in the chest, they're going to increase intrathoracic volume.

So the volume in the chest is now greater.

Now if the volume in the chest is greater let me ask you what is

that going to do to the pressure inside the chest, what is that going to do to intrathoracic pressure?

Well if the volume is greater the pressure is going to be reduced

because now the air molecules that were in the chest have got a greater volume to move around in.

So as the ribs go up and out the diaphragm goes down the volume increases but the pressure decreases

And what this means is the pressure inside the lungs

which of course are located inside the thoracic cavity is now lower than the atmospheric pressure

and as a result of this air is going to be

sucked in from the external atmosphere where the air pressure is relatively high into the lungs where the pressure is lower

as a result of the expansion of the thoracic cavity.

So air is actually sucked into the lungs.

We are negative pressure ventilators.

We ventilate the lungs because we generate a negative pressure in the lungs and the air is sucked in.

That's good because

that means fresh air comes in with plenty of oxygen ready

to be absorbed into the blood in the process of gaseous exchange.

and of course we depend on this constant supply of oxygen to support all of our energy generating processes throughout the body.

So oxygen from that inhaled air will go into the blood from the air in the lungs

and in the same way carbon dioxide generated by metabolic processes

throughout the body will diffuse from the blood into the air in the lungs.

So this means that the air that we inhaled

that was high in oxygen and low in carbon dioxide is now lower in oxygen but higher in carbon dioxide.

So we need to breathe it out again so the air can be refreshed again.

Now this process of expiration or breathing out is actually facilitated in two ways.

The first is that the lungs themselves are very elastic so

if you blow a lung up just like a balloon it will contract all on its own because it has elastic walls.

The small air sacs called the alveoli have elastic tissues in their walls

and they will naturally contract down again

and as the alveoli contract down that's going to decrease the volume of the air inside these alveolar air sacs.

If you decrease the volume that will increase the pressure causing the air to be blown out.

So it happens on the scale of the small elastic air sacs

but it also happens on the scale of the entire thoracic cavity

because when you breathe out your chest wall will move down and in.

The intercostal muscles will relax and the chest wall will naturally fall down

and in and at the same time as this the diaphragm will go up.

Now when the diaphragm goes up it's relaxing so relaxation causes the diaphragm to dome up the way.

It's the contraction of the diaphragm that causes it to flatten

and move down during the process of inspiration but now we're breathing out

so all the diaphragm has to do is relax and it will assume it's upward projecting domed position.

So the chest wall goes down and in and the diaphragm goes up

and the combination of these two movements is going to decrease the volume in the thoracic cavity.

So now we have more air molecules in a given space so that's going to increase the pressure.

So to breathe out, chest wall down and in, diaphragm up, therefore the volume is going to be decreased,

therefore the pressure is going to be increased.

And this has the effect of increasing the pressure inside the lungs in the thoracic cavity.

It increases the pressure above the atmospheric pressure

and as a result of this the air is going to be blown out of the chest.

So we We inspire by generating negative intrathoracic pressures

and we expire by generating positive intrathoracic pressures facilitated by the movement of the chest wall and the movement of the diaphragm.

That is the basic mechanism of how air gets in and out of the lungs.

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Shadowing Practice: Respiratory System 5, Podcast on breathing - Learn English Speaking with Video

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