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 Physiology, Sheet 3, Dr.yanal 7\2\2012

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Majed Sharayha



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PostSubject: Physiology, Sheet 3, Dr.yanal 722012   Sat Feb 11, 2012 12:14 am

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Physiology lec # 3
Before you start studying this lecture, I know it's too long but don't worry it's easy
Review :
The end result of the respiratory system is maintaining normal values of the arterial blood gases (ABGs), that is:
pO2 = 90 - 100
pCO2 = 40
pH = 7.4
this is the main whole result of the respiratory system, and what we're going to talk about the whole system. Till now we have taken that this is the normal values that we will get.
If we said that before we inhale, there is some volume of air inside the lungs which equals (2.2L). Means that each time we inhale we take half a litter (0.5L) which is the tidal volume (Vt) in addition to the volume that already present in the lungs the (2.2L) .
It makes sense to have some volume of air inside the lungs; because during expiration -if you don't have air in the lungs- the blood which comes to the lung will leave during expiration, without any exchange of gases. So if you have some volume of air inside the lungs, even during expiration there would be plenty of oxygen to diffuse to guarantee oxygen diffusion during inspiration and expiration.
This is the same as saying that normally we have (2.2L) and we can inhale at each breath a volume of air -this volume equals The Tidal Volume (0.5L)- and on the top of this tidal volume we still can still inhale forcefully (using all inspiratory muscles) adding extra volume of air (inspire more air).
In addition to this half litter (0.5L) we can add up to 3 litters, which means that you can bring the total volume to (5.7L).
But we don't do this all the time, usually aduring exercise you might make your tidal volume 1L, 2L, 3L or even 3.5 litters is the max. But not a half litter; because half litter is during the normal conditions (quite breathing) not during exercising.
This additional volume is called "Inspiratory Reserve Volume (IRV)"; it is reserved we don't use it all the time, we keep it for reserve.
So IRV : is the volume of air which you can inhale maximally forcefully on the top of the Tidal volume (Vt).
Now before you take the Vt (before breathing), can you exhale forcefully any volume of air inside your lungs, can you empty your lungs?
Yes, by using the expiratory muscles.
There are two types of muscles here:
1) The Inspiratory muscles.
2) The Expiratory muscles.
The volume of air that we can exhale forcefully just before taking the (Vt) is: The "Expiratory Reserve Volume (ERV)".
Can you empty your lungs to ZERO volume?
No, we cannot empty our lungs totally even after forced expiration, there must be some air resides (remains) in the lungs (residual volume of air).
"The residual volume (RV)": is the volume of air which remains in the lungs following forced expiration.
RV = 1.1L
So now we have discussed 4 volumes :
1) Tidal volume
2) IRV
3) ERV
4) RV
If you put two volumes together or three volumes or even 4 volumes together, you will come up with a new volume, but this new volume we don't call it "volume" anymore we will call it "Capacity".
So Respiratory Capacity: is the summation of the previously mentioned volumes.
The 1st capacity, before inhaling the (Vt), has 2 air volumes : RV & ERV.
These 2 volumes make up the "Functional Residual Capacity (FRC)" = 2.2L
FRC : is the volume of air present in the lungs just before taking the (Vt).
The sum of (Vt) and (IRV) is called the "Inspiratory Capacity (IC)".
If you inspired forcefully as to reach (5.7L), then expired forcefully as to reach (1.1L), how many volumes are you going to exhale?
We exhale 3 volumes; IRV, Vt & ERV.
The sum of these three volumes together is called "Vital Capacity (VC)" = 4.6L
VC : is the volume of air we can exhale forcefully following forced inspiration.
2.7
5.7
The sum of all 4 volumes: IRV, Vt, ERV & RV is called the "Total Lung Capacity (TLC)" = 5.7L
TLC : is the volume of air that both lungs can take.
1.1
2.2

There is a device which is called : "Spirometer" it measures any volume of air that you take out or in, but the residual volume remains inside the lungs so you cannot measure it using Spirometer.
Not only the residual volume, also anything that is related to it (ex: FRC, TLC), so we have to use other methods to measure them.
As we know there is something called The Anatomic Dead Space Volume = 150ml
The lungs are surrounded by negative intrapleural pressure. This negative pressure prevents the lung from collapsing.
The lungs is trying to collapse because it's an elastic structure; so to prevent this collapse we need an opposing force which is the inflation force, which is inward pressure surrounded by negative pressure.
If we removed this negative pressure, for example:
By taking the lungs outside the body
Stab wounds
Gun shots
Or anything that could take the lungs outside the body.
The lung will collapse to its Resting state.
Resting state means no more tendency to collapse or expand.
If the lung loses the negative pressure surrounding it, (the intrapleural pressure which is more negative), the lung will immediately collapse to its resting volume.
So the Resting Volume of the lung ( when the lung is not surrounded by any negative pressure) is called The Minimal Volume = 150ml .
NOTE: this is not the anatomic dead space volume, which is found in the airways (ex: nose, mouth, pharynx, larynx, trachea, the bronchi, bronchioles, the conduction zone) → all of them = 150ml .
The minimal volume is used for medicolegal purposes (اسباب طبية شرعية).
In vivo can we reach minimal volume?
No.
When do the lungs go to minimal volume?
Pneumothorax is the presence of air in the thorax cavity. Pneumothorax makes the pressure ZERO or Positive not negative. Immediately the lung will collapse to its minimal volume and → goes down to 150ml .
So we don't like pneumothorax; because the lung will collapse to minimal volume and this is not good.
The ATP we need for the muscles is less than 5% (3 or 2%) but if the lung became incompliant (stiff, غير مطاوعة) we need much more ATP.
The Inspiration Flow :
According to Ohm's law : flow =(DF/R)
The driving force (DF) for the air is not concentration is not electrochemical, it's a pressure difference.
So if (P1) is more than (P2) → Inspiration happens.
We may have:
P atm > P alv
P alv < P atm
And these cases are not the same!
P1 atm


P2 alv






Mechanics of breathing:
We have an enclosed box that can be moved by a handle. And inside this box there is a tube that is connected to a ballon.
0
How can we inflate this balloon (بدون النفخ داخل الأنبوب)?
The tube
P=0

The box
P=0

The balloon


The handle
(1)


P zero = 760 mmHg ( The atmospheric pressure).
By increasing the air volume surrounding the balloon → volume increases = pressure decreases from ZERO to negative.
This negative pressure will pull the balloon to the outside (it's an outward force).
P=0


P = -1


(2)

P = 0
When the balloon inflates the pressure inside the balloon will be negative.
(3)
P = -1
P = -1





The balloon is connected to the outside by a tube, the pressure outside = 0 ,the pressure inside = -1
There is a pressure difference → driving force → immediately air enters ( O2 & N ) → this molecules will raise the pressure in the balloon from -1 to ZERO. At the ZERO pressure there is no more driving force → no more entry of air.
Note that : inside the balloon→ P = (F/A) → the number of molecules remains the same → the area increased → the pressure decreased.
It's the inflated balloon that drives the air not the opposite.
Breathing airways
In the lungs, the same scenario happens :
The diaphragm
0
4-
0
The lung
The intra-alveolar cavity


The intrapleural cavity


The pressure in the pleural cavity = -4mmHg , this pressure is not connected to the outside.
The pressure at the lungs = 0
The pressure at the air ways = 0
The pressure at the outside = 0
Even though the lungs has 2200ml air still the pressure inside = 0
The diaphragm contracts and this contraction needs ATP (an active process).
The pressure in the intrapleural cavity drops from (-4) to (-6) (become more negative).
The lung now is surrounded by more negative pressure, immediately it will expand (inflate).
→the pressure intrapulmonary (intralungs) or what's called "inra-alveolar pressure" drops from (0) to (-1).
So now the alveolar pressure < the atmospheric pressure.
→normally, before the contraction happens the P alv = P atm .
We have 2 options :
↑ the P atm
Or ↓ the P alv
But we can't play with the (atm) pressure, so what happened is that we decreased the (alv) pressure to (-1) → therefore immediately the air will enter the lungs
When air enters, it will raise the pressure inside the lungs, from
(-1) to (0.75) to (0.5) to … to (0).
At the (0) pressure no more air enters the lung.
At the end of inspiration, what would be the intra-alveolar pressure?
ZERO, the word end means ZERO .
Our normal physiological pattern of breathing, do we call it negative pressure breathing or positive pressure breathing?
Negative; because we make one end negative (below Patm).
If a patient has a paralysis in the diaphragm (this diaphragm cannot contract for some reason), what is the solution for this case?
We do sth called "intubation", a tube that is connected to a machine, this machine is called "ventilator or respirator or resuscitator", it makes the pressure outside either positive to push the air in or negative to drive the air out.
→ here we have drived the air in when we made on end above the (P atm), but this is not a normal pattern of breathing, it's an artificial breathing. So artificial breathing is a positive pressure breathing.
The Expiration:
The expiration is passive, what does this mean?
When the diaphragm relaxes it ascends up, making the pressure in the intrapleural cavity goes from (-6) to (-4) again.
And this will compress the lung. When the lung is compressed it will become smaller because of the pressure around it, putting in mind that the number of molecules inside it still the same.
The pressure inside the lung is (+1) and the (P atm) is always (0)
So the air inside the lungs will go out. When this happens the pressure in the lungs go back to (0) → expiration stops.
At the end of expiration, what would be the intrapulmonaric pressure?
ZERO again.
So the most important word in this whole process is "the pressure difference".

How many pressure difference do we need to make half litter available to the lungs?
Example:
In the heart :
The stroke volume = 70ml
The heart rate = 70
→The cardiac output = 5 L/mint
In the lungs :
The tidal volume = 500 ml
The respiratory rate = 12
→The respiratory minute ventilation (كم تتنفس في الدقيقة ) = 6 L/mint
Systemic circulation:
(اين تبدأ و الى اين تنتهي و ما كميتها في الدقيقة الواحدة)
Starts: from the left ventricle (aorta), the pressure in the aorta = 100 .
Ends: in the right atrium, the pressure there = 0 .
The pressure difference = 100
The blood flow = 5L = 100/TPR
The air flow = 6L = 1/Rairways

Almost the same
Blood flow (per minute): To let the blood move we need a driving force, and this driving force = the pressure difference = 100 → is going to be opposed by a vascular resistance (TPR).
Air flow (per minute): the pressure difference in the respiratory system between the outside and the inside = 1 → opposed by airway resistance
The conclusion of this scenario is that the airway resistance (with all its branches) is so small negligible, why it is so small?
Because we need a small driving force = 1, but in the vascular system the driving force that we need is high = 100 so we need a high resistance (and if the driving force = 200 we need a resistance = 200 too).
Why are the airways so important?
Because the flow (either expiration or inspiration) = DF/R airways
= P alv – P atm / R airways

If the pressure difference increased three ↑↑↑, the airways resistance should increase ↑↑↑ to make sure that the flow stays the same.
How do we know that the airway resistance is very small?
We don't feel like facing any difficulties when we take a breath.
We know for a fact that we need very small driving force = 1mmHg .
Anytime you find that you need more than 1mmHg, for example you need (-10) while the P atm = 0, you have to increase the airway resistance 10 times = (-10).
Normally physiologically, do we face significant airway resistance?
No.
Where this airway resistance resides? In the large airways or in the small airways?
In the large airways; because : R α 1/A2

The area A2 = ↓ lower
The summation area of all these bronchiols = ↑ higher










In pathophysiology, in the abnormal conditions (disease conditions), where do you think most the additional resistance originates from? Normally the R=1, if the resistance were = 20, from where did the additional (19) come from?
Here the additional airways comes from the small airways; because :
The large airways (ex: trachea & the main bronchia) are surrounded by cartilage and this cartilage is very tuff, it makes this structure no collapsible.
But the small airways lack cartilage, they have a lot of smooth muscles, if they became irritated they will contract (bronco constriction) → the lumen decreases.
The bronchioles have an excessive mucus and this mucus if secreted it can block the small lumen but cannot block the large lumen (trachea).
They have a sub mucosal layer if inflamed and swollen, this causes more opstruction.
The last take home message:
If we have more airway resistance, which will be more difficult ? inhaling or exhaling?
Thank you
Done by : Rand Saqer Herzallah
Physiology Lecture #3
Date : 7/2/2012
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