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 Physiology, Sheet 2, Dr.yanal 6\2\2012 "corrected by ahmad khjail"

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Majed Sharayha

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PostSubject: Physiology, Sheet 2, Dr.yanal 622012 "corrected by ahmad khjail"   Tue Feb 07, 2012 9:54 pm

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The air around us is mainly composed of 79% N₂ ( P N₂ = 600 mmHg) , 21% O₂ (P O₂ = 160 mmHg ) and almost 0% CO₂ ( P CO₂ = 0.3 mmHg ).
If we ascend to high altitude, the total atmospheric pressure will decrease, but the percentage of each gas will be kept the same.
Example, if the total pressure at high altitude is 400 mmHg :
P N₂ = 79% of the tot. Pressure = 316 mmHg
P O₂ = 21% of the tot. Pressure = 84 mmHg
The respiratory system is composed of two portions: a. Air ways “: start at the trachea → right and left main “primary” bronchus → secondary bronchus → tertiary bronchus → …. 23 generations (divisions)
b. the lung or the alveoli

=> remember that, when a structure is divided, the surface area of the divisions is more than that of the main structure.
-the first 16 generations are the Conducting Zone “ Anatomical Dead Space “ → conduct the air without any gas exchange.
=> It is called Dead because the walls in this area are thick therefore preventing any exchange of gases. Moreover, it is not surrounded by capillaries.
- the last 7 generations are the Respiratory Zone :
→ Transitional Zone “ 17-19”
→ Respiratory Zone “ 20 – 23 “ → where the gas exchange occur across their wall.
- Trachea is generation # 0
- primary bronchus is generation # 1
- secondary bronchus is generation # 2
- tertiary bronchus is generation # 3
- the terminal bronchiole # 16
- the respiratory bronchiole # 17
- the alveolus # 23 “ which is a dead end “

The bronchial tree is surrounded by two membranes “plural membranes”, which have interplural space / cavity between them.
Usually, when we breath, we don’t breath in an empty lung “ in a totally collapsed lung “ because there is already about 2.2 L of air in the lung just before taking the tidal volume.
=> The Tidal volume: is the volume of the air that we breathe per single breath “which is estimated by 0.5 L.”
And after talking the tidal volume the total Volume of the air in the respiratory tract will be 2.7 L. (and will return back to 2.2 L again at the end of the expiration).
So, why do we take fresh air at every breath?
This is as a result of the continuous diffusing of the O₂ into the blood. And unless we renew the oxygen (alveolar oxygen) in each breath, the amount of the O₂ in the alveoli will decrease to a very low degree.
Note that the composition of the air outside the body is different from inside, and that’s why we breathe and renew the alveolar air.
The last portion of the tidal volume remains in the anatomical dead space and never reaches the alveoli, and this volume (anatomic dead space) is about 2ml/kg of the body weight. So the air in the alveoli is not 100% fresh air. Assuming a person of 75 kg body weight, this person’s anatomic dead space volume equals to (2*75 = 150 ml), when this person take 500 ml of air as tidal volume, 150 ml will remain at the anatomical dead space as atmospheric air and 350 ml will reach the alveoli as fresh air.
Remember that outside the body , P O₂ = 160 mmHg , P CO₂ = 0 mmHg and P N₂ = 600 mmHg.
The Anatomical Dead Space is lined from inside with respiratory epithelium with goblet cells, which secrete mucus that trap any foreign particles inhaled.
This epithelium also has cilia that move only in one direction “escalator” (which moves in one direction) toward the larynx and then expelled outside. (These cilia are paralyzed by smoking).
The mucus also warmth the inhaled air (bring it to 37°) preventing physical irritation and adds water vapor to change it from dry atmospheric air to humidified air.
In this dead space we have 3 major gases: O₂, H₂O and N₂.
Because the body temperature is usually fixed at 37°, the P H₂O is always equal to 47 mmHg, and this air is poly saturated, so we can’t add more H₂O “100% humidified”.
When we have a mixture of gases, each time we add a new gas it replaces another one depending on their proportions.
when the atmospheric pressure is 760 mmHg, and the P H2O is 47 mmHg, which is fixed, the rest , that is 713mmHg, will be composed of 21% O₂ (150mmHg) and 79% N₂. So at the sea level, P O₂ in the fresh air never exceeds 150 mmHg.
Note that the saturation of the air with H₂O occur in the trachea.

In the alveoli we have 4 major gases : N₂, O₂,H₂O and CO₂ which added by the blood coming to the lungs.
PA O₂ will be less than 150 mmHg , it equals to 100 mmHg “ because it is continuously transformed to the blood”.
PA H₂O will be fixed, = 47 mmHg. (Its fixed because our body temp is fixed as well)
PA CO₂ = 40 mmHg.
PA N₂ = 760 – ( 100+47+40) = 573 mmHg (N2 is a spectator molecule )
Systematic venous blood that is running toward the lung has Pv O₂ = 40 mmHg , that is why the O₂ will move from the alveoli to the blood.
Pv CO₂ = 45 mmHg, that is why the CO₂ will move from the blood to the alveoli.
When the blood leaves the lung, it will have the same PA O₂ and PA CO₂. So the ABG “ Arterial Blood Gases” pressure will be P O₂ = 100 mmHg, and P CO₂ = 40 mmHg which is the same as alveolar pressures.
At the next level, the blood will run in the capillaries, so there will be the exchange of O₂ between the capillaries and the cells.
between the capillaries and the cells there lies the Interstitial space “ with 20 µm width”.
In the capillary, P O₂= 100 mmHg, P CO₂ = 40 mmHg. While in the interstitial space , the P O₂ = 40 mmHg and the P CO₂= 45mmHg. “ these differences facilitate the movement of the O₂ from the capillary to the cell and CO₂ in the opposite direction.
At the intracellular level, P O₂ must be less than 40mmHg to permit the diffusion of O₂ to the cell; on the other hand, P CO₂ must be higher than 45 mmHg to allow the diffusion of CO₂ toward the interstitial space.

During CPR “Cardiopulmonary Resuscitation” (mouth to mouth breathing ) we give the patient our exhaled air , so are we giving him any O₂? to answer this question we have to analyze the expired “ exhaled” air.
when we exhale 500 ml, the first proportion ( 150ml ) will exit from the anatomic dead space, so it will humidified fresh air.
the second portion ( 350 ml ) will be alveolar air.
these two portions have different compositions , while the first portion has P O₂ = 150 mmHg, the second portion has P O₂= 100 mmHg.

So, the P O₂ in the mixed expired air is more than the P O₂ in the alveolar or arterial air. ( as a result of the air that is present in the anatomical dead space. and the more long this space, the exhaled air will be closer to the atmospheric air)
and therefore during CPR, we give a significant amount of O₂, that is enough to succor the patient.

Done by : Haneen Kharoob
Corrected by : ahmad khajiil

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