L4 Principles of Gas Exchange

Question 1

What is the partial pressure of a gas?

Answer 1

The partial pressure of a gas in a mixture of gases is the pressure that it would exert if it was the only gas in the container.. It is calculated using:

Partial pressure = mole fraction * total pressure

Question 2

What is Dalton’s law?

Answer 2

In chemistry and physics,Dalton’s law(also calledDalton’s lawof partial pressures) states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

Question 3

What is the partial pressure of oxygen, carbon dioxide and nitrogen at sea level?

Answer 3

Oxygen: 21.3kPa
Carbon dioxide: <0 kPa
Nitrogen: 80 kPa

Question 4

Why is the partial pressure of oxygen in our alveoli lower than than of the atmosphere?

Answer 4

• The oxygen we breath in mixes with the carbon dioxide we breath out diluting it.
• we humidify the air as we breath in. It is humidified to the saturated vapour pressure of water which is 6.03kPa.
• We consume more oxygen than we produce carbon dioxide due to western diet and so a lower respiratory quotient.

Question 5

What is the effect of humidification on the partial pressure of oxygen inspired air?

Answer 5

Partial pressure of air at sea level 101.3

(101.3-6.3)*0.21 = 19.95 kPa

Question 6

What is the effect of carbon dioxide the partial pressure of oxygen in inspired air?

How do we get to 13.3 kPa of oxygenic capillaries?

Answer 6

→ Most people produce CO2 at a rate of 5 kPa into the alveolus
If one CO2 molecule was produced for every O2 molecule consumed the alveolar partial pressure of oxygen would be:

19.95 - 5 = 14.95 kPa

However due to our diet 1.25 molecules of oxygen are consumed for every molecule of carbon dioxide produced.

14.95(51.25) = 13.7kPa

Some red blood cells by pass the alveolus without filling with oxygen and so the partial pressure of oxygen becomes 13.5kPa.

There is also a cardiac shunt, some blood from the right atria leaks into the left atria and so the partial pressure at the capillaries is 13.3 kPa. This is what we aim for as clinicians.

Question 7

How does solubility effect partial pressure?

Answer 7

Gases are differentially soluble. A gas that is less soluble requires less molecules to exert the same partial pressure of that gas in solution, to dissolve into that solution. If you have a very hydrophobic gas in water, for every time it is in water it will try and push its way out of water as quick as possible. If you have many of these molecules, you will exert a large amount of pressure as all the molecules are trying to get out. However, if you have a very hydrophilic molecules, and if you put 10 molecules in solution, they will not want to push out.

The partial pressure of a low solubility gas can be achieved quicker and with few molecules than a more soluble gas.

Question 8

Why do anaesthetics use gases with low solubility?

Answer 8

In anesthesia, they use gases with low solubility as you can get the partial pressure up in the blood quickly. When it gets into the brain, it moves out of the blood quickly. The lower the solubility, the faster the onset of the anesthesia.

Question 9

How does the solubility of carbon dioxide compare to that of oxygen?

Answer 9

Carbon dioxide is approximately 24 times more soluble in water than oxygen.

Question 10

Why does oxygen take longer to diffuse across into the blood compared to alveoli?

Answer 10

CO2 is 20 times more diffusion as diffusing across the membrane is a soluble process. Alveolar are surrounded in surfactant. The solution part allows gases to dissolve before they diffuse. CO2 is more soluble and so generates a lower partial pressure. In theory CO2 transfer is more efficient than O2. O2takes longer to diffuse across even though you can generate a large partial pressure gradient across an alveoli. For the small pressure change, you can move lots of CO2. For the O2 you generate a large pressure difference, using the same number of molecules. This takes longer and so oxygen takes longer to diffuse across and generate the partial pressures needed in the blood.

Question 11

At rest, how long does it take a red blood cell to transverse an alveoli?

Answer 11

0.75s

Question 12

At rest, how long does it oxygen to diffuse into a RBC, and carbon dioxide to diffuse out?

Answer 12

Oxygen: 0.25s

Carbon dioxide: 0.1s

Question 13

How long does it take a RBC to transverse an alveolus during exercise?

Answer 13

0.25s. A healthy person should still be able to oxygenate their blood at maximal capacity without any shunting.

Question 14

What is pulmonary fibrosis?

Answer 14

In Pulmonary Fibrosis, fibrosis tissue builds up causing restrictive lung diseases and a difficulty to breath properly. They get thickening of the alveolus basement membrane, decreasing ability for oxygen to diffuse in. Despite having enough oxygen in the alveolus they cannot shift enough in the time frame to get it equilibrated in the blood.
CO2 however can do so as it takes less time to equilibrate across in the same time period. There is normal CO2 removal but decreased oxygenation

Question 15

What is Type I respiratory failure?

Answer 15

The patient is hypoxic but not hypercapnia (normal CO2 levels).

Question 16

What is hypercapnia?

Answer 16

Hypercapnia, or hypercarbia, is when you have too much carbon dioxide (CO2) in your bloodstream. It usually happens as a result of hypoventilation, or not being able to breathe properly and get oxygen into your lungs.

Question 17

What is Type II respiratory failure?

Answer 17

The patient is hypoxic and hypercapnic.

Question 18

How is oxygen diffusing capacity measure?

Answer 18

Oxygen diffusing capacity = oxygen uptake/ alveolar partial pressure of oxygen - mean pulmonary capillary partial pressure of oxygen

It is difficult to measure mean pulmonary capillary CO2. Traditionally we use carbon monoxide. Carbon monoxide binds avidly to hemoglobin with an affinity of 250x that of oxygen and so must do a lot of competing to get rid of it.
Since carbon monoxide binds so avidly to hemoglobin, therefore carbon monoxide in the mean pulmonary capillary exerts a partial pressure of 0kPa as all the carbon monoxide will bind to hemoglobin and not be in solution. It is bound out of solution, not dissolved and so does not exert a pressure.

The carbon monoxide diffusing capacity = carbo monoxide uptake/ Alveolar PCO

Question 19

Why is compliance reduced with obesity?

Answer 19

Compliance is reduced with obesity as the chest wall will not tend to spring out, reducing compliance. This is due to the adipose tissue lying on top of the chest. They have a lower FRC.

Question 20

How can pulmonary fibrosis lead to Type I respiratory failure?

Answer 20

Pulmonary fibrosis – thickening of alveoli (sometimes occurs with cystic fibrosis, coal miner lung etc.). By thickening the membrane increase in time for CO2 to diffuse out and O2 in. This leads to a decreased rate of diffusion and increased time for diffusion. As the blood passes over, RBC not oxygenated. Initially they will show a Type I failure, if it gets worse, it can turn into type 2.

Question 21

How does pneumonia lead to respiratory failure?

Answer 21

Alveolar consolidation occurs in pneumonia. Bacteria infection fills up the lung and blocks of alveolus which can no longer participate in gas exchange. This leads to shunting as blood goes around the alveolus but does not participate in gas exchange. This can then mix together in pulmonary vein and dissolve the oxygen blood pressure.

Question 22

How can a mismatched heart lead to pulmonary oedema?

Answer 22

`frothy secretions and pulmonary edemas occur with mismatch of right heart and left heart. Left heart supplies the body and the right to the lungs. If you get a mismatch, the right heart is pumping well and the left not well. This means the right heart is pumping into the lungs and the left heart will not move the fluid out of the lungs This can lead to a build up o fluid in the lungs (left heart failure) and pulmonary odema.

Question 23

What are causes of emphysema?

Answer 23

The alveolus is destroyed through smoking or alpha1 antitrypsin deficiency disorder.

Question 24

What is Acute Respiratory Distress Syndrome (ARDS)?

Answer 24

Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath, rapid breathing, and bluish skin coloration. Among those who survive, a decreased quality of life is relatively common. It is a result of a systemic inflammatory response which can lead to ARDS or adding fluid in sepsis. Treat using positive pressure ventilation and removing the fluid.

Question 25

What is the percentage of oxygen, carbon dioixde, nitrogen and argon in the atmosphere?

Answer 25

Oxygen - 20.93% (21%)
Nitrogen - 78.09% (79%)
Carbon dioxide - 0.03%
Argon - 0.93%