Gas Exchange in Lungs

Question 1

What is the normal range of PaO2?

Answer 1

10-13.1 kPa. At this point Hb is fully saturated.

Question 2

What is Hypoxaemia?

*Difference to Hypoxia?

Answer 2

When the partial pressure of O2 in blood falls below 10kPa.

*Different to Hypoxia which means low pO2 delivery to tissues.

Question 3

CO2 regulates blood pH. The normal values for PaCO2 are?

Answer 3

Between 4.9-6.1kPa

Question 4

What is Hypocapnia?

Answer 4

When PaCO2 falls below 4.9kPa

CO2 is lost, lose acidity and pH is increased (i.e. alkalosis)

Question 5

What is Hypercapnia?

Answer 5

When PaCO2 rises above 6.1 kPa

CO2 is being retained, acid is retained, pH decreases (acidosis)

Question 6

What are the 2 types of respiratory failure and describe them?

Answer 6

Type 1: CO2 levels are okay and within range, but O2 is not good and the patient is hypoxaemic (i.e. below 10kPa in blood).
Type 2: O2 isn’t good AND CO2 is also beginning to accumulate in the body, this is hypercapnia (CO2 above 6.1kPa).

Question 7

What are the 4 things that cause you to become hypoxic?

Answer 7

1) Hypoventilation
2) Diffusion Defects
3) VQ mismatch
4) Shunt (severe VQ mismatch)

Question 8

What is Hypoventilation?

Answer 8

Not breathing enough (causing CO2 in blood to rise). Hypoventilation is also the ONLY cause of hypercapnia. hypoventilation can cause hypoxaemia due to not taking in enough O2
*May be seen in obesity and sleep apnoea.

Question 9

Fick’s Law of Diffusion tells us there is increased diffusion if there is?

Answer 9

A greater concentration gradient
Higher solubility
Decreased molecular size
Increased surface area

Question 10

What happens at the alveolus when venous blood comes through the capillary?

Answer 10

Venous blood comes along in the capillary, with 5kPa of O2 and 6kPa CO2 gets oxygenated as O2 diffuses across the alveolar wall and through capillary wall and then the blood leaves, now with 13kPa O2 and 5kPa CO2.

Question 11

What are the various factors that effect the surfaces of the alveoli and can cause Hypoxaemia?

Answer 11

Increased thickness e.g. Oedema, fibrosis
Surface area can be decreased e.g. in emphysema
Concentration gradient can be effected (a ventilation problem)

Question 12

Describe a shunt?

Answer 12

A shunt is when O2 doesn’t work.There is complete blockage of the right bronchus of the lung (for example). As a result there is no ventilation into the right lung, so venous blood enters and venous blood exits and goes back to the heart unaltered (no O2 added).

Question 13

In a shunt if the right lung is obstructed then the left one is left unobstructed (and vice versa) what does this mean?

Answer 13

Left lung is unobstructed, so ventilation is taking place, venous blood is entering and oxygenated blood is leaving and going back to the heart.
When the two mix, despite blood from the left lung being fully saturated, the venous blood from the right lung at 5kPa brings down the overall PaO2 (kPa) and the overall saturation of the blood.
Giving a hypoxic reading of arterial blood!

Question 14

Why is supplemental O2 not effective in patients with a shunt?

Answer 14

Because the right side cannot get any O2 and the left is already saturated.

Question 15

Other causes of a shunt?

Answer 15

Oedema e.g. caused by pneumonia (reduced alveolar ventilation)
Developmental problems such as with the foramen ovale or ductus arteriosus (poor/no blood flow to lung).

Question 16

What is a VQ mismatch?

Answer 16

V means ventilation and the Q means blood flow. There is about 5L/min of cardiac output from the heart going into about 5L of air (so ratio of 1).
When the ratio of alveolar ventilation to blood flow is not 1, expect dyspnoea (breathlessness).
**V/Q mismatch is not a complete blockage but still an obstruction to air flow.

Question 17

How do you identify a VQ mismatch?

Answer 17

Breathe in radioactive air and then scan the chest you can see where the ventilation is, where the air is going. You can then put a radioactive substance in the blood and scan the chest to see where the blood is flowing in the lungs.

Question 18

What is the respiratory exchange ratio?

*Write the equation

Answer 18

A measure of how much CO2 you’re producing and how much O2 you’re consuming.
*R = Vol of CO2 evolved/ Vol of O2 absorbed = 0.8 (usually)

Question 19

What is the Alveolar Gas Equation and what does it tell us about PAO2?

Answer 19

PAO2 = F1O2 x PB - (PaCO2 /R)
PAO2 = Alveolar O2 partial pressure 
F1O2 = Fraction of O2 inspired that is oxygen (0.21)
PB = Barometric pressure (atmospheric pressure) (100)
PaCO2 = Arterial partial pressure of CO2
R = Respiratory exchange ratio

PAO2 and PaO2 should be within 2 kPa. It tells us what PAO2 SHOULD be, based on PaCO2, so you can use this to see how you can get PO2 back up to normal.

Question 20

How do the lungs compensate for VQ mismatch?

*How is this relevant to COPD?

Answer 20

The lungs can usually cope to some extent by doing something called hypoxic vasoconstriction, which is where in the areas of the lungs where the ventilation is very poor, the blood vessels here sense there is little oxygen so constrict so it diverts blood away to a different area.
*In COPD/fibrosis the entire lung is hypoxic always, so there is constant hypoxic vasoconstriction which causes a massive back-pressure, meaning the right heart would eventually fail.

Question 21

What drives our reflex control of breathing?

Answer 21

PaCO2 (referred to as hypercapnia drive, too much CO2). We also have a hypoxic (when O2 is low) but this is slower and only matters when PaO2 is below 8kPa.