Gas Exchange: Oxygen

Question 1

What are the partial pressures of O2 in the following: Dry atmospheric air, Alveolar gas, Arterial blood and Mixed venous blood?

Answer 1

Dry atmospheric air
PiO2 = 21.2 kPa (159.2 mmHg)

Alveolar gas
PAO2 = 14 kPa (10.5 mmHg)

Arterial blood
PaO2 = 13.3 kPa (100 mmHg)

Mixed venous blood
PvO2 = 5.3 kPa (40 mmHg)

Question 2

What is the function of the conducting zone and which generations does it include?

Answer 2

Generations 1-16 (trachea to terminal bronchioles)

It allows bulk flow during inspiration and expiration and also humidification of the inspired air.

Question 3

What is the function of the respiratory zone and which generations does it include?

Answer 3

Generations 17-23 (respiratory bronchioles to alveolar sacs)

Gas moves down the concentration gradient via diffusion. Allows gas exchange.

Question 4

What factors affect gas exchange?

Answer 4

Dead space: The proportion of tidal volume not involved in gas exchange

Diffusing capacity: The volume of gas that can transfer across a membrane per unit time

Shunt: The proportion of the blood entering the left side of the heart that has bypassed the oxygenation process of the lungs.

Question 5

What is physiological dead space?

Answer 5

It is the combination of anatomical and alveolar dead space

Anatomical dead space: Upper airway and conducting zones. Approximately 150mls.

Alveolar dead space: The proportion of alveolar minute ventilation that does not take part in gas exchange due to entering either unperfused alveoli (where no gas exchange occurs) or under-perfused alveoli (where gas exchange is incomplete). **

** A clinical example of this would be a PE in which a proportion of the alveoli in the lung would not be perfused.

Question 6

What is the formula for alveolar minute ventilation?

Answer 6

AVM = (TV-Dead space volume) x RR

Question 7

What factors affect diffusion capacity?

Answer 7

Diffusion capacity is calculated using Fick’s law of diffusion.

Flow of gas is proportional to: A/T x D (P1-P2)

1. Surface area of the lungs (A)
2. Diffusion constant for oxygen (D)
3. Thickness of the capillary and alveolar membrane (T)
4. Partial pressure gradient between alveoli and blood. (P1 and P2 respectively)

Question 8

On average how much oxygen diffuses from the alveoli into the pulmonary capillaries per minute?

Answer 8

250ml/min

Question 9

What are the 4 different types of shunt?

Answer 9

Normal extrapulmonary shunt
- Part of the bronchial circulation
- Thebesian drainage, from heart muscle directly into the left ventricle

Normal pulmonary shunt
- Areas of lung with V/Q >0 and <1

Pathological extra-pulmonary shunt i.e. heart disease (mainly congenital)

Pathological pulmonary shunt
-Pneumonia, atelectasis, collapse etc

Question 10

What is meant by the term venous admixture?

Answer 10

The calculated amount of mixed venous blood required to be mixed with pulmonary end capillary blood to produce the observed difference between arterial and alveolar PO2.

A quantification of shunt.

Question 11

What is the V/Q ratio in a ventilated but non perfused alveoli and an unventilated but perfused alveoli?

Answer 11

Ventilated but not perfused V/Q = infinity

Unventilated but perfused V/Q=0

Question 12

What is the ~ normal V/Q ratio?

Answer 12

Alveolar Minute Ventilation ~ 4000ml *
Cardiac output ~ 5000ml **

4000/5000 = 0.8

*TV~500 RR ~14 Dead space ~150ml
(500-150) x14 =4200 ~4000

** SV=70ml Hr 70bpm CO =4900ml ~5000ml

Question 13

What is the alveolar gas equation?

Answer 13

PAO2 = PiO2 - PaCo2/R

Question 14

What are the assumptions we make when using the alveolar gas equation?

Answer 14

1. PaCO2 = PACO2
2. Alveolar and arterial CO2 are less affected by changes in the V/Q ratio
3. Water vapour effect on gas exchange
   Inspired PO2 = FiO2 x (Barometric pressure – Saturated vapour pressure of water @ 37 degrees Celsius)

Question 15

What is the formula for PiO2?

Answer 15

PiO2 = FiO2 x (Barometric pressure - water vapour pressure H2O)

Question 16

What is the A-a gradient, and what is the normal value?

Answer 16

It is the partial pressure difference between alveolar and arterial O2.

In healthy lungs it is <2kPa, in shunt the A-a gradient will increase.

Question 17

How is oxygen content calculated?

Answer 17

98% of O2 is carried in Hb and the remaining 2% is directly dissolved in the blood.

Oxygen content (CaO2) = (Hb x 1.34 x (O2 saturations/100)) + (PaO2 x 0.023)

This is usually expressed as mls/O2 per 100ml blood

Question 18

Describe the structure of haemaglobin?

Answer 18

Quaternary structure consisting of 4 subunits. 2 alpha globin chains, 2 beta globin chains.

Each subunit contains a haem group with a Fe2+ iron.
Each haem group is capable of binding one O2 molecule.

Question 19

Draw and explain the oxygen dissociation curve?

Answer 19

Y axis Hb saturation
X axis PaO2

Sigmoid shaped curve. Hb undergoes confirmational binding which makes it easier for the second and third oxygen molecule to bind, the fourth then becomes more difficult to bind.

Question 20

What is the P50?

Answer 20

P50 is the point at which the Hb saturation is 50% or 50% of binding sites are occupied and usually is around 3.5 kPa. This is a reference point to describe the position of the curve under different conditions.

Question 21

What is the Bohr effect?

Answer 21

Describes the effect of pH on the oxygen dissociation curve. The shifting of the curve to the right causes a reduction in the affinity of haemoglobin to oxygen helping oxygen delivery to tissues.

Shifts P50 to the right.

Question 22

What is the formula for tissue oxygen delivery (DO2)?

Answer 22

Blood O2 content x Cardiac output

Blood O2 content ~ 200ml/L CO~ 5L/min

DO2 ~ 1L/min

Question 23

What is the formula for Oxygen consumption (VO2)?

Answer 23

VO2 = CO x (CaO2-CvO2)

Remember CaO2 (oxygen content) = (Hb x 1.34 x (O2 sats/100)) + (PaO2 x 0.023)

Question 24

At rest calculate the VO2?

Hb 150
O2 sats 98%
PaO2 13

Answer 24

VO2 = CO x (CaO2 -CvO2)

CaO2 = (150 x 1.34 x 0.98) + (13 x 0.023) = 200ml/L

O2 Content is multiplied by the cardiac output to give a delivery rate.

DO2 = 200ml/L x 5l/min = 1000ml/min

The rate of consumption (VO2) at rest will be 25% of the DO2*

VO2 =250ml/min

*Note: This can subsequently increase to 10x this value during exercise helped by increased cardiac output, oxygen extraction by tissues and alveolar minute volume