4. Oxygen Transport

Question 1

How is oxygen transported from the lungs to the cells of the tissues?

Answer 1

1.
> Ventilation of the lungs supplies
oxygen to the alveolus.

2. > Diffusion of oxygen across
   the alveolus to the pulmonary capillaries.
3. > Oxygen carriage by blood
   (combined with haemoglobin and
   dissolved in plasma).
4. > Diffusion from capillary to mitochondria.

Question 2

What is the oxygen cascade?

Answer 2

The oxygen cascade describes the sequential reduction in PO2 from
atmosphere to cellular mitochondria.

Question 3

Describe what occurs at each step of the oxygen cascade.

Answer 3

1. > Oxygen is present in the air
   at a concentration of 21%
   (NB. this does not vary with altitude).
2. > Atmospheric pressure at sea level
   is 1 atmosphere (or 101 kPa).
3. > Inspired PO2 at sea level is
   therefore 21 kPa
   (atmospheric pressure · % oxygen in air).

4.
> Humidification of inspired air
occurs in the upper respiratory tract.

The humidity is formed by water vapour,
which as a gas exerts a pressure.

At 37oC the saturated vapour pressure (SVP)
of water in the trachea is 6.3 kPa.

Taking the SVP into account,
the PO2 in the trachea when
breathing air is

(101.3–6.3) × 0.21 = 19.95 kPa.

5.
> By the time the oxygen has
reached the alveoli the
PO2 has fallen to about 15 kPa.

This is because the PO2
of the gas in the alveoli (PAO2)
is a balance between two processes:

the removal of oxygen
by the pulmonary capillaries
and its continual supply by alveolar ventilation
(breathing) –

thus hypoventilation will result in a lower PAO2.

6.
> Blood returning to the heart 
from the tissues has a 
low PO2 (5.3 kPa) and
travels to the lungs 
via the pulmonary arteries. 

The pulmonary arteries form pulmonary capillaries, which surround the alveoli.

Oxygen diffuses
from the high pressure 
in the alveoli (15 kPa) 
to the area of lower pressure
of the blood in the pulmonary capillaries (5.3 kPa).

7.
> After oxygenation blood 
moves into the pulmonary veins, 
which return to the left side of 
the heart to be pumped 
to the systemic tissues. 

In a ‘perfect lung’ the PO2 of
pulmonary venous blood would be equal
to the PO2 in the alveolus.

Three factors may cause the PO2 in the
pulmonary veins to be less than the PAO2:

a - ventilation/perfusion mismatch,

b shunt

c and diffusion impairment.

These are the causes of an increased
Alveolar–arterial (A–a) gradient.

8. 
> Arterial blood with a PaO2 of 13.3 kPa 
passes to the tissues – 
the capillary PO2 being 
in the order of 6–7 kPa.

9. 
> Oxygen then diffuses 
to the cells in the capillary beds, 
the mitochondria receiving 
a PO2 of 1–5 kPa 
depending on the capillary bed.

10. > An increase in the size of any
    of the ‘steps’ in the oxygen cascade may
    result in hypoxia at the mitochondrial level.

Question 4

What are the causes of an increased A–a gradient?

Answer 4

Under normal circumstances the
A–a gradient is less than 2 kPa

(PAO2 15 kPa and PaO2 13.3 kPa)

and is caused

by small ventilation–perfusion
(V. /Q. )
mismatch and
shunt present in normal healthy individuals.

However, an increased A–a gradient
is present in disease states

that result in an increase in V. /Q. mismatch/shunt

or

conditions which impair diffusion.

1.
> Diffusion impairment, 
e.g. 
pulmonary oedema 
or 
pulmonary fibrosis

2
> V. /Q. mismatch,

e.g. severe hypotension, COPD, LRTI or asthma

3.
> Shunt:
• Intrapulmonary causes,
e.g. LRTI or atelectasis

• Extrapulmonary causes,
e.g. right to left cardiac shunt

Question 5

What are the causes of hypoxia?

Answer 5

> Low inspired oxygen

> Hypoventilation

> Anaemic hypoxia

> Stagnant hypoxia

> Histotoxic hypoxia

> V /Q mismatch

> Diffusion impairment

> Shunt

Question 6

What methods can be used to increase oxygen content and delivery?

Answer 6

This can be achieved by

increasing CaO2
and or

increasing cardiac output (CO).

To increase CaO2:

1
> Increase circulating haemoglobin concentration (blood transfusion).

2
> Maintain high oxygen saturations (supplemental oxygen).

3
> Increase dissolved oxygen by increasing partial pressure of oxygen,

e.g. hyperbaric oxygen (achieving a PO2 of 3 atmospheres supplies sufficient dissolved oxygen to meet oxygen demand).

To increase CO:

1
> Optimise heart rate and rhythm
(rate 60–90 bpm/sinus rhythm).

2
> Optimise stroke volume (i.e. preload and contractility).

3
> Maintain perfusion pressure to
ensure organ oxygen delivery
(i.e. afterload).

> The above can be achieved with the use of fluids and or inotropes.