Gas Exchange In Lungs

Question 1

Describe the journey of oxygen from atmospheric to rbc

Answer 1

It is first
inhaled into the lungs into the alveoli where it diffuses from alveoli into blood within pulmonary
capillaries. The oxygen is then bound to haemoglobin in RBCs and transported around the body. At
target cells, oxygen diffuses into them for use in aerobic respiration and CO2 diffusing the other way
to be returned into the lungs.
Oxygen enters the alveolar airspace from the atmosphere where it then dissolves into alveolar lining
fluid. The dissolved oxygen then diffuses through the alveolar epithelium, basement membrane and
capillary endothelial cells where it meets blood. Oxygen then dissolves in blood plasma where it is
then bound to haemoglobin in RBCs.

Question 2

What can happen if the passage of oxygen is disrupted

Answer 2

If anything disrupts this passage of oxygen from the alveoli to
haemoglobin, the rate of diffusion can potentially be impacted.

Question 3

What is one key feature of oxygenation of blood

Answer 3

This process of oxygenation of blood
must occur rapidly during the brief time taken for RBCs to flow through pulmonary capillaries (blood
flows through them very quickly in around 0.75s). The rate of diffusion of oxygen however is very
quick.

Question 4

When does oxygen levels become saturated

Answer 4

In a normal situation, oxygen partial pressure in the blood reaches saturation levels in around
0.25s

Question 5

What happens when diffusion is abnormal ( oxygen )

Answer 5

However, in situations where diffusion is abnormal (due to fibrosis or thickening of the blood-
gas barrier), the rate of diffusion is impaired and the oxygenation process of blood is slower. By the
end of the 0.75s that blood is in the capillary, a significant amount of blood may not be fully
oxygenated. This situation is exacerbated in situations where rate of movement of blood flow
through the lungs increases (e.g. during exercise) where the result is even less blood is fully
oxygenated. Patients with this condition get breathless and become hypoxic extremely quickly upon
initiation of exercise.

Question 6

What determines rate if diffusion

Answer 6

The rate of diffusion is determined by surface area and distance multiplied by the partial pressure
gradient between air and capillary blood. The surface area in this case is the alveolar surface area
and the distance is the epithelial and endothelial cell thickness, the basement membrane thickness
and the fluid layer depth (diffusion rate is proportional to the surface area divided by the square of
the distance of the diffusion multiplied by the partial pressure gradient PA -Pc).

Question 7

What happens when partial pressure gradient is increased

Answer 7

The bigger the partial
pressure gradient, the quicker the diffusion rate. To get maximum diffusion rate, partial pressure
gradient must be high, large surface area and low barrier thickness.

Question 8

How can partial pressure gradient be impaired

Answer 8

The partial pressure gradient can
be impaired by hypoventilation (type II respiratory failure), the surface area can be decreased by
emphysema and the thickness of the barrier can be increased by fibrosis (increased basement
membrane thickness) or pulmonary oedema (increased fluid layer thickness in pneumonia for
example).

Question 9

What maintains the pressure gradient between alveoli and blood

Answer 9

Pressure gradients between alveoli and blood are maintained by adequate diffusion. At the
beginning of the capillary, the partial pressures of oxygen (PvO2) and carbon dioxide (PvCO2) are
around 5 kPa and 6 kPa respectively. The respective partial pressures of oxygen and carbon dioxide in
the alveolus are 14 kPa (PAO2) and 5 kPa (PACO2).

Question 10

Describe the relationship between partial pressure of o2 and rate of ventilation

Answer 10

The relationship between the partial pressure of oxygen in the alveoli and the rate of ventilation can
be demonstrated on a graph (see right). The greater the rate of breathing, the more the partial
pressure of oxygen in alveoli will resemble atmospheric levels (hyperventilation). If the partial
pressure of oxygen in the atmosphere is around 21 kPa, that is essentially the plateau level of oxygen
that can be reached in the alveoli. During hypoventilation, oxygen will be taken out of alveoli at a
rate that is not sustainable to ventilation rates and therefore the partial pressure of oxygen in the
alveoli will decrease.

Question 11

What is the relationship between partial pressure elf co2 and ventilation

Answer 11

With carbon dioxide levels, the reverse is true. The more air is recycled in
alveoli, the lower the partial pressure of carbon dioxide will be in alveoli
(hyperventilation) as the partial pressure of carbon dioxide in the atmosphere is
almost zero. During hypoventilation, levels of carbon dioxide in the alveoli will
increase exponentially with reduced ventilation rates (see graph left).

Question 12

How is the pressure gradients of diffusion maintained

Answer 12

To maintain pressure gradients for diffusion of gases between alveoli and capillaries, the rate of
ventilation must be matched by the rate of perfusion of blood in the capillaries.

Question 13

Why is perfusion important for ventilation ( what limits it )

Answer 13

The rate of perfusion
is also important as blood can only store a finite amount of oxygen in each unit due to there being a
finite amount of haemoglobin in blood. The relationship between ventilation and perfusion is
describes as the V/Q ratio (V stands for ventilation and Q stands for perfusion). In general, around 1
litre of blood can carry around 200 ml of oxygen. 1 litre of air can in fact also carry around 200 ml of
oxygen. This means the ideal V/Q ratio should be around 1. If there is a mismatch in this ratio, gas
exchange is impaired resulting in hypoxaemia.

Question 14

How has the body adapted to match ventilation and perfusion

Answer 14

The body in fact has developed specific adaptations to ensure that ventilation and perfusion are
matched. These are homeostatic mechanisms mainly facilitated by hypoxic vasoconstriction. This is
where blood vessels are constricted and manipulated such that blood flow is diverted from poorly
ventilated, to well ventilated alveoli. Under normal circumstances, blood flow and ventilation are
matched.

Question 15

What happens when ventilation is reduced in a SPECIFIC alveoli region

Answer 15

If ventilation of specific alveoli decreases, PACO2 will rise and PAO2 will fall. This causes
reduced oxygenation of blood flowing through innervating capillaries and the body signals for
hypoxia. This induces the vasoconstriction of the blood vessels supplying this poorly ventilated
alveoli reducing blood flow through it. Blood is as a result diverted to alveoli that are better
ventilated and their capillaries have not vasoconstricted.

Question 16

Why is ventilation perfusion mismatch not very effective as an overall mechanism

Answer 16

The reason why a ventilation perfusion mismatch is bad is because gas exchange will be reduced.
This does not necessarily occur at the level of the whole lung, but rather varies substantially between
alveolar units.

Question 17

What does increased pp of co2 induce

Answer 17

The VQ ratio in theory affects both oxygen and carbon dioxide gaseous exchange.
However, in most cases, the increased partial pressure of carbon dioxide will induce a reflex
hyperventilation to clear the excess CO2 with this clearance not corresponding to an increase in the
partial pressure of oxygen in alveoli.

Question 18

Give an example of VQ mismatch - describe

Answer 18

An example of a VQ mismatch can occur during a pulmonary
embolism. This is where a clot lodges itself in one of the capillaries in the lung blocking the flow of
blood through it. This creates a region where no perfusion occurs and therefore un-perfused alveoli
(physiological dead space).

Question 19

What happens to alveoli that experience pulmonary embolism

Answer 19

These alveoli will have an increased V/Q ratio due to the fact they are still
being ventilated but not perfused at all. Other nearby alveoli to the un-perfused alveoli will
experience an increase in perfusion as the excess blood is now diverted to capillaries supplying these
alveoli. This decreases the V/Q ratio as the ventilation rate remains the same (initially). A result of
this is blood that flow through these alveoli do not have a sufficient of oxygen supply and therefore
oxygenation of this blood will decrease.

Question 20

What is the result of pulmonary embolism - treatment

Answer 20

The overall result is hypoxaemia. This situation can be
compensated for by increasing ventilation to the remaining perfused alveoli (depends on the extent
of the physiological dead space). This means oxygen therapy can be used to treat some of the
symptoms of pulmonary embolism.

Question 21

What are other causes of under perfusion of aveoli

Answer 21

Under perfusion of alveoli can also be caused by heart failure
and loss or damage of capillaries supplying the alveoli.

Question 22

Describe the shunt effect - causes of shunt

Answer 22

In the reverse situation, there is reduced ventilation of alveoli or limits to the diffusion of gasses into
the capillaries that decreases the V/Q ratio. In this situation, there is now perfusion without
ventilation and this is known as shunt. Shunt can be caused by pneumonia (mass oedema that
reduces the amount of gas that reaches capillaries), acute lung injury, respiratory distress syndrome
and atelectasis (collapse or closure of a lung).

Question 23

Why can shunt induced hypoxeamia respon poorly to oxygen supplemental treatment

Answer 23

Shunt-induced hypoxaemia responds quite poorly to
supplemental oxygen. In healthy lungs, blood arrives at alveoli with an oxygen saturation of around
60%. Gaseous exchange takes place and the blood leaves the lungs at around 95% oxygen saturation.
In the event of a shunt, gaseous exchange does not occur and enters the injured alveoli at around
60% saturation and leaving the alveoli at around 60%. It can then join up with blood that passes
through the unaffected alveoli. The overall oxygen saturation is now around 75% (bad because a
small decrease can produce a number of bad effects). Supplemental oxygen will only increase the
gaseous exchange rate at the healthy alveoli (blood will now leave at near 100% saturation as
opposed to 95%) and the injured alveoli are still not oxygenating the blood flowing through their
capillaries. This means that when the blood mixes again, it will not increase by much in oxygen
saturation (75% to 80% in this example). This increase does not prevent hypoxaemia.

Question 24

How is shunt treated

Answer 24

The way to
treat shunt is in fact to deal with the cause of the shunt. Increasing ventilation to treat shunt will still
remove carbon dioxide from the blood at a quicker rate due to the various ways that carbon dioxide
is carried in the blood.

Question 25

How can respiratory failure be determined

Answer 25

Determining the cause of respiratory failure can be done through the interpretation of the different
gas pressures in the alveoli (mainly PAO2). This is done to compare the partial pressure of gases
between alveolar air and blood. If gas exchange occurs efficiently, there will be almost no difference
between the partial pressure of oxygen in the alveoli and in capillaries. If gas exchange is impaired,
the difference can be substantial.

Question 26

How can pp of o2 be measured

Answer 26

Clinically measuring the alveolar partial pressure of oxygen is quite
challenging and not practical. Oxygen partial pressure can however be calculated from other
measurements that can be obtained practically through the alveolar gas equation. This equation
describes how alveolar oxygen partial pressure (PAO2) equals to the fraction of oxygen present in
inspired gas (FIO2) multiplied by atmospheric pressure corrected against water vapour pressure (PB –
PH2O) subtracted by arterial pressure of carbon dioxide over respiratory exchange ratio (PaCO2/RER).
This equation subtracts the amount of oxygen being used by the body from the partial pressure of
incoming oxygen. The respiratory exchange ratio divides the amount of CO2 produced by the amount
of O2 consumed.

Question 27

How is o2 consumed indicated

Answer 27

This ratio is determined by the type of metabolism taking place in the body. For
example, in the consumption of carbohydrates, 6 oxygens are consumed and 6 carbon dioxides are
produced. This gives carbohydrates an RER of 1. Fatty acids and lipids will have a slightly different RER
value. In a modern diet, the average RER is 0.8. Analysis of the diet of a patient can be used to
estimate their RER.

Question 28

What can pp of aveoli be compared to in order calculate if gas exchange is efficient

Answer 28

The partial pressure of alveoli can then be compared to the partial pressure in
the blood to calculate if gas exchange is efficient (A-a O2 gradient is the difference between alveolar
and arterial pressure and is normally under 2 kPa).
These results can then be used to determine the cause of hypoxaemia. The first question to resolve is
if hypoventilation is contributing to the hypoxaemia. This is can be resolved by working out if PaCO2 is
over 6 kPa. The second question to ask is if the oxygen reaching the alveoli is diffusing into the blood.
This question is answered by using the alveolar gas equation (AGE) and the arterial blood gas (ABG)
readings to calculate the A-a gradient (as mentioned before, this should be under 2 kPa in a healthy
individual).