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The Respiratory System > Gas Exchange in the Lungs > Flashcards

Gas Exchange in the Lungs Flashcards

1
Q

What is the oxygen requirement at rest

A

250ml/minute

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2
Q

What is the carbon dioxide production at rest

A

200ml/minute

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3
Q

What happens to the 250ml/minute oxygen that is inhaled

A

It enters the blood stream to allow perfusion of organs.

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4
Q

What happens to the 200ml/minute carbon dioxide produced by the body

A

It diffuses out of the blood and enters the lungs where it is exhaled.

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5
Q

How are the blood and gas split

A

These are split up into smaller amounts by divisions of the bronchioles and the blood vessels.

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6
Q

Why is blood split into smaller sections compared to the gas

A

Because after a certain point the bronchioles continue to divide but do not decrease in size however the blood vessels continue to decrease in size after this point.

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7
Q

What happens to the velocity of both the blood and gas as the blood vessels and bronchioles split respectively

A

The velocity of the blood/gas at each division of the blood vessels/bronchioles will decrease. This will cause slower and slower flow until a stop is reached.

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8
Q

At what point does the velocity of the gas reach a stop

A

At the alveoli where the final stage of gas transfer occurs by diffusion across alveolar membranes.

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9
Q

What does it mean by “as the total area increases the flow slows down”

A

The total area for gas exchange increases moving from the trachea to the bronchi, bronchioles and finally alveoli. As this area increases, the velocity of flow decreases until the alveoli are reaches where stopping occurs and diffusion takes place. More branches = larger surface area = slower flow.

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10
Q

What type of blood vessels are present between alveoli

A

Capillaries.

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11
Q

How does oxygen move across alveolar membranes into the blood in the capillary

A

Oxygen moves down a concentration gradient from a high concentration in the alveoli to a lower concentration in the blood.

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12
Q

When does equilibrium occur between air and liquid

A

When the partial pressure of the gas in the liquid is equal to the partial pressure of the gas in the air.

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13
Q

What equilibrium exists across the alveolar membranes

A

The partial pressure of gas in air and the partial pressure of that gas in the blood. Oxygen moves from the alveoli to the blood until equilibrium is reached.

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14
Q

In 1L of blood how much oxygen is there

A

There is 200ml of oxygen in a litre of blood. 197ml of this is bound to haemoglobin while only 3ml is dissolved. This is why we need haemoglobin to survive.

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15
Q

What is the structure of haemoglobin

A

Haemoglobin is a tetrameter protein with 4 protein subunits. Each subunit has a haem group with an Fe2+ ion with which oxygen binds.

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16
Q

Why can methaemoglobin not bind to oxygen

A

Because it contains an Fe3+ ion.

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17
Q

What happens to a molecule of haemoglobin when an oxygen molecule binds

A

It changes conformation and charge.

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18
Q

What happens to haemoglobin as you increase the partial pressure of oxygen

A

Hameoglobin becomes more saturated until all of the binding sites are filled.

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19
Q

Which factors increase the oxygen haemoglobin saturation curve

A
  • An increase in hydrogen ion concentration
  • An increase in temperature
  • An increase in 2,3 DPG
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20
Q

Why is the graph of the oxygen content versus partial pressure very similar to that of the graph of haemoglobin saturation and partial pressure

A

Because most of the oxygen in the body is bound to haemoglobin. Very little is dissolved in the blood.

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21
Q

What is the normal partial pressure of oxygen

A

Around 13.4/14 kPa

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22
Q

What is the normal oxygen content of the blood

A

Around 200ml in a litre.

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23
Q

Which two factors can decrease the partial pressure of oxygen in the blood

A
  • Hypoventilation

- Ventilation/perfusion mismatch

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24
Q

What PO2 can people with chronic respiratory diseases live with permanently

A

As low as 80%.

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25
Q

What two factors can increase the partial pressure of oxygen in the blood

A
  • Hyperventilation

- Administration of oxygen

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26
Q

Why can the partial pressure of oxygen not be increased to much higher than normal

A

Because a normal oxygen saturation is between 96% and 100%, and it cannot be made to go any higher than 100%.

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27
Q

How does the graph of increasing partial pressure of CO2 differ from that of O2

A

Because past a certain partial pressure of oxygen the gas content can no longer increase as the haemoglobin will be fully saturated. The curve therefor levels off. This does not happen for CO2. The content continues to increase as the partial pressure increases.

28
Q

What is the impact of increased CO2 on breathing

A

Hyperventilation

29
Q

What is the impact of decreased CO2 on breathing

A

Hypoventilation

30
Q

What detects the concentration of CO2

A

Chemoreceptors

31
Q

What drives ventilation - oxygen or carbon dioxide

A

Carbon dioxide.

32
Q

There is an equilibrium of the partial pressure of oxygen between each alveolus an the blood that drains it. Is this also the case for the lungs as a whole

A

No. There is no apparent equilibrium in the lungs as a whole - the partial pressure of oxygen in mixed alveolar gas is higher than that of arterial blood. This is because some of the arterial blood has not been passed the alveoli for oxygenation so there is a lower oxygen concentration in the blood.

33
Q

Why is the partial pressure of oxygen in the arterial blood lower than in the alveoli

A
  • Because some veins drain straight into the left hand side of the heart leading to deoxygenated blood in the systemic circulation.
  • Some blood does not pass the alveoli for gas exchange. This happens in the bronchial circulation.
  • V/Q mismatch.
34
Q

What is a high V/Q ratio

A

This is when there is lots of ventilation from the alveoli but not much blood to be perfused.

35
Q

Which is higher in a high V/Q ratio - PO2 or PCO2

A

PO2 is higher in a high V/Q ratio because the alveoli and blood reach an equilibrium which is closer to air.

36
Q

How does V/Q mismatch come about in hypo/hyper ventilation

A

Each part of the lung acts as a whole lung in hypo/hyperventilation. Most blood comes from areas with normal PO2 however blood from areas of low PO2 will be over represented and oxygen content will be low.

37
Q

What is a low V/Q ratio

A

A low V/Q ratio is when there is not much ventilation (V) but lots of blood (Q).

38
Q

What is the equilibrium reached in a low V/Q ratio like

A

The equilibrium between the alveoli and the blood is closer to venous blood

39
Q

What are the levels of PO2 and PCO2 like in a low V/Q ratio

A

There is lower PO2 and higher PCO2

40
Q

Oxygen and carbon dioxide for a high V/Q ratio

A

High PO2 and low PCO2

41
Q

Oxygen and carbon dioxide for a low V/Q ratio

A

Low PO2 and high PCO2.

42
Q

What does a high V/Q ratio means in terms of the mismatch between ventilation and perfusion

A

There is a lot of oxygen (high PO2) to perfuse a small amount of blood.

43
Q

What does a low V/Q ratio mean in terms of the mismatch between ventilation and perfusion

A

There is not a lot of oxygen (low PO2) to perfuse a large amount of blood.

44
Q

What formula is used to quantify to ventilation perfusion mismatch between the alveoli and the blood

A

P(A-a)O2
Where A = alveolar
and a = arterial

45
Q

What is the maximum that PO2(A-a) should be to be normal

A

2kPa.

46
Q

What does it mean if the PO2(A-a) is higher than normal

A

This means there is a greater gap between the partial pressure of oxygen in the alveoli and in the blood. The equilibrium is closer to that of blood than air. It means that although there may be a high partial pressure of oxygen in the air in the alveoli this is not diffusing across the alveoli and into the blood as there is still a low partial pressure in the blood.

47
Q

What is indicated if the PaO2 is lower than expected

A

Respiratory failure.

48
Q

What are the conditions of type I respiratory failure

A

Low PO2 but normal PCO2

49
Q

What are the conditions of type II respiratory failure

A

Low PO2 and high PCO2.

50
Q

What can cause the “won’t breathe” scenario resulting in failure of the ventilatory pump

A

If there is control failure -

  • Brain failure to command due to a drug overdose
  • This also occasionally happens in COPD.
51
Q

What are the two situations that may cause failure of the ventilatory pump

A
  • If the patient won’t breathe

- If the patient can’t breathe

52
Q

What can cause the “can’t breathe” scenario resulting in failure of the ventilatory

A

If there is a broken peripheral mechanism -

  • The nerves not working (e.g. phrenic nerve is cut)
  • The muscles are not working (e.g. muscular dystrophy)
  • The chest cannot move (e.g. severe scoliosis)
  • Gas cannot get in and out (e.g. asthma)
53
Q

What happens to the concentrations of oxygen and carbon dioxide in hypoventilation

A

There is a decrease in PO2 and an increase in PCO2.

54
Q

What are two things which can cause V/Q mismatch

A
  • Narrow airways

- Fluid in the alveoli

55
Q

How do narrowed airways and fluid filled alveoli lead to V/Q mismatch

A

Gas exchange does not occur efficiently in this part of the lung so the blood leaves the site with low oxygen and high carbon dioxide.

56
Q

If there is one set of lungs which has had a left pneumonectomy and one set of lungs which has a left basal pneumonia, which will be hypoxic?

A

The left basal pneumonia patient will be hypoxic while the patient who has had a whole lung removed will not be hypoxic. This is because the left pneumonectomy patient will have no V/Q mismatch as there is no blood to this area so no deoxygenated blood supply. Blood from the basal pneumonia part of the lung will be deoxygenated as there will not be efficient gas exchange from this area leading to V/Q mismatch.

57
Q

What will be the gaseous content of the blood leaving the area of low V/Q ratio

A

The blood will have high PCO2 and low PO2.

58
Q

What does the high PCO2 concentration of the blood leaving low V/Q areas do

A

It stimulates increased ventilation to areas of normal lung and areas of high V/Q ratio. These areas become hyperventilated to decrease the CO2 from these areas.

59
Q

What is the result of hyperventilation to the areas of the lung without high V/Q ratio

A

The blood with low CO2 (from the hyperventilated areas) and the blood with high CO2 (from the areas of high V/Q ratio) mix to create blood with a relatively normal CO2 content. However, the patient will still be hypoxic.

60
Q

When may compensation by hyperventilation by the rest of the lung not be able to make up for an area with high V/Q ratio

A

When the disease is severe and widespread.

61
Q

How do you treat patients with high V/Q mismatch such as those with pneumonia

A

Give oxygen to treat the hypoxia and bring the oxygen level back to normal.

62
Q

What are the three types of oxygen mask used to treat respiratory failure

A
  • Variable performance masks
  • Fixed function masks
  • Reservoir masks
63
Q

What do variable performance masks do

A

These are “cheap and cheerful”. The exact inspired oxygen concentration is not known and depends on respiratory pattern.

64
Q

What do fixed function (venturi) masks do

A

There is a constant, known inspired concentration of oxygen. There are different jets which give different inspired concentrations of oxygen (24%, 40%, 60% etc.)

65
Q

What do reservoir masks do

A

A high inspired concentration of oxygen is given.

66
Q

What treatment (other than giving oxygen) may a person with respiratory failure require

A

Ventilation

The Respiratory System (20 decks)

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