Gas Exchange in the lungs

Question 1

How does oxygen get from the atmosphere to the cells?

Answer 1

1. O2 inhaled from the atmosphere into the alveoli within the lungs
2. O2 diffuses from alveoli into blood within pulmonary capillaries
3. O2 transported in blood, predominantly bound to haemoglobin
4. O2 diffuses into cells/tissues for use in aerobic respiration
5. CO2 diffuses from respiring tissues to blood-exchanged at lungs

Question 2

what does gas exchange involve?

Answer 2

involves diffusion of blood gases through multiple structures and mediums

Question 3

there are a number of events that take place in order for o2 to get from alveolar air to binding to haemoglobin - what can get in the way of these steps?

Answer 3

if one of the structures gets thicker/wider/scarred, it can potentially impact on the rate of diffusion or gas exchange

Question 4

when must oxygenation occur?

Answer 4

oxygenation of blood must occur during the brief time taken for RBCs to flow through pulmonary capillaries

Question 5

does blood flow through the capillaries very quickly or very slowly?

Answer 5

very quickly

0.5 seconds

Question 6

how does thickening of the blood gas barrier affect diffusion

Answer 6

there is reduced diffusion, as it makes the oxygenation process slower, so that by 0.75 seconds when the blood is leaving the capillary, the blood may only be partially oxygenated

Question 7

what is the consequence of blood only being partially oxygenated?

Answer 7

person gets breathless (hypoxic) extremely quickly

Question 8

what occurs during exercise?

Answer 8

blood flows through capillaries at a faster rate due to the increase in CO

Question 9

what determines the rate of diffusion?

Answer 9

• alveolar surface area
• diffusion distance
• partial pressure gradient between alveolar air and capillary blood

Question 10

explain how the factors have to be in order to get maximal diffusion:

Answer 10

↑ partial pressure gradient

↑ surface area

↓ distance (barrier thickness)

Question 11

what does diffusion distance mean?

Answer 11

how thick are the structures that gases have to diffuse past to get from the alveolar air to the blood
eg. basement membranes, walls of alveoli, alveolar fluid layer

Question 12

what does alveolar surface area mean?

Answer 12

it means: what is the total surface area over which gas exchange can occur?

Question 13

how can the partial pressure gradient be negatively affected, reducing diffusion?

Answer 13

• partial pressure gradient can be impaired if you start to hypoventilate, because you don’t breathe at a sufficient rate to meet the demands of the body (breathing too slow)
• you’re not putting in enough o2 from the atmosphere to the lungs, and you’re not getting rid of enough co2 from the lungs to the atmosphere
• so, there is a reduced partial pressure gradient between the blood and the alveolar air and then reduced gas exchange.

Question 14

how can surface area be negatively affected, reducing diffusion?

Answer 14

• you want a v. high SA, which alveoli naturally have because of their 3d structure
• in emphysema because of the inflammatory pathology that is associated with chronic cigarette smoke exposure, you get damage to the overall structure of the alveoli
• alveoli start fusing together, decreasing the overall SA to volume ratio

Question 15

how are the pressure gradients between alveoli and blood maintained?

Answer 15

adequate ventilation

Question 16

how is the air inside alveoli different to atmospheric air?

Answer 16

the air inside alveoli is slightly stale because of the residual volume

Question 17

in what situation will the partial pressures of gases in alveolar air start to resemble atmospheric air?

Answer 17

as ventilation increases and the depth of each breath becomes greater

Question 18

how does increasing ventilation affect the partial pressure of o2 within alveoli?

Answer 18

partial pressure of o2 within alveoli increases as ventilation increases

Question 19

as hypoventilation increases what happens?

Answer 19

o2 will be taken out of the alveolar air at a rate that is not replaced by fresh air coming from the atmosphere

leads to decreased partial pressure of o2 within alveoli, causing causes hypoxaemia (decreased partial pressure of o2 within the arterioles)

Question 20

how does increasing ventilation affect the partial pressure of co2 within alveoli?

Answer 20

the partial pressure of co2 within alveoli decreases as ventilation increases

Question 21

what is the consequence of not breathing on co2 levels?

Answer 21

if you stop breathing co2 will just build up in the alveoli, until the partial pressure of co2 in the alveoli = the partial pressure of co2 in the capillaries

Question 22

relationship between rate of breathing and and co2 levels in arterial blood?

Answer 22

the greater the rate of breathing, the lower the level of co2 within arterial blood

the more gas exchange takes place and the more co2 you can expel from your lungs

Question 23

what is perfusion?

Answer 23

blood flow through pulmonary capillaries

Question 24

what else do you require for maintaining pressure gradients for diffusion?

Answer 24

adequate perfusion

Question 25

why does perfusion need to be matched to alveolar ventilation?

Answer 25

perfusion (Q) needs to be matched to alveolar ventilation (V) to enable efficient gas exchange, as there is a maximum amount of o2 each unit of blood can carry.

Question 26

explain what the V/Q ratio is:

Answer 26

V/Q ratio describes the relationship between perfusion and ventilation

Question 27

what should the V/Q ratio ideally be at?

Answer 27

1 (at rest, 0.8 - 1)

because 1L of blood can carry ≈200mLO2, and 1L dry air ≈ 200ml O2

Question 28

what does it mean when the V/Q ratio starts to become substantially different to 1?

Answer 28

indicates that there is impaired gas exchange overall, potentially resulting in hypoxia

Question 29

name a homeostatic mechanism that exists to reduce ventilation-perfusion mismatching:

Answer 29

hypoxic vasoconstriction of capillaries

• this is where blood flow is diverted from poor to well ventilated alveoli
• decreased partial pressure of o2 induces this vasoconstriction
• occurs when the V/Q ratio differs substantially between alveolar units

Question 30

how does the body deal with an increase in the partial pressure of co2?

Answer 30

a reflex hyperventilation is induced that clears the excess co2

Question 31

name some factors that cause an increase in the V/Q ratio (ventilation without perfusion)

Answer 31

1. Heart failure (cardiac arrest)
2. Blocked vessels (pulmonary embolism)
3. Loss/damage to capillaries (emphysema)

• reduced perfusion of lung regions
• the affected alveoli becomes physiological dead space as no gas exchange occurs

Question 32

explain what happens during a pulmonary embolism:

Answer 32

• clot forms within the vasculature, blocks one of the major blood vessels/ 1 of the pulmonary arteries serving one of the alveoli or a region of the lung, so blood cannot get to this part
• this alveoli is still being ventilated so V stays the same, but Q is decreased
• you have to increase ventilation to the alveolus that is receiving the blood to compensate and prevent hypoxaemia from occuring

Question 33

name some factors that cause an decrease in the V/Q ratio (perfusion without ventilation)

Answer 33

1. Pneumonia, acute lung injury, respiratory distress syndrome, atelectasis
2. Blood moves from the right heart to the left, without taking part in gas exchange (shunt).

Question 34

what happens in shunt to cause a decrease in the V/Q ratio?

Answer 34

• alveolar oedema occurs in one of the alveoli due to lung injury eg. the left one, leading to decreased gas exchange
• so, the blood is coming back at 60% saturated and leaving the alveoli at 60%, but in the right the blood is leaving at 100%
• the blood from these 2 sides mixes to go to systemic circulation, and instead of being at 100% the saturation is now 75% - decrease!

Question 35

how is the problem in shunt overcome? what is the important thing to remember

Answer 35

• pump lots more o2 into the working alveoli, increasing gas exchange
• deoxygenated blood mixing with the blood that’s coming from the well-ventilated places means you’ll never be able to get back to healthy oxygen saturation unless you deal with the cause of the shunt in the first place

Question 36

give 2 reasons a patient could be hypoxaemic?

Answer 36

1) not breathing at a sufficient rate/not ventilating their lungs properly
2) oxygen that’s getting into their alveoli can’t get into their blood efficiently

Question 37

what is the RER

Answer 37

respiratory exchange ratio

ratio between the amount of carbon dioxide (CO2) produced and oxygen (O2) used in metabolism

Question 38

what is the ratio between the amount of o2 the body uses and the amount of co2 it produces determined by?

Answer 38

the type of metabolism that is taking place
-e.g. when you derive energy from carbs you have a certain ratio of o2 consumed to co2 produced, and using fats and lipids mean you use a slightly diff ratio

Question 39

what 2 things can be used to investigate the causes of hypoxaemia, and how are they used?

Answer 39

1. alveolar gas equation
2. alveolar-arterial oxygen gradient
   - work out the partial pressure of oxygen within alveoli
   - compare that to partial pressure of oxygen within the blood

• see if there is a big difference between the 2, i.e. is gas exchange being efficient
• can see if hypoventilation or blood oxygenation are contributing to hypoxaemia