Respiratory Physiology 3.1- Alveolar Ventilation vs Pulmonary ventilation

Question 1

Is anatomical dead space volume fixed?

Answer 1

relatively fixed

Question 2

What does the term ventilation mean?

Answer 2

refers to movement of air in and out of lungs (breathing!)

Question 3

What two ways can ventilation be described? (both measured in L/min)

Answer 3

Pulmonary (Minute) ventilation = total air movement into/out of lungs (relatively insignificant in functional terms)

Alveolar ventilation = fresh air getting to alveoli and therefore available for gas exchange (functionally much more significant!)

Question 4

What is stale air?

Answer 4

air that has already participated in gas exchange

Question 5

How efficient is normal tidal volume breathing?

Answer 5

70% efficient- only 70% actually reaches the alveoli and 30% gets stuck in dead space and can’t participate in gas exchange

Question 6

What happens when we get worried and anxious?

Answer 6

our respiration rate tends to increase but our tidal volume goes down. Fast shallow breaths

Question 7

What happens when we are very relaxed?

Answer 7

slow breathing pattern and increased tidal volume

Question 8

How do we workout pulmonary ventilation?

Answer 8

multiply our respiration rate by our tidal volume

Question 9

How do we work out alveolar ventilation?

Answer 9

for e.g. tidal volume =500ml - anatomical dead space (as this is not reaching the alveoli)
only 350ml reaches alveoli
and multiply this by breaths a minute

Question 10

What is important in determining alveolar ventilation?

Answer 10

tidal volume
respiration rate

Question 11

What usually happens when we increase the rate in which we breathe?

Answer 11

decrease tidal volume

Question 12

What is hypoventilating?

Answer 12

getting less air to his alveoli than normal

Question 13

What gases is the air composed of?

Answer 13

Our “air” is a gas mainly a mixture of two gases; nitrogen (79%) and oxygen (21%). There is negligible quantities of CO2 in air (0.03%).

The CO2 in our cells and blood is there because we make CO2, we do NOT breathe it in.

Question 14

What can partial pressure be defined as?

Answer 14

The pressure of a gas in a mixture of gases is equivalent to the percentage of that particular gas in the entire mixture multiplied by the pressure of the whole gaseous mixture

Question 15

How do we work out the partial pressure of oxygen in the air we breathe?

Answer 15

Atmospheric Pressure = 760mmHg
Pressure of air we breathe therefore = 760mmHg
21% of air we breath = O2
Partial pressure of O2 in air we breath = 21% x 760mmHg = 160mmHg

Question 16

What would be alveolar ventilation of a healthy man?

Answer 16

4.2L/min

Question 17

Why when oxygen reaches the alveoli does partial pressure of oxygen drop from 160 - 100mmHg?

Answer 17

air we breathe in diluted by anatomical dead space and residual volume
air saturated with water vapour
pressure of gas is in equilbrium with pressure of gas in the blood

Question 18

What is the partial pressure of carbon dioxide and oxygen in the alveoli at normal alveolar ventilation ?

Answer 18

carbon dioxide: 100mmHg or 13.3kPa
oxygen: 40mmHg or 5.3kPa

Question 19

What are the values found in the alveoli the same as?

Answer 19

same values that we find in systemic arterial blood.
Partial pressures of gas in alveoli are same as partial pressures in systemic arterial blood

Question 20

What happens when you hypoventilate?

Answer 20

During hypo-ventilation (decreased alveolar ventilation) PO2 falls to 30 mmHg and PCO2 rises to 100 mmHg
reduction in ventilation of the alveoli and oxygen able to participate in gas exchange.
Oxygen levels in alveoli fall because that oxygen is taken away by blood as it’s being metabolised by the peripheral tissues faster than it is being replenished in the alveoli.
Levels of CO2 increase in alveoli

Question 21

What happens during hyperventilation?

Answer 21

During hyper-ventilation (increased alveolar ventilation) PO2 rises to about 120 mm Hg and PCO2 falls to about 20 mmHg.

getting oxygen into the alveoli faster than you are using it and so your partial pressure for oxygen rises and metabolising carbon dioxide faster than you’re producing it . Decrease in oxygen
See these changes reflected in systemic arterial blood.

Question 22

What happens when carbon dioxide levels fall?

Answer 22

removes the stimulus for us to breathe

Question 23

What happens when we breathe in for any given change in pressure?

Answer 23

bigger change in vol at base of lung than the apex of the lung
alveolar ventilation declines with height from base to the apex

Question 24

Why does alveolar ventilation decline with height from base to the apex?

Answer 24

Compliance (how much increase in volume we get in the lungs for any given change in pressure) is lower at the apex due to being more inflated at FRC.
Lungs effectively hanging within the thoracic cavity and the weight compresses alveoli at base more than alveoli at apex.
Opportunity or capacity for alveoli at apex to expand on inspiration is less than alveoli at base to expand.