Alveolar Ventilation Vs Pulmonary Ventilation

Question 1

What is pulmonary ventilation? How to calculate?

Answer 1

Total movement of air into/out of lungs (relatively insignificant as doesn’t indicate how much air makes it to the alveoli)

Pulmonary ventilation = tidal volume x respiratory rate

Question 2

What is alveolar ventilation?

Answer 2

Amount of fresh air getting to alveoli and therefore available for gas exchange (functionally significant)

Question 3

Why is breathing only 70% efficient at normal rest?

Answer 3

Due to the anatomical dead space

Tidal volume = 500ml
But 150ml of stale air in the dead space
Of the 500ml fresh air inhaled, only 350ml makes it to the alveoli (500ml - 150ml = 350ml)

Question 4

What has a greater impact on alveolar ventilation? Tidal volume or respiratory rate, and why?

Answer 4

Tidal volume has a greater impact as the volume of air in the anatomical dead space is fixed
Therefore proportion of tidal volume inhaled changes the proportion of air stuck in the dead space

Question 5

What is the best way to enhance alveolar ventilation?

Answer 5

Breathing deeply and rapidly (e.g. when exercising)

Question 6

What is Hypoventilation vs hyperventilation?

Answer 6

• Hypoventilation – getting less air to alveoli than normal (e.g. fast shallow breaths)
• Hyperventilation – getting more air to alveoli than normal (e.g. deep breaths)

Question 7

What is the partial pressure of oxygen in air?

Answer 7

160mmHg

Question 8

What is the partial pressure of oxygen at normal alveolar ventilation?

Answer 8

100mmHg or 13.3kPa

Question 9

What is the partial pressure of carbon dioxide at normal alveolar ventilation?

Answer 9

40mmHg (5.3kPa)

Question 10

Why is the partial pressure of oxygen in air 160mmHg and 100mmHg at alveolar ventilation?

Answer 10

Because the air we breathe is diluted by:

- anatomical dead space 
- residual volume (air that never leaves the alveoli)
- (further diluted by water vapour as needs to be in solution)

Question 11

What is the partial pressure of oxygen and carbon dioxide in systemic arterial blood?

Answer 11

Provided lungs are working normally and gas exchange is taking place as normal the values will be the same as in the alveoli (100mmHg O2 and 40mmHg CO2)

Question 12

How does Hypoventilation and hyperventilation affect the alveolar ventilation?

Answer 12

Hypoventilation:

• less air getting to alveoli to participate in gas exchange
• therefore oxygen levels in alveoli fall as oxygen still taken away by blood
• CO2 levels rise as being produced faster than it can be removed
• this is reflected in systemic arterial blood (constant equilibrium)

Hyperventilation:

• the exact opposite happens
• very hard to hyperventilate for long period of time as CO2 is primary stimulus for breathing

Question 13

What is the bronchial circulation?

Answer 13

Branch from systemic circulation to the lungs

• Delivers oxygenated blood, nutrients, enzymes and hormones to the lung tissues
• removes waste
• blood then drains to the left atrium slightly diluting the oxygenated blood that is coming from the lungs

Question 14

What is the pressure in the pulmonary system in comparison with the systemic?

Answer 14

Pulmonary = 25/10mmHg (high flow, low pressure system)
Systemic = 120/80

Question 15

What is the partial pressure of oxygen in the peripheral tissues? How does this aid oxygen delivery to peripheral tissue?

Answer 15

Peripheral tissue partial pressure of oxygen = 40mmHg
Therefore large partial pressure gradient which sucks oxygen out of the systemic arterial blood into peripheral tissue

Peripheral tissue constantly producing CO2 which goes down its partial pressure gradient into the systemic venous blood

Question 16

What is the PO2 and PCO2 of the blood going to the lungs?

Answer 16

PO2 = 40mmHg
PCO2 = 46mmHg

(Therefore PCO2 pulmonary artery > PCO2 in alveoli so goes down concentration gradient into alveoli)
(PO2 pulmonary artery < PO2 alveoli so oxygen travels down concentration gradient back into pulmonary artery)

Question 17

In health, what should the systemic arterial pressures and systemic venous pressures be the same as?

Answer 17

Systemic arterial pressure should be same as alveoli

Systemic venous pressure should be same as in peripheral tissues

Question 18

What influences the diffusion of gases across the alveoli?

Answer 18

The partial pressure gradient
Gas solubility
Available surface area
Thickness of membrane 
Distance to cross (fastest over short distances)

Question 19

How does partial pressure gradient affect diffusion across a membrane?

Answer 19

• the greater the gradient the greater the rate of diffusion

- PO2 gradient is 10x greater than that of CO2, but oxygen only diffuses marginally faster

Question 20

How does gas solubility affect diffusion rate across a membrane?

Answer 20

• gas has to be in solution to diffuse
  
  • therefore the more soluble, the easier diffusion
• CO2 diffuses quickly despite low partial pressure gradient because its highly water soluble (unlike O2)

Question 21

What is emphysema?

Answer 21

Destruction of alveoli —> reduced area for gas exchange
Blood flowing away from lungs has lower PO2 than normal
Breakdown of alveoli walls and loss of elasticity—> high compliance
- Very easy to inflate lungs, lots of effort to force air out
Mainly caused by smoking

Question 22

What is fibrotic lung disease?

Answer 22

Fibrous tissue laid down alongside elastic tissue
Fibrous tissue between alveolar type 1 and capillary cells (thickening of alveolar membrane)
- impedes diffusion as gas has to diffuse further
- Loss of lung compliance (fibrous tissue resists stretch)
- decreased alveolar ventilation therefore decrease PO2

Question 23

What is pulmonary oedema?

Answer 23

Fluid build up between alveoli and capillaries

• pushes the alveoli and capillaries apart leading to greater diffusion distance
• usually a result of pulmonary hypertension forcing plasma out of capillaries into interstitial space
• oxygen doesnt dissolve well in water = difficult to diffuse from alveoli to blood

Lower PO2 in blood leaving lungs

Question 24

How does asthma impact diffusion?

Answer 24

Constriction of bronchioles —> Increased air resistance —> decreased alveolar ventilation —> less oxygen in alveolar