Lecture 11 - Pulmonary Ventilation and Gas Diffusion

Question 1

what is airflow?

Answer 1

• flow of air or any other fluid, caused by a pressure differential between two points
• aka pressure gradient (high to low)

Question 2

what is the pulmonary system?

Answer 2

• the system in which you breathe with
• includes nasal cavity, pharynx (trachea), primary bronchi, bronchioles and alveoli

Question 3

what are alveoli?

Answer 3

• sacs of air (like grapes)
• thin-walled structures
• the site of transfer between the atmosphere and the internal environment
• lots of capillaries here (oxygen in and carbon dioxide out)

Question 4

what are the mechanics of ventilation?

Answer 4

• *creating pressure differentials to manipulate airflow
• inhalation and exhalation

Question 5

what happens in the body during inspiration?

Answer 5

• diaphragm contracts/flattens/moves downwards
• air in lungs expands, reducing its pressure
• caused by pressure difference between lungs and atmosphere

Question 6

what happens in the body during expiration?

Answer 6

• sternum and ribs swing down
• diaphragm moves towards thoracic cavity
• air in lungs compresses, increasing its pressure
• caused by pressure difference between lungs and atmosphere

Question 7

what is the respiratory cycle?

Answer 7

• a single breath

Question 8

where is the intrapleural space?

Answer 8

• space between the lung and the thoracic cavity

Question 9

where is the intrapulmonary space?

Answer 9

• space within the lung

Question 10

how does pressure change across the respiratory cycle?

Answer 10

• pressure has to be lower in intrapleural space than intrapulmonary space (or lung will collapse)
• pressure decreases during inspiration and increases during expiration (greater volume in the lungs = lower pressure)

Question 11

what causes the max flow-volume loop to be very high?

Answer 11

• elastic recoil

Question 12

what is ventilation?

Answer 12

• the movement of air in and out of the lungs
• VE = total volume of expired gas per minute

Question 13

what is the equation for VE?

Answer 13

• VE = respiratory rate (RR) x tidal volume (VT)

Question 14

how is VE influenced by metabolic demand?

Answer 14

• VE rises with metabolic demand
• we need to breathe more to supply muscles with oxygen
• we also need to breathe more to get rid of carbon dioxide (so we don’t become acidotic)

Question 15

what is alveolar ventilation (VA)?

Answer 15

• the volume of gas per minute that participates in gas exchange (only the air that reaches the alveoli)
• a large fraction of VE

Question 16

what is dead space ventilation (VD)?

Answer 16

• the fraction of minute ventilation that does not contribute to gas exchange
• made up of respiratory passages and non-perfused alveoli passages

Question 17

what is the relationship between VD and VE?

Answer 17

• higher VD means a higher VE is needed to maintain VA for adequate gas exchange

Question 18

what is the equation for alveolar ventilation?

Answer 18

• VA = VE - VD
• VA = RR x (VT - VD)

Question 19

how does breathing change during exercise?

Answer 19

• increase in inspiratory and expiratory reserve volume used during exercise (bigger pressure gradient, easier to breathe when lungs are more empty)
• breathing is more frequent

Question 20

why do we breathe?

Answer 20

• to take up oxygen
• to get rid of carbon dioxide
• to fulfill the metabolic needs of the body

Question 21

where does gas exchange occur?

Answer 21

• at the alveolar-pulmonary capillary interface

Question 22

how does gas exchange occur?

Answer 22

• through the process of pulmonary diffusion

Question 23

how do we know if gases are exchanging properly?

Answer 23

• partial pressure of oxygen and carbon dioxide in arterial blood

Question 24

what determines proper gas exchange?

Answer 24

• VA matches Q
• diffusion capacity

Question 25

What are the two laws of diffusion?

Answer 25

• dalton’s law of partial pressure
• henry’s law of diffusion between gases and liquids

Question 26

what is dalton’s law?

Answer 26

• individual gases in a mixture exert pressure proportional to their abundance
• more molecules = greater partial pressure
• sum of partial pressures = total pressure

Question 27

what is henry’s law?

Answer 27

• the amount of gas dissolved in a fluid depends on pressure differential and solubility
• without a gradient, gases are at equilibrium and therefore diffusion will not occur

Question 28

what is gas concentration?

Answer 28

• the amount of gas in a given volume determined by product of gas partial pressure and solubility

Question 29

what is gas pressure?

Answer 29

• force exerted by gas molecules against surfaces they encounter (mmHg)

Question 30

how are gas concentration and gas pressure connected?

Answer 30

• partial pressure difference is what allows diffusion (and therefore allows concentration to change)

Question 31

how do you calculate partial pressure?

Answer 31

PP = F (percentage concentration) x P (total pressure of gas mixture)

Question 32

what is the alveolar-capillary interface?

Answer 32

• site of pulmonary diffusion (where gases are exchanged)
• contains alveolar wall, capillary wall, and basement membranes

Question 33

what is the gas exchange between the alveoli and capillaries?

Answer 33

• air inflow = bronchial tree –> alveoli
• blood inflow = right ventricle –> pulmonary arteries –> pulmonary capillaries
• alveoli surround the capillaries

Question 34

how does oxygen exchange in the alveoli?

Answer 34

• gases diffuse from high to low pressure
• takes about 0.75 seconds for equilibrium to be established between capillaries and vein

Question 35

what is fick’s law of diffusion?

Answer 35

• the rate of diffusion (V gas) is directly proportional to surface area, differential partial pressure and diffusion constant
• it is inversely proportional to the thickness of the tissue

Question 36

what is the relationship between VA and Q?

Answer 36

• VA ~ Q (in healthy individuals)
• VA/Q = > 1 –> poorly perfused alveoli
• VA/Q = < 1 –> poorly ventilated alveoli
• VA/Q = anything but 1 –> need increased VE to satisfy requirements