16) Pulmonary ventilation

Question 1

What does tachypnoeic, tachycardic and hypoxemic mean?

Answer 1

• Tachypnoeic: Elevated breathing rate
• Tachycardic: Elevated heart rate
• Hypoxemic: Lower than normal levels of oxygen

Question 2

What is Total Lung Capacity (TLC)?

Answer 2

• The total amount of air the lungs can accommodate

Question 3

What is tidal volume?

Answer 3

• The volume of air being breathed in and out

Question 4

What is Inspiratory Reserve Volume (IRV)?

Answer 4

• It is the amount of air a person can forcefully inhale

Question 5

What is Expiratory Reserve Volume?

Answer 5

• It is the amount of air a person can forcefully exhale

Question 6

What is the Residual Volume (RV)?

Answer 6

• The amount of air remaining in a person’s lungs after fully exhaling

Question 7

What is Functional Residual Capacity (FRC)?

Answer 7

• The volume of air remaining in the lungs after passive exhalation

Question 8

What is Vital Capacity (VC)?

Answer 8

• The maximum amount of air a person can inhale after maximum exhalation

Question 9

What can lung volume and capacity depend on?

Answer 9

• Age
• Sex
• Height
• Lung properties (e.g. obstruction/damage due to disease)

Question 10

Do we use our total lung capacity all the time when breathing?

Answer 10

• No. At rest our tidal volume is much smaller than our total lung capacity

Question 11

What is pulmonary ventilation?

Answer 11

• The movement of air from the atmosphere to gas exchange surfaces within the lungs

Question 12

Why is pulmonary ventilation important?

Answer 12

• It maintains the O2 and CO2 gradients between air in the alveoli and the arterial blood
• This allows a sufficient level of gas exchange to occur which ensure adequate O2 supply to respiring tissues and CO2 removal from respiring tissues from the blood

Question 13

Describe the movement of oxygen from the atmosphere to respiring tissues.

Answer 13

• First oxygen is taken in from the atmosphere into the alveoli in our lungs in the airways through pulmonary ventilation
• In the alveoli gas exchange occurs and oxygen diffuses into the blood and is now in pulmonary circulation.
• Through blood flow it travels to the heart, then enters systemic circulation and finally reaches respiring tissues

Question 14

Describe the concentrations of oxygen and carbon dioxide in the atmosphere, alveoli and in the capilliaries.

Answer 14

• In atmospheric air (which we inhale) there is a very high conc. of oxygen and a very low conc. of CO2
• In the alveoli (site of gas exchange in the lungs) we find high levels of O2 and low levels of CO2
• In the capillaries we find low levels of O2 and high levels of CO2

Question 15

Describe the pressure gradient across the alveoli.

Answer 15

• At the venous end we find lower partial pressures of O2 and higher partial pressures of CO2 in the blood than in the alveoli
• As we get towards the arterial end the partial pressures level off and are equal in the blood and in the alveoli

Question 16

How does the level of ventilation affect levels of gas exchange?

Answer 16

• Higher levels of ventilation means there is a steeper partial pressure gradient between the alveoli and blood
• This steeper gradient means there is more gas exchange that occurs

Question 17

How does the level of ventilation affect partial pressure of O2 in the alveoli?

Answer 17

• As ventilation increases the level of oxygen in the alveoli increases
• As we decrease ventilation the level of oxygen in the alveoli decreases as the oxygen is being pulled out of the alveoli at a faster rate than it is being replenished
• However at a certain limit levels of the partial pressure starts to plateau as ventilation increases
• This is because at this point the partial pressure of oxygen in the atmosphere is the same as the partial pressure of oxygen in the alveoli

Question 18

How does the level of ventilation affect partial pressures of CO2?

Answer 18

• As ventilation increases partial pressure of CO2 decreases as more gas excchange occurs so more CO2 is being exhaled
• However as ventilation decreases partial pressure of CO2 increases. This is because it will just build up in the alveoli

Question 19

What is the formula for total ventilation?

Answer 19

• V = Vt x f
• V: minute volume (the total volume of air inhaled in all breaths over one minute)
• Vt: tidal volume (volume of air inhaled in each breathe)
• f: frequency (number of breaths per minute)

Question 20

What does air within the lungs consist of ?

Answer 20

• The air in the lungs consist of “stale” air and “fresh air”
• Stale air is air that has remained from the last breathe
• Fresh air is air that has just been inhaled
• Therefore the alveolar air is not the same as the inspired air

Question 21

Why do we have stale air?

Answer 21

• The respiratory system is a two-way system in which air enters and leaves via the airways
• The airways is known as anatomic dead space as no gas exchange takes place in this area (only in the alveoli)
• This means at the end of each expiration there is a residual volume of air which remains in the airways and never takes part in gas exchange
• When we inhale again the fresh air then mixes with this stale air

Question 22

What is the formula of alveolar ventilation?

Answer 22

• Va= (Vt - Vd) * f
• Va: Alveolar minute volume (the total volume of fresh air entering the alveoli across all breathes in a minute)
• (Vt - Vd): volume of fresh air entering the alveoli in each breathe
• Vt: Tidal volume (volume of air being inhaled/exhaled)
• Vd: Dead space volume (volume of air remaining in the respiratory system at the end of expiration)
• f: Frequency

Question 23

How does gas naturally move?

Answer 23

• From areas of higher pressure to areas of lower pressure

Question 24

What is Boyle’s law and how is it derrived?

Answer 24

• P ∝ n/v
• P: Pressure (the number of gas molecules in a given volume)
• n: No. moles
• V: Volume
• If n is constant then as volume increases pressure decreases
• It is derived from the ideal gas equation: PV=nRT
• R: gas constant
• T: Temperature

Question 25

What are pleura?

Answer 25

• These are serous membranes that surround each lung which line the chest wall

Question 26

What is the pleural cavity?

Answer 26

• A fluid filled space between the pleura (membranes which line the chest wall and the lungs)
• It helps reduces friction between lungs and chest

Question 27

What is the effect of the pleural cavity on volume?

Answer 27

• The properties of the pleural cavity (i.e. sealed, fluid-filled) means they resist changes in volume
• Hence changes in volume of the thoracic cavity (due to respiring muscle activity) results in changes in lung volume

Question 28

How is the pleural cavity put under “negative pressure”?

Answer 28

• The opposing elastic recoil of the chest wall (outward) and lungs (inward) results in the pressure within the pleural cavity being sub-atmospheric (or under negative pressure)

Question 29

What is negative and positive pressure?

Answer 29

• Differences in pressure between neighbouring spaces are unstable.
• In the absence of sufficient opposing forces, equilibrium will be re-established either through the movement of liquid/gases or the collapse/expansion of a volume.
• Negative pressure: Lower number of molecules per volume (compared to surroundings) and so generates a collapsing force which pulls surfaces together
• Positive pressure: Increased number of molecules per volume (compared to surroundings) and so generates an expanding force which pushes the surfaces apart.

Question 30

How is air flow in the lungs generated during inspiration

Answer 30

• During inspiration the diaphragm contracts and the thoracic cavity expands. This causes an increase in thoracic cavity volume.
• Intrapleural pressure becomes more negative so the lungs expand and increase lung volume
• Alveolar pressure decreases below atmospheric pressure
• This change in pressure sets up a pressure gradient between the alveoli and the atmosphere and the air flows from the atmosphere (high pressure) into the alveoli (low pressure)

Question 31

How is air flow out of the lungs generated during expiration

Answer 31

• During expiration the diaphragm relaxes and the lung recoils due to elastic fibres.
• This causes the thoracic (lung) volume to decrease (so compresses the air) and intrapleural pressure increases
• The lungs become compressed (only during forced expiration) and the volume decreases
• This causes an increase in alveolar pressure above the atmospheric pressure
• This change in pressure sets up another pressure gradient between the alveoli and the atmosphere and so the air flows from the alveoli (high pressure) to the atmosphere (low pressure)
• The lungs deflate

Question 32

What is pneumothorax?

Answer 32

• Pneumothorax is a collapsed lung

- It occurs when air enters into the pleural cavity

Question 33

What happens if either of the pleura is ruptured?

Answer 33

• If there is a rupture, the pleura cavity is no longer sealed and provides a passage way for the air to move from one compartment to another
• Due to the pressure gradient between the pleural cavity and the surrounding, air will enter (pneumothorax) until the intrapleural pressure is the same as atmospheric pressure.
• Air will enter causing pleural cavity volume to increase.
• The lung tissue then recoils and expansion of chest (during inspiration) can draw more air into the pleural space