pulmonary ventilation

Question 1

what is pulmonary ventilation ?

Answer 1

movement of air from the atmosphere to gas exchange surfaces within the lung

Question 2

why is pulmonary ventilation required ?

Answer 2

• to maintain O2 and CO2 gradients between alveolar air and arterial blood
• enables sufficient gas exchange ensuring adequate O2 supply to respiring tissue and CO2 removal.

Question 3

what is hyperventilation ?

Answer 3

• excessive levels of breathing
• increases alveolar oxygen partial pressure and decreases alveolar CO2 partial pressure
• plateaus at approx 21kpa as air in alveoli is similar to air in the atmosphere

Question 4

what is hypoventilation ?

Answer 4

• insufficient levels of breathing
• decrease O2 partial pressure and increase CO2 partial pressure
• plateaus when levels of O2 are similar to the levels in the venus blood

Question 5

how is ventilation calculated ?

Answer 5

v = Tv x f
where:
V = ventilation ( volume of fresh air reaching respiratory surfaces over a given time)
Tv = tidal volume mL ( volume of air inhaled in each breath)
f = frequency ( number of breaths per minute)

Question 6

why is the formula for ventilation not 100 % accurate ?

Answer 6

• gas exchange only occurs in the alveoli but air must pass through the airways first
• the respiratory system is a 2 way system so air leaves ad enters the same way there is also a residual volume of air that remains in the airways and and lungs after each breath
• this means that the final approx. 150ml of each breath never reaches the alveoli and takes place in gas exchange

Question 7

how do you calculate alveolar minute volume ?

Answer 7

Va = ( Vt - Vd ) x f

where:
Va= alveolar minute volume mL ( total volume of fresh air reaching the alveoli across all breaths taken in a minute )
vt= tidal volume
vd= dead space volume - volume of air remaining in the respiratory system after each breath.
f = frequency

Question 8

what is the pleural cavity and what is its function ?

Answer 8

• fluid filled space between the membranes that line the chest wall and each lung
• reduced friction between lungs and chest during breathing
• resists changes in volume so changes in thoracic cavity volume change lung volume rather than pleural cavity volume.
• pressure is slightly lower than pressure in the atmosphere due to opposing elastic recoil of chest wall outwards and lung inwards.

Question 9

describe the processes of inspiration and expiration

Answer 9

inspiration:
* diaphragm contract
* volume in thoracic cavity increases
* interpleural pressure becomes more negative
* outward force exerted on visceral pleura. becomes greater than inward recoil force
* lungs expand increasing volume
* alveolar pressure decreases below atmospheric pressure
* air moves down pressure gradient.

expiration:
* diaphragm relaxes - lungs recoil due to elastic fibres
* volume of thoracic cavity decreases
* interpleural pressure increases
* lungs compress volume decreases ( lung compression only occurs during forced expiration in quite breathing elastic recoil is enough to decrease lung volume)
* alveolar pressure increases above atmospheric pressure
* air moves down pressure gradient.

Question 10

what is a pneumothorax and what does it cause ?

Answer 10

• presence of air within the pleural space
• can occur if either pleural membrane is damaged, resulting in a passageway for air to enter (either from the atmosphere or lungs).
• Air enters the pleural space as intrapleural pressure is lower than atmospheric pressure and there is thus a volume gradient resulting in movement of air.
  *As air enters the pleural space, its volume will increase at the expense of the lungs, which will thus decrease (causing atelectasis or collapse of lung tissue).
  *Elastic recoil of the lungs and expansion of the thoracic cavity during breathing can cause further air to enter the pleural space, further reducing lung volume.