6.7 The mechanism of breathing

Question 1

What is ventilation

Answer 1

It occurs in order to maintain a concentration gradient of oxygen and CO2 across the alveolar epithelium

Air is constantly moved in and out of lungs

Question 2

What are inspiration (inhalation) and expiration (exhalation)

Answer 2

Inhalation occurs when the pressure inside the lungs is less than atmospheric pressure, so air is forced into lungs

Exhalation occurs when pressure inside lungs is greater than that of the atmosphere so air is forced out

Question 3

What muscles control the pressure changes of the lungs

Answer 3

. Diaphragm: sheet of muscle that separates thorax (contains important organs eg lungs) from the abdomen

. The intercostal muscles lie between the ribs and there are 2 different types

• Internal intercostal muscles:
  When they contract the volume of lungs decrease so expiration occurs
• External intercostal muscles:
  When they contract, the volume of lungs increases so inspiration occurs

Question 4

What is the equation for pulmonary ventilation

Answer 4

Pulmonary ventilation rate(dm3min-1) =
tidal volume (dm)3 X breathing rate(min-1)

Question 4

Describe the process of inspiration

(breathing in)

Answer 4

. The external intercostal muscles contract and the internal intercostal muscles relax

. This makes the ribs move up and outwards , increasing volume of thorax

. The diaphragm muscles contract which makes it flatten, increasing volume of thorax

. The increased volume of the thorax results in a lower pressure inside the lungs

. So the pressure is lower than atmospheric pressure, so forces air in

AIR MOVES IN

Question 4

Describe process of expiration

(breathing out)

Answer 4

. Internal intercostal muscles contract and external intercostal muscles relax

. This makes the ribs move down and inwards so decrease volume of thorax

. The diaphragm relaxes so it is pushed up by the contents of the abdomen that were compressed during inhalation.
This further decreases volume of thorax.

. This decreases volume of lungs so pressure in them increases above atmospheric pressure, so air is forced out

AIR MOVES OUT