Ventilation & Gas Exchange (AQA AS Biology): Revision Note

Ventilation & gas exchange

• Gas exchange in the lungs occurs when:

  • oxygen diffuses down its concentration gradient from the air into the blood

  • carbon dioxide diffuses down its concentration gradient from the blood into the air

• The concentration gradients required for effective gas exchange are maintained by:

  • ventilation in the lungs

  • the continuous flow of blood in the capillaries

Ventilation

• Ventilation is the process of moving air in and out of the lungs; this involves:

  • breathing in, or inhalation

  • breathing out, or exhalation

Breathing in

• The mechanism by which air is drawn into the lungs is as follows:

  1. The diaphragm contracts and flattens and the external intercostal muscles contract

  2. The ribcage moves upwards and outwards

  3. Chest volume increases, resulting in a pressure decrease

  4. Air moves into the lungs down a pressure gradient

• The process of breathing out is active; it requires energy for muscle contraction

Breathing out

• The mechanism by which air is forced out of the lungs is as follows:

  1. The diaphragm relaxes and curves upwards and the external intercostal muscles relax

  2. The ribcage moves downwards and inwards

  3. Chest volume decreases, resulting in a pressure increase

  4. Air moves out of the lungs down a pressure gradient

• When at rest breathing out is a passive process that occurs due to the elastic recoil of the lung tissue

• During forced exhalation the process can become active:

  • Internal intercostal muscles contract to pull the ribs down and the abdominal muscles contract to push organs upwards against the diaphragm

  • These actions decrease the chest volume further, increasing the pressure and forcing more air out of the lungs

Calculating pulmonary ventilation rate

• The volume of air breathed by an individual over a minute is known as the pulmonary ventilation rate (PVR); it is determined by:

  • breathing rate: the number of breaths per minute

  • tidal volume: volume of air breathed in per normal breath

• PVR can be calculated as follows:

PVR = tidal volume x breathing rate