Diffusion: (in animals, for gaseous exchange) being ventilated.

The idea

Ventilation in animals means the movement of air or water over a gas exchange surface so that oxygen is delivered and carbon dioxide is removed. In mammals, breathing movements change the volume and pressure of the thorax: the diaphragm contracts and flattens, the external intercostal muscles raise the ribs, thoracic volume increases, pressure falls slightly below atmospheric pressure, and air enters the lungs. Exhaling reverses these changes. Ventilation maintains a steep concentration gradient across the alveoli, so oxygen diffuses into the blood and carbon dioxide diffuses out. Without ventilation, the gradient would fall and diffusion would slow or stop. Fish ventilate gills by passing water over the gill filaments, and insects ventilate tracheae through spiracles and abdominal pumping.

What earns the marks

• Ventilation is the movement of air or water over a gas exchange surface, maintaining a concentration gradient.
• In mammals, inhaling involves contraction of the diaphragm and external intercostal muscles, increasing thoracic volume and lowering pressure so air enters the lungs.
• Exhaling involves relaxation of these muscles, reducing thoracic volume and raising pressure so air leaves the lungs.
• A steep concentration gradient of oxygen and carbon dioxide across the alveoli is maintained by ventilation and blood flow.
• Oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses from the blood into the alveolar air.
• Fish ventilate gills with a flow of water, and insects ventilate tracheae through spiracles and abdominal movements.

Common mistakes and how to put them right

• Thinking that breathing is the same as respiration.

  Put it right: Breathing, or ventilation, is a mechanical process that moves air; respiration is a chemical process in cells that releases energy from glucose.

• Stating that oxygen is pushed into the lungs.

  Put it right: Air enters because the pressure inside the thorax becomes lower than atmospheric pressure, so air moves down a pressure gradient.

• Believing that ventilation alone causes gas exchange.

  Put it right: Ventilation maintains the concentration gradient, but gases still move by diffusion across the alveolar surface.

Exam tips

• Use the terms concentration gradient, thoracic volume and pressure difference when explaining how ventilation maintains diffusion.
• When describing inhalation and exhalation, link each muscle action to the resulting volume and pressure change, then to air movement.
• For fish or insects, name the gas exchange surface and the medium moved over it, such as water over gills or air through tracheae.