Concentration Gradient

To keep gas exchange as a passive process, concentration gradients must be maintained at gas exchange surfaces

Dense networks of blood vessels - average adult has approximately 100,000km of blood vessels inside their body

Continuous blood flow - helps to replenish the concentration gradient by removing the substance that have recently diffused across the exchange surface

Maintains a higher concentration of those substances in the external environment

Ventilation: movement of air across an exchange surface

Helps replenish the supply of gases and maintain a concentration gradient

Ventilation (for lungs) - the lungs enable ventilation in humans

Movement ensures a continuous supply of oxygen-rich air to the alveoli and facilitates the removal of CO2

Water movement (for gills) - need circulating water to extract dissolved oxygen

In aquatic organisms gases are typically dissolved in water

Aquatic animals use movements of their mouths and opercula to actively move water over the gills

Countercurrent exchange system: water flows over the gills in a direction opposite to the flow within the gill filaments