Maintenance of Concentration Gradients at Exchange Surfaces in Animals
Definition
Concentration Gradient
The difference in concentration of a substance (e.g., oxygen or carbon dioxide) between two regions.
- Gas exchange relies on diffusion, where gases move from high to low concentration.
- For this to work, there must be a constant concentration gradient - oxygen needs to be high in the lungs and blood low, while carbon dioxide is high in the blood and low in the lungs.
- To maintain these gradients, animals use active processes like blood flow and ventilation.
Dense Networks of Blood Vessels at Exchange Surfaces
- The gas-exchange surfaces, such as the alveoli in lungs or gills, are surrounded by dense networks of blood vessels.
- These networks ensure that oxygen can quickly be transported away from the exchange surface, and carbon dioxide can be transported to the surface for removal.
- By continuously removing oxygen from the exchange surface and delivering carbon dioxide to be expelled, the blood helps maintain the concentration gradient.
- This ensures that there is always a difference in gas concentrations, promoting efficient diffusion.
- In the alveoli of the lungs, capillaries surround the alveolar sacs.
- Oxygen from the air diffuses into the blood in the capillaries, and carbon dioxide diffuses from the blood into the alveoli to be exhaled.
To remember the three main processes for maintaining concentration gradients:
- C - Capillaries (dense networks increase surface area and shorten diffusion distance)
- B - Blood Flow (continuous movement prevents equilibrium)
- V - Ventilation (constant air or water flow ensures gas replenishment)
Mnemonic: "Capillaries, Blood, and Ventilation" (CBV) – these three processes work together like a "CBV team" to maintain concentration gradients.
Continuous Blood Flow
- The continuous flow of blood through the blood vessels ensures that the gas-exchange surface is always exposed to a fresh supply of blood, which helps maintain the concentration gradient.
- Blood carrying oxygen is constantly moved through the exchange surface, while blood carrying carbon dioxide is carried away to be expelled from the body.
- Continuous blood flow ensures that oxygenated blood is constantly removed from the exchange surface, keeping the partial pressure of oxygen low in the blood.
- Similarly, by removing deoxygenated blood that contains carbon dioxide, the concentration of CO₂ near the surface is kept high, promoting its diffusion into the air or water.
In gills of fish, blood flows in the opposite direction to the flow of water (countercurrent flow), which maximizes the concentration gradient for oxygen diffusion from water to blood.
Ventilation: Moving Air or Water Across Exchange Surfaces
Definition
Ventilation
Ventilation is the process of moving air or water across the gas-exchange surface to refresh the supply of gases and maintain the concentration gradient.
