B3.1.3 Maintenance of concentration gradients at exchange surfaces in animals Notes

Maintenance of Concentration Gradients at Exchange Surfaces in Animals

1. Gas exchange relies on diffusion, where gases move from high to low concentration.
2. 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.
3. To maintain these gradients, animals use active processes like blood flow and ventilation.

Dense Networks of Blood Vessels at Exchange Surfaces

1. The gas-exchange surfaces, such as the alveoli in lungs or gills, are surrounded by dense networks of blood vessels.
2. 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.
3. By continuously removing oxygen from the exchange surface and delivering carbon dioxide to be expelled, the blood helps maintain the concentration gradient.
4. This ensures that there is always a difference in gas concentrations, promoting efficient diffusion.

Continuous Blood Flow

1. 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.
2. Blood carrying oxygen is constantly moved through the exchange surface, while blood carrying carbon dioxide is carried away to be expelled from the body.
3. Continuous blood flow ensures that oxygenated blood is constantly removed from the exchange surface, keeping the partial pressure of oxygen low in the blood.
4. 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.

Ventilation: Moving Air or Water Across Exchange Surfaces

1. In terrestrial animals, ventilation is achieved by breathing air into the lungs, while in aquatic animals, ventilation occurs when water is moved over the gills.
2. Ventilation brings in fresh oxygen to replace the oxygen that has diffused into the blood and expels carbon dioxide to maintain a high concentration of CO₂ at the gas-exchange surface.
3. This continuous exchange ensures the maintenance of the concentration gradients necessary for efficient diffusion.

Ventilation in Lungs (Air)

1. In mammals, the respiratory system (lungs) uses a system of inhalation and exhalation to ventilate the lungs.
2. Fresh air is brought into the lungs during inhalation, replenishing the oxygen available for diffusion into the blood and expelling CO₂ out during exhalation.
3. The diaphragm and intercostal muscles help drive this ventilation process.

Ventilation in Gills (Water)

1. In fish, water is passed over the gills through buccal and opercular pumps, which help draw water into the mouth and expel it through the gill slits.
2. The water flowing over the gills maintains a constant supply of oxygen, allowing efficient oxygen diffusion from the water into the blood.

Countercurrent Exchange

1. Fish utilize a countercurrent exchange mechanism, where water flows in the opposite direction to the blood flow in the gills.
2. This maintains a concentration gradient for oxygen across the entire length of the gill, maximizing oxygen absorption.

How These Mechanisms Work Together

1. Dense capillary networks increase surface area.
2. Continuous blood flow keeps gases moving, preventing equilibrium.
3. Ventilation keeps external gradients intact by constantly refreshing oxygen and removing carbon dioxide.

These mechanisms create an efficient system for gas exchange.