1. Structure:
* Filaments and Lamellae: Gills consist of thin, thread-like filaments that branch out from the gill arches. Each filament is covered in even smaller, plate-like structures called lamellae. These lamellae create a vast surface area for gas exchange.
* Capillary Network: A dense network of capillaries runs through the lamellae, bringing deoxygenated blood close to the water.
* Countercurrent Flow: The flow of water over the gills is opposite to the flow of blood within the lamellae. This countercurrent flow maintains a concentration gradient, ensuring that the water always has a higher oxygen concentration than the blood, maximizing oxygen diffusion.
2. Diffusion:
* Oxygen Gradient: The water passing over the gills contains dissolved oxygen, while the blood within the capillaries has a lower oxygen concentration. This difference in oxygen concentration creates a gradient, driving oxygen to move from the water into the blood.
* Thin Membranes: The thin membranes of the lamellae and the capillaries allow for easy diffusion of oxygen molecules, minimizing resistance.
* High Surface Area: The vast surface area of the lamellae maximizes the area for gas exchange, increasing the rate of oxygen absorption.
In summary:
* The gill filaments and lamellae provide an enormous surface area.
* The countercurrent flow system maintains a constant oxygen gradient.
* The thin membranes and capillary network facilitate rapid diffusion of oxygen into the blood.
Together, these features enable fish to extract the maximum amount of oxygen from the water, even in environments with low oxygen levels.