Overview
This lecture covers how fish are adapted for gas exchange in water, focusing on the structure and function of gills and the countercurrent flow principle.
Gas Exchange in Fish: Challenges and Adaptations
- Fish cannot diffuse oxygen through their skin due to scales and a small surface area to volume ratio.
- Fish extract oxygen from water, which contains much less oxygen than air.
- Fish possess gills as specialized gas exchange surfaces.
Essential Features of Gas Exchange Surfaces
- All gas exchange surfaces require a large surface area to volume ratio.
- Surfaces must allow a short diffusion distance for gases.
- A mechanism is needed to maintain concentration gradients for effective diffusion.
Structure of Fish Gills
- Fish have four layers of gills on each side of the head.
- Each gill is made of stacked gill filaments; gill lamellae are thin structures at right angles to filaments.
- Gill lamellae provide a large surface area for gas exchange.
- Water flows in through the mouth, passes over gills, and exits via a gap at the side of the head.
Maximizing Gas Exchange Efficiency
- The many filaments and lamellae increase surface area.
- Lamellae are thin, and capillaries are close to the surface, ensuring a short diffusion distance.
- Gas exchange occurs only at the lamellae.
Maintenance of Concentration Gradient: Countercurrent Flow
- Water flows over the gills in the opposite direction to blood in the capillaries (countercurrent exchange).
- Countercurrent flow prevents equilibrium and maintains a diffusion gradient across the entire length of each lamella.
- In concurrent (same direction) flow, equilibrium is reached early, so diffusion stops over part of the lamella.
- Countercurrent flow allows efficient oxygen uptake along the entire lamella.
Key Terms & Definitions
- Gill Filaments — Finger-like structures stacked in gills, increasing surface area.
- Gill Lamellae — Thin, flat plates perpendicular to filaments; main sites of gas exchange.
- Capillaries — Small blood vessels in the lamellae close to the surface for effective exchange.
- Countercurrent Flow — Water and blood flow in opposite directions to maximize oxygen diffusion.
Action Items / Next Steps
- Review the structure and function of fish gills and the countercurrent flow principle.
- Practice explaining how countercurrent flow maintains the concentration gradient along the entire lamella.
- Prepare for exam questions by summarizing the three key adaptations for efficient gas exchange.