Review – factors needed for efficient gas exchange

1. Review – factors needed for efficient gas exchange • Large surface area • Moist surface to dissolve gases • Concentration Gradient

2. The Gill Gas Exchange • Fish have gills over which water continually flows. • Gills are supported by water flowing over them, so are unsuitable for life on dry land. • The water has oxygen dissolved in it.

3. Fish Anatomy Mouth (Buccal Cavity) Operculum Buccal Floor

4. Behind the Operculum… …lie the gills – the fish’s equivalent of our lungs

5. Gill Structure • Gill arches • Gill filaments • Lamellae

6. Adaptations of the gills for efficient gas exchange • Large surface area : volume • allows more diffusion of gases • Permeable membranes • allows gases to diffuse through tissues • Thin (flattened cells) • short diffusion distance • Good vascular (blood) supply – • maintains concentration gradients

7. Structure of Gills • The gills are made of numerous thin filaments supported by a bony arch.

10. Each filament is made of thin delicate plates (lamellae) containing many capillaries, so they look dark red. • gills are protected by an operculum.

11. Ventilation • The fish opens its mouth and lowers its buccal floor, increasing the volume in the mouth and so decreasing the pressure • Water rushes in • The fish closes its mouth and raises its buccal floor • This decreases the volume, and so increases the pressure in the mouth • Water is forced over the gills and out of the operculum • Gas exchange happens in the gills

14. Exchange and Diffusion of Gases. • The capillaries have very thin walls that allow the O2 to move from the water into the blood and CO2 from the blood into the water by diffusion. • O2 moves from where there is a high concentration in the water to where there is a lower concentration in the blood of the capillaries. • CO2 moves from where there is a high concentration in the capillary blood to where there is a low concentration in the water.

17. Counter-Current Exchange • The blood flow in the capillaries is opposite to the flow of water over the gills. • This means that the blood in the capillary is meeting new water with a full load of oxygen. • Counter current exchange of water flow and blood stream maintains concentration gradients for diffusion.

18. Counter Current Principle Animation of counter-current

20. Efficiency of Counter-current Exchange • Counter-current exchange removes up to 80% of the oxygen in the water.

21. Gills greatly increase the surface area for gas exchange.

22. Thin Exchange Surface • Thin walls of lamellae and capillaries to ↓ barrier for diffusion of O2 into capillaries and CO2 out

23. Moist Gas Exchange Surface • Moist surface maintained by watery environment. • O2 already dissolved into watery substance for diffusion into capillaries.

24. Why don’t fish drown? • Oxygen is 21% of air, < 1% in water • More water can flow over (external) gills than in and out of (internal) lungs. • Countercurrent flow in gills extracts more of the oxygen from the water in a low O2 environment. • Fish demand for O2 supply and removal of CO2 is lower than larger animals

25. The gill gas exchange system limits fish to a watery environment. • The watery environment does not require the gas exchange surface to be internal.