6.3 - Gas exchange in fish

Question 1

describe the problem that fish have for gas exchange

Answer 1

• waterproof, gas-tight, outer covering + small SA:Vol
  –> therefore their body surface isn’t adequate
  –> therefore they need gills

Question 2

describe the structure of the gills

Answer 2

• located behind the head
• made up of gill filaments which are stacked up in a pile (like pages in a book)
• at right angles to gill filaments are gill lamellae –> increases SA of gill
• water is taken in by the mouth and forced over the gills
• the flow of water over the gill lamellae and the flow of blood within them are in opposite directions (known as counter current flow)

Question 3

describe the arrangement of gill lamellae for counter current

Answer 3

• blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen
• therefore diffusion of oxygen from the water to the blood takes place
• blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed. Again, diffusion of oxygen from the water to blood takes place

Question 4

describe the result of counter current flow

Answer 4

• a diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae
• in this way, 80% of the oxygen available in the water is absorbed into the blood of the fish (rather than 50% if it was parallel flow)