I. Respiration in Water Breathing Animals
A. Issues to overcome
1. Water has lower oxygen content than air
2. More ventilation is needed to provide the same amount of oxygen. For example, to obtain the same amount of oxygen as an air-breathing animal, the water-breathing one must move about thirty times as much water across the respiratory surface.
3. Water is higher density and higher viscosity than air
4. More energy required to move water than air
5. Diffusion rate of oxygen is slower in water than air. Per Ficks law, water is more viscous than air and has a lower diffusion coefficient. Plus, water and blood movements are driven by bulk flow which is due to pressure gradients.
B. Integumentary exchange –thin skin, large surface area, convection
C. External gills –protected or unprotected
D. Internal gills – within body contour
1. Ventilation –unidirectional– not tidal (tidal is in and out); In through mouth or spiracle and out across gills. Ram ventilation is the term used for a fish swimming with its mouth open and “ramming” the water in and across the gills. This method decreases the amount of work involved in ventilation.
E. Gas transport –involves gases moving from high to low pressure, gill perfusion, gill diffusion, circulation, tissue diffusion
1. Gill filaments are covered with gill lamellae to increase surface area
2. Countercurrent mechanism
water and blood move in parallel tubes in the opposite direction ensuring concentration gradient