Physiology of Finfish and Shellfish

2.1.16. Double pump system

When they are moving slowly or resting in water, fish can breathe by synchronously expanding and contracting the buccal cavity (the mouth and throat) and the opercular cavity or a n elaborate system of passing water over the gills while the fish stays stationary. In majority of teleosts, 2 respiratory pumps, buccal and opercular function in co-ordination with each other, to propel water through the gills. These 2 pumps are operated by a set of muscles resulting in contraction or expansion of the buccal and opercular cavities. As a result, there is increase or decrease in the pressure inside the 2 cavities so that water flows unidirectionally across the gill lamellae.

The process can be described as below

1. The mouth is opened and the buccal cavity is enlarged by lateral expansion of its walls. Enlargement of the buccal cavity lowers the water pressure in it, so that water is sucked in.
2. Mouth is closed and volume of opercular cavity is increased. But, the opercular opening remains closed, so that the low pressure is created in the opercular cavity and water flows from the buccal to opercular cavity through the gill slits.
3. When the operculum is closed, the gills must extract as much oxygen as possible. The blood (high CO₂, low O₂) comes into contact with the water (low CO₂, high O₂) through the microscopically thin membrane of the lamellae. Carbon dioxide seeps into the water and oxygen seeps into the blood vessels. After this, water pass outside due to external branchial aperture through a process called diffusion.
   
   

 

(Diffusion is a passive process (as opposed to active transport processes like that caused by pumps) where particles, such as oxygen and carbon dioxide, gradually flow from a high concentration area to a low one, resulting in both areas being equal).

Respiration occurs at the time when water passes over the gills. This means of respiration is modified in different fishes. Fast swimming fishes may leave their mouth and opercular aperture wide open, so that gills are bathed by a continuous current of water produced by swimming. This is called ‘ram ventilation’. The mechanism is also modified in species living in hill streams.

Many sharks rest regularly on the bottom, although they must constantly inhale water to provide the gills with a constant supply. These resting sharks (as well as rays) have structures called spiracles behind their eyes. These are basically holes in the head that lead to the gills, supplementing the water taken in by the mouth.