VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

RESPIRATORY PHYSIOLOGY

• The energy in the living system is achieved by the process of respiration. Respiration is an important heat and energy yielding process. During respiration the oxidation of carbon and hydrogen in the food takes place in tissue to release energy for maintaining life processes.

Mechanism of breathing

• The principal of breathing in fishes is that from the unidirectional flowing water (through mouth and gill and not back), blood in the lamellae take up O₂ and release CO₂.

Gills

Gill ventilation

• If a stationary gill removes O₂ from completely still water, the immediately adjacent layers of water will soon be depleted of O₂. Removal of this after by way of ventilation is done to overcome this. Such water flow in most fishes is maintained by a double pump mechanism constituted by the two functional cavities of the respiratory system: buccal cavity and opercular

Cavity

• The mechanism may be explained in terms of a series of pumps: a station pump drawing water into the mouth, a pressure pump in the buccal cavity, pushing water through the gills and suction pump posterly drawing water over them. As the mouth closes in the breathing rhythm, the oral valves close and prevent an outflow of water anterioly. Pressure in the buccal cavity rises and as the space is constricted, the water is found bacavity .this is the action of the pressure pump expansion of opercula. The branchiostegal membrane closes and prevents the backflow of water. In this way water is drawn from the oral into the opercula cavity. Then with the adduction of the opercula water flows out. Thus, more or less, a continues flow of water over the gills is maintained. Some fast swimming fishes like tuna ventilate their gill in a way different from the way above described pumping method. As the fish swims very fast, it keeps the mouth partly open facilitating a continuous flow of water through the mouth and gill with force. This type of ventilation is called ‘ram ventilation “or “ramjet ventilation” however when the fish swims at a lower speed, it switches over to pumping.

Ref : naturescrusaders.wordpress.com/.../gill_pic/

Gas exchange

• The highly vascularised gill lamellae provide a site for close contact between the respiratory surface and the respiratory medium (water). During the contact, O₂ diffuses from water into the blood and CO₂ moves in the opposite direction. The diffusion of gases (across the gill epithelium) occurs in accordance with the law of physical diffusion

Oxygen transport

• Blood is the medium responsible for acquiring O₂ from the respiratory medium and transporting (supplying) it to the individual cells in the body. Blood, like water is capable of physically dissolving O₂
• Hemoglobin is the respiratory pigment present in the fish blood and is incorporated in RBC. It has much affinity for O₂. At high O₂ concentration (at respiratory surface) the hemoglobin (Hb) combines with O₂ to form oxyhaemoglobin (HbO₂). At low O₂ concentration (at the cell) O₂ is given by the haemoglobin.

Hb+ O₂  Hb -----> O₂

• The respiratory pigments are protein in combination with metal ions. Iron is the metal present in Hb. Myoglobin, which is abundant in muscles is also a respiratory pigment. It serves in O₂ storage.

Carbon-di-oxide transport

• Unlike O_2, CO₂ is transported in different (chemical) forms by blood (HCO₃^-, H₂CO₃, CO₂, KHCO₃, HbNH₂COOH etc.). This is because CO₂, when dissolved in water (or any aqueous medium), results in different chemical species.

CO₂+H₂O H₂CO₃ <----->HCO^-₃+H⁺<------> CO^-₃+H⁺

• But O₂ dissolved in water has only physical binding with H₂O molecules.
• In summary, CO₂ is carried in blood in three forms:
  • As H₂CO₃ (a small amount)
  • As carbamino compound (about 20% of total CO₂ in blood)
  • As bi-carbonate of cation (K+ and Na+) (about 70%)