Physiology of Finfish and Shellfish

2.1.29. Red blood cells important to carbon dioxide transport

Most of the CO₂ entering or leaving the blood go through red blood cells for two reasons. One reason is due to the enzyme carbonic anhydrase. This enzyme is present in red blood cells and not in the plasma. The enzyme is important in the transportation of CO₂ because, within the red blood cells, it catalyzes the reaction of CO₂ with OH^- resulting in the formation of HCO₃^- ions. As the level of HCO₃^- ions increases within the erythrocytes, the HCO₃^- ions diffuse through the erythrocyte membranes into the plasma of the blood. In order to maintain electrical balance within the erythrocytes, an anion exchange occurs in a process called a chloride shift. In this process, HCO₃^- ions leave the red blood cells while a net influx of Cl^- ions from the plasma enters the red blood cells. The membrane of red blood cells is very permeable to both ions because the membrane has a high concentration of a special anion carrier protein, the band III protein. This protein allows for a passive diffusion of the Cl^- and HCO₃^- ions to and from the red blood cells and plasma. This keeps the bicarbonate from building up in the red blood cells, which would slow down or stop the reversible conversion of CO₂ to HCO₃^-_. Facilitated diffusion occurs in the movement of CO₂ across the respiratory surfaces as bicarbonate (HCO₃^-) diffuses out of the red blood cells and into the epithelium where it is converted back to CO₂. Excretion of CO₂ is limited by the rate of bicarbonate-chloride exchange across the erythrocyte membrane.

Carbon dioxide transport: When blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form carbamino compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

There are three general variations in gills found in fishes:

Pouched gills - Agnatha

• It has external and internal pores rather than gill slits
• water is drawn into the gill chambers through the mouth and then passed over the gills

Septal gills - Elasmobranchs

• have gill slits rather than pores and gill septa that help support gill filaments
• inspiration occurs through the mouth and expiration occurs through the gills - the exception is when the shark is feeding, when water moves into the pharynx through the spiracle

Opercular gills - bony fishes

• have no septa (aseptal) but gill bars anchor gill filaments
• the operculum protects the filaments and expiration occurs through a single gill slit

External gills

• develop from the skin ectoderm of the branchial area but are not directly related to the visceral skeleton or branchial chambers
• are found most often in larval or paedomorphic amphibians

Swim bladders and the origin of lungs

Lungs are found among fishes found in warm or stagnant water, as well as in primitive fishes, and allow for the fish to gulp air and undergo diffusion in an environment with relatively low dissolved oxygen

Such fishes undergo long periods of breath-holding (apnea) alternated with short periods of lung ventilation

Ventilation of respiratory structures depends on

• Ram ventilation - forward momentum contributes to flow of water across the gill membranes
• Dual pump - buccal and opercular action operating in tandem drives water in a nearly continuous unidirectional flow across the gill curtain between them
  • the suction phase begins with compressed buccal and opercular cavities and closed valves
  • as the buccal cavity expands, the internal oral valves open and water moves into the buccal cavity and across the gill curtain
  • during the force phase, the oral valve closes and water is forced out through the opercular valve
• Pulse pump - the dual pump is modified into an inhalation/exhalation phase
  • the exhalation phase begins with transfer of spent air from the lungs into the buccal cavity
  • the exhalation phase concludes with expulsion of air from the buccal cavity to the outside either through the mouth or under the operculum
  • the inhalation phase begins with the organism taking fresh air into the mouth
  • the inhalation phase concludes with transfer of air from the buccal cavity into the lungs
• Aspiration pump - air is sucked in, or aspirated, by low pressure created around the lungs
  • the lungs are located within the pump so that the force required to ventilate them is applied directly
  • a moveable diaphragm and rib cage cause pressure changes rather than the action of the buccal cavity