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Respiration in Fishes

Generalized Fish Circulation

Heart - pump
All the rest are diffusion exchangers (capillary beds)
Muscles - Oxygen used; carbon dioxide produced.
Kidney - Water and salts regulated.
Intestine - Foods picked up
Liver - Foods processed and stored; ammonia produced
Gills - Carbon dioxide and ammonia diffuse out; oxygen diffuses in.
Diffusion exchangers provide a passive exchange, operating on physical laws.
R - Rate of diffusion.
D - Diffusion constant (a function of the material the substance is diffusing through.)
A - Area of exchange
- Difference in partial pressure between the two solutions.
d - distance involved.
Of these, A, , and d can be modified to control the rate of diffusion.
The larger the surface, the more rapid the diffusion. In general the gill area is proportional to the amount of red muscle, more active fish having a proportionately greater area. A mackerel has 1000 sq mm /g body weight; a toadfish has 57 sq mm/g body weight. The problem with increasing the area is that water also diffuses and the amount of work the kidney has to do for osmotic regulation increases proportionately to the gill area.
Fish have reduced this to a minimum. One layer of epithelium; one layer of endothelium.

The secondary lamellae (thin, leaf-like protrusions from the gill filament) are best described as a sheet of blood separated by an array of posts (the pillar cells) which keep the lamellum from ballooning, maintaining it one red blood cell thick.

Fish as a group have done a great deal to maximize . The first step is to keep both blood and water moving. If either stopped, it would quickly reach an equilibrium with the other and would go to zero.
  1. The blood pump: the heart pumps in a discontinuous fashion, squeezing out a spurt, then having to relax and refill before squeezing out the next spurt. However the bulbus arteriosus is made of elastic fibers and expands to receive the entire output of the ventricle. It's elasticity keeps pressure on that blood and keeps it flowing smoothly while the ventricle refills.
  2. The Water Pump Water also moves continually across the diffusional surface. There are two pumps, a buccal force pump and an opercular suction pump and two one-way valves, an oral valve just inside the lips and an opercular valve reinforced by the branchiostegal rays. The bucccal pump consists of the hyoid apparatus, which can be depressed to draw water into the mouth, then raised to force it out. However as the water starts to exit the mouth, it inflates the cusps of the oral valve, popping it closed. The water then can only exit by way of the gill slits to the opercular cavity. As the opercals are flared out, the opercular valve is pressed against the body, preventing the entry of water so that water must enter from the buccal cavity across the gills. With the two pumps operating slightly out of phase (see illustration) a constant difference in pressure can be maintained in the two chambers resulting in a constant flow of water across the gills.

  3. Counter current exchange The tips of gill filaments from adjacent arches press together so that water must flow between the filaments. Between the filaments, the space is divided by the thin secondary lamellae, creating tiny channels for the water. Each secondary lamellum contains a sheet of blood, one cell thick, flowing in the direction opposite to the water.

    A resting teleost fish typically removes 80% of the oxygen from the water passing over it's gills. Some experimental results for the tench showed a mean efficiency of 51% oxygen removal. Reversal of the water flow reduced the efficiency to 9%. A typical mammal, such as a human removes only 10 to 20 % of the oxygen from air. Why such a contrast in efficiency? Consider the medium. Water is 800 times as dense as air. Air normally contains 20% oxygen (200,000 ppm). Water, by contrast contains only about 10 ppm oxygen and under stagnant conditions this may drop much lower. Fish have to be more efficient.

    If fish gills are so efficient and air contains so much more oxygen, why do fish suffocate out of water? Surface tension of water clinging to the gill collapses the lamellae and the effective surface area is reduced to a tiny fraction of normal. Catfish survive better than most other fish because they have cartilage supports for the primary filaments and have the secondary lamellae thickened and separated more than usual.
  4. Respiratory Pigment Oxygen diffuses into the blood according to the difference in dissolved oxygen (DO) between water and blood. Hemoglobin in the blood will react with the oxygen to form oxyhemoglobin, keeping the DO low and high. As a result one ml of blood will carry as much oxygen as 15 - 25 ml of water. The reaction is reversible; as oxygen diffuses out of the blood into the tissues, more is released from the oxyhemoglobin.
    • Effect of acid on hemoglobin/oxyhemoglobin.

      Hemoglobin exhibiting the Bohr effect will require a higher oxygen tension to fully load under conditions of low pH. Hemoglobin showing the Root effect can never be fully saturated with oxygen when pH is low.

      • Normal tissues (muscles, etc.) are high in carbon dioxide and low in oxygen.
      • Gills are normally low in carbon dioxide and high in oxygen.
      • Hemoglobin loads on the high curve and unloads on the low curve.
    • Suppose conditions deteriorate in the pond. Decomposition uses up much of the oxygen, producing abnormally high carbon dioxide levels.
        Fish with a strong Bohr effect are in triple jeopardy:
      • There is less oxygen to start with.
      • They can't load as much in the blood because they have to load on the low curve.
      • They can't unload as much to the tissues since they will be unloading on the same curve they loaded on.
      • Fish such as carp and bullheads that often live in habitats that may experience such conditions, usually show relatively small Bohr and no Root effects.

    Air Breathing

    Considering the relative commonness of organic material in water and the fact that abundant oxygen is available in the air a few feet away, we would expect that fish often encounter selective pressure for air-breathing. Air breathing has arisen independently a number of times,
      Fish have modified in diverse ways:
    • gills
    • buccal cavity
    • intestine
    • pharyngeal cavity
    • swim bladder
    Circulatory modifications are also necessary if the fish breathes with any structure other than gills. The original plumbing would bring the oxygenated blood to the gills, where the oxygen would diffuse out and be lost.