Pisces: Respiration | Zoology Optional Notes for UPSC PDF Download

Gas Exchange and Respiration

Biochemical Definition of Respiration: Cellular Respiration

• Cellular respiration is a metabolic process through which an organism obtains energy by reacting oxygen with glucose, resulting in the production of water, carbon dioxide, and ATP (energy).
• Physiologic respiration is distinct, involving the bulk flow and transport of metabolites between the organism and the external environment.
• Although separate, physiologic respiration is essential to sustain cellular respiration and life in animals.

Respiratory Organs in Fishes

• Fish primarily respire through gills, which are tissues with feathery structures called gill filaments.
• Gill filaments provide a large surface area for gas exchange, crucial in aquatic organisms due to the limited dissolved oxygen in water.
• Filaments in fish gills are organized in rows within gill arches, each comprising lamellae, discs supplied with capillaries.
• Blood circulates through these capillaries, facilitating the exchange of gases.
• Despite occupying a small body section, fish gills efficiently provide extensive respiratory surfaces for gas exchange.

Structure of Gill in Bony Fish

• Gills are located in gill chambers on either side of a fish's head.
• Elasmobranchs (Sharks and Rays) have gill slits, while bony fishes have an operculum covering a large opening to the water.
• Gill arches bear two rows of gill filaments forming a complete gill or holobranch, with each row called a hemibranch.
• Most teleosts have four holobranches (8 hemibranches), while elasmobranchs have five holobranches (10 hemibranches) on each side.
• Gill arches also have gill rakers on the inner side, covered by an epithelial layer with taste buds and mucus-secreting cells.
• Gill filaments, arranged in a V-shaped manner, bear lamellae, flat leaf-like structures increasing the surface area for gas exchange.
• Lamellae, rich in blood capillaries, ensure the barrier between capillaries and water is minimal, enhancing the efficiency of gas exchange.

Gill Area and Diffusion Capacity

• The gill area in fish is influenced by the number and size of gill filaments and lamellae.
• Fast-swimming and active fish exhibit more gill area and a higher number of gill lamellae/mm compared to sedentary species.
• Air-breathing fish, relying on accessory respiratory organs, have a gill area approximately half that of species dependent on aquatic respiration.
• The total gill area increases with the fish's body weight during growth.
• The efficiency of gills as respiratory organs is determined by the oxygen diffusion capacity, inversely proportional to the blood-water barrier's thickness (diffusion distance).
• The lamellar epithelium, basement membrane, and pillar cells form this barrier, with pillar cells preventing lamellar collapse.

Mechanism of Respiration or Ventilation of Gills

• Respiration involves maintaining a continuous water current through the buccopharyngeal cavity.
• Water is drawn into the buccal cavity and expelled through the external branchial aperture.
• Fish use two methods for ensuring a constant supply of new water:
  1. Double Pump System:
     • Unidirectional system with water moving across gills and out operculum.
     • Coordination of buccal and opercular pumps in most teleosts.
     • Phase I involves expanding buccal and opercular cavities with closed opercula.
     • Phase II sees the closure of the mouth and opening of opercula, forcing water across gills.

1. Ram Ventilation:
   • Observed in fast-moving species like tuna, where gill covers are immobile.
   • Requires high water flow over gills through mantle cavity intake and jet propulsion expulsion.
   • Fast-swimming fish with larger gill areas meet increased oxygen demands for their activity.
   • Mouth and opercular apertures are left wide open during swimming for continuous water flow, known as 'ram ventilation.'
   • Rhythmic operculum movement is controlled by respiratory centers in the medulla oblongata area.

Counter-Current Flow Mechanism

• Counter-current flow involves the opposite direction of water flow over the gill and blood flow within the gill.
• This mechanism maximizes oxygen removal efficiency from water, utilized by fish gills.
• During counter-current flow, fluids with different concentrations (blood and water) move in opposite directions, separated by thin membranes.
• This promotes the diffusion of substances, like oxygen, down their concentration gradients.
• Fish gills employing counter-current flow can deplete up to 80-90% of the initial oxygen content in the water.

Circulation of Blood Through Gill Filament and Lamellae

• Teleost gills typically feature one afferent unit bringing deoxygenated blood to the gill and one efferent unit collecting oxygenated blood from the gill.

Transportation of Respiratory Gases in the Blood

• The gas exchange system's primary function is to meet cell metabolic requirements for O₂ and remove CO₂ produced by cellular metabolism.
• Blood carries O₂ to tissues and removes CO₂ from respiring tissues, transporting them to the gas exchange surface.
• Blood's adaptation for gas transport includes the presence of the respiratory pigment hemoglobin (Hb) within red blood cells (RBCs).
• Hemoglobin significantly increases blood's O₂ carrying capacity, up to 20 times that of physically dissolved O₂.
• Hemoglobin's H+ binding capacity aids in the transportation of CO₂.

Hemoglobin

• Hemoglobin is a tetrameric molecule with four polypeptide chains and four heme groups in most teleost fishes.
• Agnathans (lampreys & hagfishes) possess monomeric hemoglobin.
• Antarctic fishes (ice fish) lack hemoglobin.