Breathing

Problem

Oxygen is the terminal proton acceptor for almost all sustained metabolism that powers the majority of fish activities, directly or through a recovery period. Oxygen is limiting in the aquatic environment because water-oxygen solubility is low. In addition, aerobic metabolism produces CO2 which must be transported and excreted.

Transfer of oxygen from the water to the tissues involves four steps:

    1. gill ventilation,
    2. branchial diffusion,
    3. blood-oxygen transport,
    4. tissue diffusion.

Basic models are modified Fick Equations for diffusion and for convective transport:

Diffusion: MO2 = k.S.D pO2/D

Convection: MO2 = VG.D [O2]water = Q.D [O2]blood

These show the design parameters involved in meeting the metabolic rate, MO2 .

Water Breathing

Gill Ventilation

Basic mechanism for water irrigation of the gills is essentially the same in all species (except for tidal ventilation in feeding lampreys). Details vary greatly among species.

Ventilatory flow is generated by the combined action of a buccal pressure pump and an opercular suction pump operating out-of-phase. Interspecific variation is associated with modes of living.

number of inhalant and exhalent openings

Spiracle in elasmobranchs for ventilation while on the substratum to avoid debris entering the gill cavity.

Reduced exhalent openings in benthic teleosts (e.g. flatfish).

contributions of buccal and opercular pumps

Pressure pump in active swimmers and large or exclusive ram ventilation component.

Enhanced suction pump with well developed branchiostegal apparatus and intermittent closure to prevent backflow in benthic fish. Presumably keeps branchial cavities free of debris.

fusion of filaments and lamellae

Fusion of adjacent secondary lamellae at their tips to eliminate dead space that would arise from separation of the filament tips under high dynamic pressures at high swimming speeds.

Air breathers.

Ventilatory flow can be increased 10 to 15-fold during activity by active fish. Ventilation costs are believed to be of the order of 10% of routine metabolism, but 70% to total at metabolic rates three times routine levels. With ram ventilation, costs of breathing remains roughly independent of exercise level.

Branchial Diffusion

Gills have a fractal structure of branching plates that results in package of a large area within a limited space. Gas exchange occurs at the secondary lamellae. Water and blood flow in opposite directions (counter-current flow). Functional area, that area available for gas exchange, modulated according to metabolic needs and to minimize ion and water exchanges.

Air-Breathing Fishes

Most air-breathing fish are found in tropical swamps where high water temperature, stagnant conditions and abundant micro-organisms combine to make the water very acid, with high pCO2 and low pO2. Amia, common in temperate swamps, is an exception, but is indicative of low water quality. Most fish living in high acid-low oxygen waters utilize air for oxygen, using accessory respiratory organs. These are all hollow spaces with richly vascularized walls which can be ventilated periodically. These include swimbladders, lungs, branchial cavities and the gut.

Cutaneous Gas Exchange

Cutaneous gas exchange remains important for most fish, at least at some point during their life history. Eggs and larvae lack functional gills. Many adults also use cutaneous gas exchange.