Summary: Acid Base Balance



Summary: Acid Base Balance

I. Physical chemical principles

Neutral pH. The pH of water decreases with increasing temperature.

The pH of blood is also temperature dependent but is always more alkaline.

CO2 + H2O ↔ H2CO3↔ HCO3− + H+

If concentration of any one of the three reactants/products is changed the equilibrium shifts so the concentrations of the other two reactants change in a compensatory way according to the dissociation constant K.

For example if H+ increases, equilibrium shifts to left: this reduces elevation of H+ and decreases HCO3−; resulting pH is partially restored and bicarbonate reserve is partially used up.

In fish adjustment of blood pH in response to changing temperature is accomplished by adjustment of blood bicarbonate concentrations.

II. Conditions that disturb acid base balance

Metabolic acidosis

Production of large amounts of lactic acid

CO2 + H2O ↔ H2CO3↔ HCO3− + H+

Compensation: Active H+ excretion and/or HCO3− uptake

Respiratory acidosis

High production of respiratory CO2

CO2 + H2O ↔ H2CO3↔ HCO3− + H+

Compensation: rapidly compensated by CO2 diffusion from blood to water.

Environmental hypercapnia

CO2 + H2O ↔ H2CO3↔ HCO3− + H+

Because environmental P C02 is elevated, this condition cannot be corrected by CO2 diffusion from blood to water.

Compensation: Active uptake of HCO3− (driving equilibrium back to the left) and /or active excretion of H+ is required. Because the increase in bicarbonate reserves in limited in fish (reserves are approximately one fourth that of air-breathers), bicarbonate buffering is also lower, and small changes in environmental PCO2 can have relatively large effects on the blood pH of fish.

Environmental acidosis:

Acidification of water from natural or human changes.

CO2 + H2O ↔ H2CO3↔ HCO3− + H+

Compensation:

Non-bicarbonate buffers (Nbbs) are important for buffering of pH disturbances, providing about half of the buffering power of the blood and almost all of the buffering power of the intracellular fluid. Because of the relatively large mass of white muscle intracellular fluid, it provides approximately 80% of the total buffering capacity of the body; blood provides about 5%.

Nbbs in the intracellular fluid include organic and inorganic phosphates, amino acid residues, and proteins. Nbbs in the blood are mainly hemoglobin in the red blood cells and plasma proteins.

III. Longer term compensation for acid base disturbance

Although buffering by bicarbonate and Nbbs reduces the magnitude of acid-base changes, balance is ultimately maintained by uptake or elimination of H+ and /or HCO3− . Most of this exchange occurs across the gill epithelium; the role of urinary excretion is relatively small in fish because urine flow is small in comparison to gill water flow.

A number of lines of experimental evidence indicate that electroneutral exchanges of Na+ for H+ and Cl− for HCO3− occur across the gill epithelium. The exact mechanisms are still speculative: H+ excretion may involve 1) apical (water side) Na+ /H+ antiporter (direct, “classical” exchange), or the exchange may be 2) indirect, with H+ excretion driven by an H+-ATPase, accompanied by inward diffusion of Na+ down the resulting electrochemical gradient through apical Na+ channels.

These Na+ for H− exchanges appear to occur across the respiratory epithelium on the secondary lamellae (pavement cells) rather than involving the chloride cells, which are located predominantly on the gill filament at the base of the secondary lamellae.

The chloride cell is, however, the site of Cl−/HCO3−exchange. Changes in rates of HCO3− exchange may be accompanied by changes in gill morphology, so that more or less of the apical side of the chloride cell is exposed to the water.

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