ACID-BASE BALANCE IN FISHES

FEROSEKHAN S FISHERIES SCIENCE CIFE,MUMBAI

Introduction

• Acid –base balance involves the maintenance of internal pH.

• Regulation of acid –base homeostasis is of critical importance to fishes.

• In order to achieve homeostasis, there must be a balance between the intake or production of hydrogen ions and the net removal of hydrogen ions from the body.

CONT….

• A delicate balance of fluids, electrolytes, and acids and bases is required to maintain good health.

• This balance is called Homeostasis

pH Review

• pH = - log [H+]

• H+ is really a proton

• Range is from 0 - 14• If [H+] is high, the solution is acidic; pH < 7

• If [H+] is low, the solution is basic or alkaline ; pH > 7

• Acids are H+ donors.

• Bases are H+ acceptors, or give up OH- in solution.

• Acids and bases can be:

– Strong – dissociate completely in solution

• HCl, NaOH

– Weak – dissociate only partially in solution

• Lactic acid, carbonic acid

Body Fluids

• Intracellular fluid (ICF)– found within the cells of the body– constitutes 2/3 of total body fluid – major cation is potassium

• Extracellular fluid (ECF)– found outside the cells– accounts of 1/3 of total body fluid– major cation is sodium

The Body and pH

• Homeostasis of pH is tightly controlled

• Intracellular fluid = 7.2 – 7.5

• Blood pH = 7.7 – 8.0• < 7.0 or > 8.5 death occurs

• Acidosis (acidemia) below 7.4

• Alkalosis (alkalemia) above 8.3

7.7 8.0

Normal blood pHAlkalosisAcidosis

8.2 8.47.47.27.0

pH of Fish Blood

Small changes in pH can produce major disturbances

• Most enzymes function only with narrow pH ranges

• Acid-base balance can also affect electrolytes (Na+, K+, Cl-)

• Can also affect hormones

The body produces more acids than bases

• Acids take in with foods

• Acids produced by metabolism of lipids and proteins

• Cellular metabolism produces CO2.

• CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

How the Body defends against fluctuations in pH

Three Systems in the body:

1.Buffers in the blood

2.Respiration through the gills

3.Excretion by the kidneys

Buffers in the Blood

• Buffers are substances that neutralize acids or bases

• Bicarbonate which is a base and carbonic acid in the body fluids protect the body against changes in acidity

• These buffer systems serve as a first line of defense against changes in the acid-base balance

Respiration through the gills

• Carbon Dioxide which is formed during cellular metabolism forms carbonic acid in the blood decreasing the pH

• When the pH drops respiration rate increases this hyperventilation increases the amount of CO2 exhaled thereby lowering the carbonic acid concentration and restoring homeostasis

Excretion by the Kidneys

• The kidneys play the primary role in maintaining long term control of Acid-Base balance

• The kidney does this by selecting which ions to retain and which to excrete

• The kidneys adjust the body’s Acid-Base balance

Control of Acids

1. Buffer systems:

Take up H+ or release H+ as conditions change

Buffer pairs – weak acid and a base

Results in a much smaller pH change

Cont…

1. Extracellular buffer

- Bicarbonate , Ammonia

2.Intracellular buffer

- Protein, Phosphate

Bicarbonate buffer

• Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3)

• Maintain a 20:1 ratio : HCO3- : H2CO3

HCl + NaHCO3 ↔ H2CO3 + NaCl

NaOH + H2CO3 ↔ NaHCO3 + H2O

Phosphate buffer

• Major intracellular buffer

• H+ + HPO42- ↔ H2PO4-

• OH- + H2PO4- ↔ H2O + H2PO4

2-

Protein Buffers

• Non-bicarbonate buffers (Nbbs) – plasma proteins and hemoglobin.

• Includes hemoglobin, work in blood

• Carboxyl group gives up H+

• Amino Group accepts H+

2. Respiratory mechanisms

• Exhalation of carbon dioxide

• CO2 solubility in water is 25 times higher than that of oxygen ,so CO2 is easily lost to the envt.

• CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

• Body pH can be adjusted by changing rate and depth of breathing.

3. Kidney excretion

• Can eliminate large amounts of acid

• Can also excrete base

• Can conserve and produce bicarb ions• Most effective regulator of pH

• If kidneys fail, pH balance fails

Rates of correction

• Buffers function almost instantaneously

• Respiratory mechanisms take several minutes to hours

• Renal mechanisms may take several hours to days

Acid-Base Imbalances

• pH< 7.4acidosis

• pH > 8.3 alkalosis

• The body response to acid-base imbalance is called compensation

• Partial compensation if range is still outside norms.

Compensation

• If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation.

• If problem is respiratory, renal mechanisms can bring about metabolic compensation.

7–8.0

There are 4 Types of Acid-base Imbalances

1.Respiratory Alkalosis

2.Respiratory Acidosis

3.Metabolic Alkalosis

4.Metabolic Acidosis

Respiratory Alkalosis

• Is a decrease in CO2, decrease in H+ (Hydrogen ions) and in increase in pH

• This condition can be caused by continuous fish movement or anxiety reaction

• The body compensates by the gills slowing the respiration rate and kidneys excreting more bicarbonate

Respiratory Acidosis

• Is an increase in CO2, an increase in H+ and a decrease in pH

• This condition can be caused by , restrictive or obstructive gill diseases

• The body compensates by the gills increasing respiration rate and the kidneys by conserving bicarbonate ions and increasing renal net acid excretions

Metabolic Alkalosis

• Is a decrease in CO2, a decrease in H+ and an increase in pH

• This condition can be caused by , increased ingestion of alkali

• The body compensates by the gills creating a slow respiration rate and the kidneys excreting more bicarbonate

Metabolic Acidosis

• Is an increase in CO2, and increase in H+ and a decrease in pH

• This condition can be caused by alkaline loss, excess acid production or ingestion

• The body compensates by the gills increasing respiration rate and the kidneys increasing renal net acid excretion

Conclusion

• Fishes are capable of regulating their internal pH using both internal buffering and transepithelial exchanges across the gills.

• Though their Pco2 and HCO3- are well

below those found in mammals, they can tolerate a variety of acid-base challenges.

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