Introduction

• The regulation of acid-base balance is a very important component of homeostasis.
• Normal metabolic processes produce vast quantities of carbon dioxide (CO₂) (volatile acid) and hydrogen ions (H⁺) (fixed acid) every day.
• CO₂ is combined with water to form carbonic acid (H₂CO₃) which dissociates to generate H⁺ and HCO₃^-. This is described by the carbonic anhydrase equation which is crucial to the understanding of acid-base physiology:
  • H⁺ + HCO3^- ⇆ H₂CO₃ ⇆ CO₂ + H₂O
• What is measured in acid-base analysis is pH. pH can be described by the folliwing 2 equations:
  • pH = -log10[H⁺]
  • pH = 6.1 + log([HCO₃^-]/0.03 x PCO₂)
• As you can see, pH is determined by the H⁺ in the carbonic anhydrase equation.
• And, pH can also be determined by HCO₃ (which is excreted and absorbed by the gastrointestinal tract and kidneys); and CO₂ (which is regulated by the respiratory tract).
• Acid-base regulation also involves integrated normal activity of the lungs, kidney and liver. The lungs excrete CO₂, the kidneys excrete H⁺ as part of the ammonium ion (NH4⁺), and the liver produces H⁺ from metabolism of protein.
• The body uses 3 principle ways of buffering changes in [H⁺], thereby keeping pH as near  normal as possible:
  • Chemical buffers such as bicarbonate, phosphate and proteins act within seconds to resist changes in pH.
  • Respiratory system → responds within minutes to resist changes in [H⁺] by altering the partial pressure of carbon dioxide and eliminating excess CO₂ (volatile acid), thereby excreting excess H+ ions.
  • The kidney excretes excess H⁺ in the urine and also synthesizes new HCO₃^- over a period of hours or days.

Clinical aspects of acid-base balance

Interpreting blood gas analysis

Primary acid-base disturbances

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Hopper K & Haskins S C (2008) A case-based review of a simplified quantitative approach to acid-base analysis. JVECC 18 (5), 467-476 VetMedResource.

Other sources of information

• Silverstein D C & Hopper K (2015) Small Animal Critical Care Medicine. 2nd edn. Elsevier.
• Di Bartola S P (2011) Fluid, Electrolyte, and Acid-Base Disorders. 4th edn. Elsevier.
• Rose B D & Post T (2001) Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th edn. McGraw-Hill.