Acid-base balance

From Iusmphysiology

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Revision as of 14:48, 19 April 2011

• started here on 03/30/11.

Contents 1 Acid-base balance 1.1 Objectives 1.2 Normal pH in arterial blood 1.3 Threats to pH of extracellular fluid

Acid-base balance

Objectives

• Define the following: acid, base, buffer, pH. Give the normal range of arterial blood pH and the limits compatible with life. Explain why constancy of pH is important.
• State the isohydric principle. List the important chemical buffers present in extracellular fluid, intracellular fluid, and bone.
• Write the Henderson-Hasselbalch equation for the bicarbonate/CO2 system. Write Henderson's equation for calculating [HCO3-] from [H+] and PCO2 measurements.
• Explain why the bicarbonate/CO2 system is so important.
• List the four simple acid-base disturbances. Describe for each: 1) the primary defect, 2) changes in arterial blood chemistry (pH, PCO2, and plasma [HCO3-]), 3) some common causes, 4) chemical buffering processes, and 5) respiratory and renal compensations.
• Given plasma electrolyte concentrations, calculate and interpret the anion gap.
• Given values for arterial blood pH, plasma [HCO3-], and PCO2 (or any two of the three), be able to identify the type of acid-base disturbance present.

Normal pH in arterial blood

• A normal pH is 7.4 (7.38-7.42), at which point the concentration of H+ is 40 (38 - 42) nmol / liter.
  • Survivable pH is 7.0-7.6 which is considered acidosis and alkalosis, respectively.
  • Note that this is a four fold change in H+ concentration.
• Death by change in pH occurs as a result of the changes in intracellular proteins upon change in pH.

Threats to pH of extracellular fluid

• There are two major forces that affect extracellular pH: oxidative phosphorylation and protein metabolism.
  • Both of these processes produce sources of acid: oxidative phosphorylation produces CO2 and protein metabolism produces H2SO4 and HCl.

• Oxidative phosphorylation as a source of extracellular acid:
  • Recall that oxphos dumps electrons onto oxygen and secretes this waste as CO2.
  • Recall that CO2--via carbonic anhydrase--affects the extracellular levels of H+ and HCO3-.
  • That is, as CO2 rises, more H2CO3 and H+ are found in the blood; the pH decrease.
  • We say CO2 is a volitile acid because it indirectly affects H+ levels.
  • 13-20 moles of CO2 are produced each day by oxphos.

• Protein metabolism as a source of extracellular acid:
  • Recall that proteins are digested for energy and as a source of amino acids.
  • Metabolism requires the removal and storage of the many hydrogens on the proteins and thus can generate H+.
    • Met or Cys metabolism generates H2SO4.
    • Lys or Arg metabolism generates HCl.
  • We say protein metabolism generates fixed acids because it directly affects H+ levels.
  • 40-60 moles of fixed acid are produced each day by protein metabolism.