Acid-Base Balance and Buffer Systems

• Acid-base balance, and physiological buffer systems are key to homeostasis in the body
• Assoc. Prof. Dr. Victor Markus, Near East University Medical Faculty

Normal pH of Body Fluids

• Arterial blood pH is normally 7.4 (7.35-7.45)
• Venous blood and interstitial fluid pH is 7.35 (CO2→H2CO3)
• Intracellular fluid pH is 7.0
• Alkalosis or alkalemia: Arterial blood pH >7.45
• Acidosis or acidemia: Arterial blood pH <7.35
• pH range compatible with life: 6.8-8.0

Acidosis

• A condition in which there is too much acid or too little base
• Frequently results in a decrease in blood pH below 7.35 i.e increase in H+

Alkalosis

• A condition in which there is too much base or too little acid
• Results in an increase in blood pH above 7.45 i.e increase in OH-

Ratio of Bicarbonate to Carbonic Acid

• A pH of 7.4 corresponds to a 20:1 ratio of HCO3- and H2CO3
• HCO3- concentration is 24 mEq/liter.
• H2CO3 concentration is 1.2 mEq/liter.
• Deviations from this ratio lead to acid-base imbalances.
• Acid-base homeostasis is vital for hydrogen ion (H+) concentration for the operation of many cellular enzymes and function of vital organs, such as the brain and the heart.

Acid-Base Regulation

• Buffer systems react very rapidly (less than a second).
• Respiratory responses react rapidly (seconds to minutes).
• Renal responses react slowly (hours to days) but are the most effective.
• Intracellular shifts of ions also regulate acid-base balance.

Physiological Buffer Systems

• Buffering systems provide an immediate response (within seconds) to fluctuations in pH
• Phosphate buffer- intracellular
• Amino acid and protein (hemoglobin, albumin) buffer- both intra- and extracellular
• Bicarbonate buffer- extracellular
• Chemical buffer minimizes the pH changes within solution
• The body uses chemical buffers in the blood to guard against sudden pH changes
• Components include intracellular fluid (ICF) and extracellular fluid (ECF)
  • Intracellular: Phosphate buffer system and protein buffers, including hemoglobin buffer system (RBCs only)
  • Extracellular: Protein buffer systems, including amino acid buffers (all proteins), carbonic acid-bicarbonate buffer system and Plasma protein buffers

Phosphate Buffer System

• Regulates pH within cells and kidney
• Phosphate concentrations are higher intracellularly and within the kidney tubules
• Alternately switches Na+ with H+
• Na2HPO4 + H+ <-> NaH2PO4 + Na+

Protein Buffer System

• Proteins are excellent buffers because they contain both acid (carboxyl) and base (amino) groups that can give or take H+ (amphoteric molecules).
• Proteins are extremely abundant in the cell
  • Hemoglobin is the most important protein buffer.
  • Most powerful buffer system in the body
• 75% of the body's buffer capacity is controlled by proteins.

Hemoglobin Buffer System

• H+ generated at tissue level from the dissociation of H2CO3 binds to Hb (hemoglobin).
• Hemoglobin can accept H+ as it has histidine, which is a basic amino acid.
• H+ bound to Hb does not contribute to the acidity of the blood.
• As H+Hb picks up O2 from the lungs, it releases H+ (Hb has a higher affinity for O2).
• Liberated H+ combines with HCO3-
  • HCO3 + H+ <-> H2CO3 <-> H2O + CO2 (exhaled)

Carbonic Acid-Bicarbonate Buffer System

• Consists of H2CO3 and its salt, sodium bicarbonate (NaHCO3).
• If strong acid is added, hydrogen ions released combine with HCO3- and form carbonic acid (a weak acid) by carbonic anhydrase.
• The pH of the solution decreases slightly.

Limitations of Buffer Systems

• Provide only temporary solutions to acid-base imbalances.
• Do not eliminate H+ ions.
• Buffer molecules are limited.

Respiratory Regulation

• Respiratory control centers and lungs regulate blood pH by adjusting the speed and depth of breathing.
• Hyperventilation increases breathing rate and depth, which decreases blood carbon dioxide, and the blood becomes more basic.
• Hypoventilation decreases breathing rate and depth, which increases blood carbon dioxide, and the blood becomes more acidic.

Respiratory Response

• Neurons constitute the respiratory center in the medulla oblongata and pons.
• Control is accomplished by responding to CO2 and H+ concentrations in the blood.
• Chemosensitive areas of the respiratory center detect blood concentration levels of CO2 and H+.
• Increases in CO2 and H+ stimulate the respiratory center
  • This effect is to raise respiration rates
    • The effect diminishes in 1 - 2 minutes.
• Chemoreceptors are also present in the carotid and aortic arteries, which respond to changes in partial pressures of O2 and CO2 or pH.
• Overall compensatory response:
  • Hyperventilation happens in response to increased CO2 or H+ (low pH)
  • Hypoventilation happens in response to decreased CO2 or H+ (high pH).

Renal Mechanisms of Acid-Base Balance

• Chemical buffers can tie up excess acids or bases, but they cannot eliminate them.
• The lungs can eliminate carbonic acid by eliminating carbon dioxide.
• The kidneys can rid the body of metabolic acids like phosphoric, uric, and lactic acids and ketones, and prevent metabolic acidosis.
• The kidneys are the ultimate acid-base regulatory organs.

Renal Response

• Compensates within 24 hours and is responsible for long-term control.
• Response to acidosis:
  • Retains and generates bicarbonate ions, adding them to blood.
  • Excretes equal amounts of hydrogen ions.
• Response to alkalosis:
  • Eliminates bicarbonate ions.
  • Retains hydrogen ions.

Hyper and Hypokalemia

• Hyperkalemia (elevated serum K+) is associated with acidosis
  • Acidosis is compensated through the shift of H+ ions into cells buffered intracellularly, and K+ ions exit the cell to maintain electroneutrality
• Hypokalemia (low serum K+) is associated with alkalosis
  • Alkalosis is compensated by a shift of H+ ions into the plasma and K+ ions enter into the cell to maintain electroneutrality

Electrolyte Shifts

• Acidosis: H+ buffered intracellularly; hyperkalemia.
• Alkalosis: Tendency to correct alkalosis; hypokalemia.

Deviations From Normal Acid-Base Status

• These fall into four general categories depending on the source and direction of the abnormal change in H+ concentrations
  • Respiratory acidosis: develops when the lungs don't expel CO2 adequately
  • Respiratory alkalosis: caused by hyperventilation that eliminates excessive CO2
  • Metabolic acidosis
  • Metabolic alkalosis

Respiratory Acidosis and Alkalosis

• Respiratory acidosis is the most common cause of acid-base imbalance
  • Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema
• Respiratory alkalosis is a common result of hyperventilation.

Respiratory Acidosis - Causes

• Severe lung diseases prevent the lungs from expelling CO2 adequately.
• Diseases of the nerves or muscles of the chest impair mechanics of breathing.
• Narcotics and strong sleeping medications slow respiration.

Respiratory Alkalosis

• Cause: hyperventilation.
• Usually the only treatment needed is to slow down the rate of breathing.
• Breathing into a paper bag or holding the breath as long as possible may help raise the blood CO2 content as the person breathes CO2 back in after breathing it out.

Metabolic Acidosis

• Occurs when there is a decrease in the normal 20:1 ratio.
• Any acid-base imbalance not attributable to CO2 is classified as metabolic.
  • Metabolic overproduction of acids, such as in diabetic ketoacidosis.
  • Loss of bases.
  • Excessive H+ or decreased HCO3-.
• As the blood pH drops, breathing becomes deeper and faster as the body attempts to rid the blood of excess acid by decreasing the amount of CO2.

Metabolic Alkalosis

• Develops when the body loses too much acid or gains too much base.
• Elevation of pH due to an increased 20:1 ratio.
  • Caused by: An increase of bicarbonate or a decrease in hydrogen ions.
• pH elevation has a non-respiratory origin.
• Can be the result of:
  • Ingestion of alkaline substances like excessive antacids and cleaning solutions.
  • Vomiting, which causes a loss of HCl.

Alkalosis

• Anxiety, overdose of certain drugs, high altitudes, prolonged vomiting, ingestion of excessive alkaline drugs, excess aldosterone can all lead to alkalosis.
• This can lead to either respiratory or metabolic alkalosis

Acidosis

• Decreased removal of CO2 from lungs, kidney failure to excrete acids, metabolic acid production, production of keto acids, absorption of metabolic acids from the GI tract, prolonged diarrhea all lead to acidosis
• This can lead to either respiratory or metabolic acidosis

Arterial Blood Gas (ABG)

• Components: pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
• Desired ranges:
  • pH: 7.35 - 7.45
  • PaCO2: 35 - 45 mmHg
  • PaO2: 80 - 100 mmHg
  • HCO3: 21 - 27
  • O2sat: 95 - 100%
  • Base excess: +/-2 mEq/L

Base Excess

• Base excess beyond the reference range indicates a metabolic problem.
  • Excessively high base excess means metabolic alkalosis.
  • Excessively low base excess means metabolic acidosis.
• Comparing the base excess with the reference range assists to determine whether an acid-base disturbance is caused by a respiratory or metabolic problem.

Interpreting ABGs

• Review the pH to determine if it is the primary problem acidosis (low pH) or alkalosis (high pH)
• Check the CO2 respiratory indicator to see if it less than 35 (alkalosis) or more than 45 (acidosis)
• Check the HCO3 to see if it less than 22 (acidosis) or more than 26 (alkalosis)
• This determines the which is the primary disorder (Resp. or Metabolic)
  • If the pH is low acidosis look to see if CO2 or HCO3. if acidosis is the primary problem
  • If the pH is high alkalosis look to see if CO2 or HCO3 if alkalosis is primary problem
  • The one that matches the pH (acidosis or alkalosis), is the primary disorder.
• pH 7.79, CO2 24, HCO3- 21. HCO3 is acidic. Respiratory Alkalosis.
• pH 7.17, CO2 35, HOs- 12. pH HCO3 is acidic. Metabolic Acidosis.