Acids, Bases, and pH Balance

Vital Roles of Maintaining H

• Enzyme activities are affected
• Movement across cell membranes is affected
• Muscle contraction is affected
• Nerve conduction is affected
• Blood flow is affected

Expressing H Ion Concentration

• Directly as [H+]
• Indirectly as pH
• pH equals log 1 divided by H concentration.
• pH and [H+] are inversely related

Acids and Bases in the Body

• Acid is a molecule that liberates H+ ions.
• A strong acid, like HCl, dissociates into H+ and Cl-.
• A weak acid, like H2CO3, reversibly dissociates into H+ and HCO3-.
• A base is a molecule that accepts H+ ions.
• Strong bases tightly bind to H+ ions.
• Weak bases weakly bind to H+ ions.

Sources of Acids

• Volatile acids, such as H2CO3, result from complete oxidation.
• Fixed acids, such as ketone bodies and lactic acid, come from incomplete oxidation.

Sources of Bases

• Vegetables
• Metabolism of amino acids like glutamate and aspartate

Metabolic Reactions

• Metabolic reactions produce more acids in the body

Compatible Range of H

• The compatible range of [H+] for life is 20-126 nmol/L
• Normally, [H+] is kept within a tight range; = 40 nmol/L (pH 7.4).

Lines of Defense Against pH Changes

• First line of defense involves buffers, acting within seconds to remove or release H+
• Second line is the respiratory system, acting within 1–3 minutes to eliminate volatile acid (CO2 as H2CO3)
• The third line is the kidneys, acting within hours to eliminate fixed acids and H+

Buffers

• Chemical solutions act to decrease changes in pH

Buffer Components

• Weak acid
• Conjugate base

PK of Buffer

• pH at which the buffer is maximally effective
• Ideal buffer has a pK nearer to the pH of body fluids

Buffer Systems in the Body

• Bicarbonate, phosphate, protein, and hemoglobin act as buffer systems
• The carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3) system is regulated easily by respiration to control H2CO3
• This is also regulated by the kidneys to control NaHCO3
• The sodium hydrogen phosphate (NaH2PO4) and sodium phosphate (Na2HPO4) buffer system is important for urinary and intracellular buffering with a PK of 6.8, near plasma pH
• Protein buffering is the most plentiful
• Oxyhemoglobin and reduced hemoglobin is important for O2 and CO2 transport
• Bicarbonate has disadvantages such as, PK is 6.1, away from plasma pH
• Phosphate has a disadvantage as little amount

How Buffers Work

• A strong acid such as HCL: HCL + NaHCO3 yields NaCl + H2CO3
• A strong base such as sodium hydroxide (NaOH): NaOH + H2CO3 yields H2O + NaHCO3

Bicarbonate Buffer

• The two components are easily controlled

Main Buffers

• Interstitial fluid is mainly buffered by bicarbonate
• Blood uses bicarbonate then hemoglobin
• Intra-cellular fluid uses phosphate
• Urine uses phosphate

Role of the Respiratory System

• Buffering power of the respiratory system is approximately two times that of chemical buffers
• Effectiveness of the respiratory is approximately 50-75% to return pH to normal
• Increased PCO2 stimulates respiration, which decreases PCO2 back to normal
• Decreased PCO2 inhibits respiration, which increases PCO2 back to normal

Role of the Kidney

• The control of acid-base balance: excretion of acidic urine (eliminating acids from ECF)
• The control of acid-base balance: excretion of basic urine (eliminating bases from ECF)
• Kidney can deal with fixed acids.

Role of the Liver

• The liver is a metabolically active organ that produces or consumes hydrogen ions
• Roles of the liver: carbon dioxide production, metabolism of organic acids, and ammonia, and production of plasma proteins, especially albumin

Acid-Base Disorders

• Described by the Hendresson-Haselbalch Equation: pH = pK + log (Base/Acid)

Acidosis

• Respiratory acidosis involves increased H+ and decreased pH, and increased PCO2
• Causes of respiratory acidosis :
  • breathing excess CO2
  • obstruction of airway
  • respiratory center depression
  • increased CO2 production
• Effects of respiratory acidosis: depression in CNS, disorientation, coma, and death
• Metabolic acidosis involves increased H+ and decreased pH, and decreased HCO3
• Causes of metabolic acidosis:
• excess consumption of HCO3 for example: ingestion of excess protein and acidifying salts, renal failure, and circulatory failure
• excess loss of HCO3, such as diarrhea (loss in stool) and loss in urine
• The most common disturbance is metabolic acidosis, seen as DKA in diabetes mellitus

Alkalosis

• Respiratory alkalosis involves decreased H+ and increased pH, and decreased PCO2
• Causes of respiratory alkalosis:
• prolonged hyperventilation
• high altitude
• voluntary
• meningitis
• salicylate poisoning
• Metabolic alkalosis involves decreased H+ and increased pH, and increased HCO3
• Causes of metabolic alkalosis :
• ingestion of excess NaHCO3
• ingestion of excess vegetables
• prolonged vomiting
• gastric secretion
• Effects of alkalosis: over excitability of the central nervous system (CNS), muscle spasm or tetany, convulsion, and death

Normal Values

• Normal pH: 7.4
• Normal HCO3: 24
• Normal CO2: 40