Biochemistry of acidobasic regulations Alice Skoumalová

Body water compartments:

Derived values: AG (anion gap) (Na + + K + ) - (Cl - + HCO 3 - ) mmol/l SID (strong ion difference) ( Na + + K + ) – Cl - 42 mmol/l Diagram showing chemical constituents of the three fluid compartmens:

pH lemon 2,3 orange 3,7 sceletal muscle c. 6,9 prostatic c. 4,5 erythrocytes 7,3 trombocytes 7,0 osteoblasts 8,5 blood 7,36 – 7,44 gastric juice 1,2 – 3 pancreat. juice 7,5 – 8 bile 6,9 – 7,7 urine 4,8 - 8

Acids in the blood:

Buffer The Henderson-Hasselbalch equation Titration curve for acetic acid Equivalents of OH - added

Body buffers: BloodISFICF HCO 3 - /H 2 CO 3 + CO 2 64% HCO 3 - hemoglobin 29% - - proteins 6% proteins HPO 4 2- /H 2 PO 4 - 1% phosphates

Plasma:  mixed buffer system BufferpKconcentration HCO 3 /CO 2 6,124 mmol/l HPO 4 2- /H 2 PO 4 - 6,81 mmol/l Proteins histidine N-term. amino groups ,6 – 7,0 7,6 – 8,4 15 mmol/l Major physiological buffers:

The bicarbonate buffer system: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - erythrocytes (carbonic anhydrase) in the body – an open system The Henderson-Hasselbalch equation for the bicarbonate buffer system: The pK a of carbonic acid – 3,8 But: carbonic acid is replenished from CO 2 in body fluids and air (the concentration of dissolved CO 2 is 400 times greater than that of carbonic acid) dissolved CO 2 is in equilibrium with the CO 2 in the alveoli (the availability of CO 2 - breathing) the pK a combines the hydratation constant of CO 2 with the chemical pKa - a modified version of the H.-H. equation: s - a conversion factor (0,23 in kPa 0,03 in Torr)

Phosphate buffer:  intracellular fluids (0,1M) Protein buffer:  intracellular fluids (and also extracellular) Hemoglobin buffer:  in erythrocytes  + Bohr effect

Co-operation of the body buffers: Buffers in an organism

Carbon dioxide transport: CO 2 in the blood: 1. as HCO 3 - (ionization of H 2 CO 3 )75-85 % 2. as carbamino groups (CO 2 reacts with amino groups of proteins)10-15 % 3. dissolved CO % Partial pressures air-inspiration air-expiration arterial blood venous blood pO 2 (kPa) 21 15,3 12 – 13,3 4,6 - 6 pCO 2 (kPa) 0,03 4,4 4,6 – 6 5,3 – 6,6 CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - erythrocytes (carbonate dehydratase)

O 2 and CO 2 transport: Bohr effect (the increase in acidity of hemoglobin as it binds O 2, releases H + ) Isohydric carriage of CO 2 (Hb‘s ability to take up H + with no change in pH through Bohr effect) Chloride shift (the exchange of Cl - and HCO 3 - between the plasma and the erythrocyte)

Structure of a nephron: Urine pH 4,8 – 8 (most acids must be in some form other than H + ) 60 mmol H + / day Urine buffers: phoshate NH 3 The physiological levels of the metab. acids: lactate – 0,6-2,4 mmol/l ketone bodies – 3-20 mg/l (0,2mmol/l)

Possibilities of H + excretion: 1. Reaction with HCO 3 - (reabsorption of NaHCO 3 ) 2. Reaction with HPO 4 2- (titratable acidity of the urine) 3. Reaction with NH 3 The kidney in acid-base balance:

The liver in acid-base balance: In acidosis: induction of the glutamine synthesis and renal glutaminase (increased excretion of NH 4 + ) In alkalosis: induction of the urea synthesis, excretion of HCO 3 -

The major indicators of acid-base imbalance (arterial blood): Measured: pH = 7,4 ± 0,04 [H + ] = 40 nmol/l pCO 2 = 5,3 ± 0,5 kPa = 40 torr = 1,2 mmol/l Hb, pO 2 Calculated: [HCO 3 - ] = 24 ± 3 mmol/l BE (base excess) = 0 ± 2,5 mmol/l (the amount of acid that would have to be added to the blood to titrate it to pH 7,4 at a pCO 2 of 5,3 kPa at 37 °C) NBB (buffer base) (the concentration of all bases in the blood at the standard conditions) plasma 42 ± 3 mmol/l blood 48 ± 3 mmol/l AG (anion gap) = mmol/l AG = [Na + ] + [K + ] - [Cl - ] - [HCO 3 - ]

acute stage x compensated four main disorders x mixed Classification of the acid-base disorders: acidemia x acidosis alkalemia x alkalosis Acidosis: a process leading to the accumulation of H + in the body Alkalosis: a process leading to a decrease in H + concentration in the body Two components of acid-base balance: respiratory, metabolic

Correction of the acid-base disorders: Buffer reactions Compensations -processes in which one system compensates the alteration of the other one Corrections The maintenance of pH:

Diagram of Henderson-Hasselbalch equation showing compensations for acid-base disorders:

Metabolic acidosis (MAc): 1. Increased production of H + :-lactasidosis (hypoxia, intensive muscular work, ethanol) -ketoacidosis (starvation, diabetes) -acid retention (renal failure) 2. Exogenous intake of H + :- methanol, ethylene glycol intoxication, salicylate poisoning 3. Loss of HCO 3 - :-diarrhoea, burns, renal tubular disorders, diuretics 4. Relative dilution of HCO 3 - :-excessive infusion of isotonic solutions !

physiologicalacutecompensation -lungs -hyperventilation [HCO 3 - ]24 mmol/l↓ pCO 2 5,3 kPaN↓ [HCO 3 - ]/[H 2 CO 3 +CO 2 ]20 : 1< 20 : 1≤ 20 : 1 pH7,34 – 7,44< 7,34≤ 7,4

Metabolic alkalosis 1. Loss of H + : - vomiting, gastric lavage 2. Input of HCO 3 - : - bicarbonate overdosing 3. Loss of Cl - and K + : - diuretics

physiologicalacutecompensation -lungs -hypoventilation [HCO 3 - ]24 mmol/l pCO 2 5,3 kPaN [HCO 3 - ]/[H 2 CO 3 +CO 2 ]20 : 1> 20 : 1≥ 20 : 1 pH7,34 – 7,44> 7,44≥ 7,4

Respiratory acidosis Hypoventilation: - depression of the respiratory center (opiates, sedatives, narcotics, CO 2 ) - failures -ventilation, diffusion, perfusion (respiratory diseases) -gass transport (anemia, circulatory failure, CO intoxication) -gass exchange between the blood and tissues (cyanide intoxication) -neuromuscular junction (pharmaceuticals, nikotine, botulin intoxication) -neural transmission (spinal cord injuries)

physiologicalacutecompensation -kidney - excretion of H 2 PO 4 - and NH 4 + - resorption of HCO 3 - [HCO 3 - ]24 mmol/lN,  pCO 2 5,3 kPa [HCO 3 - ]/[H 2 CO 3 +CO 2 ]20 : 1< 20 : 1≤ 20 : 1 pH7,34 – 7,44< 7,34≤ 7,4

Respiratory alkalosis Hyperventilation: - mechanical ventilation - respiratory center stimulation: from CNS (hysteria, anxiety, infection), drugs (salicylates), from thermoregulation center (fever, physical effort)

physiologicalacutecompensation -kidney - excretion of HCO 3 - [HCO 3 - ]24 mmol/lN, ↓↓ pCO 2 5,3 kPa↓ [HCO 3 - ]/[H 2 CO 3 +CO 2 ]20 : 1> 20 : 1≥ 20 : 1 pH7,34 – 7,44> 7,44≥ 7,4

Mixed acid-base disorders 1. Antagonistic – metabolic acidosis + metabolic alkalosis  acid-base indicators are often physiological (hypochloremia discovers MAlk) 2. Synergic – e.g. metabolic acidosis + respiratory acidosis Diagnosis: electrolytes, proteins, lactate, calculation from the iontogram, symptoms Study material: Liebrman and Marks, Mark‘s Basic Medical Biochemistry a Clinical Approach, 2009