Acid-Base Balance Basics

Slide 1: 

- C.S.N.Vittal Acid Base Balance

ACIDS, BASES AND SALTS: 

ACIDS, BASES AND SALTS CHEMICAL COMPOUNDS CAN BE PROTON DONORS OR ACCEPTORS PROTON DONORS ARE ACIDS PROTON ACCEPTORS ARE BASES ACIDS AND BASES REACT TO NEUTRALIZE EACH OTHER FORMING SALTS

H+ ion & pH SCALE: 

H + ion & pH SCALE H+ ion conc. of plasma: 0.000 000 04 mol/L or 40 nmol/L pH is the negative logarithm of hydrogen ion conc. Normal : 7.35 – 7.45

Acid Base Balance Introduction: 

Acid Base Balance Introduction Metabolic processes continually produce acid and, to a lesser degree, base. H + : can attach to negatively charged proteins & in high concentrations, alter their overall charge, configuration, and function.

Acid Base Balance Introduction: 

Acid Base Balance Introduction To maintain cellular function, the body has elaborate mechanisms that maintain blood H + concentration within a narrow range— typically : 37 to 43 nmol/L (pH 7.35 to 7.45), & ideally : 40 nmol/L (pH = 7.4) Disturbances of these mechanisms can have serious clinical consequences.

Slide 6: 

Volatile acid Can leave solution and enter the atmosphere (e.g. carbonic acid) Produced by aerobic metabolism Fixed acids Acids that do not leave solution (e.g. sulfuric and phosphoric acids) Generated during catabolism of amino acids Organic acids Participants in or by-products of aerobic and anaerobic metabolism Metabolic byproducts such as lactic acid, ketone bodies Types of acids in the body

Acid-Base Physiology : 

Acid-Base Physiology Most acid comes from carbohydrate and fat metabolism (15,000 to 20,000 mmol of CO 2 daily) CO 2 combines with water (H 2 O) in the blood to create carbonic acid (H 2 CO 3 ), which in the presence of the enzyme carbonic anhydrase dissociates into H + and HCO 3 − . The H + binds with Hb in the blood and is released with oxygenation in the alveoli, the above reaction is reversed, creating H 2 O and CO 2 , which is exhaled Very little metabolic acid is produced - which is eliminated by kidney and liver.

Acid-Base Physiology : 

Acid-Base Physiology Most base comes from metabolism of anionic amino acids (glutamate and aspartate) and from oxidation and consumption of organic anions such as lactate and citrate, which produce HCO 3 −

Slide 9: 

Terminology

pH: 

pH : the negative logarithm of the hydrogen ion concentration a "decrease" in pH means an "increase" in acidity. Standard pH: (Hasselbalch, 1916) the pH under standard conditions: PCO 2 =40 mmHg, and 37 o C, and saturated with oxygen Arterial pH = 7.4 Venous pH = 7.36 pH

PaCO2: 

PaCO 2 : the partial pressure of carbon dioxide. The normal value in arterial blood is 40 mm Hg (or 5.33 kPa ) PaCO 2 ∝ CO 2 production + inspired CO 2 Low PaCO 2 reflects the rate of CO 2 elimination Principal physiological cause of hypocapnia is hyperventilation Intentional, incidental (HFV, ECMO) PaCO 2

Bicarbonate: 

HCO 3 - : concentration (in mEq/L) of the bicarbonate ion is not measured, it is calculated from the PCO 2 and pH Standard Bicarbonate : (Jorgensen and Astrup, 1957) bicarbonate concentration under standard conditions: PCO 2 =40 mmHg, and 37 o C, and saturated with oxygen. an excellent measurement of the metabolic component. = 21-27 mmol/l Bicarbonate

Base Escess (Astrup and Siggard-Andersen, 1958) : 

a better method of measuring the metabolic component. In essence the method calculated the quantity of Acid or Alkali required to return the plasma in-vitro to a normal pH under standard conditions. Base Escess (Astrup and Siggard-Andersen, 1958)

Base Excess & Base Deficit (Astrup and Siggard-Andersen, 1958) : 

Base Excess & Base Deficit (Astrup and Siggard-Andersen, 1958) Amount of strong acid or base that has to be added to a sample of blood to produce a pH of 7.4 under the specified conditions fro standard bicarbonate. a more accurate in assessing metabolic component of acid-base status. Normal Buffer Base = 48mMol/L (41.8 + 0.4 X Hb in g/dL)

Base Excess & Base Deficit: 

Base Excess & Base Deficit Base excess – 3 mmol/l : means 3 mmol of strong acid had to be added to each litre of original sample to get a pH of 7.4 while kept at 37 0 C and a PaCO 2 of 40 mm Hg. Base deficit – 3 mmol/l : means 3 mmol of strong base had to be added to each litre of original sample to get a pH of 7.4 while kept at 37 0 C and a PaCO 2 of 40 mm Hg.

Base Excess & Base Deficit: 

Base Excess & Base Deficit A base excess below -2.0 mmol/l : Metabolic acidosis A base excess above +2.0 mmol/l : Metabolic alkalosis Normal Range

Anion Gap: 

the difference between major plasma cations and major plasma anions. Anion gap = ([Na + ] +[K + ]) - ([Cl - -] +[HCO 3 - ]) Gap = Na+ + K+ - Cl- - HCO3- [ 15 = 140 + 5 - 105 - 25 mMol/L] Anion Gap Normal Anion Gap Children : 9mo. 19 yrs = 8 + 2 mMol /L Adults : 12 + 2 mMol /L

Metabolic Acidosis: Types “Normal Anion Gap”, “ h Anion Gap”: 

Metabolic Acidosis: Types “Normal Anion Gap”, “ h Anion Gap”  [Na + ] - ([Cl - ] + [HCO 3 - ]) Na + Cl - HCO 3 - Alb - Na + Cl - HCO 3 - Alb - A - Na + Cl - HCO 3 - Alb - No Anion gap M acidosis High Anion gap M acidosis

ACID/BASE BALANCE AND THE BLOOD: 

ACID/BASE BALANCE AND THE BLOOD Acidic Alkaline (Basic) [OH - ] [H + ] Neutral pH 0 14 7 Acidosis Alkalosis Normal 7.35-7.45 Venous Blood Arterial Blood 6.8 8.0 7.4 DEATH DEATH

Abnormal acid-base balance: 

Abnormal acid-base balance Acid-base imbalances can be defined as acidosis or alkalosis. Acidosis is a state of excess H + Acidemia results when the blood pH is < 7.35 Alkalosis is a state of excess HCO 3- Alkalemia results when the blood pH is > 7.45 You can have acidosis without acidemia but You can not have acidemia without an acidosis!

Regulation of arterial pH: 

Regulation of arterial pH Respiratory Buffer System Renal

Slide 22: 

Acid-Base Homeostasis Kidneys Output Lungs Maintenance of Normal [H + ] Buffers Metabolism Input

CHEMICAL BUFFER SYSTEMS: 

CHEMICAL BUFFER SYSTEMS H 2 CO 3 HCO 3 - H + Na + Cl - Add HCl Na + Cl - H + Cl - Unbuffered Salt Solution All protons are free H 2 CO 3 : HCO 3 - Buffer Add HCl H 2 CO 3 HCO 3 - + H + Protons taken up as Carbonic Acid

Buffer: 

Buffer

CHEMICAL BUFFER SYSTEMS: 

Weak acid/salt systems act as a “sponge” for protons As acidity tends to increase they take protons up As acidity tends to decrease they release protons CHEMICAL BUFFER SYSTEMS

CHEMICAL BUFFER SYSTEMS: 

Extracellular Buffers : Carbonic acid/Bicarbonate: Primary buffer against non-carbonic acid changes Serum Proteins (albumin) Ammonia ( in renal tubules) Intracellular Buffers : Hemoglobin Intracellular proteins Phosphates CHEMICAL BUFFER SYSTEMS

Handerson Hasselbalch Equation: 

Handerson Hasselbalch Equation pH = 6.1 + log HCO 3 - PaCO 2 X 0.0301 pK

Kassirer and Bleich Equation (Handerson Equation): 

Kassirer and Bleich Equation (Handerson Equation) H + = 24 X pCO 2 HCO 3 - With this formula, any 2 values (usually H+ and Pco2) can be used to calculate the other (usually HCO3 −).

Slide 30: 

Saturation of carbonic acid – bicarbonate buffer does not occur because carbonic acid is continuously breaking down into carbon dioxide and water.

Regulation of arterial pH: 

Regulation of arterial pH Respiratory Control: The power of the lungs to excrete large quantities of carbon dioxide enables them to compensate rapidly, i.e. metabolic acidosis and metabolic alkalosis normally elicit characteristic partial respiratory compensation almost immediately. Respiratory Buffer System Renal Not so efficient (50%) Less in preterm babies Control of respiratory centre A CO 2 conc. of > 9% depresses centers and causes CO 2 narcosis

Regulation of arterial pH: 

Regulation of arterial pH Buffer System: Act within seconds Act at cellular level ¾ of body’s buffering system from intracellular proteins and phosphates. Respiratory Buffer System Renal

Regulation of arterial pH: 

Regulation of arterial pH Renal Control: HCO 3 - Reclamation of almost (80%) all the filtered HCO 3 - (5000 mEq) Substantial task: 180 L x 24 mmol/L = 4320 mmol bicarbonate filtered/day Generation of new HCO 3 - with net secretion of H + (energy dependant) (1 - 1.5 mmol/kg/day) H + Increased excretion of acid as phosphate buffer and as ammonia Na + re-absorption during the formation of H + Respiratory Buffer System Renal

Response of body to increase in acid load Overview: 

Response of body to increase in acid load Overview Induces extra-cellular buffering by HCO 3 - Within minutes Respiratory Compensation with decrease in pCO 2 and H 2 CO 3 [to maintain a ratio of HCO 3 - : H 2 CO 3 ] of 20 : 1 Intracellular buffering – in 1 to 4 hours Renal acid excretion and production of new HCO 3 - formation : in hours to days

Acid-base disturbance : 

Acid-base disturbance Disorder type Primary change in HCO 3 - → Metabolic disorder Primary change in blood pCO 2 → Respiratory disorder Simple Mixed

Abnormal acid-base balances: 

Acid-base imbalance Plasma pH Primary disturbance Compensation Respiratory acidosis Low Increased pCO 2 Increased renal net acid excretion with resulting increase in serum bicarbonate Respiratory alkalosis High Decreased pCO 2 Decreased renal net acid excretion with resulting decrease in serum bicarbonate Metabolic acidosis- Low Decreased HCO3 - Hyperventilation with resulting low pCO 2 Metabolic alkalosis- High Increased HCO3 - Hypoventilation with resulting increase in pCO 2 Abnormal acid-base balances

Conclusions: 

Conclusions Acid Base Homeostasis is a Dynamic Process Buffers form the first line of Defence Bicarbonate buffers are by far the most important Lungs, Kidneys and Liver play important role in Acid Base Homeostasis

Slide 42: 

Vittal Than Q