Acid base balance - Acid-Base reactions PDF

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Acid-Base reactions...

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4/21/2020

Acid-Base Balance for Allied Health Majors Using the Henderson-Hasselbach Equation Chapter 30 (12th ed.), Chapter 31 (13th ed.)

pH = pK + log

HCO3 -

α =0.03

pCO 2 (α)

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Outline I. Acids and bases II. Buffering systems III. Compensation for deviation IV. Practice problems V. Anion gap VI. Clinical correlation: Abuse of diuretics or Conn’s disease

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I. Acids and Bases ACID: a substance that will disassociate a H+ and become more negatively charged (electron acceptor). •When hydrogen ions accumulate in a solution, it becomes more acidic ([H+] increases = more acidity).

HCl 2 H+ H+

H+ H+

ClH+

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Cl-

Cl-

pH ClCl-

Hydronium ions in solution Concentration of hydrogen ions increases, pH drops

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I. Acids and Bases BASE: a chemical that will remove hydrogen ions from the solution •The base has a negative charge (or extra electrons) to donate to hydrogen ions and thus create a bond with hydrogen

NaOH

Na+ H+

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OH-

H+ ClClNa+ OHH+ Cl-

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Cl- Na+ OHH+ ClNa+ OH-

H+

Acids and basis neutralize eachother

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A change of 1 pH unit corresponds to a 10-fold change in hydrogen ion concentration

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Na+ H+

Na+

ClNa+

Cl-

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pH

H2O

OH-

Na+ Cl-

Cl-

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Acids are being created constantly through metabolism • Inorganic phosphates (mostly from ATP, etc.) • Anaerobic respiration of glucose produces lactic acid • Fat metabolism yields organic acids and ketone bodies • Carbon dioxide ultimately causes H+ to be released from carbonic acid (see slide 8)

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II. Buffering systems A buffer is a substance that can reversibly bind H+ . Acids must be buffered, transported away from cells, and eliminated from the body

Phosphate: important renal tubular buffer HPO 4-+ H+ H2PO 4 Ammonia: important renal tubular buffer NH 3 + H+ NH4+ Proteins: important intracellular and plasma buffers H + + Hb HHb Histidine in proteins is particularly good at neutralizing hydrogen ions

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Bicarbonate: most important ECF buffer

CA

From metabolism

Hydrogen is buffered by hemoglobin

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Buffering is good, but it is a temporary solution. Excess acids and bases must be eliminated from the body. gas

H2O + CO 2

aqueous CA

H2CO 3

H+ + HCO 3 Kidneys can remove excess non-volatile acids and bases

Lungs eliminate carbon dioxide

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Excessive acids and bases can cause pH changes (denature proteins) • N mal p • Alkalosis or alkalemia – arterial blood pH rises above 7.45 • Acidosis or acidemia – arterial pH drops below 7.35 (physiological acidosis) • For our class, we will stick to 7.40 as normal

Acidosis: Too much acid or too little base

Alkalosis: Too much base or too little acid

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III. Compensation for deviation • Lungs (only if not a respiratory problem) – If too much acid (low pH)—respiratory system will ventilate more (remove CO 2 ) and this will raise pH back toward set point – If too little acid (high pH)—respiratory system will ventilate less (trap CO 2 in body) and this will lower pH back toward set point

• Kidneys – If too much acid (low pH)—intercalated cells will secrete more acid into tubular lumen and make NEW bicarbonate (more base) and raise pH back to set point. – If too little acid/excessive base (high pH) proximal convoluted cells will NOT reabsorb filtered bicarbonate (base) and will eliminate it from the body to lower pH back toward normal.

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How would your ventilation change if you had excessive acid? Alveolar Ventilation

[H+ ]

pCO 2 CO 2 vented out

This is too high and this means the buffer system swings this way!

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How would your ventilation change if you had too little acid?

Alveolar Ventilation

[H+ ]

pCO 2

CO 2 trapped

This is too low and this means the buffer system swings this way!

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How can the kidneys control acids and bases? • Bicarbonate is filtered and enters nephron at Bowman’s capsule • Proximal convoluted tubule – Can reabsorb all bicarbonate (say, when you need it to neutralize excessive acids in body) OR

– Can reabsorb some or NONE of the bicarbonate (maybe you have too much base in body and it needs to be eliminated)

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How can the kidneys control acids and bases? • Acidosis • Intercalated cells – Secrete excessive hydrogen – Secreted hydrogen binds to tubular buffers (ammonia and phosphate bases) – Secretion of hydrogen leads to gain of bicarbonate (NEW!)

HPO4NH3

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What would happen if the respiratory system had a problem with ventilation? Respiratory Acidosis and Alkalosis P CO2 levels-Normal

PCO2 fluctuates between 35 and 45 mmHg • Respiratory Alkalosis (less than 35mmHglowered CO2) • Hyperventilation syndrome/ psychological (fear, pain) • Overventilation on mechanical respirator • Ascent to high altitudes • Fever

• Respiratory Acidosis (elevated CO 2 greater than 45mmHg) • Depression of respiratory centers via narcotic, drugs, anesthetics • CNS disease and depression, trauma (brain damage) • Interference with respiratory muscles by disease, drugs, toxins • Restrictive, obstructive lung disease (pneumonia, emphysema)

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What if your metabolism changed? • Metabolic acidosis • bicarbonate ion levels below normal (22 mEq/L)

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• Ingestion, infusion or production of more acids (alcohol) • Carbonic anhydrase inhibitors (decreased H+ secretion) • Salicylate overdose (aspirin) • Diarrhea (loss of intestinal HCO 3- ) • Accumulation of lactic acid in severe diabetic ketoacidosis • Starvation

• • • •

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• Metabolic alkalosis • bicarbonate ion levels higher (greater than 26mEq/L) Excessive loss of fixed acids due to ingestion, infusion, or renal reabsorption of bases Loss of gastric juice during vomiting Intake of stomach antacids (Leisure world syndrome) Diuretic abuse (loss of H+ ions) Severe potassium depletion (increased aldosterone stimulates H+ secretion by intercalated cells) Steroid therapy (mineralcorticoid excess)

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Mechanisms of Acidosis and Alkalosis H2 O + CO 2

H+ + HCO 3 -

H2 CO3

pH = pK + log

HCO3 -

α =0.03

pCO 2 (α) Acidosis: pH < 7.4 - metabolic: - respiratory:

HCO3 pCO2

Alkalosis: pH > 7.4 - metabolic: HCO 3 - respiratory: pCO2

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Summary of causes of deviation and compensatory mechanims Acidosis (pH7.40)

Problem

Too much acid (High CO2)

Too little acid (low CO2)

Respiratory

Too little base (low HCO3-)

Too much base Metabolic (high HCO3-)

Compensation

Kidneys Respiratory (quick!) Kidneys (long-term)

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What’s the pH? pH > 7.4 Alkalosis

How did it happen?

How is it compensated? Kidneys: Proximal convoluted tubule does not release H+ at all or releases just a little into tubular lumen, so not all filtered HCO3- will be neutralized and some will leave via urine. Urine has excess HCO3-.

Respiratory alkalosis Too little acid (low CO2) pCO2 < 35mmHg

Respiratory: Hypoventilation (keep more CO2,pH goes down) Metabolic alkalosis Too much base (high HCO3-) Kidneys: Same as with respiratory alkalosis - Proximal convoluted tubule does not release H+ at all or releases just a little into tubular lumen, so not HCO3- > 26mEq/L all filtered HCO3- will be neutralized and some will leave via urine. Urine has excess HCO3-.

pH < 7.4 Acidosis

Respiratory acidosis

Kidneys:

Too much acid (high CO2) pCO2 > 45mmHg

(1) Proximal convoluted tubule releases H+ into tubular lumen to eventually reclaim filtered HCO3-; and (2) Intercalated cells in distal convoluted tubule secrete H+ (buffered by phosphate and ammonia bases. NEW HCO3- is formed and released in interstitial fluid.

Metabolic acidosis

Respiratory: Hyperventilation (release more CO2, pH goes up) Kidneys: Same as with respiratory acidosis (1) Proximal convoluted tubule releases H+ into tubular lumen to eventually reclaim filtered HCO3-; and (2) Intercalated cells in distal convoluted tubule secrete H+ (buffered by phosphate and ammonia bases. NEW HCO3- is formed and released in

Too little base (low HCO3-) HCO3- < 22mEq/L

interstitial fluid. 20

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Analysis of Simple Acid-Base Disorders

Step One:

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