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STEP Physiology > Acid Base Physiology > Flashcards

Acid Base Physiology Flashcards

1
Q

Basic definition of acid and base.

A

Acid: substance that can donate hydrogen ionsBase: substance that can accept hydrogen ions

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2
Q

What are the two main types of acids?

A

1) Carbonic acid- Volatile acids that can be converted to CO2 from carbohydrate and fat metabolism (eliminated from lungs)2) Noncarbonic acids- nonvolatile acids such as phosphoric and sulfuric acids that cannot be converted to CO2 and are made mostly from protein and phospholipids metabolism

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3
Q

How are Noncarbonic acids excreted?

A

these combine with buffers and are excreted by the kidneys

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4
Q

What are the pH extremes that are compatible with life (only for a short time)?

A

6.8-7.8

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5
Q

How is pH usually measured in the body?

A

arterial blood gas samples (also measures CO2 and hemoglobin and calculates HCO3- from the Henderson eqn)

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6
Q

How is HCO3- more directly measured?

A

venous samples as total CO2 conc (dissolved CO2 plus HCO3- conc)so the venous estimation of HCO3- is always about 1-.1.5 me/l greater than actual HCO3 due to the extra dissolved CO2

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7
Q

So what is normal plasma HCO3-?

A

24 mEq/l (arterial)

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8
Q

Bicarbonate buffer system eqn

A

CO2+H2O - H2CO3 - H+ + HCO3-

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9
Q

Henderson eqn revised

A

(pH) H+ = 24(CO2/HCO3)pH must be converted to H+ (nEq/L) pH= 7.4= 40 nEq/L7.40=40=24(40/24)

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10
Q

Relationship between pH and [H+]

A

7.0=1007.1=807.2=647.3=507.4=407.5=327.6=257.7=207.8= 16[H+]= 80- decimal digits of pH

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11
Q

What is the definition of metabolic acidosis?

A

some process that lowers bicarbonate, while alkalosis is some process that increases HCO3-

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12
Q

What is the definition of respiratory acidosis?

A

increased CO2, while respiratory alkalosis is decreased CO2

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13
Q

T or F. HCO3- is slightly increased with respiratory acidosis (increased CO2)

A

T. Think of the eqn.

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14
Q

Do buffers act quickly or slowly?

A

very quickly- almost an immediate onset

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15
Q

What is the isohydric principle?

A

all buffers change in the same direction

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16
Q

Where is HCO3- used as a buffer?

A

ECFV

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17
Q

What buffers are used in the urine?

A

phosphate (HPO42- and H2PO4-) and ammonia (NH3 and NH4+)

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18
Q

How does the body compensate for metabolic disorders?

A

by altering CO2 (via the lungs, rapid onset, minutes)likewise, respiratory disorders are compensated with HCO3- changes (via the kidney, slower onset, 1-2 days)

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19
Q

Why would there be a large pH change in acute respiratory disorders than chronic?

A

because the compensatory mechanisms haven’t kicked in yet

20
Q

What happens to pH, HCO3- and pCO2 in metabolic acidosis?

A

all decrease, pCO2 as a compensatory mechanism

21
Q

What happens to pH, HCO3- and pCO2 in metabolic alkalosis?

A

all increase, pCO2 as a compensatory mechanism

22
Q

What happens to pH, HCO3- and pCO2 in respiratory acidosis?

A

pH decreases and HCO3- increases (compensatory) and pCO2 increases

23
Q

What happens to pH, HCO3- and pCO2 in respiratory alkalosis?

A

pH increases and pCO2 and HCO3- decrease

24
Q

How much does HCO3- compensate in acute respiratory acidosis?

A

HCO3- increases 1 mEq for each 10mm increase in PCO2

25
Q

How much does HCO3- compensate in chronic respiratory acidosis?

A

HCO3- increases 4mEq for each 10 mm increase in PCO2 because the kidney has had time to react

26
Q

How much does HCO3- compensate in acute respiratory alkalosis?

A

HCO3- decreases 2mEq for each 10mm decrease in PCO2

27
Q

How much does HCO3- compensate in chronic respiratory alkalosis?

A

HCO3- decreases 5mEq for each 10mm decrease in PCO2

28
Q

Golden rules of simple acid-base disorders

A

1) pCO2 and HCO3 always change in the same direction2) the secondary physiologic mechanism must be present (if not, its mixed)3) compensatory mechanisms never fully correct pH and they never over-shoot

29
Q

What are some possible causes of metabolic acidosis?

A

-decreased renal acid excretion of H+ in the form of NH4+-direct HCO3- losses (GI tract- diarrhea or intestinal fistulas (everywhere but stomach) or urine)-Increased acid generation (exogenous or endogenous)

30
Q

What are some causes of increased acid generation?

A

-lactic acidosis, -ketoacidosis, -ingestion of acids (aspirin, ethylene glycol, methanol)

31
Q

What is Type 1 renal tubular acidosis?

A

transport defect in H+ ATPases in the CD

32
Q

What is Type 4 renal tubular acidosis?

A

low renin production- hypoaldosteronism

33
Q

What is the main mechanism of respiratory acidosis?

A

-induced by hypercapnia (decreased alveolar ventilation)

34
Q

What is the compensatory mechanism for respiratory acidosis?

A

HCO3- will increase rapidly but only about 1-2mEq/Lover time the kidney will work to increase acid excretion (NH4+) generating new bicarbonate ions (delayed response, 2-3 days)

35
Q

What are some causes of acute respiratory acidosis?

A

-general anesthesia-sedative overdose-cardiac arrest-pneumothorax-aspiration of foreign body -blockage of airways

36
Q

What are some causes of chronic respiratory acidosis?

A

-COPD-Brain tumor-respiratory nerve damageetc.

37
Q

What are some causes of acute respiratory alkalosis?

A

-anxiety-fever-salicylate intoxication-CNS diseases-CHF-Hypoxia etc.

38
Q

What are some causes of chronic respiratory alkalosis?

A

-pregnancy -cirrhosis

39
Q

T or F. In respiratory disorders, plasma Cl changes equally and inversely with plasma HCO3

A

T. The plasma anion gap does not change with respiratory disorders

40
Q

NOTE: Plasma Na is not directly altered by acid base disorders

A

NOTE: Plasma Na is not directly altered by acid base disorders (and K+ levels usually)

41
Q

What are the potential etiologies of metabolic alkalosis?

A

loss of H+ from the GI tract (vomiting- removes gastric secretions) or into the urine (diuretic therapy-mainly loop and thiazides)- will be hypotensive ORif the kidney is excessively excreting acid into urine (primary hyperaldosteronism) - will have HTN

42
Q

What is plasma anion gap?

A

only relevant to metabolic acidosis. If you add a strong acid to the system at pH=7.4, it will fully dissociate into H+ and A-. H+ will be buffered by HCO3- (thus, HCO3- goes down when an acid is added to the system)

43
Q

What happens to the A-?

A

It is either excreted into the urine causing Cl- to increase to maintain electrical neutrality (normal anion gap)or A- is reabsorbed by the kidney as an unmeasured anion (causes increased plasma anion gap, minimal change in plasma Cl- conc.)

44
Q

What is the eqn for PAG?

A

PAG= Na+- (Cl- + HCO3-)Normal PAG is ~12

45
Q

What does increase in PAG suggest?

A

due to unmeasured negative charge. So if you see increase in PAG, you know the kidney is reabsorbing an unmeasured anion such as lactate or keto-anion

STEP Physiology (68 decks)

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