Acid-Base Physiology Flashcards
What are the extremes of the pH range that is compatible with life?
pH 7.80 and 6.80
What do clinical laboratories measure in arterial samples?
• venous?
Arterial
• pH
• Carbon Dioxide
• Oxygen
Venous
• measure total CO2 from which you can calculate Bicarbonate
What is a normal level for total CO2 plus bicarbonate?
• How does Bicarb concentration correspond to this?
~25-26 meq/L
• Bicarbonate Concentration Typically 24 mEq/L
So actual CO2 is 1.0 to 1.5 mEq/L
What pH range do you typically see in patients in most clinical scenarios?
• what hydrogen concentration does this correspond to?
pH = 7.1 - 7.4
[H+] = 80 at 7.1 [H+] = 40 at 7.4
What is normal pH in a patient?
• normal pCO2
• Normal HCO3-
Normal pH:
7.35 - 7.45
Normal pCO2
• 36-44 mmHg (40mmHg)
Normal HCO3-
• 22 - 26 meq/L (24 meq/L)
What is the simplified H/H equation used clinically for acidosis and alkyosis?
pH = 24 (CO2)/(HCO3)
What causes Metabolic Disorders of acidosis and alkylosis?
• Respiratory?
Metabolic:
Changes in Bicarbonate
Respiratory:
Changes in CO2
What is the Buffer effect?
• is this seen in metabolic or respiratory disorders?
Respiratory disorders have buffer effect aka:
• Increased HCO3- with acidosis
• Decreased HCO3- with alkalosis
Where does bicarbonate manifest most of its buffering effects?
EXTRACELLULARLY
• Bicarb. is an extracellular Buffer
What is the Isoydric principle?
It says that all buffers change in the same direction
If bicarbonate is the main extracellular buffer then what buffers are active in the urine?
• which is the body better at regulating?
Phosphate and Ammonia
Body more effectively uses AMMONIA to buffer urine pH
What does it mean if Compensatory (secondary) mechanisms ARE NOT used in acid-base disorders?
the Disorder is MIXED
• with both respiratory and renal aspects of buffer system messed up there is no way to compensate
What compensatory mechanisms are used for metabolic and respiratory acidosis and alkylosis?
Metabolic Dysfunction = Respiratory Compensation.
• Acidosis = Hyperventilation
• Alkylosis = Hypoventilation
Respiratory Dysfunction = Metabolic Compensation in kidney
• Acidosis = HCO3- reabsorption increased
• Alkylsosis = HCO3- secretion increased
From fastest to slowest what systems react when the H+ and HCO3- balance is thrown off?
Immediate:
• Extracellular Fluids
• HCO3- + H+ H2CO3 CO2 + H2O is adjusted
Minutes to Hours:
• Lungs
• BREATHING changes
2-4 Hours:
• Intracellular Fluids
• Phosphate and Protein Buffer Changes
Hours to Days:
• Kidneys
• Hyrogen ion excretion, bicarb reabsorption and Bicarbonate generation
What are the 3 GOLDEN rules of simple acid-base disorders?
1) PCO2 and HCO3 always change in the same direction.
2) The secondary physiologic compensatory mechanisms must be present.
3) The compensatory mechanisms never fully correct pH.
What is the general cause of metabolic acidosis?
• what are some things that cause increased acid?
General Cause: any increase in acid or loss (failed reabsorption) of bicarbonate
increased acid generation:
Lactic acidosis, Ketoacidosis, ingestion of acids (aspirin, ethylene glycol, methanol), dietary protein intake (animal source)
What is the general cause of metabolic acidosis?
• what are some things that cause decreased bicarbonate?
General Cause: any increase in acid or loss (failed reabsorption) of bicarbonate
loss of bicarbonate:
Gastrointestinal
•(diarrhea, intestinal fistulas)
Renal: type 2 proximal renal tubular acidosis
• decreased acid excretion (impaired NH4+ excretion)
• Renal failure (reduced GFR) decreased ammonium excretion
• Type I (distal) renal tubular acidosis
•Type 4 renal tubular acidosis (hypoaldosteronism)
What is the root cause of Respiratory Acidosis?
• what rapid compensation is always associated with this type of acidosis?
• how effective is this compensation?
Induced Hypercapnia (Decreased Alveolar Ventilation)
- Rapid Increase in Plasma Bicarbonate concentration via Buffer Mechanism
- only a limited response of ~1-2 mEq/L
What is ultimate compensation made in Respiratory Acidosis?
• how long does this take?
- Increasing acid Excretion by NH4+ to GENERATE BICARB
* Takes 2-3 days
What is the most common cause of chronic respiratory acidosis?
Chronic Obstructive Pulmonary Disease
What is the Root cause of Respiratory Alkalosis?
• what are the short term and long term fixes to this problem?
• Reduced CO2 due to INCREASED Alveolar Ventilation
Short Term:
• Minimal Change in Buffer to lower serum Bicarb
Long Term (1-2 days):
• More Bicarb excreted
• Less NH4+ excreted
What are some diseases that cause Respiratory Alkalosis?
ANYTHING that causes HYPOXIA
- Anxiety, Hysteria
- Fever
- Salicylate Intoxication
- CNS diseases (trauma, stroke, etc.)
- Congestive Heart Failure
- Hepatic Insufficiency
- Pregnant Women
Would you expect an acute or chronic respiratory disorder to have a greater change in pH associated with it?
why?
Acute, because there is less time for the long term NH4+ excretion compensatory mechanism to kick in
Are respiratory or Metabolic Disorders associated with an ion gap?
Metabolic disorders is where you will see ion gap
• In respiratory disorders the Cl changes equally and inversely with plasma HCO3
T or F: Acid Base disorders cause radical changes in Plasma Sodium
False
Metabolic Alkalosis
• General Cause
• Specific Causes
Processes that raise plasma bicarbonate concentration
Etiology: Loss of hydrogen ion from the GI tract (vomiting) or into the urine (diuretic therapy)
Excessive urinary net acid excretion (primary hyperaldosteronism)
Differentiate between a chloride responsive and chloride resistant Alkylosis.
Chloride Responsive Metabolic Alkylosis is usually associated with Hypovolemic states
Chloride Responsive:
• Urine Cl less than 20 meq/l (usually less than 10 meq/l) - indcates that the kidney’s are doing what they should and excreting more HCO3- and taking up more Cl- into the serum
Chloride Resistant:
• Urine Cl greater than 20 meq/l (usually more than 50 meq/l) - indicates that you are still taking up a lot of bicarb and excreting a lot of Cl-
Differentiate between Acute and Chronic respiratory Acidosis and Alkylosis.
Acute Respiratory Acidosis:
HCO3- increases 1 mEq for each 10 mm increase in PCO2
Chronic Respiratory Acidosis:
HCO3- increases 4 mEq for each 10 mm increase in PCO2
Acute Respiratory Alkalosis:
HCO3- decreases 2 mEq for each 10 mm decrease in PCO2
Chronic Respiratory Alkalosis:
HCO3- decreases 5 mEq for each 10 mm decrease in PCO2
In what two places in the nephron can you work to correct HCO3- concentrations?
- Proximal Tubule
* Collecting Duct (alpha or beta intercalated cells)
In what Acid-Base disorder do we see an ion gap?
• why does this happen/explain the mechanism of the ion gap?
- Strong Acid HA fully dissociates at physiologic pH (into H+ and A-)
- H+ component is removed via HCO3-
- A- is either reabsorbed or secreted
- Reabsorption happens if A- is something that the body doesn’t want to get rid of (e.g. Lactate etc.)
Absorbed anion takes the place of the normal increase in Chloride that would replace decreased HCO3- (w/ less competition in the PT
What is the formula for Ion Gap?
• what unmeasured ions may account for this gap?
• Ion Gap = Na - (Cl + HCO3-)
- Protein
- PO4
- SO4
- Organic Acids
What are some major causes of an increased ion gap?
- Renal Failure
- Lactic Acidosis
- Ketoacidosis
- Ingestions: Asprin, Ethylene Glycol, Methanol
T or F: you can have metabolic acidosis without an ion gap.
True, this occurs in situations of Renal Tubular Acidosis, Diarrhea, and some cases of chronic renal failure
Where is the main place that bicarbonate is filtered?
- Proximal Tubules
* Final Excretion of ~50-100 meq/d occurs primarily in the collecting duct
How is phosphate excretion maintained?
• what role does phosphate play in acid-base balance of urine?
Urinary Excretion of Dietary Phosphate
• It is a good urinary buffer especially at low urinary pH (levels can be raised up to 40 mEq/d)
Why is Ammonium a better buffer of urinary acid load than phosphate?
- It can be increased to MUCH greater amounts (300 mEq/d) if necessary
- NH4+ is also Lipid Insoluble and thus is TRAPPED in the Urinary Lumen
What is the Urine Anion Gap?
• what is it used for?
Urine Anion Gap
• Indirect estimate of NH4+ excretion
Calculation:
• (Na + K) - Cl
What values are you looking for with Urine Anion Gap?
Probably looking for correction of respiratory acidosis
UAG = (Na + K) - Cl
Normally POSITIVE at ~10 mEq/l, becomes more positive and possibly even negative with increased urinary NH4+ excretion
What is the major way the Na and Cl differ when it comes to acid/base disorders?
Na ALWAYS remains constant
Cl CHANGES
***If you see a normal Na with messed up Cl then think about acid-base disorders
