Acid-Base Balance Flashcards
What is the Henderson-Hasselbach equation?
pH = pK + log ([HCO3-]/[H2CO3})
What is the ratio of bicarbonate (salt) to carbonic acid (acid) which results in a normal pH of 7.4?
20:1
What is the equilibrium equation which describes the bicarbonate-carbonic acid system?
CO2 + H2O ↔ H2CO3 ↔ H + HCO3
What enzyme catalyzes the reversible reaction in the equilibrium equation?
Carbonic anhydrase
What two cell types contain carbonic anhydrase?
RBCs and renal epithelium cells only
What is the modified Henderson-Hasselbach equation as used in the bicarbonate:carbonic acid buffer system?
pH = pK + log([HCO3]/(alpha){pCO2])
Modified Henderson-Hasselbach
- What are the values for pK n the blood and alpha, the solubility coefficient?
- pK = 6.1
- alpha = 0.031
Two reasons why hemoglobin is an important whole blood buffer, regulating acid-base balance both in the lungs and the tissues
- RBCs contain carbonic anhydrase (enzyme that converts the three forms of CO2)
- Has 9 histidine residues on each of its four chains that can accept CO2 molecules forming stable amide bonds
Physiologic importanceof the isohydric shift in RBCs
It’s important b/c it’s a set of chemical rxns by which O2 is released into the tissues and CO2 is taken up WHILE the blood remains at a constant pH
Process of the isohydric shift in RBCs
- CO2 is generated from metabolism
- it joins with H2O to become H2CO3 (by carbonic anhydrase)
- It then splits to become an H+ ion and HCO3-
- H+ ion attaches to hemoglobin to become reduced hemoglobin (HHb)
- when that happens, oxygen is given to the tissues
Chloride shift
- Movement of HCO3- and Cl-
HCO3 goes out of the cell and Cl- goes into the cell
Chloride shift
- How is this shift responsible for the hyper/hypochloremia noted in acid-base disturbances for which the body is compensating?
This regulates how much Co- is getting into the cell so if its too low, then its hypochloremia
Protein buffer system
- Specific sites of action (blood, tissue, and/or organs)
2/3 buffering power in blood and most of the buffering power intracellularly
Phosphate buffer system
- Specific sites of action (blood, tissue, and/or organs)
Minor component of blood but great importance in the kidneys and RBCs
Protein buffer system
- Processes involved
It accepts H+ ions b/c of its histidine residues
Phosphat buffer system
- Processes involved
H+ ions are added to filtrate in the forming urine. Dibasic phosphate picks up a H+ ion to become monobasic
Rank the body’s buffer systems in order of their importance
- Hemoglobin (most important in the whole blood)
- Bicarbonate (most important in the plasma)
- Proteins
- Phosphate
Organ which regulates the respiratory component of acid-base balance
Lungs
Organ which regulates the metabolic component of acid-base balance
Kidneys
Pulmonary hyperventilation
- How does it regulate acid-base balance according to how it alters the bicarbonate:carbonic acid ratio, thus compensating for acidosis or alkalosis?
Hyperventilation ↑ CO2 release, ↓ denominator ( in of the Henderson-Hasselbach equation
Pulmonary hypoventilation
- How does it regulate acid-base balance according to how it alters the bicarbonate:carbonic acid ratio, thus compensating for acidosis or alkalosis
Hypoventilation ↓ CO2 release, ↑ the denominator of the Henderson-Hasselbach equation
Four specific mehcanisms by which the kidney regulates acid-base balance
- Reabsorption of bicarbonate
- Excreting excess H+ by exchanging Na+ for H+
- Forming titratable acids w/ phosphate
- Excreting excess H+ as NH4+
Reabsorption of HCO3
- How does it correct for acidosis or alkalosis?
In filtrate:
- HCO3 in filtrate + H ions from renal cells form carbonic acid (H2CO3), this breaks down into H2O and CO2 which enter the renal tubular cells
In renal cells:
- H2O and CO2 come together to form H2CO3. This breaks down into H+ ion and HCO3-. Bicarb goes into the interstitial fluid
Excreting excess H+ by exchanging Na+ for H+
- How does it correct for acidosis or alkalosis?
In renal cells:
- H2CO3 (carbonic acid) breaks down into H+ ion and bicarbonate. H+ ions are exchanged (out) for a sodium (in)
Forming titratable acids w/ phosphate
- How does it correct for acidosis or alkalosis?
In filtrate:
-NaHPO4 joins with the secreted H+ ion to form NaH2PO4 which is a titratable acid that is excreted in the urine
Excreting excess H+ as NH4+
- How does it correct for acidosis or alkalosis?
H+ ion plus NH3 are secreted by the renal cells. They come together to form NH4+ which is excreted in the urine
How do kidneys excrete acid via Na+/H+ ion exchange?
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Kidney excretion of acid via Na+/H+ ion exchange
- How does this mechanism maintain the Gibbs-Donnan equilibrium?
Electoneutrality
- Total anions = total cations
Kidney excretion of acid via Na+/H+ ion exchange
- Interrelationship of hydrogen, sodium, and potassium ions’ reabsorption and secretion
- K+ and H+ “pair up”
- Na+ is the opposite direction
Kidney excretion of acid via Na+/H+ ion exchange
- The reabsorption of HCO3- when Na+ is reabsorbed
HCO3- is reabsorbed w/ Na+ into the blood
Reabsorption of HCO3 by the kidney
- Role of carbonic anhydrase in renal epithelial cells
CO2 is reabsorbed into the renal cell and is joined with H2O to become H2CO3 (by carbonic anhydrase)
Reabsorption of HCO3 by the kidney
- How does the rxn of H+ and HCO3 form CO2 and H2O aid in the reabsorption of HCO3?
TUBULAR LUMEN
- H + HCO3 → H2CO3 –(CA)→ H2O + CO2 (reabsorbed into renal cell)
RENAL CELL
- CO2 + H2O –(CA)→ H2CO3 → HCO3 (reabsorbed into blood) + H (exchanged for Na+)
Reabsorption of HCO3 by the kidney
- Importance of Na+ reabsorption in HCO3 reclamation
If the Na/H exchange doesn’t work properly, HCO3- doesn’t get reabsorbed into the blood b/c we’re missing the secreted H+
Process of renal formation of NH3 and the excretion of NH4+ in maintaining acid-base balance, including how NH4+ formation assists in the reabsorption of Na+?
Secreted H+ binds to NH3 from the liver which also allows for the exchange of Na+
Importance of renal formation of NH3 and the excretion of NH4+ in maintaining acid-base balance, including how NH4+ formation assists in the reabsorption of Na+?
Ammonium is more excretable than ammonia???????????
How does the kidney excrete acid via the formation of titratable acids (principally phosphoric acid)?
Acts as a buffer
Mechanism of compensation in maintaining a normal bicarbonate:carbonic acid ratio
If either the [HCO3-] or the [H2CO3] becomes out of balance, the body will alter the concentration of the other component in order to return the ratio to 20:1
How do lab values for pH, pCO2, and HCO3 concentration change upon partial and complete compensation?
Partial compensation
- When the organ NOT responsible for the initial imbalance BEGINS to correct the ratio
Complete compensation
- When the pH has returned to normal
Metabolic imbalance
- Which compound, HCO3 or carbonic acid, is abnormal?
Change in [HCO3-] or [H+]
Respiratory imbalance
- Which compound, HCO3 or carbonic acid, is abnormal?
Change in [H2CO3]
Metabolic acidosis (primary HCO3 deficit) - 3 general causes
- Increased addition of H+ ions, either via excess production of organic acids that exceeds the rate of elimination or addition of metabolic acids
- Reduced excretion of acids
- Excessive loss of bicarbonate
How does an increased addition of H+ ions cause metabolic acidosis?
H+ causes pH to decrease making it more acidic
How does the reduced excretion of acids cause metabolic acidosis?
↑ in H+ making the urine more acidic
How does excessive loss of bicarbonate cause metabolic acidosis?
↓ in HCO3- → ↑ H+ making the urine more acidic
How is the bicarbonate: carbonic acid ratio affected in metabolic acidosis?
In all causes of metabolic acidosis, the normal 20:1 ratio is decreased b/c of the addition of acid and ↓ in HCO3
Two conditions associated w/ increased addition of H+ ions, either via excess production of organic acids that exceeds the rate of elimination or addition of metabolic acids
- Ketacidosis
- Lactic acidosis
- Additon of acids in poisonings (e.g., volatile metabolites)
Two conditions associated w/ reduced excretion of acids
- Renal failure
- Renal tubular acidosis
- Kidney is malfunctioning and can no longer secrete sufficient H+ ions
Two conditions associated w/ excessive loss of bicarbonate
- ↑ renal excretion of HCO3-
- ↓ renal absorption of HCO3- due to ↓ renal absorption of Na+ (e.g., Addison’s disease w/ ↓ aldosterone)
- Excessive loss of duodenal fluid that contains HCO3- (e.g., diarrhea)
Metabolic acidosis
- One specific respiratory compensatory mechanisms
Respiratory component begins partial compensation via HYPERVENTILATION (“blow off” CO2) to quickly fix the problem → ↓ [HCO3-] → ↑ pH
Metabolic acidosis
- Four specific renal compensatory mechanisms
- ↑ HCO3- reabsorption
- ↑ NH4+ formation
- ↑ titratable acid formation
- ↑ Na+/H+ exchange
Metabolic alkalosis
- 3 general causes
- Administration of excess alkali
- Hydrogen ion loss
- Potassium depletion
How does the administration of excess alkali cause metabolic alkalosis?
Alkali increases the pH making it more basic
How does hydrogen ion loss cause metabolic alkalosis?
Kidneys respond to H+ loss by reabsorbing more Na+ in the PCT causing more renal reabsorption of HCO3
How does potassium depletion cause metabolic alkalosis?
Diuretic therapy and/or hyperaldosteronism force the kidneys to secrete more H+ ions in exchange for Na+ and K+; HCO3 is reabsorbed w/ Na+
How is the bicarbonate: carbonic acid ratio affected in metabolic alkalosis?
In all causes of metabolic alkalosis, the 20:1 ratio is INCREASED b/c the primary increase is in HCO3
Two conditions associated conditions w/ administration of excess alkali
- Excessive HCO3- intake
- Multiple blood txns (citrate anticoagulant in blood products)
- Milk-alkali syndrome (excess intake of calcium carbonate antacids)
Two conditions associated w/ hydrogen loss
- Prolonged vomiting w/ HCl loss
- Nasogastric suctioning
Two conditions associated w/ potassium depletion
- Diuretic therapy
- Hyperaldosteronism (e.g., Cushing’s syndrome or licorice abuse)
Metabolic alkalosis
- One specific respiratory compensatory mechanism
Respiratoyr component begins partial compensation via HYPOVENTILATION (retain more CO2) to quickly try to fix the problem → ↑ [HCO3-] → ↓ pH
Metabolic alkalosis
- Four specific renal compensatory mechanisms
- ↓ HCO3- reabsorption
- ↓ NH4+ formation
- ↓ titratable acid formation
- ↓ Na+/H+ exchange
Respiratory acidosis
- General cause
Inadequate ventilation or exchange in the lungs
Three general causes of acute respiratory acidosis
- Respiratory chemoreceptor depression (CNS trauma, narcotics such as morphine, general anesthesia)
- Neuromuscular system disorders (Guillain-Barre, myasthenia gravis)
- Acute pulmonary edema (pneumothorax, multiple rib fractures)
Four general causes of chronic respiratory acidosis
- COPD (most common), asthma, pneumonia, emphysema, pulmonary fibrosis
- Cardiac disease (less blood provided to lungs for gas exchange)
- RDS in infants
How is the bicarbonate: carbonic acid ratio affected in respiratory acidosis?
In all causes of respiratory acidosis, the normal 20:1 ratio is DECREASED b/c of primary increase of carbonic acid (measured as CO2 in lab)
Respiratory acidosis
- One specific respiratory compensatory mechanism
HYPERVENTILATION
Respiratory acidosis
- Four specific renal compensatory mechanisms
- ↑ HCO3- reabsorption
- ↑ NH4+ formation
- ↑ titratable acid formation
- ↑ Na+/H+ exchange
Respiratory alkalosis
- Two general causes
- Psychogenic stimulation
- Hypoxia/impaired CNS control of respiration
Causes associated w/ psychogenic stimulation
- Anxiety, nervousness
- Excessive crying
- Pregnancy
At least three causes of hypoxia/impaired CNS control of respiration
- Pulmonary embolism
- Excessive use of mechanical respirators
- Thyrotoxicosis
- GN septicemia
- CNS lesions (menigitis)
- Alcoholism and DT’s
- Strenuous exercises
- Epinephrine
- High altitude
- Chronic liver disease
- Anemia
- CHF
Respiratory alkalosis
- How do the causes of psychogenic stimulation and hypoxia/impaired CNS control of respiration affect the bicarbonate: carbonic acid ratio?
In all causes of respiratory alkalosis, the normal 20:1 ratio of HCO3- to H2CO3 is INCREASED b/c the primary decrease is in H2CO3 (measured in the lab as CO2)
Respiratory alkalosis
- One specific respiratory compensatory mechanism
Respiratory component compensates via HYPOVENTILATION (sometimes patient needs help and sedatives or breathing into a paper bag are used) → ↑ [HCO3-] → ↓ pH
Respiratory alkalosis
- Four specific renal compensatory mechanisms
- ↓ HCO3 reabsorption
- ↓ NH4+ formation
- ↓ titratable acid formation
- ↓ Na+/H+ exchange
Mechanism for mixed acid-base disturbance that occurs in salicylate poisoning
Respiratory alkalosis followed by metabolic acidosis
Oxygen dissociation curve
- X-axis
- Y-axis
- P50
- X-axis → pO2 (mmHg)
- Y-axis → % O2 saturation
- P50 → pO2 at which Hb is 50% saturation w/ O2
Where do right shifts of the O2 dissociation curve normally occur?
Tissues (↓ affinity)
Where do left shifts of the O2 dissociation curve normally occur?
Lungs (↑ affinity)
Four normal physiological factors which causes a shift to the right of the O2 dissociation curve
↓ pH
↑ pCO2, 2,3-DPG, temperature
Four normal physiological factors which causes a shift to the left of the O2 dissociation curve
↑ → pH
↓ → pCO2, 2,3-DPG, temperature
What is the clinical use of the p50 value?
- To assess the affinity of O2 for hemoglobin
- To assess whether a right or left shift is occurring in a patient
What are the effects on blood gas results causes by delays in testing?
↓ pH
↑ pCO2
↓ pO2
What are the effects on blood gas results caused by exposure to room air?
↑ pH
↓ pCO2
↑ pO2
Reference range of pH
- Arterial blood
- Venous blood
- Measured or calcualted parameter in blood gas analyzer?
- Arterial blood: 7.37-7.44
- Venous blood: 7.35-7.45
- Measured parameter in blood gas analyzers
Reference range for pCO2
- Arterial blood
- Venous blood
- Measured or calcualted parameter in blood gas analyzer?
- Arterial: 35-45 mmHg
- Venous: 40-55 mmHg
- Measured parameter in blood gas analyzers
Reference range for pO2
- Arterial blood
- Venous blood
- Measured or calcualted parameter in blood gas analyzer?
- Arterial: 80-90 mmHg
- Venous: 30-50 mmHg
- Measured parameter in blood gas analyzers
Reference range for bicarbonate
- Venous blood
- Measured or calcualted parameter in blood gas analyzer?
- Venous: 22-27 mEq
- Calculated parameter in blood gas analyzers
Reference range for base excess
- Arterial blood
- Venous blood
- Measured or calcualted parameter in blood gas analyzer?
- Arterial: 0 +/- 2
- Venous: 0 +/- 2
- Calculated parameter in blood gas analyzers
Reference range for %O2 saturation
- Arterial blood
- Venous blood
- Measured or calculated parameter in blood gas analyzer?
- Arterial: 95-99%
- Venous: 50-70%
- Calculated parameter in blood gas analyzers
Reference range for CO2
- Venous blood
- Measured ro calculated parameter in blood gas analyzer?
- Venous: 40-55%
- Measured parameter in blood gas analyzers
Three forms of CO2 in plasma
- dCO2
- HCO3
- H2CO3
Total CO2 formula
TCO2 = (alpha)(pCO2) + HCO3
Number of hydrogen ions needed to raise or lower 1 L of whole blood to a pH of 7.4
Base excess
What is the usefulness of base excess in determining IV therapies?
Used to assess the metabolic component of acid-base status and helps determine whether patient should be given sodium bicarb or ammonium chloride intravenously to help correct the problem
Three parameters which can be obtained using the Siggaard-Anderson nomogram
- [HCO3]
- Total CO2
- Base excess
Three abnormal (pathological) causes of a right shift
- Heart and lung
- Severe anemia
- High altitude (↓ 2,3-DPG production)
Three abnormal (pathological) causes of a left shift
- Abnormal Hbs present
- CO/methemoglobinemia
- Massive txn of 2,3-DPG-depleted blood
↑ pH, N pCO2, ↑ HCO3
↑ pH, ↑ pCO2, ↑ HCO3
N pH, ↑ pCO2, ↑ HCO3
Uncompensated metabolic alkalosis
Partially compensated metabolic alkalosis
Compensated metabolic alkalosis
↑ pH, ↓ pCO2, N HCO3
↑ pH, ↓ pCO2, ↓ HCO3
N pH, ↓ pCO2, ↓ HCO3
Uncompensated respiratory alkalosis
Partially compensated resp. alkalosis
Compensated resp. alkalosis
↓ pH, N pCO2, ↓ HCO3
↓ pH, ↓ pCO2, ↓HCO3
N pH, ↓ pCO2, ↓HCO3
Uncompensated metabolic acidosis
Partially compensated met. acidosis
Compensated metabolic acidosis
↓ pH, ↑ pCO2, N HCO3
↓ pH, ↑ pCO2, ↑ HCO3
↓ pH, ↑pCO2, ↑ HCO3
Uncompensated respiratory acidosis
Partially compensated resp. acidosis
Compensated respiratory acidosis
↑ pCO2 indicates what?
Respiratory acidosis
↑ HCO3 indicates what?
Metabolic alkalosis
↓ HCO3 indicates what?
Metabolic acidosis
↓ pCO2 indicates what?
Respiratory alkalosis
