Free ASCP MLS Biochemistry Mock Test: Part 54 – Acid-Base Balance & Blood Gas Analysis Practice Exam

Hemoglobin plays a crucial role as a buffer in the blood, and this is its primary function in acid-base balance.

• As red blood cells travel through tissues, CO₂ diffuses into them and is converted to carbonic acid (H₂CO₃), which quickly dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃⁻).

• Hemoglobin acts as a buffer by binding these hydrogen ions, which prevents a significant drop in pH.

• This binding also facilitates the conversion of hemoglobin to its deoxygenated form, which has a higher affinity for H⁺ (this is part of the Bohr effect).

Why the other options are incorrect:

• a) Produce CO₂: CO₂ is a waste product of cellular metabolism, not produced by hemoglobin.

• c) Transport amino acids: This is not a function of hemoglobin. Amino acids are transported freely in the plasma.

• d) Maintain osmotic pressure: Albumin is the primary protein responsible for maintaining plasma oncotic (osmotic) pressure.

pH = pKa + log ( [HCO₃⁻] / [H₂CO₃] )

The key is that the concentration of carbonic acid ([H₂CO₃]) is directly proportional to the partial pressure of carbon dioxide (pCO₂) in the blood. The proportionality constant is the solubility coefficient of CO₂ in blood, which is approximately 0.03 mmol/L/mm Hg.

Therefore, the formula is:
[H₂CO₃] = pCO₂ × 0.03

Let’s look at why the other options are incorrect:

• a) HCO₃⁻ / 2.8: This is a misrepresentation of the Henderson-Hasselbalch equation. The ratio in the log term is [HCO₃⁻] / [H₂CO₃], and the pKa is 6.1, not 2.8.

• b) HCO₃⁻ / H₂CO₃: This is the ratio used within the Henderson-Hasselbalch equation, not the formula for calculating the H₂CO₃ concentration itself.

• d) tCO₂ × 0.03: Total CO₂ (tCO₂) is primarily a measure of bicarbonate (HCO₃⁻), not carbonic acid. Multiplying it by 0.03 does not give the H₂CO₃ concentration.

• On ice: Placing the sample on ice (0-4°C) slows down cellular metabolism, which can significantly alter the PaO₂ and PaCO₂ values if the sample is left at room temperature.

• No clots: Clots within the sample will consume oxygen and release carbon dioxide, producing inaccurate results.

• No air bubbles: Air bubbles can equilibrate with the blood, falsely increasing PaO₂ and decreasing PaCO₂.

The other options are incorrect because they allow for fibrin strands or air bubbles, which compromise the sample’s integrity.

Let’s carefully analyze the data step by step.

Given:

• Na⁺ = 136 mEq/L

• K⁺ = 4.4 mEq/L

• Cl⁻ = 92 mEq/L

• HCO₃⁻ = 40 mEq/L

• pH = 7.32 (↓, acidemia)

• pCO₂ = 79 mm Hg (↑)

Step 1: Determine primary disorder

• pH < 7.35 → acidemia

• pCO₂ ↑ → respiratory origin (CO₂ retention)

• So the primary disorder is respiratory acidosis.

Step 2: Check for compensation

• HCO₃⁻ = 40 mEq/L (elevated) → indicates renal compensation (chronic respiratory acidosis)

Step 3: Match with options

• a) Respiratory Alkalosis → pH ↑, pCO₂ ↓ →

• b) Respiratory Acidosis → pH ↓, pCO₂ ↑ →

• c) Metabolic Alkalosis → pH ↑, HCO₃⁻ ↑ →

• d) Metabolic Acidosis → pH ↓, HCO₃⁻ ↓ →

Metabolic acidosis is characterized by a primary decrease in bicarbonate (HCO₃⁻) levels in the blood.
This occurs due to either the accumulation of acids (which consume bicarbonate) or the direct loss of bicarbonate, leading to a lower pH.

Option breakdown:

• a) Increased bicarbonate → Seen in metabolic alkalosis, not acidosis.

• b) Decreased bicarbonate →  Characteristic of metabolic acidosis.

• c) Increased CO₂ → Seen in respiratory acidosis.

• d) Increased pH → Indicates alkalosis, not acidosis.

The Henderson–Hasselbalch equation relates pH to the ratio of the concentration of a weak acid and its conjugate base.
It is commonly used for calculating the pH of buffer solutions and, in medical contexts, for understanding acid-base balance in blood.

Respiratory alkalosis is defined by a primary decrease in the partial pressure of carbon dioxide (pCO₂) in the blood, which is caused by excessive elimination of CO₂ through the lungs.

• Hyperventilation is the direct cause. By breathing too rapidly or too deeply, a person “blows off” too much CO₂.

• Since CO₂ in the blood forms carbonic acid (H₂CO₃), a loss of CO₂ reduces the acid level and raises the blood pH, leading to alkalosis.

Why the other options are incorrect:

• a) Vomiting: This causes a loss of gastric acid (HCl), leading to metabolic alkalosis.

• b) Starvation: This can lead to ketoacidosis, which is a form of metabolic acidosis.

• c) Asthma: During an acute asthma attack, impaired airflow leads to CO₂ retention (hypercapnia), which causes respiratory acidosis, not alkalosis.

• Blood gas analyzers directly measure pH, pCO₂, pO₂, and often calculate HCO₃⁻ and base excess.

• They use electrochemical sensors for precise measurements.

• Other instruments:

  • Flame photometer → measures ions like Na⁺, K⁺.

  • Ion-selective electrode → measures specific ions (e.g., Na⁺, K⁺, Cl⁻).

  • Spectrophotometer → measures hemoglobin derivatives and other analytes, not gases.

Why the other options are incorrect:

• a) Flame photometer: This is an older technology used to measure sodium and potassium levels, not blood gases.

• b) Ion-selective electrode: While blood gas analyzers contain ion-selective electrodes (specifically for pH, pCO₂, and often for electrolytes like K⁺, Na⁺, Cl⁻, Ca²⁺), the instrument as a whole is called a blood gas analyzer. The term “ion-selective electrode” describes the technology for a single ion, not the complete instrument for blood gas measurement.

• d) Spectrophotometer: This is used to measure the concentration of substances by their light absorption, such as bilirubin, hemoglobin, and many clinical chemistry assays, but not the partial pressures of blood gases.

Heparin is the only acceptable anticoagulant for blood gas analysis because it does not interfere with the measurement of pH, carbon dioxide (PCO₂), or oxygen (PO₂). Other anticoagulants like EDTA or Oxalate can alter the blood pH and bind essential ions like calcium, making the results invalid.

The total CO₂ measured in serum (often in a chemistry panel) is primarily bicarbonate (HCO₃⁻), but technically it includes:

• HCO₃⁻ (~95% of total CO₂)

• Dissolved CO₂ (pCO₂ × 0.03 mmol/mmHg)

• Carbonic acid (H₂CO₃) (very small amount, often combined with dissolved CO₂ as “H₂CO₃*”)

Lab “total CO₂” ≈ [HCO₃⁻] + [dissolved CO₂] + [H₂CO₃]
Dissolved CO₂ + H₂CO₃ together = H₂CO₃* in some formulas, but in common clinical terms:

Total CO₂ = HCO₃⁻ + H₂CO₃ (where H₂CO₃ here includes dissolved CO₂).

From the given options, the most accurate is: a) H₂CO₃ + HCO₃⁻

• Metabolic acidosis → primary problem is low HCO₃⁻ → pH ↓.

• Respiratory compensation occurs rapidly: hyperventilation → blows off CO₂ → reduces carbonic acid → helps raise pH.

• The kidneys also compensate over hours to days by retaining HCO₃⁻ and excreting H⁺, but the immediate response is respiratory.

Why the other options are incorrect:

• a) Decreasing ventilation: This would retain CO₂ and make the acidosis worse. This is a compensatory mechanism for metabolic alkalosis.

• c) Increasing bicarbonate excretion: The kidneys would do the opposite—they would conserve bicarbonate and generate new bicarbonate to compensate for metabolic acidosis (renal compensation).

• d) Decreasing H⁺ excretion: The kidneys would increase H⁺ excretion to help correct the acidosis.

When an arterial blood gas sample is exposed to room air, it equilibrates with the atmospheric gases, leading to predictable changes:

1. pO₂ (Partial Pressure of Oxygen): The pO₂ of room air (≈ 160 mmHg) is much higher than that of arterial blood (≈ 80-100 mmHg). Therefore, oxygen will diffuse into the sample, increasing the pO₂.

2. pCO₂ (Partial Pressure of Carbon Dioxide): The pCO₂ of room air (≈ 0.3 mmHg) is much lower than that of arterial blood (≈ 40 mmHg). Therefore, carbon dioxide will diffuse out of the sample, decreasing the pCO₂.

3. pH: Since CO₂ in solution forms carbonic acid (H₂CO₃), a decrease in pCO₂ means a decrease in carbonic acid. This loss of acid causes the pH of the sample to increase (become more alkaline).

The kidneys regulate bicarbonate (HCO₃⁻) concentration by reabsorbing filtered bicarbonate and generating new bicarbonate, primarily in the renal tubules.
The lungs primarily regulate carbon dioxide (CO₂) levels.

This combination of values points directly to a primary respiratory alkalosis.

• pH > 7.45: This defines alkalosis.

• pCO₂ < 35 mmHg: This indicates that the alkalosis is being driven by the respiratory system. A low pCO₂ (hypocapnia) means too much carbon dioxide is being exhaled, which reduces the carbonic acid in the blood and raises the pH.

This condition is caused by hyperventilation, which can be due to anxiety, pain, fever, or hypoxia.

Why the other options are incorrect:

• a) Respiratory acidosis: This is the opposite pattern: decreased pH and increased pCO₂.

• c) Metabolic acidosis: This shows a decreased pH with a normal or low pCO₂ (as the lungs compensate by hyperventilating).

• d) Mixed disorder: A mixed disorder involves more than one primary process. The given values (increased pH, decreased pCO₂) are a clear, classic match for a single primary disorder—respiratory alkalosis.

This is the classic blood gas pattern for a primary metabolic acidosis.

• pH < 7.35: This defines acidosis.

• Decreased HCO₃⁻ (Bicarbonate): This indicates that the acidosis is being driven by a metabolic cause. A decrease in bicarbonate, which is a base, directly lowers the blood pH.

This condition is caused by either a gain of strong acid (e.g., lactic acid, ketoacids) or a loss of bicarbonate (e.g., diarrhea).

Why the other options are incorrect:

• b) Respiratory acidosis: This shows a decreased pH but an increased pCO₂, with bicarbonate being normal or increased as compensation.

• c) Metabolic alkalosis: This is the opposite pattern: increased pH and increased bicarbonate.

• d) Respiratory alkalosis: This shows an increased pH and decreased pCO₂, with bicarbonate being normal or decreased as compensation.

Respiratory acidosis is characterized by a primary increase in carbon dioxide (pCO₂) and a decrease in blood pH. The body’s compensatory response is metabolic compensation, which is carried out by the kidneys.

Why the other options are incorrect:

• a) Increased respiratory rate: This would be the correction for the problem, not compensation. In respiratory acidosis, the lungs are unable to effectively increase ventilation—that is the cause of the disorder.

• b) Decreased ammonia formation: Ammonia (NH₃) in the kidneys is crucial for excreting hydrogen ions (as ammonium, NH₄⁺). In acidosis, the kidneys increase ammonia formation to get rid of more acid.

• c) Increased blood pCO₂: This is the primary cause of respiratory acidosis, not a compensatory mechanism. Compensation works to counter the primary problem.

The general form of the Henderson-Hasselbalch equation is:
pH = pK + log([A⁻] / [HA])

Where:

• [A⁻] is the concentration of the conjugate base (bicarbonate, HCO₃⁻).

• [HA] is the concentration of the weak acid (carbonic acid, H₂CO₃).

Therefore, for the bicarbonate buffer system, the correct equation is:
pH = pK + log( [HCO₃⁻] / [H₂CO₃] )

Metabolic alkalosis occurs when there is a primary increase in bicarbonate (HCO₃⁻).
This can happen from either:

• Loss of acid (e.g., vomiting stomach acid)

• Gain of bicarbonate (e.g., ingesting antacids)
  Both mechanisms increase the blood pH.

This is a classic blood gas pattern. To diagnose an acid-base disorder, you look at the relationship between pH, pCO₂ (the respiratory component), and HCO₃⁻ (the metabolic component).

• pH < 7.35: This defines acidosis.

• pCO₂ > 45 mmHg: This indicates a respiratory cause, as CO₂ dissolved in the blood forms carbonic acid.

Why the other options are incorrect:

• b) Metabolic Acidosis: This would show a low pH with a low or normal pCO₂ (as the lungs try to compensate by blowing off CO₂).

• c) Respiratory Alkalosis: This would show a high pH and a low pCO₂.

• d) Metabolic Alkalosis: This would show a high pH and a high or normal pCO₂ (as the lungs try to compensate by retaining CO₂).

We can use the Henderson-Hasselbalch equation for blood pH:

pH=pK+log⁡([HCO3−][H2CO3])pH=pK+log([H2​CO3​][HCO3−​]​)

Given:

• [HCO3−]=24 mEq/L[HCO3−​]=24 mEq/L

• [H2CO3]=1.2 mEq/L[H2​CO3​]=1.2 mEq/L

• $\text{pK}=6.1$

• $\log 20=1.3$

Step 1: Calculate the ratio:

[HCO3−][H2CO3]=241.2=20[H2​CO3​][HCO3−​]​=1.224​=20

Step 2: Apply the equation:

$$\text{pH}=6.1+\log (20)=6.1+1.3=7.4$$

Step 3: Match with options:
That corresponds to c) 7.40.

Modern blood gas analyzers have three main electrodes that directly measure:

• pH (potentiometric)

• pCO₂ (potentiometric, via pH change in bicarbonate solution)

• pO₂ (amperometric)

HCO₃⁻ and %O₂ saturation are calculated or derived (HCO₃⁻ from Henderson–Hasselbalch; %O₂ saturation from pO₂ and using oxygen dissociation curve or co-oximetry if available — but co-oximetry is a separate direct measurement if included).

Approximately 70-80% of the carbon dioxide (CO₂) transported in the blood is converted into bicarbonate ion (HCO₃⁻) within the red blood cells. This is the primary transport mechanism for CO₂.

• Dissolved CO₂ makes up only about 5-10%.

• Carbamino compounds (where CO₂ binds to hemoglobin) account for another 15-20%.

• Carbonic acid and carbonate are present in negligible amounts.

Respiratory acidosis occurs when the lungs cannot remove enough CO₂, leading to an increase in blood CO₂ (hypercapnia) and a subsequent decrease in pH.
This is often due to hypoventilation from conditions like lung disease, airway obstruction, or depression of the respiratory center.

Option breakdown:

• a) Excess CO₂ retention → Main cause of respiratory acidosis.

• b) Loss of bicarbonate → Seen in metabolic acidosis, not respiratory.

• c) Increased ventilation → Causes CO₂ loss → respiratory alkalosis.

• d) Increased hydrogen ion excretion → A compensatory mechanism by kidneys, not a cause.

The anion gap (AG) is calculated using measured electrolytes to check for consistency and detect errors in lab results or underlying metabolic disorders. The formula is usually:

$$\text{AG}=(\text{Na⁺}+\text{K⁺})-(\text{Cl⁻}+\text{HCO₃⁻})$$

• Sometimes potassium (K⁺) is omitted: $\text{AG}=\text{Na⁺}-(\text{Cl⁻}+\text{HCO₃⁻})$

• It is useful for quality control of electrolyte measurements and for evaluating metabolic acidosis.

Why the other options are incorrect:

• a) Amino acids and proteins: While proteins (especially albumin) contribute to the unmeasured anions that define the gap, the anion gap itself is not used for QC of their specific analytical methods.

• b) Blood gas analyses: Blood gas analyzers measure pH, pCO₂, and pO₂ directly. Bicarbonate is a calculated value on a blood gas report. The anion gap uses the bicarbonate from the electrolyte panel, not the blood gas analyzer, for its QC purpose.

• d) Calcium, phosphorus, and magnesium: These are not part of the anion gap calculation. They are important electrolytes, but their results are verified through other QC means.

The normal arterial bicarbonate (HCO₃⁻) concentration range is 22–26 mmol/L.
This range is maintained by the kidneys and is a key component of the blood’s acid-base balance, acting as the primary metabolic buffer

If a blood gas sample is left at room temperature and not iced, cells in the blood (especially leukocytes, erythrocytes, platelets) continue metabolizing glucose → produce lactic acid and CO₂.

Effects:

• Lactic acid → lowers pH (fall in pH).

• Metabolism consumes O₂ → pO₂ falls.

• CO₂ production from aerobic/anaerobic metabolism → pCO₂ rises.

So: pH falls, pCO₂ rises, pO₂ falls.

• The normal blood pH in a healthy adult is approximately 7.35 – 7.45, with an average of 7.4.

• This slightly alkaline pH is tightly regulated by buffer systems, lungs, and kidneys to maintain proper enzyme function and metabolic balance.

Option breakdown:

• a) 6.8 → Too acidic; life-threatening acidosis.

• b) 7.0 → Neutral, but still too low for blood; indicates severe acidosis.

• c) 7.4 → Normal physiological pH.

• d) 7.8 → Too alkaline; incompatible with life.

Respiratory acidosis is caused by a primary failure of ventilation, leading to an increase in pCO₂. The body’s compensatory mechanism for this is metabolic compensation, which is carried out by the kidneys.

• The kidneys respond by:

  1. Increasing the reabsorption of bicarbonate (HCO₃⁻) from the renal tubules back into the blood.

  2. Increasing the excretion of hydrogen ions (H⁺) in the urine, often in the form of titratable acid and ammonium (NH₄⁺).

• This process increases the plasma bicarbonate concentration, which helps to neutralize the acid and raise the pH back toward the normal range.

Why the other options are incorrect:

• a) Increased renal excretion of bicarbonate: This would worsen the acidosis. This is a compensatory mechanism for respiratory alkalosis.

• c) Increased ventilation: This would correct the primary problem (high pCO₂), not compensate for it. In respiratory acidosis, the lungs are unable to increase ventilation effectively—that’s the cause of the disorder.

• d) Decreased CO₂ production: The body has limited direct control over CO₂ production, which is primarily a result of metabolic processes. This is not a major compensatory mechanism.

• The lungs are primarily responsible for regulating carbon dioxide (CO₂) levels in the blood.

• CO₂ combines with water to form carbonic acid (H₂CO₃), which affects blood pH:

$$\text{CO₂}+\text{H₂O}\leftrightarrow \text{H₂CO₃}\leftrightarrow \text{H⁺}+\text{HCO₃⁻}$$

• When blood CO₂ increases, pH drops (acidosis), and the respiratory center in the brain increases the respiratory rate to expel more CO₂.

• When blood CO₂ decreases, pH rises (alkalosis), and breathing slows down.

Option breakdown:

• a) Liver → Metabolic functions, not gas regulation.

• b) Lungs →  Regulate CO₂ (and therefore carbonic acid) — respiratory control of pH.

• c) Kidney → Regulates bicarbonate (HCO₃⁻), not CO₂.

• d) Stomach → Produces gastric acid, unrelated to CO₂ balance.

• Diabetic ketoacidosis (DKA) is characterized by the accumulation of ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone).

• Ketone bodies are organic acids, which increase hydrogen ion concentration, leading to metabolic acidosis.

The other options

• b) High pH: This would be seen in a state of alkalosis, which is the opposite of DKA.

• c) Low pO₂ & d) High pO₂: Oxygen levels (pO₂) are not directly related to the metabolic acidosis of DKA. A patient’s pO₂ would depend on their respiratory function and lung status, not the ketoacidosis itself. While not a primary feature, a severely ill DKA patient could develop respiratory complications that might affect pO₂, but a low or high pO₂ is not an expected or diagnostic finding for the condition itself.

• Metabolic alkalosis → primary problem is excess bicarbonate (HCO₃⁻) → pH ↑.

• TCO₂ (total CO₂) mainly reflects bicarbonate levels in blood.

• Therefore, in metabolic alkalosis:

  • HCO₃⁻ ↑

  • TCO₂ ↑

• Low TCO₂ or H₂CO₃ would be consistent with metabolic acidosis instead.

Why the other options are incorrect:

• b) Low TCO₂, increased HCO₃⁻: This is contradictory. If HCO₃⁻ is increased, TCO₂ cannot be low.

• c) High TCO₂, decreased H₂CO₃: A decreased H₂CO₃ (which is proportional to pCO₂) indicates a respiratory alkalosis, not a metabolic alkalosis. In a compensated metabolic alkalosis, pCO₂ may be slightly elevated, not decreased.

• d) Low TCO₂, decreased H₂CO₃: This pattern is consistent with metabolic acidosis (low TCO₂/HCO₃⁻) with respiratory compensation (low H₂CO₃/pCO₂).

A blood pH below 7.35 defines a state of acidosis, where the blood has an excess of acid or a deficiency of base. This disrupts normal physiological processes and can be caused by respiratory or metabolic problems.

Option breakdown:

• a) Acidosis →  pH < 7.35

• b) Alkalosis → pH > 7.45

• c) Neutrality → pH = 7.0

• d) Normal pH → 7.35–7.45

• The chloride shift (or Hamburger phenomenon) occurs in systemic capillaries:

  • CO₂ enters RBCs → converted to HCO₃⁻ by carbonic anhydrase.

  • HCO₃⁻ exits the RBC into plasma.

  • To maintain electrical neutrality, Cl⁻ moves into the RBC.

Why the other options are incorrect:

• b) Conversion of CO₂ into carbonic acid: This is the action of carbonic anhydrase, not the chloride shift itself.

• c) Movement of sodium out of cells / d) Exchange of potassium for sodium: These are not part of this specific gas exchange mechanism.

This ratio is derived from the Henderson-Hasselbalch equation for the bicarbonate buffer system:

pH = pKa + log₁₀([HCO₃⁻] / [H₂CO₃])

Where:

• The pKa for the bicarbonate/carbonic acid system is 6.1.

• The concentration of carbonic acid ([H₂CO₃]) is proportional to the pCO₂. A normal pCO₂ of 40 mmHg corresponds to a [H₂CO₃] of approximately 1.2 mmol/L.

• A normal bicarbonate ([HCO₃⁻]) level is approximately 24 mmol/L.

Plugging these values into the equation:
pH = 6.1 + log₁₀(24 / 1.2)
pH = 6.1 + log₁₀(20)
pH = 6.1 + 1.3
pH = 7.40

This calculation confirms that at a normal pH of 7.40, the ratio of bicarbonate to carbonic acid is 20:1. This 20:1 ratio is fundamental to understanding acid-base physiology.

• Arterial blood is preferred because it accurately reflects:

  • Oxygenation (pO₂)

  • Carbon dioxide levels (pCO₂)

  • Acid–base status (pH, HCO₃⁻)

• Venous blood can be used in some situations, but it does not reliably measure oxygenation.

• Capillary samples are sometimes used in neonates, and serum/plasma are not suitable for gas measurements.

Why the other options are incorrect:

• a) Venous blood: Venous blood gas can be used to assess metabolic acid-base status, but it cannot evaluate oxygen status (pO₂) because the oxygen has already been extracted by the tissues. It is not the sample of choice for a full blood gas analysis.

• c) Capillary plasma: While capillary blood (often from a heel stick in infants) can be used as a substitute when arterial sampling is difficult, it requires proper warming of the site to “arterialize” the sample and is still less accurate than a direct arterial sample.

• d) Serum: Serum is the liquid fraction of clotted blood and cannot be used for blood gas analysis.

This combination of values is the defining laboratory finding for a primary metabolic alkalosis.

• pH > 7.45: This indicates alkalosis.

• Increased HCO₃⁻ (Bicarbonate): This indicates that the alkalosis is being driven by a metabolic cause. An increase in bicarbonate, which is a base, directly raises the blood pH.

This condition is caused by a primary increase in bicarbonate concentration, which can occur due to loss of acid (e.g., vomiting) or gain of base (e.g., antacid ingestion).

Why the other options are incorrect:

• b) Metabolic acidosis: This is the opposite pattern: decreased pH and decreased bicarbonate.

• c) Respiratory acidosis: This shows a decreased pH and increased pCO₂, with bicarbonate being normal or increased only as compensation.

• d) Respiratory alkalosis: This shows an increased pH and decreased pCO₂, with bicarbonate being normal or decreased as compensation

• hronic renal failure → kidneys cannot excrete hydrogen ions (H⁺) or reabsorb bicarbonate (HCO₃⁻) effectively.

• This leads to accumulation of acids and decreased bicarbonate → metabolic acidosis.

• The respiratory system may partially compensate by hyperventilation (Kussmaul breathing) to blow off CO₂.

Why the other options are incorrect:

• b) Respiratory Acidosis: This is caused by impaired lung function (hypoventilation), not kidney failure.

• c) Metabolic Alkalosis: This would involve a high pH and high HCO₃⁻, which is the opposite of what occurs in renal failure.

• d) Respiratory Alkalosis: This is caused by hyperventilation and results in a high pH and low pCO₂, but with a normal or low HCO₃⁻. It is not the primary pattern of chronic renal failure.

Prolonged vomiting causes metabolic alkalosis by losing large amounts of gastric acid (hydrochloric acid), which removes hydrogen ions from the body.

• Severe diarrhea typically causes metabolic acidosis (loss of bicarbonate).

• Starvation and diabetic ketoacidosis cause metabolic acidosis (ketoacid accumulation).

• Emphysema → destruction of alveolar walls → impaired gas exchange.

• Fluid accumulation in alveoli further reduces CO₂ elimination → CO₂ retention.

• CO₂ retention → increased pCO₂ → decreased pH → respiratory acidosis.

• Chronic cases may show renal compensation with HCO₃⁻ retention.

Why the other options are incorrect:

• b) Respiratory Alkalosis: This is caused by hyperventilation and CO₂ loss, the exact opposite of what is happening here.

• c) Metabolic Acidosis & d) Metabolic Alkalosis: These are driven by changes in bicarbonate (HCO₃⁻) levels due to renal or metabolic issues, not by a primary failure of ventilation. While the kidneys will attempt to compensate for the respiratory acidosis by retaining bicarbonate, the primary disorder in this scenario is respiratory.

Severe diarrhea causes Metabolic Acidosis through a specific mechanism:

• The lower gastrointestinal tract, especially the colon, secretes bicarbonate (HCO₃⁻) into the intestines.

• Normally, this bicarbonate is reabsorbed. However, during severe diarrhea, large amounts of bicarbonate are lost in the watery stool.

• This loss of bicarbonate, which is a base, reduces the blood’s buffering capacity and leads to an accumulation of hydrogen ions (H⁺), resulting in acidosis.

This is often referred to as a normal anion gap metabolic acidosis (or hyperchloremic metabolic acidosis) because the primary problem is the direct loss of bicarbonate.

Why the other options are incorrect:

• b) Metabolic Alkalosis: This is typically caused by loss of acid, such as from vomiting (loss of gastric HCl).

• c) Respiratory Acidosis: This is caused by impaired lung function and CO₂ retention (hypoventilation).

• d) Respiratory Alkalosis: This is caused by hyperventilation and excessive loss of CO₂.

• pH = 7.32 (low → acidemia)

• pCO₂ = 78 mm Hg (high → respiratory acidosis)

• HCO₃⁻ = 39 mEq/L (high → metabolic compensation)

Step 1: Identify primary disorder

• Low pH + high pCO₂ → primary respiratory acidosis

Step 2: Look for compensation

• Kidneys compensate for chronic respiratory acidosis by retaining bicarbonate (HCO₃⁻) to buffer the blood.

• Elevated HCO₃⁻ confirms metabolic (renal) compensation.

Step 3: Conclusion

• This is chronic respiratory acidosis with metabolic compensation.

• The pH electrode uses a glass membrane that is selectively permeable to hydrogen ions (H⁺).

• The voltage difference generated across the membrane is proportional to the H⁺ concentration, which is then converted to pH.

• Other ions (Na⁺, Cl⁻, HCO₃⁻) do not directly affect the pH electrode under normal conditions.

We use the Henderson–Hasselbalch equation for pH in blood:

pH=pK+log⁡([HCO3−]0.03×pCO2)pH=pK+log(0.03×pCO2​[HCO3−​]​)

Where:

• pK = 6.1

• [HCO₃⁻] = 18 mmol/L

• pCO₂ = 60 mm Hg

• 0.03 = solubility coefficient of CO₂ in mmol/L/mmHg

pH=6.1+log⁡(180.03×60)pH=6.1+log(0.03×6018​)$$0.03\times 60=1.8$$181.8=101.818​=10$$\log (10)=1$$$$\text{pH}=6.1+1=7.1$$

• Metabolic acidosis occurs when there is a primary decrease in bicarbonate (HCO₃⁻), leading to a decreased blood pH.

• The body compensates by hyperventilating (Kussmaul respirations) to blow off CO₂, which lowers pCO₂.

• So, in metabolic acidosis:

  • pH: decreased

  • HCO₃⁻: decreased

  • pCO₂: decreased (compensatory)

Aldosterone secretion is directly stimulated by Angiotensin II, which is the active component of the Renin-Angiotensin-Aldosterone System (RAAS).

• Renin (a) converts angiotensinogen to angiotensin I.

• Angiotensinogen (b) is the precursor protein.

• Angiotensin I (c) is converted to angiotensin II by angiotensin-converting enzyme (ACE).

An arterial blood gas (ABG) sample must be analyzed within 15-30 minutes of collection if kept at room temperature, or within 1-2 hours if kept strictly on ice. A sample received at 11:00 AM that was drawn at 8:30 AM is 2.5 hours old.

Even if it was on ice, this exceeds the generally accepted maximum stability time. Cellular metabolism (glycolysis) continues slowly, consuming oxygen and producing acids (like lactic acid and CO₂), which significantly alters the critical pH, pCO₂, and pO₂ results, making them unreliable for clinical decisions. The only correct action is to reject the specimen and request a new one.

• The bicarbonate–carbonic acid (HCO₃⁻/H₂CO₃) buffer system is the primary extracellular buffer in plasma.

• It maintains blood pH around 7.35–7.45 by neutralizing acids or bases.

• Other buffers:

  • Phosphate → important in renal tubules

  • Hemoglobin/Imidazole → major intracellular buffer in red blood cells

  • Sulfate/Bisulfate → minor role

Why the other options are incorrect:

• a) Phosphate/Biphosphate (HPO₄²⁻/H₂PO₄⁻): This is an effective buffer, but its concentration in the blood plasma is too low to play a major role. It is the primary buffer system within cells and in the urine.

• b) Hemoglobin/Imidazole: This is an extremely important buffer inside red blood cells for managing CO₂ transport and buffering hydrogen ions, but it is not the primary buffer in the plasma itself.

• d) Sulfate/Bisulfate: This pair is not a significant physiological buffer system in the human body.

• Metabolic acidosis occurs when there is an accumulation of acids or a loss of bicarbonate in the body.

• In starvation or diabetic ketoacidosis (DKA), the body breaks down fats for energy, producing ketone bodies (acids) such as acetoacetic acid and β-hydroxybutyric acid — leading to acidosis.

Option breakdown:

• a) Vomiting → causes metabolic alkalosis (loss of stomach acid)

• b) Starvation or diabetic ketoacidosis → causes metabolic acidosis

• c) Anxiety → causes respiratory alkalosis due to hyperventilation

• d) High altitude → causes respiratory alkalosis due to increased ventilation

The buffering capacity of blood is largely maintained by the bicarbonate–chloride exchange, known as the chloride shift:

• In red blood cells, CO₂ combines with H₂O → H₂CO₃ → H⁺ + HCO₃⁻.

• To maintain electroneutrality, bicarbonate (HCO₃⁻) exits the RBC into plasma, and chloride (Cl⁻) moves into the RBC.

• This reversible exchange helps buffer pH and transport CO₂ efficiently from tissues to lungs.

Why the other options are incorrect:

• a) Sodium & b) Potassium: While these are major cations in the blood, they are not directly involved in this specific, reversible exchange process for pH buffering in the same way chloride is.

• c) Calcium: Calcium homeostasis is critical for many functions, but it is not part of the rapid acid-base buffering system involving bicarbonate exchange.

The normal ratio of bicarbonate (HCO₃⁻) to carbonic acid (H₂CO₃) in blood is 20:1.
This ratio is key to maintaining the normal arterial pH of approximately 7.4, as described by the Henderson–Hasselbalch equation for the bicarbonate buffer system:
pH = 6.1 + log( [HCO₃⁻] / [H₂CO₃] )

• rterial blood gas (ABG) samples must remain unclotted to accurately measure pH, pCO₂, pO₂, and electrolytes.

• Heparin is the anticoagulant of choice because:

  • It does not significantly alter pH or electrolytes.

  • Available as liquid or dry-coated in syringes.

The bicarbonate–carbonic acid (H₂CO₃/HCO₃⁻) system is the primary buffer in the blood plasma. It is the most important because it is an open system: the lungs can rapidly adjust the level of carbonic acid (by exhaling CO₂), and the kidneys can regulate the level of bicarbonate.

Option breakdown:

• a) Hemoglobin buffer → Important for buffering in RBCs, but not the main system.

• b) Phosphate buffer → Important in urine and intracellular fluids, not blood.

• c) Bicarbonate–carbonic acid buffer →  Main blood buffer system.

• d) Protein buffer → Plays a secondary role.

A blood pH above 7.45 defines a state of alkalosis, where the blood has an excess of base or a deficiency of acid. This is the opposite of acidosis and can be caused by conditions like hyperventilation or vomiting.

Option breakdown:

• a) Acidosis → pH < 7.35

• b) Alkalosis →  pH > 7.45

• c) Hypoxia → Low oxygen level, not defined by pH.

• d) Hypercapnia → High CO₂, usually causes acidosis, not alkalosis.

• The pCO₂ electrode (Severinghaus electrode) is essentially a modified pH electrode.

• How it works:

  1. CO₂ from the blood diffuses through a CO₂-permeable membrane into a bicarbonate-containing buffer.

  2. CO₂ reacts with water → H₂CO₃ → H⁺ + HCO₃⁻

  3. The change in H⁺ concentration is detected by the pH electrode.

• So, the pCO₂ measurement is indirect, relying on the pH change caused by dissolved CO₂.

• Metabolic alkalosis → primary problem is high HCO₃⁻ → pH ↑.

• Respiratory compensation: the lungs slow down ventilation → retain CO₂ → forms more carbonic acid → helps lower pH toward normal.

• This is a partial compensation; the kidneys also eventually excrete bicarbonate to restore balance.

Why the other options are incorrect:

• b) Increasing CO₂ elimination: This is the compensatory mechanism for metabolic acidosis, not alkalosis. It would make the alkalosis worse.

• c) Increasing bicarbonate excretion: This is a renal compensatory mechanism, not a pulmonary one. The kidneys would excrete more bicarbonate to correct metabolic alkalosis.

• d) Increasing hydrogen ion excretion: The kidneys would actually decrease H⁺ excretion to help correct alkalosis. Retaining H⁺ ions adds acid to the blood.

Respiratory alkalosis occurs when excessive breathing (hyperventilation) removes too much CO₂ from the blood.
This loss of CO₂ reduces the concentration of carbonic acid, causing the blood pH to rise. Common causes include anxiety, fever, or hypoxia.

Option breakdown:

• a) CO₂ retention → Causes respiratory acidosis, not alkalosis.

• b) Hyperventilation →  Causes CO₂ loss → respiratory alkalosis.

• c) Increased acid production → Causes metabolic acidosis.

• d) Bicarbonate loss → Causes metabolic acidosis.

The pH of arterial blood is tightly regulated. The normal reference range is 7.35 to 7.45.

• A pH below 7.35 is defined as acidemia (the blood is too acidic).

• A pH above 7.45 is defined as alkalemia (the blood is too alkaline).

• The narrow range of 7.35-7.45 is crucial for optimal functioning of cellular enzymes and physiological processes.

Why the other options are incorrect:

• a) 7.28-7.34: This range is too low and would be considered acidemic.

• b) 7.33-7.37: This range is slightly low and does not include the upper part of the normal range.

• d) 7.45-7.50: This range is too high and would be considered alkalemic.

The pO₂ electrode (Clark electrode) measures current produced when oxygen is reduced at the cathode.

• Potentiometry → measures voltage at zero current (used for pH, ion-selective electrodes).

• Amperometry → measures current at constant voltage (used for pO₂ electrode).

• Coulometry → measures total charge over time.

• Conductometry → measures electrolyte conductivity.

Respiratory acidosis is caused by the inability to remove enough CO₂ from the blood.
Hypoventilation (reduced breathing) is the primary mechanism, leading to CO₂ retention (hypercapnia) and a decrease in blood pH. Common causes include COPD, drug overdose, and severe asthma.

Option breakdown:

• a) Hyperventilation → causes respiratory alkalosis

• b) Hypoventilation → causes respiratory acidosis

• c) Vomiting → causes metabolic alkalosis

• d) Dehydration → unrelated to CO₂ retention

The partial pressure of carbon dioxide (pCO₂) is a measure of the tension of carbon dioxide dissolved in the arterial blood. It is the respiratory component of the acid-base balance.

• The normal, healthy range for arterial pCO₂ is 35 to 45 mmHg.

• Values below 35 mmHg indicate hypocapnia (often from hyperventilation).

• Values above 45 mmHg indicate hypercapnia (often from hypoventilation).