Case 3 - Complete Answers

Patient Summary

• 45-year-old male, Type II DM, unconscious
• Deep and labored (Kussmaul) breathing
• Dehydrated
• Blood glucose: 400 mg%
• ABG: pH 7.0, HCO3- 20 mEq/L, PCO2 40 mmHg

Q1. Comment on His Acid-Base Status

• Severe hyperglycemia (400 mg%) with insufficient insulin leads to increased fat breakdown and ketone body production (acetoacetic acid, beta-hydroxybutyric acid, acetone)
• Ketoacids accumulate, consuming bicarbonate, dropping pH
• The deep, labored (Kussmaul) breathing is the respiratory compensatory response - the body tries to blow off CO2 to raise pH
• Normally in metabolic acidosis, PCO2 should fall below 35 mmHg (respiratory compensation). Here PCO2 is still 40 mmHg, suggesting compensation is insufficient or early, contributing to the very low pH of 7.0
• The anion gap is elevated (due to accumulation of ketoacid anions)

Q2. Define Buffers

• When acid (H+) is added: the conjugate base accepts the H+ to form the weak acid - pH change is minimized
• When alkali (OH-) is added: the weak acid donates H+ to neutralize it - pH change is minimized

NaOH + H2CO3 → NaHCO3 + H2O

• Mulholland and Greenfield's Surgery, 7e; Basic Medical Biochemistry - A Clinical Approach, 6e

Q3. Enumerate the Important Buffer Systems of the Body

1. Bicarbonate-Carbonic Acid Buffer System (HCO3- / H2CO3)

• Location: Extracellular fluid (ECF), blood plasma
• The most important extracellular buffer
• CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (catalyzed by carbonic anhydrase)
• pKa = 6.1 (not ideal), but highly effective because:
  • Large amounts of bicarbonate are available
  • CO2 (the acid form) is rapidly excreted by the lungs
  • Metabolic CO2 provides an inexhaustible supply
• Regulated by both lungs (PCO2) and kidneys (HCO3-)

2. Hemoglobin (Hb) Buffer System

• Location: Red blood cells (RBCs)
• Hb is a protein buffer that accepts and releases H+
• Deoxygenated Hb is a better H+ acceptor (buffer) than oxygenated Hb
• Closely linked to O2 transport and CO2 transport (chloride shift)
• At the tissues: CO2 enters RBCs → carbonic acid → H+ buffered by Hb; HCO3- exits into plasma
• At the lungs: process reverses; CO2 is exhaled

3. Phosphate Buffer System (H2PO4- / HPO4 2-)

• Location: Intracellular fluid (ICF), renal tubular fluid, and some in plasma
• pKa = 7.2 - ideal for intracellular buffering (close to intracellular pH of ~7.1)
• Organic phosphates (ATP, glucose-6-phosphate) also contribute intracellularly
• Important in renal regulation of acid-base balance (urinary buffering)

4. Protein Buffer System

• Location: Plasma proteins (especially albumin) and intracellular proteins
• Proteins contain histidine and other amino acid side chains that can accept or donate H+
• Albumin is the major plasma protein buffer
• Intracellular proteins are important ICF buffers

"The major buffer systems in the body are the bicarbonate-carbonic acid buffer system, which operates principally in ECF; the hemoglobin (Hb) buffer system in red blood cells; the phosphate buffer system in all types of cells; and the protein buffer system of cells and plasma." - Basic Medical Biochemistry - A Clinical Approach, 6e

Q4. Why Is pH of Venous Blood Less Than Arterial Blood?

1. CO2 production from metabolism: Cells continuously produce CO2 during oxidative metabolism (TCA cycle). This CO2 diffuses into the blood.
2. CO2 dissolves and forms carbonic acid: CO2 + H2O → H2CO3 → H+ + HCO3-
3. Accumulation of H+ ions: The dissociation of carbonic acid releases H+ ions into venous blood, lowering the pH.
4. Higher PCO2 in venous blood: Venous PCO2 averages 6-8 mmHg higher than arterial PCO2. Per the Henderson-Hasselbalch equation, a higher PCO2 directly lowers pH.
5. Lactic acid and other metabolic acids from actively metabolizing tissues also contribute small amounts of H+ to venous blood.

"Venous blood gas: PvCO2 averages 6 to 8 mmHg higher than PaCO2; venous pH is slightly lower than arterial pH." - The Harriet Lane Handbook, 23rd ed.

"Venous blood pH is usually 0.05 lower than arterial blood pH." - Morgan and Mikhail's Clinical Anesthesiology, 7e

Q5. Define the Terms Acid and Base

Acid (Bronsted-Lowry Definition)

• Strong acids dissociate completely (e.g., HCl → H+ + Cl-)
• Weak acids dissociate partially (e.g., H2CO3 ⇌ H+ + HCO3-)
• In the body, carbonic acid (H2CO3), ketoacids, lactic acid, etc. are physiologically important acids

Base (Bronsted-Lowry Definition)

• Strong bases fully accept protons (e.g., NaOH → Na+ + OH-; OH- accepts H+ to form water)
• Weak bases partially accept protons (e.g., HCO3- + H+ ⇌ H2CO3)
• In the body, bicarbonate (HCO3-), hemoglobin, proteins, and phosphates act as important bases

Normal Blood pH

• Normal arterial blood pH: 7.35-7.44
• pH < 7.35 = Acidosis
• pH > 7.45 = Alkalosis
• The body's average metabolic activity produces approximately 22,000 mEq of acid per day, all of which must be buffered and excreted to maintain normal pH.

"The pH of the blood is normally maintained between 7.36 and 7.44." - Basic Medical Biochemistry - A Clinical Approach, 6e

Case - Complete Answers

Patient Summary

• 35-year-old female
• Increased menstrual blood loss x 3 months
• Breathlessness, loss of appetite, apathy, easy fatigability x 2 months
• Pallor, koilonychia (spoon-shaped nails), tachycardia, systolic murmur

Q1. State the Diagnosis

• Cause: Chronic menstrual blood loss (menorrhagia) for 3 months - the most important cause of IDA in women of reproductive age
• Symptoms: Breathlessness, fatigue, apathy (due to reduced O2 carrying capacity), koilonychia (nail changes characteristic of IDA), pallor
• Tachycardia and systolic murmur: Cardiac compensation for severe anemia (flow murmur)
• Lab confirmation:
  • Low Hb (6 g% vs. normal 12-14 g%)
  • Low ferritin (9 ng/ml) = depleted iron stores
  • Low MCV = microcytic cells
  • Low MCHC = hypochromic cells
  • Low RBC count
  • PBF: Microcytic and hypochromic RBCs

"Iron deficiency is the most common nutritional deficiency in the world... The most common sources of bleeding are the gastrointestinal tract and the female genital tract (e.g., menorrhagia, metrorrhagia)." - Robbins & Kumar Basic Pathology

Q2. State the Morphological Classification of Anemia in General

1. Microcytic Hypochromic Anemia (MCV < 80 fl, MCHC < 32%)

• Small, pale RBCs
• Causes:
  • Iron deficiency anemia (most common)
  • Thalassemia
  • Sideroblastic anemia
  • Lead poisoning
  • Anemia of chronic disease (some cases)

2. Normocytic Normochromic Anemia (MCV 80-100 fl, MCHC 32-36%)

• Normal-sized RBCs, normal color
• Causes:
  • Acute blood loss
  • Hemolytic anemia
  • Aplastic anemia
  • Renal failure (reduced EPO)
  • Anemia of chronic disease (most cases)

3. Macrocytic Anemia (MCV > 100 fl)

• Megaloblastic (oval macrocytes + hypersegmented neutrophils):
  • Vitamin B12 deficiency
  • Folate deficiency
• Non-megaloblastic (round macrocytes):
  • Liver disease
  • Hypothyroidism
  • Alcohol excess
  • Reticulocytosis

"Based on the size of the red blood cells (RBCs) or the mean corpuscular volume (MCV), anemia can be classified as microcytic, normocytic, or macrocytic." - Creasy & Resnik's Maternal-Fetal Medicine

Q3. Why Is the MCHC Value Less Than Normal in This Case?

1. Iron is essential for heme synthesis: Heme = protoporphyrin + Fe2+. Without adequate iron, heme cannot be synthesized.
2. Reduced heme → reduced hemoglobin: Hemoglobin = 4 heme groups + 4 globin chains. When iron stores are depleted (ferritin = 9 ng/ml, very low), erythroblasts in bone marrow cannot make adequate hemoglobin.
3. RBCs continue to be produced but are poorly filled with Hb: The cells undergo extra divisions (to "fill up"), resulting in smaller cells (microcytic), but the hemoglobin concentration per cell remains below normal (hypochromic).
4. Result: Each RBC has a lower concentration of hemoglobin than normal → MCHC falls below 32% (hypochromic).

"Regardless of cause, iron deficiency develops insidiously... ultimately, the capacity to synthesize hemoglobin, myoglobin, and other iron-containing proteins is diminished, leading to microcytic anemia." - Robbins & Kumar Basic Pathology

Q4. What Would Be the Line of Management in This Case?

A. Treat the Underlying Cause

• Investigate and manage the menorrhagia (gynecology referral, hormonal therapy, etc.)
• Rule out other causes of blood loss

B. Iron Replacement Therapy (First-line)

• Ferrous sulfate 325 mg once daily on an empty stomach (maximizes absorption)
• Contains ~65 mg elemental iron per tablet
• Traditional dosing: 150-200 mg elemental iron/day (up to 3 times daily), but once-daily dosing shown to be equally effective and better tolerated
• Duration: Continue for 3-6 months after Hb normalizes to replenish iron stores
• Ascorbic acid (Vitamin C): Give with iron - increases absorption by at least 30%
• Side effects: Nausea, heartburn, constipation - start low and increase gradually
• Avoid: Taking with food, antacids, tea, coffee (reduce absorption)

• Ferric carboxymaltose, iron sucrose, iron dextran
• Used when rapid correction is needed

C. Blood Transfusion

• Consider if Hb < 7 g/dL with severe symptoms (this patient Hb = 6 g% - may need transfusion)
• Packed RBC transfusion for symptomatic severe anemia

D. Dietary Advice

• Increase dietary iron (red meat, leafy vegetables, legumes)
• Avoid iron inhibitors (tea, calcium) around meal times

E. Monitor Response

• Reticulocytosis expected within 7-10 days of starting iron
• Hb rises ~1-2 g/dL every 3-4 weeks

"Once-daily administration of ferrous sulfate 325 mg on an empty stomach is a typical dosage that maximizes absorption while maintaining high tolerance." - Goodman & Gilman's Pharmacological Basis of Therapeutics

Q5. Enumerate the Steps of Erythropoiesis

Steps of Erythropoiesis (in sequence):

• Common precursor for all blood cells in bone marrow

• BFU-E (Burst Forming Unit - Erythroid) → CFU-E (Colony Forming Unit - Erythroid)
• Responsive to EPO

• Earliest morphologically recognizable RBC precursor
• Large cell; nucleus occupies most of cell volume; contains nucleoli
• No hemoglobin yet; cytoplasm is basophilic

• Smaller than proerythroblast
• Cytoplasm strongly basophilic - ribosomes actively making hemoglobin
• Nucleus smaller; no nucleoli
• Hemoglobin synthesis begins

• Hemoglobin accumulates - cytoplasm shows blue-gray (mixed) color
• Nucleus still present, smaller, coarser chromatin
• Last stage capable of mitosis (cell division)

• Nucleus very condensed, small, pyknotic
• Cytoplasm predominantly eosinophilic (pink) - hemoglobin nearly complete
• No longer capable of division

• Nucleus extruded (expelled from cell)
• Still contains residual ribosomes - slight basophilia
• Can still synthesize small amounts of hemoglobin
• Released into blood sinuses; matures within 1-2 days in circulation

• No nucleus, no organelles
• Biconcave disc, 7-8 µm diameter
• Filled with hemoglobin
• Lifespan: ~120 days

Summary Table:

"The earliest recognizable precursor of the red blood cell is the proerythroblast... In the next stage, the polychromatophilic erythrocyte, also more commonly called a reticulocyte, has lost its nucleus and is ready to pass into the blood sinuses of the red bone marrow." - Histology: A Text and Atlas, 8e