CUET UG Biology Animal Physiology Breathing and Gas Exchange Lung Volumes Oxygen Dissociation Curve

Must‑Know (15–20 detailed bullets)

• Total Lung Capacity (TLC) is the maximum volume of air in the lungs after forced inhalation; in adults, it averages about 5000–6000 mL.
• Tidal Volume (TV) is the volume of air inhaled or exhaled during normal breathing; it is approximately 500 mL in a healthy adult.
• Inspiratory Reserve Volume (IRV) is the additional volume of air that can be inhaled forcibly after a normal inspiration; it is about 2500–3000 mL.
• Expiratory Reserve Volume (ERV) is the additional volume of air that can be exhaled forcibly after a normal expiration; it averages 1000–1100 mL.
• Residual Volume (RV) is the volume of air remaining in the lungs after maximum expiration; it is about 1100–1200 mL and cannot be measured by spirometry.
• Vital Capacity (VC) = TV + IRV + ERV; it represents the maximum volume of air a person can exhale after maximum inhalation; normal value is about 4000–4600 mL.
• Inspiratory Capacity (IC) = TV + IRV; it is the total volume of air that can be inhaled after a normal expiration; approximately 3000–3500 mL.
• Functional Residual Capacity (FRC) = ERV + RV; it is the volume of air remaining in the lungs after normal expiration; about 2100–2400 mL.
• FRC cannot be measured directly by spirometry because it includes Residual Volume.
• The Oxygen Dissociation Curve is sigmoid (S-shaped) due to cooperative binding of oxygen to hemoglobin.
• At PO₂ of 100 mm Hg (in alveoli), hemoglobin is about 98% saturated with oxygen.
• At PO₂ of 40 mm Hg (in tissues), hemoglobin saturation drops to about 75%, allowing release of oxygen to tissues.
• A right shift in the oxygen dissociation curve indicates decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading in tissues.
• Factors causing right shift: increased CO₂, decreased pH (Bohr effect), increased temperature, increased 2,3-BPG.
• A left shift indicates increased affinity of hemoglobin for oxygen, seen in decreased temperature, decreased CO₂, increased pH, fetal hemoglobin (HbF).
• Fetal hemoglobin (HbF) has higher affinity for oxygen than adult hemoglobin (HbA), enabling oxygen transfer from maternal to fetal blood.
• Carbon dioxide is transported in blood as bicarbonate (70%), carbaminohemoglobin (20–25%), and dissolved in plasma (7%).
• The chloride shift (Hamburger phenomenon) occurs in RBCs: HCO₃⁻ diffuses out and Cl⁻ moves in to maintain ionic balance during CO₂ transport.
• In the lungs, reverse chloride shift occurs: HCO₃⁻ re-enters RBCs and combines with H⁺ to form H₂CO₃, which dissociates into CO₂ and H₂O.
• verify from NCERT: Exact percentage of oxygen dissolved in plasma vs bound to hemoglobin.

Difficulty Level

Intermediate — Requires understanding of lung volumes, their interrelationships, and physiological significance of the oxygen dissociation curve with numerical values.

Common CUET Traps (3 bullets)

• Trap: Assuming Residual Volume can be measured by spirometry. Avoid: RV cannot be measured by spirometry; requires body plethysmography or gas dilution techniques.
• Trap: Confusing right shift with increased oxygen affinity. Avoid: Right shift means decreased affinity, promoting oxygen unloading in tissues.
• Trap: Thinking tidal volume includes reserve volumes. Avoid: Tidal volume is only the air moved during normal breathing; IRV and ERV are extra volumes used during forced efforts.

Practice MCQs (5 questions)

Q1. What is the typical tidal volume in a healthy adult human?
A. 1200 mL
B. 500 mL
C. 2300 mL
D. 4600 mL
Answer: B
Explanation: Tidal volume is approximately 500 mL during normal breathing.
Why others fail: Option D (4600 mL) refers to vital capacity, often confused with tidal volume.

Q2. Which lung volume cannot be measured directly using a spirometer?
A. Tidal Volume
B. Expiratory Reserve Volume
C. Residual Volume
D. Inspiratory Reserve Volume
Answer: C
Explanation: Residual volume remains in the lungs after maximal expiration and cannot be assessed by spirometry.
Why others fail: Students assume all lung volumes are spirometry-measurable; RV requires indirect methods.

Q3. A right shift in the oxygen dissociation curve occurs due to:
A. Decreased temperature
B. Decreased CO₂ levels
C. Increased pH
D. Increased 2,3-BPG
Answer: D
Explanation: Increased 2,3-BPG decreases hemoglobin’s affinity for oxygen, causing a right shift.
Why others fail: Options A, B, and C cause a left shift; students mix up directionality of shifts.

Q4. In tissues where pH is low and CO₂ is high, hemoglobin:
A. Binds oxygen more readily
B. Shows increased affinity for oxygen
C. Releases oxygen more easily
D. Forms more carbaminohemoglobin only
Answer: C
Explanation: Low pH and high CO₂ cause Bohr effect, promoting oxygen unloading.
Why others fail: Option A is tempting due to misunderstanding of Bohr effect; low pH reduces affinity.

Q5. If a person has a tidal volume of 500 mL, IRV of 3000 mL, and ERV of 1000 mL, what is their vital capacity?
A. 3500 mL
B. 4000 mL
C. 4500 mL
D. 5000 mL
Answer: C
Explanation: VC = TV + IRV + ERV = 500 + 3000 + 1000 = 4500 mL.
Why others fail: Students may omit one component (e.g., forget ERV) and choose 3500 mL (TV + IRV only).

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ TLC = IRV + TV + ERV + RV — all four lung volumes combined.
• ⚠️ VC = TV + IRV + ERV — does not include residual volume.
• ⚠️ FRC = ERV + RV — volume in lungs after normal expiration.
• ⚠️ IC = TV + IRV — maximum air inhaled after normal expiration.
• ⚠️ RV ≈ 1100–1200 mL — not measurable by spirometry.
• ⚠️ TV = 500 mL — volume per breath at rest.
• ⚠️ Sigmoid O₂ dissociation curve due to cooperative binding in hemoglobin.
• ⚠️ At 100 mm Hg PO₂, Hb is 98% saturated.
• ⚠️ At 40 mm Hg PO₂, Hb saturation is ~75%.
• ⚠️ Right shift = more O₂ release; caused by ↑CO₂, ↑temp, ↓pH, ↑2,3-BPG.
• ⚠️ Left shift = less O₂ release; seen in fetal Hb, ↓temp, ↓CO₂, ↑pH.
• ⚠️ Bohr effect: CO₂ and H⁺ reduce Hb’s O₂ affinity.
• ⚠️ HbF has higher O₂ affinity than HbA — crucial for placental O₂ transfer.
• ⚠️ 70% CO₂ transported as HCO₃⁻ in plasma.
• ⚠️ Chloride shift: Cl⁻ enters RBC as HCO₃⁻ leaves — maintains electrical neutrality.
• ⚠️ Reverse chloride shift occurs in lungs — HCO₃⁻ re-enters RBCs.
• ⚠️ CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ — catalyzed by carbonic anhydrase in RBCs.
• ⚠️ Mnemonic: “Right for Release” — right shift promotes O₂ unloading.
• ⚠️ Haldane effect: Deoxygenated Hb carries more CO₂.
• ⚠️ verify from NCERT: Exact numerical values for O₂ content in arterial vs venous blood.