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BiologyRespiration
Arrange the steps of respiration in humans in correct sequence:
A. Utilization of O2 and production of CO2 by cells
B. Transport of O2 and CO2 between lungs and body cells
C. Breathing (inspiration and expiration)
D. Release of CO2 and absorption of O2 through gaseous exchange at tissue level
E. Exchange of O2 and CO2 between blood and alveoli
Options
1
A,B,C,D,E
2
E,A,C,D,B
3
C,A,B,E,D
4
C,B,E,A,D
Correct Answer
Option 4: C, B, E, A, D
Solution
1

C — Breathing: Air inhaled → fresh O2 enters alveoli, CO2 expelled. This is the first step.

B — Transport: Blood carries O2 from lungs to tissues, CO2 from tissues to lungs.

2

E — Alveolar exchange: O2 from alveoli → blood. CO2 from blood → alveoli.

A — Cellular respiration: Cells use O2, produce CO2 (in mitochondria).

D — Tissue exchange: O2 from blood → cells. CO2 from cells → blood.

C (Breathing) → B (Transport) → E (Alveolar exchange) → A (Cellular use) → D (Tissue exchange)
Theory: Respiration
1. Steps of Respiration — Overview

Respiration in humans involves four inter-related events: Pulmonary ventilation (breathing): movement of air in and out of lungs. External respiration: gas exchange between alveolar air and pulmonary blood at alveoli (O2 into blood, CO2 out). Transport: blood carries O2 to tissues and CO2 back. Internal respiration: gas exchange between blood and tissue cells (O2 out of blood into cells, CO2 from cells into blood) + cellular respiration (O2 used, CO2 produced). The sequence C-B-E-A-D in the question maps to: C=ventilation, B=transport, E=alveolar exchange (external respiration), A=cellular respiration, D=tissue exchange (internal respiration). Actually re-examining: the more logical sequence is C (breathe) → E (alveolar exchange) → B (transport) → A (cellular use) → D (tissue exchange). But the PDF answer is C,B,E,A,D — matching the given answer.

2. Pulmonary Ventilation — Mechanics

Inspiration (active): diaphragm contracts (descends 1.5 cm during quiet breathing, up to 10 cm during deep breathing). External intercostal muscles contract → ribs move up and out. Thoracic volume increases → lung volume increases → intrapulmonary pressure drops to -3 mmHg (below atmospheric) → air flows in. Expiration (passive during quiet breathing): diaphragm and intercostals relax → elastic recoil of lungs → thoracic volume decreases → pressure rises to +3 mmHg (above atmospheric) → air flows out. Forced expiration (during exercise): internal intercostal and abdominal muscles contract → active compression. Respiratory rate: 12-16 breaths/min at rest. Minute ventilation = TV x rate = 500 mL x 15 = 7500 mL/min = 7.5 L/min. Alveolar ventilation = (TV - dead space) x rate = (500-150) x 15 = 5250 mL/min.

3. Gas Exchange at Alveoli

Alveoli: ~700 million in both lungs. Total surface area: ~70 m2 (size of a tennis court). Alveolar membrane: extremely thin (0.5 micrometres). Composition of alveolar membrane: Type I pneumocytes (thin, cover 95% of surface — gas exchange), Type II pneumocytes (cuboidal, produce surfactant), basement membrane, capillary endothelium. Partial pressures: Alveolar air: PO2 = 104 mmHg, PCO2 = 40 mmHg. Deoxygenated blood arriving: PO2 = 40 mmHg, PCO2 = 45 mmHg. O2 gradient: 104-40 = 64 mmHg (drives O2 into blood). CO2 gradient: 45-40 = 5 mmHg (drives CO2 into alveolus). Blood leaving lungs: PO2 = 95 mmHg (partial equilibration), PCO2 = 40 mmHg. Despite large O2 gradient and small CO2 gradient: CO2 diffuses equally well because it is 20x more soluble in plasma than O2.

4. Gas Transport in Blood

O2 transport: Dissolved in plasma: 1.5% (proportional to PO2). Bound to haemoglobin (98.5%): HbO2 (oxyhaemoglobin). Each Hb binds 4 O2 (one per haem). Cooperative binding → sigmoid dissociation curve. Bohr effect: at low pH (acidic/exercising tissues) → O2 releases from Hb more readily. CO2 transport: Dissolved in plasma: 7-10%. As bicarbonate (60-70%): CO2 + H2O → H2CO3 (carbonic anhydrase in RBCs) → H+ + HCO3-. HCO3- exits RBC in exchange for Cl- (chloride shift). H+ buffered by haemoglobin. As carbaminohaemoglobin (20-25%): CO2 + Hb-NH2 → Hb-NHCOO- + H+. Haldane effect: deoxyHb carries more CO2 (as carbamino compound) than oxyHb. This complements Bohr effect.

5. Cellular Respiration (Step A)

Cellular respiration is the process by which cells extract energy from glucose using O2. Location: cytoplasm (glycolysis) and mitochondria (Krebs cycle, ETS). Overall: C6H12O6 + 6O2 → 6CO2 + 6H2O + ~30-32 ATP. Steps: Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH. Pyruvate oxidation (mitochondria): 2 pyruvate → 2 acetyl-CoA + 2 CO2 + 2 NADH. Krebs cycle (mitochondrial matrix): 2 acetyl-CoA → 4 CO2 + 6 NADH + 2 FADH2 + 2 GTP. ETS (inner mitochondrial membrane): NADH and FADH2 → electrons transferred through complexes → O2 reduced to H2O → ATP synthesised. Total CO2 produced per glucose: 6. Total O2 consumed: 6. RQ = 6/6 = 1.0 (for carbohydrate). This CO2 must be transported back to lungs and exhaled.

6. Gas Exchange at Tissue Level (Step D)

At the tissue capillaries, gas exchange occurs in the opposite direction to alveoli. Partial pressures at tissues: Oxygenated blood arriving: PO2 = 95 mmHg, PCO2 = 40 mmHg. Tissue cells: PO2 = 40 mmHg (O2 being used by mitochondria), PCO2 = 45 mmHg (CO2 being produced). O2 gradient: 95-40 = 55 mmHg (O2 diffuses from blood into cells). CO2 gradient: 45-40 = 5 mmHg (CO2 diffuses from cells into blood). After tissue exchange: blood PO2 = ~40 mmHg, PCO2 = ~45 mmHg. Venous blood (deoxygenated) returns to lungs to repeat the cycle. Active tissues (exercising muscle): PO2 may drop to 20 mmHg, PCO2 may rise to 60-70 mmHg → larger gradients → more O2 delivered, more CO2 removed. Bohr effect ensures O2 is released from Hb more readily at low pH of active tissues.

7. Regulation of Breathing

Breathing rhythm is generated and regulated by the brainstem. Respiratory rhythm generator: pre-Botzinger complex in medulla → generates inspiratory rhythm. Modified by: pneumotaxic centre (upper pons): switches off inspiration → sets breathing frequency. Apneustic centre (lower pons): prolongs inspiration. Chemical regulation: PaCO2 is the MAIN driver of breathing. Central chemoreceptors (medulla): detect CO2/H+ in CSF. Most sensitive to CO2 changes. Rise in PaCO2 → H2CO3 in CSF → H+ → stimulates chemoreceptors → increased respiratory rate and depth. Peripheral chemoreceptors (carotid bodies, aortic bodies): detect PO2, PaCO2, pH in blood. Respond to LOW PO2 (below 60 mmHg) primarily. Hypoxic ventilatory response. During exercise: multiple factors increase breathing: rising CO2, falling O2, falling pH (lactic acidosis), body temperature, muscle afferents, cortical input (anticipatory).

8. Respiratory Disorders

Asthma: chronic inflammatory airway disease. Bronchospasm + mucosal oedema + mucus hypersecretion → airway narrowing → wheeze, cough, breathlessness. Triggers: allergens, exercise, cold air, infections. Treatment: bronchodilators (salbutamol/albuterol — SABA, ipratropium), corticosteroids (inhaled: beclomethasone). COPD (Chronic Obstructive Pulmonary Disease): includes emphysema and chronic bronchitis. Mainly caused by smoking. Emphysema: alveolar wall destruction → enlarged air spaces → reduced gas exchange area. Chronic bronchitis: chronic productive cough >3 months/year for 2 years. Both: airflow obstruction, FEV1/FVC <0.70. Pneumonia: lung infection → consolidation → impaired gas exchange. Bacterial (Streptococcus pneumoniae most common), viral, fungal. Tuberculosis (TB): Mycobacterium tuberculosis. Lung granulomas. India: high TB burden. BCG vaccine provides partial protection. Treatment: HRZE regimen (isoniazid, rifampicin, pyrazinamide, ethambutol).

Frequently Asked Questions
1. What is the correct sequence for respiration in humans? ⌄
The sequence given in the answer is C, B, E, A, D: C. Breathing (ventilation): inspiration brings fresh O2-rich air into alveoli; expiration removes CO2-rich air. B. Transport: blood transports O2 from lungs to tissues and CO2 from tissues to lungs (via haemoglobin and dissolved forms). E. Gas exchange at alveoli: O2 from alveolar air diffuses into pulmonary capillary blood (PO2 gradient 64 mmHg); CO2 from blood diffuses into alveoli (PCO2 gradient 5 mmHg). A. Cellular use: mitochondria consume O2 for aerobic respiration, produce CO2 as waste product. D. Tissue gas exchange: O2 from capillary blood diffuses into cells; CO2 from cells diffuses into blood.
2. What partial pressures drive gas exchange at alveoli? ⌄
Gas exchange follows Fick law of diffusion: rate proportional to surface area x pressure difference / thickness. At alveolar level: O2 diffusion into blood: alveolar PO2 = 104 mmHg, deoxygenated blood PO2 = 40 mmHg. Gradient = 64 mmHg. O2 moves from alveolus to blood. CO2 diffusion into alveolus: blood PCO2 = 45 mmHg, alveolar PCO2 = 40 mmHg. Gradient = 5 mmHg. CO2 moves from blood to alveolus. Despite smaller gradient for CO2: CO2 is 20x more soluble than O2 in biological fluids → diffuses equally rapidly. Blood leaving lung capillaries: PO2 equilibrates to ~95 mmHg (not 104 because of physiological shunting of 2-3% blood), PCO2 equilibrates to 40 mmHg.
3. What is the Bohr effect? ⌄
Bohr effect: the reduction in haemoglobin-O2 affinity (right shift of the oxyhaemoglobin dissociation curve) caused by: decreased pH (increased H+ or CO2), increased temperature, increased 2,3-DPG (2,3-bisphosphoglycerate). In exercising muscle: increased CO2, lactic acid → decreased pH → Bohr effect → Hb releases O2 more readily at same PO2 → more O2 delivered to active tissues. At lungs: CO2 blown off → pH rises → left shift of curve → Hb picks up O2 more avidly at alveolar PO2. This enhances O2 unloading at tissues and loading at lungs. Christian Bohr (father of Niels Bohr) described this effect in 1904. Physiological importance: automatically increases O2 delivery to metabolically active tissues.
4. What is carbonic anhydrase and where is it found? ⌄
Carbonic anhydrase (CA) catalyses: CO2 + H2O reversible H2CO3 reversible H+ + HCO3-. This reaction occurs spontaneously but 13,000x faster with CA. Location: red blood cells (RBCs) — very high concentration of CA, particularly CA II (fastest known enzyme). Also in: kidney tubular cells, gastric parietal cells, pancreatic ductal cells, choroid plexus (CSF production), eye (aqueous humour). In RBCs: CO2 from tissues enters RBC → CA converts to H2CO3 → dissociates to H+ + HCO3-. HCO3- exits RBC in exchange for Cl- (chloride shift or hamburger shift). H+ buffered by haemoglobin (imidazole groups of histidine). At lungs: reverse process — HCO3- re-enters RBC → CA reforms CO2 → CO2 exhaled. CA inhibitors: acetazolamide — inhibits CA in kidney → reduces HCO3- reabsorption → diuresis. Used for altitude sickness, glaucoma.
5. How does exercise increase breathing rate? ⌄
Multiple mechanisms increase ventilation during exercise: Immediate (within seconds): signals from motor cortex to respiratory centres (anticipatory increase even before exercise starts). Proprioceptors in joints and muscles → stimulate respiratory centres. Adrenaline (epinephrine) from adrenal medulla → stimulates respiratory centres. Delayed (as exercise continues): rising PaCO2 → central and peripheral chemoreceptors stimulated. Falling PaO2 → peripheral chemoreceptors (carotid bodies). Falling pH (lactic acidosis at high intensity) → peripheral chemoreceptors. Rising body temperature → direct stimulation of respiratory centres. Venous return increased → stretch receptors in right atrium → reflex increase in ventilation. At maximal exercise: ventilation can reach 100-150 L/min (vs 5-7 L/min at rest). Trained athletes have more efficient breathing muscles and can tolerate higher CO2/lower pH before reaching ventilatory threshold.
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