Exercises
Define vital capacity. What is its significance?
Answer
Vital capacity (VC) is the maximum volume of air that can be expelled from the lungs after a maximum inspiratory effort, i.e., it is the sum of Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV). In a healthy adult, VC is approximately 4500–5000 mL. Significance: It indicates the functional capacity of the respiratory system. A higher VC means greater ability to supply O2 during strenuous exercise. It decreases with age, respiratory disease, or poor physical fitness, making it a useful clinical diagnostic parameter.
State the volume of air remaining in the lungs after a normal breathing.
Answer
After a normal (tidal) expiration, the volume of air remaining in the lungs is the Functional Residual Capacity (FRC), which equals Expiratory Reserve Volume (ERV) + Residual Volume (RV). In a healthy adult: ERV ≈ 1000–1200 mL and RV ≈ 1100–1200 mL, so FRC ≈ 2300 mL. If the question refers only to residual volume (air that cannot be expelled even after forceful expiration), that is approximately 1100–1200 mL.
Diffusion of gases occurs in the alveolar region only and not in the other parts of the respiratory system. Why?
Answer
Gas exchange by diffusion requires a very thin membrane, a large surface area, a rich blood supply, and a steep concentration gradient. The alveoli uniquely provide all these: their walls are a single cell thick (squamous epithelium, ~0.2 µm), they have an enormous collective surface area (~70 m²), they are richly supplied with pulmonary capillaries, and the partial pressure gradients of O2 and CO2 between alveolar air and blood are steep. The trachea, bronchi, and bronchioles (conducting zone) have thick, cartilage-reinforced walls, no rich capillary supply, and function only to conduct air; they do not allow efficient diffusion. Hence diffusion occurs only in the alveoli.
What are the major transport mechanisms for CO2? Explain.
Answer
CO2 is transported from tissues to lungs by three mechanisms:
(i) Dissolved in plasma (~7%): A small fraction of CO2 dissolves directly in the blood plasma and is carried as dissolved gas.
(ii) As carbaminohaemoglobin (~23%): CO2 combines with the amino groups of haemoglobin (and plasma proteins) to form carbamino compounds. This reaction is rapid and reversible: CO2 + Hb ⇌ HbCO2. Deoxygenated Hb has a higher affinity for CO2 (Haldane effect).
(iii) As bicarbonate ions (~70%): CO2 diffuses into RBCs and combines with water (catalysed by carbonic anhydrase) to form carbonic acid (H2CO3), which dissociates into H+ and HCO3-. HCO3- moves out of RBCs into plasma (in exchange for Cl- — chloride shift). At the lungs, the reactions reverse, releasing CO2 for exhalation.
What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?
Answer
The correct answer is (ii) pO2 higher, pCO2 lesser. Atmospheric air has pO2 ≈ 159 mmHg and pCO2 ≈ 0.3 mmHg. Alveolar air has pO2 ≈ 104 mmHg (lower, because O2 is constantly being taken up by blood) and pCO2 ≈ 40 mmHg (higher, because CO2 is continuously released from blood). Thus, atmospheric air has higher pO2 and lower pCO2 compared to alveolar air.
Explain the process of inspiration under normal conditions.
Answer
Inspiration (inhalation) is an active process driven by muscle contractions that increase the volume of the thoracic cavity, decreasing intrapulmonary pressure below atmospheric pressure, causing air to flow in:
1. The diaphragm contracts and flattens (moves downward).
2. External intercostal muscles contract, lifting the ribs upward and outward.
3. These actions increase the volume of the thoracic cavity in all three dimensions.
4. The lungs (attached to the thoracic wall by pleura) expand, increasing lung volume.
5. Intrapulmonary pressure drops below atmospheric pressure (by ~1–3 mmHg).
6. Air flows in from outside (high pressure to low pressure) until pressures equalise.
During deep inspiration, accessory muscles (sternocleidomastoid, scalene muscles) are also recruited to further expand the thorax.
How is respiration regulated?
Answer
Respiration is regulated primarily by the respiratory rhythm centre in the medulla oblongata, which generates the basic rhythm of breathing. The pneumotaxic centre in the pons moderates the rhythm, limiting the duration of inspiration to prevent over-inflation. Regulation involves:
Chemical regulation: Specialised chemoreceptors in the medulla (central) and in the aortic arch and carotid bodies (peripheral) sense changes in blood pCO2, pH, and pO2. A rise in CO2 or fall in pH is the most potent stimulus, sending signals to the medullary centre to increase rate and depth of breathing (hyperventilation). A fall in pO2 stimulates peripheral chemoreceptors.
Neural regulation: The Hering–Breuer reflex (stretch receptors in lungs) prevents over-inflation; when lungs are sufficiently inflated, they send inhibitory signals to stop inspiration.
What is the effect of pCO2 on oxygen transport?
Answer
Increased pCO2 reduces the affinity of haemoglobin for O2, causing the oxygen dissociation curve to shift to the right (Bohr effect). This means that at a given pO2, less O2 is bound to Hb, facilitating O2 unloading in active tissues (where CO2 is high). Increased CO2 also decreases blood pH (by forming H2CO3), which further reduces Hb–O2 affinity. Conversely, low pCO2 (in the lungs) promotes O2 loading onto Hb.
What happens to the respiratory process in a man going up a hill?
Answer
At higher altitudes, atmospheric pressure decreases, resulting in a lower partial pressure of O2 (hypoxia). To compensate: (i) breathing rate and depth increase (hyperventilation) to bring more O2 into the lungs; (ii) heart rate increases to deliver more oxygenated blood to tissues; (iii) over time (acclimatisation), the number of RBCs and haemoglobin concentration increase, improving O2-carrying capacity; (iv) more 2,3-BPG is produced in RBCs, reducing Hb–O2 affinity and improving O2 release to tissues. Initially, symptoms of altitude sickness (headache, dizziness) may occur due to hypoxia.
What is the site of gaseous exchange in an insect?
Answer
In insects, gaseous exchange occurs through a system of tracheae and tracheoles — a network of air-filled tubes that open to the outside through pores called spiracles on the body surface. The tracheoles penetrate deep into the tissues, and gas exchange (O2 in, CO2 out) occurs by diffusion directly between the tracheoles and the cells, bypassing the need for a blood-based respiratory pigment for O2 transport.
Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
Answer
The oxygen dissociation (saturation) curve is a graph plotting the percentage saturation of haemoglobin with O2 (y-axis) against the partial pressure of O2 (pO2, x-axis). It is sigmoid (S-shaped) because of the cooperative binding of O2 to haemoglobin. Haemoglobin has four subunits; when the first O2 binds to one subunit, it causes a conformational change that increases the affinity of the remaining subunits for O2 (positive cooperativity). This makes subsequent O2 binding progressively easier, giving the steep middle portion of the curve. At low pO2, binding is slow (flat lower portion); at high pO2, saturation is nearly complete (flat upper portion). The sigmoid shape ensures effective O2 loading in the lungs and unloading in the tissues.
Have you heard about hypoxia? Try to gather information about it.
Answer
Hypoxia is a condition where the body or a region of the body is deprived of adequate O2 supply. It can be: (i) Hypoxic hypoxia — low pO2 in inspired air (high altitude, drowning); (ii) Anaemic hypoxia — reduced O2-carrying capacity of blood (anaemia, CO poisoning); (iii) Ischaemic (stagnant) hypoxia — inadequate blood flow to tissues; (iv) Histotoxic hypoxia — cells unable to use O2 (cyanide poisoning). Symptoms include breathlessness, fatigue, headache, confusion, and in severe cases, coma. Treatment depends on the cause (O2 therapy, acclimatisation, treating underlying disease).
Distinguish between: (a) IRV and ERV (b) Inspiratory capacity and Expiratory capacity (c) Vital capacity and Total lung capacity
Answer
(a) IRV (Inspiratory Reserve Volume) is the additional volume of air that can be forcefully inhaled after a normal tidal inspiration (~2500–3000 mL). ERV (Expiratory Reserve Volume) is the additional volume of air that can be forcefully exhaled after a normal tidal expiration (~1000–1200 mL).
(b) Inspiratory capacity (IC) = Tidal Volume + IRV — the maximum amount of air that can be inspired from the resting expiratory level (~3500–3600 mL). Expiratory capacity (EC) = Tidal Volume + ERV — the maximum amount of air that can be expired from the resting inspiratory level (~1500–1700 mL).
(c) Vital capacity (VC) = TV + IRV + ERV — the maximum volume of air that can be moved in or out of the lungs with a single forced breath (~4500–5000 mL). Total Lung Capacity (TLC) = VC + Residual Volume — the total volume of air the lungs can hold after maximum inspiration (~6000 mL). TLC > VC because the lungs always retain some air (RV) that cannot be expelled.
What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
Answer
Tidal volume (TV) is the volume of air inspired or expired with each normal (resting) breath — approximately 500 mL per breath in a healthy adult. Normal breathing rate is about 12–16 breaths per minute. Tidal volume per hour = 500 mL × 15 breaths/min × 60 min = 450,000 mL (450 litres) per hour.
Important Extra Questions
What is the Bohr effect?
Answer
The Bohr effect describes the influence of CO2 and pH on haemoglobin's affinity for O2. An increase in pCO2 or a decrease in pH (more acidic) reduces Hb-O2 affinity, shifting the oxygen dissociation curve to the right and facilitating O2 unloading in metabolically active tissues. Conversely, low pCO2 and high pH in the lungs increase Hb affinity for O2, promoting O2 loading.
What is the role of carbonic anhydrase in CO2 transport?
Answer
Carbonic anhydrase is an enzyme present in RBCs that catalyses the rapid, reversible conversion of CO2 + H2O ⇌ H2CO3. This reaction would proceed too slowly without the enzyme. H2CO3 then dissociates into H+ and HCO3-; HCO3- is transported in plasma. In the lungs, the reverse reaction releases CO2 for exhalation. Carbonic anhydrase enables ~70% of CO2 to be transported as bicarbonate.
Name the volumes that together constitute the total lung capacity.
Answer
Total Lung Capacity (TLC) = TV (500 mL) + IRV (~2500 mL) + ERV (~1000 mL) + RV (~1100 mL) ≈ 6000 mL. Alternatively, TLC = Vital Capacity + Residual Volume.
Why is haemoglobin a better O2 carrier than plasma?
Answer
Only about 3% of O2 is dissolved in plasma; at normal pO2 levels, plasma alone cannot carry sufficient O2. Haemoglobin in RBCs can carry 97% of the total O2, with each Hb molecule binding up to 4 O2 molecules cooperatively, enormously increasing the O2-carrying capacity of blood. Without Hb, blood could carry only ~0.3 mL O2/100 mL; with Hb, it carries ~20 mL O2/100 mL.
Explain the chloride shift.
Answer
In tissues, CO2 enters RBCs and is converted to HCO3- and H+. HCO3- leaves the RBC into plasma in exchange for Cl- ions entering the RBC (via the band 3 protein anion exchanger). This movement of Cl- into RBCs to balance the outward movement of HCO3- is the chloride shift (Hamburger shift). It maintains electrical neutrality and allows more HCO3- to form, enhancing CO2 transport capacity.