Chapter 14 Solutions — Breathing and Exchange of Gases

Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration, or equivalently, the maximum volume of air a person can breathe out after a forced inspiration.

It is the sum of Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV).

Significance:

1. Vital capacity is an index of the maximum ventilation capacity of the lungs — the greater the VC, the more efficiently a person can ventilate the lungs during strenuous exercise.

2. It is measured by a spirometer and used clinically to assess pulmonary function and detect restrictive or obstructive lung diseases (e.g., fibrosis, emphysema).

3. Athletes and physically fit individuals generally have a higher vital capacity, reflecting stronger respiratory muscles and greater lung compliance.

4. A reduced vital capacity may indicate conditions like pleural effusion, pneumonia, or muscular weakness affecting respiration.

After a normal (quiet) expiration, the volume of air remaining in the lungs is the Functional Residual Capacity (FRC).

FRC = Expiratory Reserve Volume (ERV) + Residual Volume (RV)

FRC = approximately 1000–1100 mL (ERV) + 1100–1200 mL (RV) = approximately 2100–2300 mL.

Note: The Residual Volume (RV) alone (about 1100–1200 mL) remains even after a forcible expiration, but after a normal expiration the lungs retain the FRC (ERV + RV).

Carbon dioxide produced at the tissues is transported to the alveoli by three mechanisms:

1. As bicarbonate ions (HCO3-) — approximately 70%:

CO2 diffuses from tissues into RBCs and plasma. Inside RBCs, the enzyme carbonic anhydrase catalyses:

CO2 + H2O -→ H2CO3 -→ HCO3- + H+

HCO3- moves out into plasma in exchange for Cl- (chloride shift). At the alveoli, the reaction reverses (carbonic anhydrase again), regenerating CO2 which diffuses out.

2. As carbamino-haemoglobin — approximately 20-25%:

CO2 binds directly to haemoglobin (at amino groups) to form carbamino-haemoglobin. This binding is favoured by high pCO2 and low pO2 (tissue conditions). At the alveoli, low pCO2 and high pO2 cause dissociation and CO2 is released.

3. Dissolved in plasma — approximately 7%:

A small fraction of CO2 remains dissolved as a gas in the blood plasma and is carried in this state to the alveoli.

Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO2 to the alveoli.