CBSE Class 11 Biology Chapter 17: Breathing and Exchange of Gases NCERT Solutions

Solution: Vital capacity refers to the maximum volume of air that can be forcefully exhaled from the lungs following a maximal inhalation. In a healthy human adult, this volume typically ranges from 3.5 to 4.5 litres. Its primary significance lies in its role in promoting efficient gas exchange. By allowing for the expulsion of stale air and the intake of fresh air, vital capacity ensures that the lungs are adequately ventilated, thereby maximizing the exchange of oxygen and carbon dioxide between the alveoli and the bloodstream.

Solution: The volume of air that remains in the lungs after a normal expiration is known as the Functional Residual Capacity (FRC). This capacity is composed of two other lung volumes: the Expiratory Reserve Volume (ERV) and the Residual Volume (RV).

The Expiratory Reserve Volume (ERV) is the additional volume of air that can be forcibly exhaled after a normal expiration, typically around 1000 mL to 1500 mL.

The Residual Volume (RV) is the volume of air that always remains in the lungs even after a maximum possible expiration, preventing lung collapse. This is usually about 1100 mL to 1500 mL.

Therefore, the Functional Residual Capacity (FRC) can be calculated as:

$$FRC\; =\; ERV\; +\; RV$$

Using the approximate maximum values:

$$FRC\; \backslash approx\; 1500\; \backslash text\{\; mL\}\; +\; 1500\; \backslash text\{\; mL\}$$

$$FRC\; \backslash approx\; 3000\; \backslash text\{\; mL\}$$

Thus, the functional residual capacity of the human lungs is approximately 2500 to 3000 mL.

Solution: The diffusion of respiratory gases (oxygen and carbon dioxide) is restricted to the alveolar region primarily because of the specialized structure of the alveoli and the surrounding capillaries, which are optimized for gas exchange.

Each alveolus is a tiny air sac lined with a very thin layer of squamous epithelial cells, making it highly permeable. Similarly, the walls of the pulmonary capillaries that surround the alveoli are also composed of a single layer of squamous epithelial cells.

When oxygen-rich air reaches the alveoli from the respiratory passages, and deoxygenated blood (rich in carbon dioxide) arrives from the body via the pulmonary artery, the conditions are set for diffusion. The barrier between the alveolar air and the capillary blood is extremely thin (the respiratory membrane), consisting of the alveolar epithelium, the capillary endothelium, and their fused basement membranes.

Gas exchange occurs via simple diffusion, driven by differences in partial pressures (or concentrations). The partial pressure of oxygen is higher in the alveolar air than in the deoxygenated blood, causing oxygen to diffuse from the alveoli into the blood. Conversely, the partial pressure of carbon dioxide is higher in the deoxygenated blood than in the alveolar air, leading to the diffusion of carbon dioxide from the blood into the alveoli.

Other parts of the respiratory tract, such as the trachea and bronchi, have thicker walls and are primarily involved in conducting air, warming, humidifying, and filtering it, but they lack the extensive surface area and thin diffusion barrier characteristic of the alveoli, making significant gas exchange impossible there.

Solution: Carbon dioxide (CO₂) is transported from the tissues to the lungs via the blood through three primary mechanisms:

1. Transport as dissolved CO₂ in plasma: A small amount of CO₂, approximately 7% of the total, is transported dissolved directly in the blood plasma. Carbon dioxide is soluble in water and reacts with water to form carbonic acid ($H\_2CO\_3$). This reaction is relatively slow in the plasma itself.

   $$CO\_2\; +\; H\_2O\; \backslash longrightarrow\; H\_2CO\_3$$

2. Transport via Red Blood Cells (RBCs) as carbaminohaemoglobin: About 20-25% of the CO₂ is transported by binding to haemoglobin within the red blood cells. CO₂ binds to the amino groups (-NH₂) on the polypeptide chains of haemoglobin, forming a compound called carbaminohaemoglobin.

   $$Hb\; +\; CO\_2\; \backslash longrightarrow\; HbCO\_2$$ (Carbaminohaemoglobin)

3. Transport as bicarbonate ions: The majority of CO₂, about 70%, is transported in the blood in the form of bicarbonate ions ($HCO\_3^-$). When CO₂ enters the red blood cells, it reacts with water in the presence of the enzyme carbonic anhydrase. This enzyme greatly accelerates the formation of carbonic acid ($H\_2CO\_3$). Carbonic acid then dissociates into a hydrogen ion ($H^+$) and a bicarbonate ion ($HCO\_3^-$). The bicarbonate ions then move out of the RBCs into the plasma, while the hydrogen ions are buffered by haemoglobin.

   $$CO\_2\; +\; H\_2O\; \backslash xrightarrow\{\backslash text\{Carbonic\; anhydrase\}\}\; H\_2CO\_3\; \backslash longrightarrow\; H^+\; +\; HCO\_3^-$$

   This mechanism is highly efficient due to the presence and action of carbonic anhydrase, which is found in much higher concentrations within RBCs than in plasma.