CBSE Class 11 Biology Chapter 19: Breathing and Exchange of Gases NCERT Solutions

Solution: Vital capacity (VC) is defined as the maximum volume of air that can be forcefully exhaled from the lungs following a maximal inhalation. In healthy adult humans, this volume typically ranges from about 3.5 to 4.5 liters. Its significance lies in its role in efficient respiration; a larger vital capacity generally indicates better lung function and capacity for gas exchange, facilitating the intake of fresh oxygen and the expulsion of carbon dioxide.

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), which is the additional volume of air that can be forcibly exhaled after a normal exhalation, and the Residual Volume (RV), which is the volume of air that always remains in the lungs even after a maximal exhalation.

The ERV is approximately 1000 mL to 1500 mL, and the RV is approximately 1100 mL to 1500 mL.

Therefore, the FRC can be calculated as:

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

Using the approximate maximum values:

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

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

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

Solution: Gas diffusion, specifically the exchange of oxygen (O2) and carbon dioxide (CO2), is primarily confined to the alveolar region of the lungs due to specialized structural adaptations that facilitate this process. Each alveolus is a tiny air sac lined with a very thin layer of squamous epithelial cells, which are highly permeable. Similarly, the capillaries surrounding the alveoli also have walls composed of a single layer of squamous epithelial cells. This creates an extremely thin barrier (the respiratory membrane) between the alveolar air and the blood.

Oxygen-rich air reaches the alveoli through inhalation. Deoxygenated blood, returning from the body and carrying a high concentration of CO2 and a low concentration of O2, is pumped by the heart to the lungs. The exchange occurs via simple diffusion, driven by differences in partial pressures. The partial pressure of O2 is higher in the alveolar air than in the venous blood, causing O2 to diffuse into the blood. Conversely, the partial pressure of CO2 is higher in the venous blood than in the alveolar air, causing CO2 to diffuse out of the blood into the alveoli. Other parts of the respiratory tract, such as the trachea and bronchi, are primarily involved in conducting air and are not adapted for efficient gas exchange due to thicker walls and lack of extensive capillary networks.

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

1. Dissolved in Plasma: A small amount of CO2, approximately 7%, dissolves directly in the blood plasma and is transported in this free state. CO2 reacts with water to form carbonic acid ($H\_2CO\_3$), a reaction that is relatively slow without a catalyst.

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

2. Transported by Red Blood Cells (RBCs) as Carbaminohaemoglobin: About 20-25% of the CO2 transported in the blood binds to the amino groups ($-NH\_2$) present on the hemoglobin molecules within the red blood cells. This forms a compound called carbaminohaemoglobin.

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

3. Transported as Bicarbonate Ions: The majority of CO2, around 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, which greatly accelerates the reaction. Carbonic anhydrase is a zinc-containing enzyme. The resulting carbonic acid ($H\_2CO\_3$) then dissociates into a hydrogen ion ($H^+$) and a bicarbonate ion ($HCO\_3^-$). The bicarbonate ions then move out into the plasma, while the hydrogen ions are buffered by hemoglobin.

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

   This mechanism is highly efficient for transporting large quantities of CO2 from the tissues to the lungs.