Q: Q5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?\n(a) pO2 lesser, pCO2 higher\n(b) pO2 higher, pCO2 lesser\n(c) pO2 higher, pCO2 higher\n

The correct option is **(b) pO2 higher, pCO2 lesser**.\n\n**Explanation:**\n\n1. **Partial Pressure of Oxygen (pO2):**\n * **Atmospheric Air:** The pO2 in atmospheric air is approximately 159 mmHg (at sea level). This is the highest pO2 in the respiratory pathway.\n * **Alveolar Air:** As atmospheric air enters the lungs, it mixes with the 'residual volume' of air already present in the alveoli (which has a lower pO2 due to continuous oxygen absorption into the blood). Additionally, the air gets humidified, which further reduces the partial pressure of oxygen. Due to these factors and the continuous diffusion of O2 into the blood, the pO2 in alveolar air is significantly lower, typically around 104 mmHg.\n * Therefore, pO2 in atmospheric air is **higher** than in alveolar air.\n\n2. **Partial Pressure of Carbon Dioxide (pCO2):**\n * **Atmospheric Air:** The pCO2 in atmospheric air is very low, almost negligible, typically around 0.3 mmHg.\n * **Alveolar Air:** CO2 is continuously diffusing from the pulmonary capillaries into the alveoli to be exhaled. This constant influx of CO2 from the blood raises the pCO2 in the alveolar air to approximately 40 mmHg.\n * Therefore, pCO2 in atmospheric air is **lesser** than in alveolar air.\n\nCombining these, atmospheric air has a higher pO2 and a lesser pCO2 compared to alveolar air.

Question 1

Q1. Define vital capacity. What is its significance?

Accepted Answer

### Definition of Vital Capacity
Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced exhalation, or the maximum volume of air a person can breathe out after a forced inhalation. It represents the total amount of air involved in a single maximal breath.

Mathematically, Vital Capacity is the sum of:
*   **Inspiratory Reserve Volume (IRV):** The additional volume of air a person can inhale by a forceful inspiration after a normal inspiration.
*   **Tidal Volume (TV):** The volume of air inspired or expired during a normal respiration.
*   **Expiratory Reserve Volume (ERV):** The additional volume of air a person can exhale by a forceful expiration after a normal expiration.

Therefore, **VC = IRV + TV + ERV**

### Significance of Vital Capacity
1.  **Indicator of Respiratory Muscle Strength:** A higher vital capacity generally indicates stronger respiratory muscles (diaphragm and intercostal muscles) and better lung elasticity.
2.  **Assessment of Lung Function:** It is a crucial parameter used in pulmonary function tests to assess the overall health and capacity of the lungs. It helps in diagnosing and monitoring various respiratory diseases.
3.  **Diagnosis of Respiratory Disorders:**
    *   **Restrictive Lung Diseases:** Conditions like pulmonary fibrosis, sarcoidosis, or scoliosis, which restrict lung expansion, typically lead to a reduced vital capacity.
    *   **Obstructive Lung Diseases:** While the primary issue in obstructive diseases (e.g., asthma, COPD) is airflow limitation, severe cases can also show reduced vital capacity due to air trapping and hyperinflation.
4.  **Physical Fitness:** Athletes and individuals with higher physical fitness often have a greater vital capacity, reflecting efficient oxygen uptake and delivery.
5.  **Prognostic Value:** Changes in vital capacity over time can indicate the progression or improvement of a respiratory condition.
6.  **Anesthesia and Surgery:** Vital capacity is assessed before surgery, especially abdominal or thoracic surgeries, to predict post-operative respiratory complications.

Question 2

Q2. State the volume of air remaining in the lungs after a normal exhalation.

Accepted Answer

The volume of air remaining in the lungs after a normal exhalation is called **Functional Residual Capacity (FRC)**.

Functional Residual Capacity (FRC) is the sum of:
*   **Expiratory Reserve Volume (ERV):** The additional volume of air a person can exhale by a forceful expiration after a normal expiration.
*   **Residual Volume (RV):** The volume of air remaining in the lungs even after a forceful expiration.

Therefore, **FRC = ERV + RV**.

Typically, FRC is approximately 2300 mL (ERV ≈ 1000-1100 mL, RV ≈ 1100-1200 mL).

Question 3

Q3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?

Accepted Answer

Diffusion of gases (oxygen and carbon dioxide) primarily occurs in the alveolar region of the respiratory system due to specific structural and functional adaptations that maximize gas exchange. Other parts of the respiratory system, such as the nasal passages, pharynx, larynx, trachea, bronchi, and bronchioles, are primarily involved in conduction, humidification, filtration, and warming of air, and lack the necessary features for efficient gas exchange.

Here's why gas diffusion is restricted to the alveolar region:

### Adaptations of the Alveolar Region for Gas Exchange:
1.  **Extremely Thin Diffusion Membrane:** The alveolar membrane (also called the respiratory membrane) is incredibly thin, typically less than 1 mm (or even 0.2 mm in some areas). It consists of three main layers:
    *   The thin squamous epithelial cells (alveolar cells or pneumocytes) of the alveoli.
    *   The basement membrane of the alveolar epithelium.
    *   The basement membrane of the capillary endothelium.
    *   The thin endothelial cells of the alveolar capillaries.
    This minimal thickness allows for rapid diffusion of gases across the membrane.

2.  **Large Surface Area:** The human lungs contain approximately 300-500 million alveoli, providing an enormous total surface area (around 70-100 square meters) for gas exchange. This vast surface area ensures that a large quantity of gases can be exchanged simultaneously.

3.  **Rich Blood Supply:** Each alveolus is surrounded by a dense network of pulmonary capillaries. This extensive vascularization ensures that a large volume of blood is constantly flowing past the alveoli, maintaining steep partial pressure gradients for oxygen and carbon dioxide between the alveolar air and the capillary blood.

4.  **Partial Pressure Gradients:** The continuous ventilation of alveoli and blood flow through capillaries maintains optimal partial pressure gradients. Alveolar pO2 is higher than capillary pO2, facilitating O2 diffusion into the blood. Conversely, capillary pCO2 is higher than alveolar pCO2, facilitating CO2 diffusion into the alveoli.

5.  **Moist Surface:** The inner surface of the alveoli is lined with a thin layer of fluid, which allows gases to dissolve before diffusing across the membrane. Diffusion occurs more efficiently when gases are dissolved in a liquid.

### Reasons Why Other Respiratory Parts Do Not Facilitate Gas Diffusion:
1.  **Thick Walls:** The walls of the conducting airways (trachea, bronchi, bronchioles) are much thicker, composed of cartilage, smooth muscle, and connective tissue, which would impede gas diffusion.

2.  **Lack of Capillary Network:** These airways do not have the dense capillary network characteristic of alveoli. Their primary blood supply is for nourishing the tissues themselves, not for gas exchange with the external environment.

3.  **Primary Function is Conduction:** The main role of the conducting zone is to transport air to and from the alveoli, filter out particulate matter, humidify, and warm the inhaled air. They are not designed for gas exchange.

4.  **Epithelial Lining:** The conducting airways are lined with ciliated pseudostratified columnar epithelium with goblet cells, which is adapted for mucociliary clearance, not for gas diffusion.

In summary, the unique structural design of the alveoli, characterized by an extremely thin, large, moist surface area with a rich blood supply, is perfectly optimized for efficient gas exchange, a function not shared by the thicker-walled, less vascularized conducting airways.

Question 4

Q4. What are the major transport mechanisms for CO2? Explain.

Accepted Answer

Carbon dioxide (CO2) is transported from the tissues, where it is produced as a metabolic waste product, to the lungs for exhalation. There are three major mechanisms by which CO2 is transported in the blood:\n\n### 1. As Dissolved CO2 in Plasma (7-10%)\n*   A small percentage of CO2 (about 7-10%) dissolves directly in the plasma and is transported in this form. CO2 is about 20-25 times more soluble in plasma than oxygen. However, due to the relatively low partial pressure gradient and the need to transport large quantities, this mechanism alone is insufficient.\n\n### 2. As Bicarbonate Ions (HCO3-) (70%)\n*   This is the most significant mechanism for CO2 transport, accounting for approximately 70% of the total CO2 transported.\n*   **In Tissues (at the systemic capillaries):**\n    *   CO2 produced by cellular respiration diffuses from the tissue cells into the systemic capillaries.\n    *   Inside the red blood cells (RBCs), CO2 rapidly reacts with water (H2O) to form carbonic acid (H2CO3). This reaction is catalyzed by the enzyme **carbonic anhydrase**, which is present in high concentration within RBCs.\n    *   H2CO3 is unstable and quickly dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-).\n    *   The bicarbonate ions (HCO3-) then diffuse out of the RBCs into the plasma. To maintain electrical neutrality, chloride ions (Cl-) move from the plasma into the RBCs. This exchange is known as the **chloride shift** or Hamburger phenomenon.\n    *   The hydrogen ions (H+) produced within the RBCs are buffered by binding to hemoglobin (Hb), forming deoxyhemoglobin (HHb). This buffering prevents a significant drop in blood pH and also facilitates the release of oxygen from hemoglobin (Bohr effect).\n    \n    **Reactions in Tissues:**\n    CO2 + H2O $\xrightarrow{\text{Carbonic Anhydrase}}$ H2CO3\n    H2CO3 $\rightleftharpoons$ H+ + HCO3-\n    HCO3- (diffuses into plasma)\n    H+ + Hb $\rightleftharpoons$ HHb (buffered by hemoglobin)\n\n*   **In Lungs (at the pulmonary capillaries):**\n    *   As blood reaches the lungs, the partial pressure of CO2 (pCO2) in the alveoli is lower than in the pulmonary capillaries.\n    *   The reactions described above are reversed. Oxygen binds to hemoglobin, causing the release of H+ ions (Haldane effect).\n    *   These H+ ions combine with bicarbonate ions (HCO3-) that diffuse back into the RBCs from the plasma (reversing the chloride shift).\n    *   Carbonic anhydrase then catalyzes the conversion of H2CO3 back into CO2 and H2O.\n    *   The newly formed CO2 diffuses out of the RBCs, across the alveolar membrane, and into the alveoli to be exhaled.\n\n    **Reactions in Lungs:**\n    HHb $\rightleftharpoons$ H+ + HbO2 (oxygenation of hemoglobin releases H+)\n    H+ + HCO3- $\rightleftharpoons$ H2CO3\n    H2CO3 $\xrightarrow{\text{Carbonic Anhydrase}}$ CO2 + H2O\n    CO2 (diffuses into alveoli)\n\n### 3. As Carbaminohemoglobin (20-25%)\n*   About 20-25% of CO2 is transported by binding directly to the amino groups of hemoglobin (and to a lesser extent, plasma proteins), forming a compound called carbaminohemoglobin (HbCO2).\n*   CO2 does not bind to the iron atom of the heme group (like O2 does) but to the globin part of hemoglobin.\n*   The binding of CO2 to hemoglobin is influenced by the partial pressure of CO2 (pCO2) and the partial pressure of O2 (pO2).\n*   **In Tissues:** High pCO2 and low pO2 (due to O2 dissociation from Hb) promote the binding of CO2 to hemoglobin, forming carbaminohemoglobin.\n*   **In Lungs:** Low pCO2 and high pO2 (due to O2 binding to Hb) promote the dissociation of CO2 from carbaminohemoglobin, releasing CO2 into the alveoli for exhalation. This effect of oxygen on CO2 transport is known as the **Haldane effect**.\n\nThese three mechanisms work in concert to efficiently transport CO2 from the body tissues to the lungs.

Question 5

Q5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?
(a) pO2 lesser, pCO2 higher
(b) pO2 higher, pCO2 lesser
(c) pO2 higher, pCO2 higher

Accepted Answer

The correct option is **(b) pO2 higher, pCO2 lesser**.

**Explanation:**

1.  **Partial Pressure of Oxygen (pO2):**
    *   **Atmospheric Air:** The pO2 in atmospheric air is approximately 159 mmHg (at sea level). This is the highest pO2 in the respiratory pathway.
    *   **Alveolar Air:** As atmospheric air enters the lungs, it mixes with the 'residual volume' of air already present in the alveoli (which has a lower pO2 due to continuous oxygen absorption into the blood). Additionally, the air gets humidified, which further reduces the partial pressure of oxygen. Due to these factors and the continuous diffusion of O2 into the blood, the pO2 in alveolar air is significantly lower, typically around 104 mmHg.
    *   Therefore, pO2 in atmospheric air is **higher** than in alveolar air.

2.  **Partial Pressure of Carbon Dioxide (pCO2):**
    *   **Atmospheric Air:** The pCO2 in atmospheric air is very low, almost negligible, typically around 0.3 mmHg.
    *   **Alveolar Air:** CO2 is continuously diffusing from the pulmonary capillaries into the alveoli to be exhaled. This constant influx of CO2 from the blood raises the pCO2 in the alveolar air to approximately 40 mmHg.
    *   Therefore, pCO2 in atmospheric air is **lesser** than in alveolar air.

Combining these, atmospheric air has a higher pO2 and a lesser pCO2 compared to alveolar air.

Question 6

Q6. Define vital capacity. What is its significance?

Accepted Answer

Vital capacity (VC) is the maximum volume of air a person can breathe in after a forced exhalation, or the maximum volume of air a person can breathe out after a forced inhalation. It is the sum of Inspiratory Reserve Volume (IRV), Tidal Volume (TV), and Expiratory Reserve Volume (ERV).

**Formula:** Vital Capacity (VC) = ERV + TV + IRV

**Significance:**
1.  **Indicator of Lung Function:** Vital capacity is a crucial measure of the strength of respiratory muscles and the overall functional capacity of the lungs. A higher vital capacity generally indicates healthier and more efficient lungs.
2.  **Assessment of Respiratory Health:** It is used in clinical settings to diagnose and monitor various respiratory diseases such as asthma, emphysema, bronchitis, and restrictive lung diseases. A reduced vital capacity can indicate impaired lung function.
3.  **Physical Fitness:** Athletes, especially those involved in endurance sports, often have a higher vital capacity, which allows for greater oxygen uptake and carbon dioxide expulsion, enhancing their performance.
4.  **Gas Exchange Efficiency:** A good vital capacity ensures efficient ventilation of the alveoli, facilitating optimal exchange of oxygen and carbon dioxide between the lungs and the blood.

Question 7

Q7. State the volume of air remaining in the lungs after a normal exhalation.

Accepted Answer

The volume of air remaining in the lungs after a normal exhalation is called **Functional Residual Capacity (FRC)**. It is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV).

**Formula:** Functional Residual Capacity (FRC) = ERV + RV

*   **Expiratory Reserve Volume (ERV):** The additional volume of air a person can exhale by a forcible expiration (approx. 1000 mL to 1100 mL).
*   **Residual Volume (RV):** The volume of air remaining in the lungs even after a forcible exhalation (approx. 1100 mL to 1200 mL).

Therefore, FRC typically ranges from 2100 mL to 2300 mL.

Question 8

Q8. Define the following terms: (a) Tidal volume (b) Residual volume (c) Inspiratory reserve volume (d) Expiratory reserve volume (e) Inspiratory capacity (f) Functional residual capac

Accepted Answer

Here are the definitions for the given respiratory volumes and capacities:

**(a) Tidal Volume (TV)**
*   **Definition:** It is the volume of air inspired or expired during a normal respiration. It represents the amount of air that moves in and out of the lungs with each normal breath.
*   **Typical Value:** Approximately 500 mL (in a healthy adult).

**(b) Residual Volume (RV)**
*   **Definition:** It is the volume of air remaining in the lungs even after a forcible exhalation. This volume cannot be expelled from the lungs.
*   **Typical Value:** Approximately 1100 mL to 1200 mL.

**(c) Inspiratory Reserve Volume (IRV)**
*   **Definition:** It is the additional volume of air a person can inspire by a forcible inspiration (i.e., beyond a normal inspiration).
*   **Typical Value:** Approximately 2500 mL to 3000 mL.

**(d) Expiratory Reserve Volume (ERV)**
*   **Definition:** It is the additional volume of air a person can exhale by a forcible expiration (i.e., beyond a normal expiration).
*   **Typical Value:** Approximately 1000 mL to 1100 mL.

**(e) Inspiratory Capacity (IC)**
*   **Definition:** It is the total volume of air a person can inspire after a normal expiration. It includes the normal tidal volume and the inspiratory reserve volume.
*   **Formula:** Inspiratory Capacity (IC) = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV)
*   **Typical Value:** Approximately 3000 mL to 3500 mL (500 mL + 2500-3000 mL).

**(f) Functional Residual Capacity (FRC)**
*   **Definition:** It is the volume of air that will remain in the lungs after a normal expiration. It is the sum of expiratory reserve volume and residual volume.
*   **Formula:** Functional Residual Capacity (FRC) = Expiratory Reserve Volume (ERV) + Residual Volume (RV)
*   **Typical Value:** Approximately 2100 mL to 2300 mL (1000-1100 mL + 1100-1200 mL).

Question 9

Q9. Explain the process of transport of O2 and CO2 in the blood.

Accepted Answer

The transport of respiratory gases, oxygen (O2) and carbon dioxide (CO2), in the blood is a complex process crucial for cellular respiration and maintaining acid-base balance. These gases are transported between the lungs and the body tissues.

### **Transport of Oxygen (O2)**
Oxygen is transported from the lungs to the tissues primarily in two forms:

1.  **As Oxyhemoglobin (about 97%):**
    *   Red blood cells (RBCs) contain a red-coloured iron-containing pigment called hemoglobin (Hb).
    *   Oxygen binds reversibly with hemoglobin in a cooperative manner to form oxyhemoglobin (HbO2). Each hemoglobin molecule can carry a maximum of four molecules of O2.
    *   The binding of oxygen with hemoglobin is primarily dependent on the partial pressure of oxygen (pO2). Other factors like pCO2, H+ concentration (pH), and temperature also influence this binding.
    *   **In the Alveoli (Lungs):** High pO2, low pCO2, lower H+ concentration (higher pH), and lower temperature favour the formation of oxyhemoglobin. Thus, O2 binds to hemoglobin in the lungs.
    *   **In the Tissues:** Low pO2, high pCO2, higher H+ concentration (lower pH), and higher temperature favour the dissociation of oxygen from oxyhemoglobin. This allows O2 to be released to the tissues for cellular activities.
    *   A sigmoid (S-shaped) oxygen dissociation curve illustrates the relationship between pO2 and the percentage saturation of hemoglobin with oxygen.

2.  **In Dissolved State (about 3%):**
    *   A small amount of oxygen (about 3%) is transported in a dissolved state through the plasma to the tissues.

### **Transport of Carbon Dioxide (CO2)**
Carbon dioxide is transported from the tissues to the lungs in three main forms:

1.  **As Bicarbonate Ions (about 70%):**
    *   This is the most significant mode of CO2 transport.
    *   In the tissues, CO2 diffuses into the RBCs. Inside the RBCs, an enzyme called carbonic anhydrase (present in high concentration) facilitates the reaction of CO2 with water to form carbonic acid (H2CO3).
    *   H2CO3 is unstable and quickly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).
    *   The bicarbonate ions then diffuse out of the RBCs into the plasma. To maintain electrical neutrality, chloride ions (Cl-) move from the plasma into the RBCs. This phenomenon is known as the **chloride shift** or Hamburger phenomenon.
    *   The H+ ions formed inside the RBCs are buffered by binding to hemoglobin, preventing a significant drop in pH.
    *   **In the Lungs:** When blood reaches the alveoli, the pCO2 is low. This causes the reverse reactions to occur. Bicarbonate ions move back into the RBCs from the plasma (with chloride shift in reverse), combine with H+ to form H2CO3, which then breaks down into CO2 and H2O. The CO2 diffuses out of the RBCs, into the plasma, and then into the alveoli for exhalation.

2.  **As Carbaminohemoglobin (about 20-25%):**
    *   CO2 binds directly to the amino groups of hemoglobin (and plasma proteins) to form carbaminohemoglobin (HbCO2).
    *   This binding is reversible and is primarily dependent on the partial pressure of CO2 (pCO2). It is also influenced by pO2.
    *   **In the Tissues:** High pCO2 and low pO2 (as in tissues) favour the formation of carbaminohemoglobin.
    *   **In the Alveoli:** Low pCO2 and high pO2 (as in alveoli) favour the dissociation of CO2 from carbaminohemoglobin, releasing CO2 for exhalation.
    *   The binding of CO2 to hemoglobin is inversely related to O2 binding (Haldane effect).

3.  **In Dissolved State (about 7%):**
    *   A small amount of CO2 (about 7%) is transported in a dissolved state through the plasma.

Question 10

Q10. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?

Accepted Answer

The partial pressures of oxygen (pO2) and carbon dioxide (pCO2) differ significantly between atmospheric air and alveolar air due to gas exchange in the lungs and the presence of water vapour.

1.  **Partial Pressure of Oxygen (pO2):**
    *   **Atmospheric Air:** The pO2 in atmospheric air is approximately **159 mm Hg**.
    *   **Alveolar Air:** The pO2 in alveolar air is significantly lower, approximately **104 mm Hg**.
    *   **Reason for Difference:** This reduction is primarily due to the humidification of inspired air (addition of water vapour, which dilutes other gases) and the continuous uptake of oxygen by the blood from the alveoli, while CO2 is simultaneously released into the alveoli.

2.  **Partial Pressure of Carbon Dioxide (pCO2):**
    *   **Atmospheric Air:** The pCO2 in atmospheric air is very low, approximately **0.3 mm Hg**.
    *   **Alveolar Air:** The pCO2 in alveolar air is significantly higher, approximately **40 mm Hg**.
    *   **Reason for Difference:** This increase is because CO2 is continuously diffusing from the pulmonary capillaries into the alveoli from the venous blood, which has a higher pCO2 (45 mm Hg), and is then exhaled.

**Summary Table:**
| Gas | Atmospheric Air (mm Hg) | Alveolar Air (mm Hg) |
| :-- | :---------------------- | :------------------- |
| O2  | 159                     | 104                  |
| CO2 | 0.3                     | 40                   |

Chapter 17: Breathing and Exchange of Gases