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Chapter 14 Breathing And Exchange Of Gases
Breathing is essential for life because it facilitates the exchange of gases necessary for cellular respiration, the process by which organisms derive energy from food. Oxygen (O$_2$) is utilized to break down simple molecules like glucose, releasing energy and producing harmful carbon dioxide (CO$_2$) as a byproduct. Therefore, O$_2$ must be continuously supplied to cells, and CO$_2$ produced must be removed. Breathing is the process of exchanging O$_2$ from the atmosphere with CO$_2$ produced by the cells. This chapter describes the respiratory organs and the mechanisms involved in breathing and gas exchange in humans and other animals.
Respiratory Organs
The mechanisms of breathing vary among different animals depending on their habitat and level of organization.
- **Lower invertebrates** (sponges, coelenterates, flatworms): Exchange gases by simple diffusion over their entire body surface.
- **Earthworms:** Use their moist cuticle for cutaneous respiration (gas exchange through the skin).
- **Insects:** Have a network of tracheal tubes to transport atmospheric air within the body.
- **Aquatic arthropods and molluscs:** Use **gills** (branchial respiration) for gas exchange in water.
- **Terrestrial forms** (most vertebrates): Use **lungs** (pulmonary respiration) for gas exchange with air.
- **Amphibians** (frogs): Can respire through their moist skin (cutaneous respiration) in addition to lungs.
- **Fishes** (vertebrates): Use gills.
- **Reptiles, birds, mammals:** Primarily respire through lungs.
Human Respiratory System
The human respiratory system (Fig 14.1 in textbook) includes air passages and a pair of lungs. Air enters through the **external nostrils**, leading to nasal chambers via nasal passages. The nasal chamber opens into the **pharynx**, a common passage for food and air. The pharynx leads to the **larynx** (sound box), which opens into the **trachea**. The epiglottis, an elastic cartilaginous flap, covers the glottis (opening of larynx) during swallowing to prevent food from entering the trachea.
The **trachea** is a straight tube extending into the mid-thoracic cavity, dividing into right and left primary bronchi at the level of the 5th thoracic vertebra. Each primary bronchus repeatedly divides into secondary and tertiary bronchi, and finally into thin terminal bronchioles. The trachea and bronchioles are supported by incomplete cartilaginous rings.
Each terminal bronchiole gives rise to numerous very thin-walled, vascularized, bag-like structures called **alveoli**. The branching network of bronchi, bronchioles, and alveoli form the **lungs**.
The lungs are covered by a double-layered membrane called the **pleura**, with pleural fluid between the layers to reduce friction during breathing. The outer pleura contacts the thoracic lining; the inner pleura contacts the lung surface.
The respiratory pathway can be divided into:
- **Conducting part:** From external nostrils up to terminal bronchioles. Transports air to alveoli, clears it of foreign particles, humidifies it, and brings it to body temperature.
- **Respiratory or Exchange part:** Alveoli and their ducts. This is the site of actual diffusion (exchange) of O$_2$ and CO$_2$ between blood and atmospheric air.
The lungs are situated in the **thoracic chamber**, which is an air-tight chamber formed by the vertebral column (dorsally), sternum (ventrally), ribs (laterally), and diaphragm (lower side). Changes in thoracic volume cause corresponding changes in lung (pulmonary) volume, essential for breathing, as we cannot directly alter lung volume.
Respiration involves these steps:
- **Breathing (pulmonary ventilation):** Drawing in atmospheric air and releasing CO$_2$-rich alveolar air.
- **Diffusion of gases:** O$_2$ and CO$_2$ exchange across the alveolar membrane.
- **Transport of gases:** O$_2$ and CO$_2$ transported by blood.
- **Diffusion of gases:** O$_2$ and CO$_2$ exchange between blood and tissues.
- **Cellular respiration:** Utilisation of O$_2$ by cells for catabolic reactions and release of CO$_2$.
Mechanism Of Breathing
**Breathing** involves two stages : **inspiration** (inhaling atmospheric air) and **expiration** (exhaling alveolar air). These movements are driven by creating pressure gradients between the lungs and the atmosphere.
- **Inspiration:** Atmospheric air is drawn in. Occurs when the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure (negative pressure in lungs). This pressure difference is created by increasing the volume of the thoracic cavity. The diaphragm contracts, increasing volume along the antero-posterior axis. External intercostal muscles contract, lifting ribs and sternum, increasing volume along the dorso-ventral axis. Increased thoracic volume leads to increased pulmonary volume, lowering intra-pulmonary pressure below atmospheric, causing air inflow.
- **Expiration:** Alveolar air is released out. Occurs when intra-pulmonary pressure is higher than atmospheric pressure. This pressure difference is created by decreasing the volume of the thoracic cavity. Relaxation of diaphragm and external intercostal muscles returns them to normal positions, reducing thoracic volume and pulmonary volume. This increases intra-pulmonary pressure above atmospheric, causing air expulsion. Forced inspiration/expiration can be aided by additional muscles (internal intercostals, abdominal muscles).
A healthy human breathes 12-16 times per minute. The volume of air involved in breathing movements can be measured using a **spirometer** for clinical assessment of pulmonary functions.
Respiratory Volumes And Capacities
Various volumes and capacities describe the amount of air involved in breathing:
- **Tidal Volume (TV):** Volume of air inspired or expired during a normal quiet respiration. Approx. 500 mL.
- **Inspiratory Reserve Volume (IRV):** Additional volume of air, a person can inspire by a forcible inspiration (beyond normal TV). Avg. 2500-3000 mL.
- **Expiratory Reserve Volume (ERV):** Additional volume of air, a person can expire by a forcible expiration (beyond normal TV). Avg. 1000-1100 mL.
- **Residual Volume (RV):** Volume of air remaining in the lungs even after a forcible expiration. Avg. 1100-1200 mL.
Pulmonary capacities are combinations of these volumes:
- **Inspiratory Capacity (IC):** TV + IRV (Total volume of air a person can inspire after a normal expiration).
- **Expiratory Capacity (EC):** TV + ERV (Total volume of air a person can expire after a normal inspiration).
- **Functional Residual Capacity (FRC):** ERV + RV (Volume of air that will remain in the lungs after a normal expiration).
- **Vital Capacity (VC):** ERV + TV + IRV (The maximum volume of air a person can breathe in after a forced expiration or breathe out after a forced inspiration).
- **Total Lung Capacity (TLC):** RV + ERV + TV + IRV = VC + RV (Total volume of air accommodated in the lungs at the end of a forced inspiration).
Question 1. Define vital capacity. What is its significance?
Answer:
**Vital Capacity (VC)** is the maximum volume of air a person can breathe in after a forced expiration, or the maximum volume of air a person can breathe out after a forced inspiration. It is the sum of Expiratory Reserve Volume (ERV), Tidal Volume (TV), and Inspiratory Reserve Volume (IRV).
Its **significance** lies in representing the maximum amount of air that can be moved in and out of the lungs with conscious effort. It reflects the functional capacity of the lungs and the strength of respiratory muscles. VC is an important parameter used in clinical diagnosis to assess pulmonary function and detect respiratory disorders like asthma, emphysema, or restrictive lung diseases, as reduced VC can indicate impaired lung function.
Question 2. State the volume of air remaining in the lungs after a normal breathing.
Answer:
The volume of air remaining in the lungs after a normal expiration is called the **Functional Residual Capacity (FRC)**. It is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV). Its average value is around 2100-2300 mL (1000-1100 mL ERV + 1100-1200 mL RV).
If "normal breathing" implies a normal expiration and asking for the air remaining in the lungs, that would be FRC. If it meant after a normal inspiration, that would be TLC minus TV and IRV.
Assuming it means after a normal expiration, the volume is the Functional Residual Capacity (FRC), which is approximately **2100-2300 mL**.
Question 13. Distinguish between (a) IRV and ERV (b) Inspiratory capacity and Expiratory capacity. (c) Vital capacity and Total lung capacity.
Answer:
(a) IRV and ERV:
| Feature | Inspiratory Reserve Volume (IRV) | Expiratory Reserve Volume (ERV) |
|---|---|---|
| Definition | Additional volume of air that can be inspired by a forced inspiration after a normal tidal inspiration. | Additional volume of air that can be expired by a forced expiration after a normal tidal expiration. |
| Volume (approx.) | 2500-3000 mL | 1000-1100 mL |
| Breathing phase | Volume inspired *above* normal inspiration. | Volume expired *below* normal expiration. |
(b) Inspiratory capacity and Expiratory capacity:
| Feature | Inspiratory Capacity (IC) | Expiratory Capacity (EC) |
|---|---|---|
| Definition | Total volume of air that can be inspired after a normal expiration. | Total volume of air that can be expired after a normal inspiration. |
| Formula | IC = TV + IRV | EC = TV + ERV |
| Volume (approx.) | 3000-3500 mL | 1500-1600 mL |
| Represents | Total air that can be *inhaled* starting from end of normal exhalation. | Total air that can be *exhaled* starting from end of normal inhalation. |
(c) Vital capacity and Total lung capacity:
| Feature | Vital Capacity (VC) | Total Lung Capacity (TLC) |
|---|---|---|
| Definition | Maximum volume of air that can be moved in and out of the lungs with a single maximal effort (after forced expiration to after forced inspiration, or vice versa). | Total volume of air that can be accommodated in the lungs at the end of a maximal (forced) inspiration. |
| Formula | VC = TV + IRV + ERV | TLC = VC + RV = TV + IRV + ERV + RV |
| Volume (approx.) | 4000-4600 mL | 5100-5800 mL |
| Represents | The maximum *usable* volume of air in the lungs that can be exchanged. | The *absolute maximum* volume of air the lungs can hold. Includes the air that cannot be exhaled (RV). |
Question 14. 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 during a normal, quiet respiration. Its approximate value for a healthy human is about **500 mL**.
To find the approximate total Tidal Volume for a healthy human in an hour, we need to know the breathing rate. The average breathing rate for a healthy human is 12-16 times/minute. Let's use an average rate of 14 times/minute.
Number of minutes in an hour = 60 minutes.
Total number of breaths in an hour = (Breathing rate per minute) $\times$ (Number of minutes in an hour) = 14 breaths/minute $\times$ 60 minutes = 840 breaths.
Total Tidal Volume in an hour = (Tidal volume per breath) $\times$ (Total number of breaths in an hour) = 500 mL/breath $\times$ 840 breaths = 420000 mL.
Converting to litres: 420000 mL / 1000 mL/L = 420 L.
The approximate total Tidal Volume for a healthy human in an hour is **420 L**.
Exchange Of Gases
The primary sites for gas exchange are the **alveoli** in the lungs and the tissues throughout the body. Gas exchange (O$_2$ and CO$_2$) occurs by **simple diffusion** across the respiratory membrane. Diffusion is driven primarily by **pressure/concentration gradients** of the gases.
The pressure contributed by an individual gas in a mixture is its **partial pressure** (pO$_2$ for oxygen, pCO$_2$ for carbon dioxide).
Partial pressures of O$_2$ and CO$_2$ in atmospheric air, alveolar air, deoxygenated blood, oxygenated blood, and tissues (in mm Hg):
| Respiratory Gas | Atmospheric Air | Alveoli | Blood (Deoxygenated) | Blood (Oxygenated) | Tissues |
|---|---|---|---|---|---|
| O$_2$ | 159 | 104 | 40 | 95 | 40 |
| CO$_2$ | 0.3 | 40 | 45 | 40 | 45 |
Gas diffusion occurs down the partial pressure gradient:
- **O$_2$:** From alveoli (pO$_2$ 104) to deoxygenated blood (pO$_2$ 40); From oxygenated blood (pO$_2$ 95) to tissues (pO$_2$ 40).
- **CO$_2$:** From tissues (pCO$_2$ 45) to deoxygenated blood (pCO$_2$ 45); From deoxygenated blood (pCO$_2$ 45) to alveoli (pCO$_2$ 40).
Other factors affecting diffusion rate:
- **Solubility of gases:** CO$_2$ is about 20-25 times more soluble in blood than O$_2$. This higher solubility allows CO$_2$ to diffuse much faster than O$_2$ for a given partial pressure difference.
- **Thickness of the diffusion membrane:** The membrane across which gases diffuse is very thin (much less than a millimetre), facilitating rapid diffusion. The diffusion membrane (respiratory membrane) is composed of three layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance between them.
Question 3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
Answer:
Diffusion of gases (exchange between air and blood) occurs primarily in the alveolar region because the alveoli are specifically designed for efficient gas exchange. Their structure provides a very large surface area (millions of alveoli) with very thin walls (single layer of cells) that are richly supplied with blood capillaries. This creates a vast and thin diffusion membrane that allows rapid exchange of O$_2$ and CO$_2$ between the alveolar air and the blood. Other parts of the respiratory system (nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles) are primarily involved in conducting air to the alveoli, clearing it, humidifying it, and bringing it to body temperature; their walls are thicker, and they lack the extensive network of capillaries necessary for significant gas exchange.
Question 5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air ?
(i) pO2 lesser, pCO2 higher
(ii) pO2 higher, pCO2 lesser
(iii) pO2 higher, pCO2 higher
(iv) pO2 lesser, pCO2 lesser
Answer:
Comparing the partial pressures from Table 14.1:
- Atmospheric air: pO$_2$ = 159 mm Hg, pCO$_2$ = 0.3 mm Hg
- Alveolar air: pO$_2$ = 104 mm Hg, pCO$_2$ = 40 mm Hg
Comparing atmospheric air to alveolar air: pO$_2$ is higher in atmospheric air (159 vs 104), and pCO$_2$ is lesser in atmospheric air (0.3 vs 40).
The correct option is **(ii) pO2 higher, pCO2 lesser**.
Transport Of Gases
**Blood** is the medium that transports O$_2$ and CO$_2$ between the lungs and the body tissues.
Transport Of Oxygen
About **97% of O$_2$** is transported by **haemoglobin**, a red-coloured iron-containing pigment present in red blood cells (RBCs). Oxygen binds reversibly with haemoglobin to form **oxyhaemoglobin**.
Factors affecting O$_2$ binding with haemoglobin:
- **Partial pressure of O$_2$ (pO$_2$):** The primary factor. Higher pO$_2$ (like in alveoli) favours O$_2$ binding to haemoglobin. Lower pO$_2$ (like in tissues) favours dissociation of oxyhaemoglobin.
- Partial pressure of CO$_2$ (pCO$_2$), hydrogen ion concentration (H$^+$ or pH), and temperature: These factors can influence the binding. High pCO$_2$ (like in metabolically active tissues), high H$^+$ concentration (low pH), and higher temperature favour the dissociation of O$_2$ from haemoglobin.
An **Oxygen dissociation curve** is a sigmoid curve showing the percentage saturation of haemoglobin with O$_2$ plotted against pO$_2$. This curve is useful for understanding the effect of factors like pCO$_2$, H$^+$, and temperature on O$_2$-haemoglobin binding.
In the alveoli (high pO$_2$, low pCO$_2$, low H$^+$, lower temperature), conditions favour oxyhaemoglobin formation. In the tissues (low pO$_2$, high pCO$_2$, high H$^+$, higher temperature), conditions favour O$_2$ dissociation from oxyhaemoglobin.
Under normal physiological conditions, every 100 mL of oxygenated blood can deliver around **5 mL of O$_2$** to the tissues.
The remaining **3% of O$_2$** is transported in a dissolved state in the plasma.
Transport Of Carbon Dioxide
CO$_2$ is transported by blood in different ways:
- About **20-25% of CO$_2$** is transported bound to haemoglobin as **carbamino-haemoglobin**. This binding is favoured by high pCO$_2$ and low pO$_2$ (as in tissues). In the alveoli (low pCO$_2$, high pO$_2$), carbamino-haemoglobin dissociates, releasing CO$_2$.
- About **70% of CO$_2$** is transported as **bicarbonate ions (HCO$_3^-$)**. CO$_2$ diffuses into RBCs (and some plasma) and reacts with water to form carbonic acid (H$_2$CO$_3$), a reaction facilitated by the enzyme **carbonic anhydrase** (present in high concentration in RBCs and minute quantities in plasma). Carbonic acid dissociates into H$^+$ and HCO$_3^-$. Bicarbonate ions move into the plasma (in exchange for chloride ions, chloride shift).
At the tissues (high pCO$_2$), CO$_2$ diffuses into the blood and is converted to bicarbonate. At the alveoli (low pCO$_2$), the reaction reverses, bicarbonate is converted back to CO$_2$ and water, and CO$_2$ is released for exhalation.
CO$_2$ + H$_2$O $\rightleftharpoons$ H$_2$CO$_3$ $\rightleftharpoons$ HCO$_3^-$ + H$^+$
Carbonic Anhydrase catalyses both forward and reverse reactions.
- About **7% of CO$_2$** is transported in a dissolved state through plasma.
Every 100 mL of deoxygenated blood delivers approximately **4 mL of CO$_2$** to the alveoli.
Question 4. What are the major transport mechanisms for CO2? Explain.
Answer:
The major transport mechanisms for CO$_2$ in the blood are:
- **As Bicarbonate Ions (HCO$_3^-$):** Approximately 70% of CO$_2$ is transported in this form. In tissues (high pCO$_2$), CO$_2$ diffuses into blood and reacts with water (catalyzed by carbonic anhydrase in RBCs) to form carbonic acid, which quickly dissociates into H$^+$ and HCO$_3^-$. Bicarbonate ions then move into the plasma. In the lungs (low pCO$_2$), the process reverses: HCO$_3^-$ re-enters RBCs, combines with H$^+$ to form H$_2$CO$_3$, which dissociates into CO$_2$ and H$_2$O (catalyzed by carbonic anhydrase). CO$_2$ diffuses out into the alveoli.
- **As Carbamino-haemoglobin:** Approximately 20-25% of CO$_2$ binds to the amino groups of haemoglobin to form carbamino-haemoglobin. This binding is favoured by high pCO$_2$ and low pO$_2$ (in tissues) and dissociates in the lungs (low pCO$_2$, high pO$_2$).
- **In Dissolved State in Plasma:** Approximately 7% of CO$_2$ is transported simply dissolved in the blood plasma.
Question 8. What is the effect of pCO2 on oxygen transport?
Answer:
The partial pressure of CO$_2$ (pCO$_2$) affects oxygen transport by influencing the binding of oxygen with haemoglobin. High pCO$_2$ (as found in metabolically active tissues) favors the dissociation of oxyhaemoglobin, meaning it reduces the affinity of haemoglobin for oxygen. This effect, known as the **Bohr effect**, facilitates the unloading of oxygen from blood to the tissues where it is needed. Conversely, low pCO$_2$ (as in the alveoli) favors the binding of oxygen to haemoglobin.
Regulation Of Respiration
Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues, controlled by the **neural system**. Specific centers in the brain are involved:
- **Respiratory rhythm center:** Located in the **medulla** region of the brain. Primarily responsible for regulating the basic respiratory rhythm (involuntary breathing rate).
- **Pneumotaxic center:** Located in the **pons** region of the brain. Can moderate the function of the rhythm center by sending signals that reduce the duration of inspiration, thereby altering the respiratory rate.
- **Chemosensitive area:** Situated adjacent to the rhythm center in the medulla. Highly sensitive to increases in **CO$_2$ and hydrogen ions (H$^+$)** in the blood. Activation by these substances signals the rhythm center to adjust breathing (increase rate and depth) to eliminate CO$_2$ and reduce H$^+$ concentration.
Receptors in the aortic arch and carotid artery also detect changes in CO$_2$ and H$^+$ concentration and send signals to the rhythm center for regulation. The role of oxygen in regulating respiratory rhythm is relatively insignificant compared to CO$_2$ and H$^+$, unless oxygen levels are very low.
Question 7. How is respiration regulated?
Answer:
Respiration is primarily regulated by the neural system. A specialized **respiratory rhythm center** in the medulla oblongata region of the brain controls the basic rate and rhythm of breathing. The **pneumotaxic center** in the pons can modulate the rhythm center, influencing the duration of inspiration and thus the respiratory rate. A **chemosensitive area** near the rhythm center is highly sensitive to levels of CO$_2$ and H$^+$ in the blood; increased levels of these substances stimulate the chemosensitive area, signaling the rhythm center to increase breathing rate and depth. Receptors in the aortic arch and carotid artery also monitor CO$_2$ and H$^+$ levels and send signals to the rhythm center for necessary adjustments. Oxygen plays a less significant role in regulating rhythm unless its concentration becomes critically low.
Question 9. What happens to the respiratory process in a man going up a hill?
Answer:
As a man goes up a hill (especially to higher altitudes), the atmospheric pressure decreases, which also means the partial pressure of oxygen (pO$_2$) in the air decreases. This leads to a lower pO$_2$ in the alveoli and blood. To compensate for the reduced availability of oxygen, the body increases the rate and depth of breathing (hyperventilation). This increased ventilation aims to increase the oxygen uptake and maintain sufficient oxygen levels in the blood and tissues, despite the lower pO$_2$ in the atmosphere. Over time, physiological adaptations like increased red blood cell production occur to enhance oxygen transport efficiency at higher altitudes.
Disorders Of Respiratory System
The respiratory system is susceptible to various disorders. Some common ones include:
- **Asthma:** A chronic condition characterized by inflammation of the bronchi and bronchioles, causing difficulty in breathing (wheezing) due to airway constriction.
- **Emphysema:** A chronic disorder, often caused by cigarette smoking, where the alveolar walls are damaged. This reduces the total surface area for gas exchange, leading to breathing difficulties and reduced lung function.
- **Occupational Respiratory Disorders:** Disorders resulting from long-term exposure to irritants (like dust, chemicals) in workplaces, especially in industries like grinding or stone-breaking. Prolonged inhalation of dust particles can cause inflammation, leading to fibrosis (proliferation of fibrous tissues) in the lungs and severe lung damage. Wearing protective masks is recommended for workers in such industries.
Exercises
Question 1. Define vital capacity. What is its significance?
Answer:
Question 2. State the volume of air remaining in the lungs after a normal breathing.
Answer:
Question 3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
Answer:
Question 4. What are the major transport mechanisms for $CO_2$? Explain.
Answer:
Question 5. What will be the $pO_2$ and $pCO_2$ in the atmospheric air compared to those in the alveolar air ?
(i) $pO_2$ lesser, $pCO_2$ higher
(ii) $pO_2$ higher, $pCO_2$ lesser
(iii) $pO_2$ higher, $pCO_2$ higher
(iv) $pO_2$ lesser, $pCO_2$ lesser
Answer:
Question 6. Explain the process of inspiration under normal conditions.
Answer:
Question 7. How is respiration regulated?
Answer:
Question 8. What is the effect of $pCO_2$ on oxygen transport?
Answer:
Question 9. What happens to the respiratory process in a man going up a hill?
Answer:
Question 10. What is the site of gaseous exchange in an insect?
Answer:
Question 11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
Answer:
Question 12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.
Answer:
Question 13. Distinguish between
(a) IRV and ERV
(b) Inspiratory capacity and Expiratory capacity.
(c) Vital capacity and Total lung capacity.
Answer:
Question 14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
Answer: