Breathing and Exchange of Gases Class 11 Biology Chapter 14 Notes
Oxygen (O2) is essential for breaking down molecules like glucose, amino acids, and fatty acids to produce energy in organisms.
During this process, carbon dioxide (CO2) is released and needs to be removed.
The exchange of O2 from the atmosphere with CO2 produced by cells is called breathing or respiration.
Respiratory Organs
Different animal groups have varying mechanisms of breathing based on their habitats and organizational levels.
Lower invertebrates rely on simple diffusion over their body surface to exchange O2 and CO2.
Earthworms use their moist cuticle, while insects have tracheal tubes to transport atmospheric air.
Aquatic arthropods and mollusks use gills (branchial respiration), while terrestrial animals use lungs (pulmonary respiration).
Among vertebrates, fishes use gills, while amphibians, reptiles, birds, and mammals respire through lungs. Some amphibians like frogs can also respire through their moist skin (cutaneous respiration).
Human Respiratory System
The human respiratory system starts with a pair of external nostrils, leading to the nasal chamber and opening into the pharynx, which is a common passage for food and air.
The pharynx connects to the larynx, a cartilaginous box responsible for sound production.
The trachea is a straight tube extending to the mid-thoracic cavity and divides into right and left primary bronchi. These bronchi further branch into secondary and tertiary bronchi and bronchioles, ending in alveoli.
The lungs, covered by a double-layered pleura with pleural fluid, consist of a conducting part responsible for transporting, humidifying, and clearing air, and an exchange part where O2 and CO2 diffuse between blood and atmospheric air.
The thoracic chamber, formed by the vertebral column, sternum, ribs, and diaphragm, is anatomically airtight and essential for breathing.
Respiration involves five steps: pulmonary ventilation (breathing), diffusion of gases across alveolar membranes, transport of gases by the blood, diffusion of gases between blood and tissues, and utilization of O2 by cells for catabolic reactions with the release of CO2.
Mechanism of Breathing
Breathing comprises two stages: inspiration (inhalation) and expiration (exhalation).
Air movement into and out of the lungs depends on the pressure gradient between the lungs and the atmosphere.
Inspiration occurs when intra-pulmonary pressure is lower than atmospheric pressure, creating a negative pressure in the lungs. Expiration occurs when intra-pulmonary pressure is higher.
The diaphragm, external intercostal muscles, and internal intercostal muscles play a role in generating these pressure gradients.
During inspiration, the diaphragm contracts, increasing thoracic chamber volume in the antero-posterior and dorso-ventral axes, leading to a volume increase in the lungs.
This decrease in intra-pulmonary pressure causes air from outside to move into the lungs.
During expiration, relaxation of the diaphragm and intercostal muscles reduces thoracic and pulmonary volume, leading to an increase in intra-pulmonary pressure, which forces air out of the lungs.
Additional muscles in the abdomen can enhance the strength of inspiration and expiration.
A healthy human typically breathes 12-16 times per minute, and spirometry is used to estimate the volume of air involved in breathing for clinical assessment of pulmonary function.
Respiratory Volumes and Capacities
Tidal Volume (TV): The volume of air inspired or expired during normal respiration, approximately 500 mL.
Inspiratory Reserve Volume (IRV): Additional air that can be inspired by forcible inspiration, averaging 2500 mL to 3000 mL.
Expiratory Reserve Volume (ERV): Additional air that can be expired by forcible expiration, averaging 1000 mL to 1100 mL.
Residual Volume (RV): The volume of air remaining in the lungs even after forcible expiration, averaging 1100 mL to 1200 mL.
Pulmonary Capacities:
Inspiratory Capacity (IC): Total air a person can inspire after a normal expiration (TV+IRV).
Expiratory Capacity (EC): Total air a person can expire after a normal inspiration (TV+ERV).
Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration (ERV+RV).
Vital Capacity (VC): The maximum air a person can breathe in after a forced expiration (ERV+TV+IRV) or breathe out after a forced inspiration.
Total Lung Capacity (TLC): Total volume of air in the lungs at the end of a forced inspiration (RV+ERV+TV+IRV) or vital capacity + residual volume. These measurements are useful for clinical diagnosis and assessment of lung function.
Gas Exchange in the Respiratory System
Alveoli are the primary sites for the exchange of gases in the respiratory system.
Gas exchange occurs between blood and tissues in addition to the exchange in alveoli.
Oxygen (O2) and carbon dioxide (CO2) are exchanged primarily through simple diffusion based on pressure and concentration gradients.
The solubility of gases and the thickness of the membranes involved in diffusion affect the rate of diffusion.
Partial pressure represents the pressure contributed by an individual gas in a gas mixture and is denoted as pO2 for oxygen and pCO2 for carbon dioxide.
There is a concentration gradient for O2 from alveoli to blood and blood to tissues. Conversely, there is a gradient for CO2 in the opposite direction, from tissues to blood and blood to alveoli.
CO2 has higher solubility than O2, allowing a greater amount of CO2 to diffuse per unit difference in partial pressure.
The diffusion membrane comprises three major layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them.
Despite its multiple layers, the total thickness of the diffusion membrane is much less than a millimeter, facilitating efficient gas exchange.
Transport of Gases in Blood
Blood serves as the medium for transporting oxygen (O2) and carbon dioxide (CO2).
Approximately 97% of O2 is transported by red blood cells (RBCs) in the blood, while the remaining 3% is carried in a dissolved state in the plasma.
Approximately 20-25% of CO2 is transported by RBCs, and around 70% is carried in the form of bicarbonate ions (HCO3-) in the blood.
About 7% of CO2 is transported in a dissolved state through the plasma.
Transport of Oxygen
Hemoglobin, a red iron-containing pigment found in red blood cells (RBCs), can bind with oxygen (O2) in a reversible manner to form oxyhemoglobin.
Each hemoglobin molecule can carry up to four molecules of O2.
The binding of O2 to hemoglobin primarily depends on the partial pressure of O2 (pO2). Other factors that can affect this binding include partial pressure of CO2 (pCO2), hydrogen ion concentration (H+), and temperature.
The Oxygen Dissociation Curve shows the relationship between the percentage saturation of hemoglobin with O2 and pO2. It helps study the impact of factors like pCO2, H+ concentration, etc., on O2 binding to hemoglobin.
In the alveoli, with high pO2, low pCO2, lower H+ concentration, and lower temperature, conditions favor the formation of oxyhemoglobin.
In the tissues, with low pO2, high pCO2, high H+ concentration, and higher temperature, conditions are favorable for the dissociation of O2 from oxyhemoglobin.
This indicates that O2 binds to hemoglobin in the lungs and dissociates in the tissues. Under normal physiological conditions, every 100 mL of oxygenated blood can deliver about 5 mL of O2 to the body’s tissues.
Transport of Carbon Dioxide
Carbon dioxide (CO2) is transported by hemoglobin as carbamino-hemoglobin, constituting approximately 20-25% of CO2 in the blood. The binding is influenced by the partial pressure of CO2 (pCO2), with pO2 being a major factor affecting this binding.
At the tissues, where pCO2 is high and pO2 is low, more CO2 binding occurs. In contrast, at the alveoli, where pCO2 is low and pO2 is high, dissociation of CO2 from carbamino-hemoglobin takes place. This allows the CO2 bound to hemoglobin from the tissues to be released at the alveoli for expiration.
Red blood cells (RBCs) contain a high concentration of the enzyme carbonic anhydrase, which facilitates the reversible reaction between CO2 and H2O, forming carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen ions (H+).
At the tissue level, where the partial pressure of CO2 is high due to catabolism, CO2 diffuses into the blood (RBCs and plasma) and forms HCO3- and H+. At the alveolar level, where pCO2 is low, the reverse reaction occurs, leading to the formation of CO2 and H2O for expiration.
Approximately 4 ml of CO2 is delivered to the alveoli by every 100 ml of deoxygenated blood. This mechanism aids in the transportation and release of CO2 during respiration.
Regulation of Respiration
Human beings can adjust their respiratory rhythm to meet the body’s demands, and this regulation is primarily controlled by the neural system.
The respiratory rhythm center, located in the medulla region of the brain, is responsible for regulating breathing.
The pneumotaxic center, located in the pons region of the brain, can influence the respiratory rhythm center and alter the duration of inspiration to adjust the respiratory rate.
There is a chemosensitive area near the rhythm center that is highly sensitive to carbon dioxide (CO2) and hydrogen ions (H+). An increase in these substances can activate the chemosensitive area, which then signals the rhythm center to make necessary adjustments in the respiratory process to eliminate them.
Receptors associated with the aortic arch and carotid artery can also detect changes in CO2 and H+ concentrations and send signals to the rhythm center for corrective actions.
The role of oxygen in regulating respiratory rhythm is relatively insignificant compared to CO2 and H+.
Respiratory System Disorders
Asthma: Asthma is a respiratory disorder characterized by difficulty in breathing and wheezing. It is caused by the inflammation of the bronchi and bronchioles.
Emphysema: Emphysema is a chronic respiratory disorder in which the walls of the alveoli in the lungs are damaged, leading to a decrease in the respiratory surface. Cigarette smoking is one of the major causes of emphysema.
Occupational Respiratory Disorders: Certain industries, such as those involving grinding or stone-breaking, produce a significant amount of dust. Prolonged exposure to such dust can overwhelm the body’s defense mechanisms, leading to inflammation and fibrosis (proliferation of fibrous tissues) in the lungs. This can cause serious lung damage, and workers in these industries should wear protective masks to prevent respiratory issues.
