Breathing and Exchange of Gases Chapter Notes | Biology Class 11 - NEET PDF Download

Breathing

Breathing is the fundamental physiological process by which organisms exchange gases with their environment. It primarily involves the intake of oxygen-rich air and the expulsion of carbon dioxide-rich air. This exchange occurs in the respiratory system, specifically in the lungs.

Respiration

Respiration is a broader biochemical process that occurs within cells, involving the breakdown of organic molecules (such as glucose) to release energy. This process requires oxygen and produces carbon dioxide as a byproduct. While breathing facilitates the exchange of gases necessary for respiration, respiration itself occurs at the cellular level and is essential for the production of adenosine triphosphate (ATP), the energy currency of cells.

Some Examples of Respiratory Organs

Respiratory Organs

In unicellular organisms, gas exchange often occurs directly through the cell membrane via diffusion. However, in multicellular organisms, specialized respiratory organs are responsible for facilitating the exchange of gases. These organs include the lungs in mammals, gills in fish, and tracheal systems in insects.

Human Respiratory System

The human respiratory system is divided into the upper and lower respiratory tracts, each comprising several organs with distinct functions.

The Upper Respiratory Tract:
The upper respiratory tract includes the nasal cavity, pharynx, and larynx. These structures are involved in the initial processes of breathing, such as air filtration, humidification, and vocalization.

The Lower Respiratory Tract

Human has two lungs, enveloped by a double-layered membrane, known as pleura. In between the pleura, pleural fluid is present, which reduces friction on the surface of the lungs. The outer pleural membrane lies close to the thoracic lining, while the inner pleural membrane lies close to the surface of the lungs.

Steps Involved in Respiration

• Breathing or pulmonary ventilation, in which oxygen from the atmosphere is drawn in and CO₂-rich air from the alveoli releases.
• Diffusion of gases (O₂ and CO₂) occurs at the alveolar membrane.
• Transportation of gases by the blood.
• Diffusion of O₂ and CO₂ occurs between blood and tissues.
• The utilisation of O₂ by the cells and release of CO₂ (cellular respiration).

Mechanism of Breathing

Breathing involves two main processes: inhalation (or inspiration) and exhalation (or expiration). During inhalation, the diaphragm contracts and the ribcage expands, creating negative pressure in the chest cavity, which draws air into the lungs. Exhalation occurs when the diaphragm and ribcage relax, causing the chest cavity to decrease in volume and expel air from the lungs.

Respiratory Volumes and Capacities

Respiratory volumes and capacities are measurements used to assess lung function. These include tidal volume (the volume of air inhaled or exhaled during normal breathing), inspiratory reserve volume, expiratory reserve volume, and vital capacity (the maximum volume of air that can be exhaled after a maximum inhalation).

Exchange of Gases

The exchange of gases, namely oxygen and carbon dioxide, occurs in the alveoli of the lungs. Oxygen from the inhaled air diffuses across the alveolar membrane into the bloodstream, where it binds to hemoglobin in red blood cells for transport to tissues. Meanwhile, carbon dioxide produced by cellular metabolism diffuses from the bloodstream into the alveoli to be exhaled.

Transport of Gases

Blood serves as the medium for transporting gases between the lungs and tissues. Oxygen is primarily transported by binding to hemoglobin in red blood cells, while carbon dioxide is transported in various forms, including dissolved in plasma, bound to hemoglobin, or as bicarbonate ions.

Transport of Oxygen

About 97 per cent of the O₂ is transported by RBCs in the blood, and the remaining 3 per cent is transported in the dissolved state through plasma.

• O₂ can be bound with haemoglobin to form oxyhemoglobin.
• The partial pressure of CO₂, hydrogen ion concentration and temperature are the factors which interfere with this binding.
• A sigmoid curve is formed when the percentage of the saturation of haemoglobin with O₂ is plotted against the pO₂. This formed curve is known as the Oxygen dissociation curve.

• Factors Favourable for the Formation of Oxyhemoglobin: In the alveoli, pO₂ is high, pCO₂ is low, H⁺ concentration is less and temperature is lower.
• Factors Favourable for the Dissociation of Oxyhemoglobin: In the tissues, pO₂ is low, pCO₂ is high, H⁺ concentration is high and temperature is high.
• This shows that O₂ is bound to haemoglobin in the lung (surface) and becomes dissociated in the tissues.

Transport of Carbon Dioxide

Approx. 20-25 per cent of the CO₂ is transported by RBCs by binding with it, to form carbamino-haemoglobin, 70 per cent is carried as bicarbonates and around 8-9 per cent of CO2 is transported in a dissolved state through plasma.

• The formation of carbamino-hemoglobin is related to pCO₂ which is a major factor that affects the binding.
• Factors Favourable for the Binding of Carbon Dioxide: In the tissues, high pCO₂ and low pO₂ are required.
• Factors Favourable for the Dissociation of Carbon Dioxide: In the alveoli, low pCO₂ and high pO₂ are required.
• CO₂ bound to haemoglobin from the tissues is transported to the alveoli.
• RBCs have a high concentration of carbonic anhydrase enzyme and low quantities of it are present in the plasma.

• In the tissues with high pCO₂ due to catabolism, CO₂ diffuses into the blood (RBCs and plasma) and forms HCO^3– and H⁺.
• In the alveoli, with low pCO₂, the reaction is followed in the reverse direction which leads to the formation of CO₂ and H₂O.
• CO₂ is formed as the bicarbonates in the tissues and transported to the alveoli where it is released as CO₂.

Regulation of Respiration

Respiration is regulated by both neural and chemical mechanisms. Respiratory centers located in the brainstem, particularly the medulla oblongata and pons, monitor the levels of oxygen, carbon dioxide, and pH in the blood and adjust breathing rate and depth accordingly. Additionally, chemical receptors sensitive to changes in blood chemistry provide feedback to regulate respiration.

Disorders of the Respiratory System