IB BIO - 6.4 Gas exchange

• 6.4.1 

  • Distinguish between ventilation, gas exchange and cell respiration. 

• Ventilation, gas exchange, and cell respiration are interconnected processes involved in the respiratory system, but they serve distinct functions:

  • 1. Ventilation:

    •    - Ventilation refers to the process of breathing, which involves the movement of air into and out of the lungs.

    •    - It includes two phases: inspiration (inhalation) and expiration (exhalation).

    •    - During inspiration, the diaphragm and intercostal muscles contract, causing the volume of the thoracic cavity to increase and the air pressure inside the lungs to decrease. This decrease in pressure allows air to flow into the lungs, filling the alveoli with fresh oxygen.

    •    - During expiration, the diaphragm and intercostal muscles relax, causing the thoracic cavity to decrease in volume and the air pressure inside the lungs to increase. This increase in pressure forces air out of the lungs, expelling carbon dioxide-rich air.

  • 2. Gas Exchange:

    •    - Gas exchange is the process by which oxygen from the air is taken up by the blood and carbon dioxide is released from the blood into the air.

    •    - It occurs in the alveoli, small air sacs in the lungs where the respiratory surface is located.

    •    - During gas exchange, oxygen diffuses from the alveoli into the bloodstream across the thin walls of the capillaries surrounding the alveoli. At the same time, carbon dioxide diffuses from the bloodstream into the alveoli to be exhaled.

    •    - The diffusion of gases across the respiratory membrane is facilitated by differences in partial pressures of oxygen and carbon dioxide between the air in the alveoli and the blood in the capillaries.

  • 3. Cell Respiration:

    •    - Cell respiration, also known as cellular respiration, is the process by which cells break down glucose and other organic molecules to produce energy in the form of ATP (adenosine triphosphate).

    •    - It occurs in the mitochondria of cells and involves three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain).

    •    - During glycolysis, glucose is converted into pyruvate, producing a small amount of ATP and NADH.

    •    - In the citric acid cycle, pyruvate is further oxidized to generate more ATP, NADH, and FADH2.

    •    - In oxidative phosphorylation, electrons carried by NADH and FADH2 are transferred to the electron transport chain, leading to the production of ATP through chemiosmosis.

    •    - The overall equation for cell respiration is:

    •      Glucose + Oxygen → Carbon Dioxide + Water + ATP

• In summary, ventilation involves the movement of air into and out of the lungs, gas exchange occurs in the alveoli where oxygen is taken up by the blood and carbon dioxide is released into the air, and cell respiration is the process by which cells produce energy by breaking down glucose and other organic molecules. These processes work together to ensure the delivery of oxygen to cells and the removal of carbon dioxide from the body.

• 6.4.2 

  • Explain the need for a ventilation system.

• A ventilation system is necessary to ensure the proper functioning of the respiratory system and to meet the physiological requirements of gas exchange in the body. Here's why a ventilation system is needed:

  • 1. Supply of Oxygen: Oxygen is essential for cellular respiration, the process by which cells produce energy (ATP) from glucose. Oxygen is obtained from the air during inspiration and delivered to the bloodstream through gas exchange in the lungs. Without a ventilation system, the body would not receive an adequate supply of oxygen, leading to cellular hypoxia (oxygen deficiency) and impaired energy production.

  • 2. Removal of Carbon Dioxide: Carbon dioxide (CO2) is a waste product of cellular metabolism that must be eliminated from the body to maintain proper pH balance and prevent respiratory acidosis. During expiration, carbon dioxide is removed from the bloodstream and exhaled into the atmosphere. A ventilation system facilitates the removal of carbon dioxide-rich air from the lungs, ensuring efficient gas exchange and the elimination of metabolic waste.

  • 3. Maintenance of Respiratory Surface: The lungs contain millions of tiny air sacs called alveoli, where gas exchange between air and blood occurs. Adequate ventilation is necessary to maintain the patency of the airways and prevent the collapse of alveoli. Without proper ventilation, areas of the lungs may become poorly ventilated or collapse, impairing gas exchange and compromising respiratory function.

  • 4. Regulation of pH: The respiratory system plays a crucial role in regulating the pH of the blood by controlling the levels of carbon dioxide and bicarbonate ions. Ventilation helps to regulate the partial pressure of carbon dioxide (PCO2) in the blood, which in turn influences the bicarbonate ion (HCO3-) concentration and pH. Proper ventilation ensures the maintenance of acid-base balance and homeostasis in the body.

  • 5. Elimination of Airborne Pollutants: A ventilation system helps to remove airborne pollutants, allergens, and pathogens from indoor environments, reducing the risk of respiratory infections, allergic reactions, and respiratory irritation. Adequate ventilation is particularly important in enclosed spaces to ensure air quality and promote respiratory health.

• In summary, a ventilation system is essential for supplying oxygen to the body, removing carbon dioxide and metabolic waste, maintaining respiratory surface integrity, regulating pH balance, and promoting air quality and respiratory health. Without proper ventilation, the body's ability to perform gas exchange and maintain homeostasis would be compromised, leading to impaired cellular function and overall health.

• 6.4.3 

  • Describe the features of alveoli that adapt them to gas exchange.

• Alveoli are the primary sites of gas exchange in the respiratory system, where oxygen is taken up by the bloodstream and carbon dioxide is released into the air. The features of alveoli are specifically adapted to facilitate efficient gas exchange. Here are the key features of alveoli that enable them to carry out this essential function:

  • 1. Large Surface Area: Alveoli have an enormous combined surface area, estimated to be around 70 square meters in the adult human lungs. This large surface area provides ample space for gas exchange to occur, allowing for the efficient diffusion of oxygen and carbon dioxide across the alveolar membrane.

  • 2. Thin Respiratory Membrane: The walls of alveoli are composed of a single layer of epithelial cells known as type I pneumocytes. These cells are extremely thin, averaging only about 0.2 to 0.5 micrometers in thickness. The thinness of the respiratory membrane minimizes the diffusion distance for gases, allowing for rapid exchange of oxygen and carbon dioxide between the alveoli and capillaries.

  • 3. Rich Capillary Network: Alveoli are surrounded by an extensive network of pulmonary capillaries, which come into close contact with the alveolar walls. This close association ensures that blood flow is in close proximity to air within the alveoli, maximizing the efficiency of gas exchange by maintaining a steep concentration gradient for oxygen and carbon dioxide.

  • 4. Moist Environment: The inner surface of alveoli is coated with a thin layer of moisture, which helps to keep the alveolar surface moist and facilitates the dissolution of gases. This moisture is essential for the diffusion of gases across the respiratory membrane and ensures that the alveolar surface remains conducive to gas exchange.

  • 5. Surfactant Production: Type II pneumocytes within the alveoli secrete a substance called pulmonary surfactant. Surfactant reduces the surface tension of the alveolar fluid, preventing the collapse of alveoli during expiration and maintaining their patency. This ensures that alveoli remain open and available for gas exchange throughout the respiratory cycle.

  • 6. Elastic Recoil: Alveoli are surrounded by elastic fibers that allow them to stretch and recoil during the breathing cycle. This elastic recoil helps to maintain the structural integrity of the alveoli and assists in the expulsion of air during expiration. It also helps to ensure efficient gas exchange by promoting the movement of gases in and out of the alveoli.

• In summary, alveoli are highly specialized structures adapted for efficient gas exchange. Their large surface area, thin respiratory membrane, rich capillary network, moist environment, surfactant production, and elastic recoil collectively contribute to their ability to facilitate the rapid exchange of oxygen and carbon dioxide, ensuring the proper oxygenation of blood and removal of carbon dioxide from the body.

• 6.4.4 

  • Draw and label a diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli.

• 6.4.5 

  • Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostal muscles, the diaphragm and abdominal muscles.

• The mechanism of ventilation, or breathing, involves a coordinated interplay of muscles and changes in thoracic volume and pressure to facilitate the movement of air into and out of the lungs. This process can be broken down into two phases: inspiration (inhalation) and expiration (exhalation). Here's how it works:

  • 1. Inspiration:

    •    - During inspiration, the diaphragm and external intercostal muscles contract.

    •    - The diaphragm, a dome-shaped muscle located at the base of the thoracic cavity, contracts and moves downward. This increases the volume of the thoracic cavity vertically.

    •    - The external intercostal muscles, located between the ribs, contract and elevate the ribcage. This increases the volume of the thoracic cavity laterally.

    •    - As the thoracic cavity expands, the intra-alveolar pressure (pressure inside the lungs) decreases below atmospheric pressure (pressure outside the body).

    •    - The pressure gradient between the higher atmospheric pressure and the lower intra-alveolar pressure causes air to flow into the lungs, filling the expanded alveoli with fresh oxygen-rich air.

  • 2. Expiration:

    •    - During expiration, the diaphragm and external intercostal muscles relax.

    •    - The diaphragm returns to its relaxed dome-shaped position as the diaphragmatic muscles relax, causing it to move upward.

    •    - The external intercostal muscles also relax, allowing the ribcage to return to its resting position.

    •    - As the thoracic cavity decreases in volume, the intra-alveolar pressure increases above atmospheric pressure.

    •    - The pressure gradient now favors the movement of air out of the lungs, and expiration occurs passively as the lungs recoil and air is expelled from the alveoli.

  • Additionally, during forced expiration, such as during vigorous exercise or coughing, the internal intercostal muscles and abdominal muscles may contract to further decrease thoracic volume and increase intra-alveolar pressure, aiding in the expulsion of air from the lungs.

• In summary, the mechanism of ventilation involves the contraction and relaxation of the diaphragm and intercostal muscles to alter thoracic volume and pressure, resulting in the inhalation and exhalation of air. This process ensures the continuous exchange of oxygen and carbon dioxide in the lungs to support cellular respiration and maintain homeostasis in the body.