DETAILED NOTES – Chapter 14 -Breathing and Exchange of Gases

Chapter 14 Breathing and Exchange of Gases 

Introduction

1. Role of Oxygen (O2):

   • Oxygen is utilized by organisms to break down simple molecules like glucose, amino acids, and fatty acids.
   • This breakdown process provides energy for various cellular activities.

2. Release of Carbon Dioxide (CO2):

   • Carbon dioxide is produced as a byproduct during the catabolic reactions mentioned above.
   • CO2 is harmful and needs to be removed from the body.

3. Need for Continuous Oxygen Supply:

   • Oxygen must be continuously provided to cells for cellular respiration to occur.

4. Process of Breathing (Respiration):

   • Breathing is the process of exchanging oxygen from the atmosphere with carbon dioxide produced by cells.
   • It’s commonly known as respiration.

5. Observing Breathing:

   • You can feel your chest moving up and down when you place your hands on it.
   • This movement is due to breathing.

6. Description of Breathing Mechanism:

   • Further details about respiratory organs and the mechanism of breathing will be covered in subsequent sections of the chapter.

14.1 RESPIRATORY ORGANS

1. Variation in Breathing Mechanisms:

   • Breathing mechanisms vary among different groups of animals.
   • These variations are primarily influenced by their habitats and levels of organization.

2. Breathing in Lower Invertebrates:

   • Lower invertebrates such as sponges, coelenterates, and flatworms exchange oxygen (O2) and carbon dioxide (CO2) by simple diffusion across their entire body surface.
   • Earthworms utilize their moist cuticle for gas exchange.
   • Insects have a network of tubes called tracheal tubes that transport atmospheric air within their bodies.

3. Aquatic Arthropods and Molluscs:

   • Most aquatic arthropods and molluscs use special vascularized structures called gills for gas exchange, a process known as branchial respiration.

4. Terrestrial Forms:

   • Terrestrial animals utilize lungs for gas exchange, a process known as pulmonary respiration.
   • Lungs are vascularized bags specialized for the exchange of gases.

5. Respiration in Vertebrates:

   • Fishes respire primarily through gills.
   • Amphibians, reptiles, birds, and mammals respire through lungs.
   • Amphibians like frogs can also respire through their moist skin, a process known as cutaneous respiration.

14.1.1 Human Respiratory System

1. External Nostrils and Nasal Passage:

   • The respiratory system begins with a pair of external nostrils located above the upper lips.
   • Nostrils lead to the nasal passage, which opens into the nasal chamber.

2. Pharynx and Larynx:

   • The nasal chamber connects to the pharynx, which is a common passage for both food and air.
   • The pharynx opens into the larynx region, which contains the vocal cords and aids in sound production.

3. Epiglottis and Trachea:

   • The glottis can be covered by the epiglottis during swallowing to prevent food from entering the larynx.
   • The trachea, also known as the windpipe, is a straight tube extending from the larynx to the mid-thoracic cavity.

4. Bronchial Tree:

   • The trachea divides into right and left primary bronchi at the level of the 5th thoracic vertebra.
   • These primary bronchi further divide into secondary and tertiary bronchi and bronchioles.
   • Cartilaginous rings support the trachea, bronchi, and initial bronchioles.

5. Alveoli and Lungs:

   • Terminal bronchioles lead to alveoli, which are thin, irregular-walled, and vascularized structures responsible for gas exchange.
   • The lungs consist of a branching network of bronchi, bronchioles, and alveoli.
   • Each lung is covered by a double-layered pleura with pleural fluid between them, reducing friction.

6. Conducting and Respiratory Parts:

   • The conducting part of the respiratory system transports air to the alveoli, clears it from foreign particles, humidifies it, and brings it to body temperature.
   • The alveoli and their ducts form the respiratory or exchange part, where actual diffusion of oxygen (O2) and carbon dioxide (CO2) occurs between blood and atmospheric air.

7. Thoracic Chamber and Breathing:

   • The lungs are situated in the thoracic chamber, formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and inferiorly by the diaphragm.
   • Changes in the volume of the thoracic cavity are reflected in the lung cavity, facilitating breathing.

8. Steps of Respiration:

   • Respiration involves several steps: pulmonary ventilation (breathing), gas diffusion across alveolar membranes, gas transport by blood, gas diffusion between blood and tissues, and cellular respiration within cells.

14.2 MECHANISM OF BREATHING

1. Two Stages of Breathing:

   • Breathing involves two stages: inspiration (inhalation) and expiration (exhalation).
   • Inspiration involves drawing atmospheric air into the lungs, while expiration involves releasing alveolar air out of the lungs.

2. Pressure Gradient:

   • The movement of air into and out of the lungs is facilitated by creating a pressure gradient between the lungs and the atmosphere.
   • Inspiration occurs when the intra-pulmonary pressure is less than atmospheric pressure, creating a negative pressure in the lungs.
   • Expiration occurs when the intra-pulmonary pressure is higher than atmospheric pressure.

3. Muscles Involved:

   • The diaphragm and a specialized set of muscles, including the external and internal intercostal muscles between the ribs, help in generating pressure gradients for breathing.

4. Inspiration:

   • Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis.
   • Contraction of the external intercostal muscles lifts up the ribs and sternum, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
   • The overall increase in thoracic volume leads to an increase in pulmonary volume, reducing intra-pulmonary pressure below atmospheric pressure and forcing air into the lungs.

5. Expiration:

   • Expiration occurs when the diaphragm and intercostal muscles relax, returning the diaphragm and sternum to their normal positions and reducing thoracic volume.
   • This reduction in thoracic volume increases intra-pulmonary pressure slightly above atmospheric pressure, causing air to be expelled from the lungs.

6. Additional Muscles and Breathing Rate:

   • Additional muscles in the abdomen can aid in increasing the strength of inspiration and expiration.
   • On average, a healthy human breathes 12-16 times per minute.

7. Clinical Assessment:

   • The volume of air involved in breathing movements can be assessed clinically using a spirometer, which helps in evaluating pulmonary functions.

14.2.1 Respiratory Volumes and Capacities

1. Tidal Volume (TV):

   • The volume of air inspired or expired during a normal respiration.
   • Approximately 500 mL.
   • A healthy individual can inspire or expire around 6000 to 8000 mL of air per minute.

2. Inspiratory Reserve Volume (IRV):

   • Additional volume of air a person can inspire by forcible inspiration.
   • Averages 2500 mL to 3000 mL.

3. Expiratory Reserve Volume (ERV):

   • Additional volume of air a person can expire by forcible expiration.
   • Averages 1000 mL to 1100 mL.

4. Residual Volume (RV):

   • Volume of air remaining in the lungs even after a forcible expiration.
   • Averages 1100 mL to 1200 mL.

5. Pulmonary Capacities:

   • Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume (TV + IRV).
   • Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV + ERV).
   • Functional Residual Capacity (FRC): Volume of air that remains in the lungs after a normal expiration. This includes expiratory reserve volume and residual volume (ERV + RV).
   • Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration. This includes expiratory reserve volume, tidal volume, and inspiratory reserve volume (ERV + TV + IRV), or the maximum volume of air a person can breathe out after a forced inspiration.
   • Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume (RV + ERV + TV + IRV), or vital capacity plus residual volume.