Respiratory system

• Energy for activities is derived from the catabolism of food components like proteins, carbohydrates, and fats
• Oxygen is required for catabolic processes and carbon dioxide is released in the process
• The body needs a continuous exchange of gases: oxygen is taken in from the atmosphere and carbon dioxide is released
• The process of gaseous exchange is called breathing or respiration
• Respiratory organs and examples:

  • Cutaneous respiration:
  • Occurs through the skin
  • Examples: poriferans, coelenterates, flatworms
  • Moist cuticle:
  • Found in earthworms
  • Moist skin:
  • Found in amphibians like frogs
  • Tracheal respiration:
  • Involves spiracles
  • Found in arthropods like cockroaches
  • Branchial respiration:
  • Uses gills
  • Found in aquatic animals like arthropods, molluscs, and fishes
  • Pulmonary respiration:
  • Involves lungs
  • Found in terrestrial vertebrates including humans
• Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation
• Lower invertebrates like sponges, coelenterates, flatworms exchange O2 with CO2 by simple diffusion over their entire body surface
• Earthworms use their moist cuticle for gas exchange
• Insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body
• Aquatic arthropods and molluscs use gills for gas exchange (branchial respiration)
• Terrestrial forms use lungs for gas exchange (pulmonary respiration)
• Fishes use gills for respiration
• Amphibians, reptiles, birds, and mammals respire through lungs
• Amphibians like frogs can respire through their moist skin (cutaneous respiration)
• Human Respiratory System
• External nostrils lead to nasal chamber, then to pharynx, which opens into the larynx region and trachea
• Larynx helps in sound production and is covered by epiglottis during swallowing
• Trachea divides into right and left primary bronchi, further dividing into secondary and tertiary bronchi and bronchioles
• Bronchi and bronchioles are supported by incomplete cartilaginous rings
• Alveoli are thin, irregular-walled, vascularised bag-like structures where gas exchange occurs
• Lungs are covered by a double-layered pleura with pleural fluid between them
• Conducting part of the respiratory system transports air to alveoli, clears foreign particles, humidifies, and brings air to body temperature
• Exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air
• Respiration involves breathing, diffusion of gases across alveolar membrane, transport of gases by the blood, diffusion of O2 and CO2 between blood and tissues, and utilisation of O2 by cells for catabolic reactions
• Breathing involves two stages: inspiration and expiration
• Inspiration: atmospheric air is drawn in

  Expiration: alveolar air is released out
• Movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere
• Inspiration occurs when intra-pulmonary pressure is less than atmospheric pressure, creating a negative pressure in the lungs
• Expiration takes place when intra-pulmonary pressure is higher than atmospheric pressure
• Diaphragm and a specialized set of muscles (external and internal intercostals) help in generating pressure gradients
• Inspiration is initiated by the contraction of the diaphragm, increasing thoracic chamber volume in the antero-posterior axis
• Contraction of external inter-costal muscles lifts up the ribs and sternum, increasing thoracic chamber volume in the dorso-ventral axis
• Increase in thoracic volume causes a similar increase in pulmonary volume
• Increase in pulmonary volume decreases intra-pulmonary pressure, allowing air from outside to move into the lungs (inspiration)
• Relaxation of diaphragm and inter-costal muscles reduces thoracic volume and pulmonary volume, leading to an increase in intra-pulmonary pressure above atmospheric pressure, causing air expulsion from the lungs (expiration)
• Additional muscles in the abdomen help increase the strength of inspiration and expiration
• Healthy humans breathe 12-16 times per minute
• Volume of air involved in breathing movements can be estimated using a spirometer for clinical assessment of pulmonary functions
• Respiratory volumes and capacities:

  • Tidal Volume (TV): approx. 500 mL, air inspired or expired during normal respiration
  • Inspiratory Reserve Volume (IRV): 2500-3000 mL, additional air inspired by forcible inspiration
  • Expiratory Reserve Volume (ERV): 1000-1100 mL, additional air expired by forcible expiration
  • Residual Volume (RV): 1100-1200 mL, air remaining in the lungs after forcible expiration
• Pulmonary capacities:

  • Inspiratory Capacity (IC): TV + IRV, total air inspired after normal expiration
  • Expiratory Capacity (EC): TV + ERV, total air expired after normal inspiration
  • Functional Residual Capacity (FRC): ERV + RV, air remaining in lungs after normal expiration
  • Vital Capacity (VC): maximum air breathed in after forced expiration, includes ERV, TV, and IRV
  • Total Lung Capacity (TLC): total air accommodated in lungs at the end of forced inspiration, includes RV, ERV, TV, and IRV