Human Respiratory System: Structure, Function and Mechanisms of Breathing

The respiratory system is responsible for the exchange of oxygen and carbon dioxide in the body, a process vital for sustaining life. It consists of various organs and structures, each playing a unique role in this essential function. From lower invertebrates to vertebrates like humans, organisms have evolved diverse mechanisms to facilitate respiration according to their habitats and needs.

Understanding Breathing: From Invertebrates to Humans

1. Mechanisms of Breathing in Different Animal Groups

• Definition of Respiration: The process of exchange of O2 from the atmosphere with CO2 produced by the cells is called breathing, also known as respiration. 

• Breathing Mechanisms across Animal Groups: Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and level of organisation:
  • Lower Invertebrates: like sponges, coelenterates, flatworms, etc., breathe through simple diffusion over their entire body surface. 
  • Earthworms: use moist cuticles.
  • Insects: have a network of tubes called trachea.
  • Arthropods(aquatic) and Molluscs(aquatic): have vascularised structures called gills.
  • Arthropods(terrestrial) and Molluscs(terrestrial): have vascularised bags called lungs.
  • Vertebrates: fishes use gills; amphibians, reptiles, birds and mammals respire through lungs. 
    • Some amphibians, like frogs, can respire through their moist skin (cutaneous respiration) too.

2. Human Respiratory System

• Components of the Human Respiratory System: It structurally extends from the nose, and pharynx, all the way to the alveoli of the lungs. 
  • The conducting part extends from the external nostrils to the terminal bronchioles.
  • It transports atmospheric air to the alveoli.
  • It clears the air from foreign particles.
  • It humidifies the air.
  • It brings the air to body temperature.
  • The respiratory or exchange part includes the alveoli and their ducts.
  • It is the site of the actual diffusion of O2 and CO2 between blood and atmospheric air. 
• Nose: Consists of a pair of external nostrils opening out above the upper lips, which leads to a nasal chamber through the nasal passage.
• Pharynx: The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air. 
  • The pharynx opens through the larynx region into the trachea.
• Larynx: Also called ‘sound box’ is a cartilaginous box which helps in sound production. 
  • During swallowing, the glottis (thin opening at the top of the larynx) can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.

• Trachea: It is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the fifth thoracic vertebra into a right and left primary bronchi.
• Bronchi: Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. 
  • The trachea, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
• Alveoli: Numerous vascularised bag like structures which rise from terminal bronchioles.
• Lungs: The branching network of bronchi, bronchioles and alveoli comprise the lungs. 
  • Humans have two lungs which are covered by a double-layered pleura (with pleural fluid between them), which reduces friction on the lung surface. 
• Diaphragm: It is a dome-shaped, muscular and membranous structure that separates the thoracic (chest) and abdominal cavities in humans (mammals) and is the principal muscle of respiration.

3. Mechanism of Breathing

• Breathing Process Overview: Breathing involves two stages: inspiration (atmospheric air is drawn in) and expiration (alveolar air is released out). 
• Inspiration Mechanism: Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure. 
• Role of Diaphragm and Intercostal Muscles: The diaphragm and a specialised set of muscles– external and internal intercostals between the ribs help in the generation of such gradients. 
  • Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber. 
• Exhalation Mechanism: The contraction of external intercostal muscles lifts up the ribs and the sternum, also increasing the volume of the thoracic chamber. 
• Adjustments in Thoracic and Pulmonary Volumes: The overall increase in the thoracic volume causes a similar increase in pulmonary volume, decreasing the intra-pulmonary pressure below the atmospheric pressure, forcing air outside to move into the lungs, i.e., inspiration. 
  • Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions, reducing the thoracic volume and the pulmonary volume. 
  • This increases intra-pulmonary pressure slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration. 

Conclusion

• The respiratory system is a complex network of organs and tissues that work together to ensure the body receives oxygen and expels carbon dioxide. 
• Understanding its structure and function helps us appreciate the remarkable adaptability of living organisms to their environments.