Respiratory System in Humans (Introduction)

Respiratory system in human 



  • Organisms utilize oxygen (O2) to break down simple molecules (e.g., glucose, amino acids) to derive energy, producing carbon dioxide (CO2) as a byproduct. This exchange of gases is crucial for cellular function.


Respiratory Organs:


  • In invertebrates:

- Lower Invertebrates: (e.g., sponges, coelenterates, flatworms) exchange gases through simple diffusion across their body surfaces. 

- Earthworms: Utilize their moist cuticle. 

- Insects: Have a network of tubes called tracheal tubes for air transport. 

- Aquatic Arthropods and Molluscs: Utilize gills for gas exchange. 

  •  In Vertebrates:

- Fishes: Respire through gills. 

- Amphibians: Can respire through gills, lungs, and moist skin (cutaneous respiration). 

- Reptiles, Birds, and Mammals: Respire exclusively through lungs. 

  • Breathing mechanisms differ based on habitat and organism complexity. 
  • Invertebrates: Often rely on diffusion or specialized structures like tracheal tubes or gills. 
  • Vertebrates: Utilize lungs for gas exchange. 
  • Amphibians: Employ both lung and skin respiration. 
  • The physical sensation of chest movement signifies the breathing process. 
  • Oxygen must be continuously provided to cells, while carbon dioxide produced must be released. 
  • The exchange of oxygen from the atmosphere with carbon dioxide produced by cells is termed breathing or respiration.


Human Respiratory System 



  • Nostrils: Paired openings above the upper lips leading to the nasal chamber via the nasal passage. Nostrils help in the intake of air and are the entry point for the respiratory system.
  • Nasal Chamber: A hollow space in the nose lined with mucous membranes and tiny hairs called cilia. It warms, moistens, and filters the air entering the respiratory system, protecting the delicate lung tissues from harmful particles. 
  • Pharynx: Also known as the throat, it serves as a common pathway for both air and food. The pharynx plays a crucial role in swallowing and directing food to the esophagus while ensuring that air enters the trachea. 
  • Larynx: Often referred to as the voice box, the larynx is a cartilaginous structure located at the top of the trachea. It houses the vocal cords and is essential for sound production and phonation. The epiglottis, a flap-like structure, covers the glottis during swallowing to prevent food from entering the airway. 
  • Trachea (Windpipe): A rigid tube composed of C-shaped cartilage rings, the trachea connects the larynx to the bronchi. It serves as a conduit for air to travel between the pharynx and the lungs, ensuring the passage of air remains open and protected. 
  • Bronchial Tree: The branching network of airways originating from the trachea, including primary, secondary, and tertiary bronchi, as well as bronchioles. These structures progressively divide and become narrower as they extend into the lungs, ultimately leading to the alveoli. 
  • Alveoli: Tiny, grape-like sacs clustered at the ends of the bronchioles within the lungs. Alveoli are the primary sites of gas exchange, where oxygen from inhaled air diffuses into the bloodstream and carbon dioxide from the bloodstream diffuses into the alveolar air for exhalation. 
  •  Lungs: Paired organs situated within the thoracic cavity, enclosed by the rib cage. Lungs are composed of spongy tissue and are responsible for exchanging oxygen and carbon dioxide with the bloodstream during respiration. 
  • Thoracic Chamber: An enclosed space within the chest cavity, comprising the vertebral column, sternum, ribs, and diaphragm. Changes in thoracic cavity volume, facilitated by the diaphragm and intercostal muscles, allow for breathing by altering lung volume and creating pressure differentials for air movement. 
  • Conducting Part: From nostrils to terminal bronchioles, transports atmospheric air, clears foreign particles, humidifies, and adjusts air to body temperature. 
  • Respiratory/Exchange Part: Alveoli and their ducts where actual gas exchange (O2 and CO2) occurs between blood and atmospheric air. 
  • Thorax Anatomy: Any change in thoracic cavity volume affects lung volume, crucial for breathing regulation as direct alteration of pulmonary volume is not possible.


Steps of Respiration:


1. Breathing (Pulmonary Ventilation):


  • Process of inhaling atmospheric air into the lungs and exhaling CO2 rich alveolar air out.
  • During inhalation, the diaphragm contracts, and the rib cage expands, creating negative pressure in the lungs, causing air to rush in. 
  • Exhalation occurs when the diaphragm relaxes and the rib cage returns to its resting position, causing air to be pushed out of the lungs. 


2. Gas Diffusion Across Alveolar Membrane:


  • Oxygen (O2) and carbon dioxide (CO2) exchange occurs between the alveoli and the bloodstream.
  • O2 moves from the alveoli into the bloodstream, while CO2 moves from the bloodstream into the alveoli due to concentration gradients.


3. Gas Transport by Blood:


  • Oxygen is transported from the lungs to tissues via red blood cells, primarily bound to hemoglobin.
  • Carbon dioxide is transported from tissues to the lungs in three forms: dissolved in plasma, bound to hemoglobin, or as bicarbonate ions (HCO3-) in the plasma.


4. Gas Diffusion Between Blood and Tissues:


  • Oxygen diffuses from capillaries into tissues, where it is utilized for cellular respiration to produce energy (ATP). 
  • Carbon dioxide diffuses from tissues into capillaries to be transported back to the lungs for elimination.


5. Cellular Respiration:


  • Cells utilize oxygen for catabolic reactions, breaking down glucose and other molecules to produce ATP energy. 
  • As a result of cellular respiration, carbon dioxide is released as a byproduct and transported back to the lungs for exhalation during breathing.