Overview of the Respiratory System

Respiratory System Overview

• Respiration is the process of supplying tissue cells with oxygen and eliminating CO2
• Oxygen consumption in a young adult is 250 ml/minute
• Carbon dioxide production is roughly 200 ml/minute
• Respiration includes three processes
  • External respiration: absorption of oxygen and removal of CO2 from the body
  • Transport of oxygen and carbon dioxide
  • Internal respiration: utilization of oxygen and production of CO2 by cells, and gaseous exchange between cells and their fluid
• External (pulmonary) respiration includes two processes:
  • Pulmonary ventilation: respiratory rate (12-16 breaths per minute in adults, 40 breaths per minute in infants and neonates), inspiration, expiration, and expiratory pause
  • Pulmonary gas exchange: oxygen diffuses from alveolar air to pulmonary capillaries, and CO2 diffuses in the opposite direction
• The respiratory system consists of:
  • Respiratory tract (airways or passages)
  • Thoracic cage and respiratory muscles
  • Nerve centers controlling respiration
• Respiratory unit components:
  • Respiratory bronchioles
  • Alveolar ducts
  • Alveolar sacs
  • Antrum
  • Alveoli
• Alveolus characteristics:
  • Pouch-like structure
  • Diameter of approximately 0.2 to 0.5 mm
  • Lined with epithelial cells
• Structure of respiratory system: conducting zone, transition zone, respiratory zone

Function of the Respiratory System

• Primary Functions
  • Provides oxygen for tissue metabolism
  • Removes carbon dioxide, a metabolic waste product.
• Secondary Functions
  • Facilitates the sense of smell
  • Produces speech.
  • Aids in regulating acid-base balance
  • Maintains body water level
  • Regulates body temperature
  • Protects against inhaled pathogens and irritants
• During expiration, besides water loss, heat is also lost.
• In the lungs, carbon dioxide is removed from the blood into the alveoli. Increased metabolic activity leads to more carbon dioxide production, resulting in acidosis.
• Breathing movements support venous return and lymph flow.
• Pulmonary capillaries filter venous blood to prevent emboli.
• Metabolic & Endocrine Functions:
  • Type II pneumocytes secrete surfactant.
  • Lungs remove substances from the blood (e.g., serotonin, bradykinin, acetylcholine, norepinephrine).
  • Secretion of heparin from mast cells.
  • Angiotensin I conversion to Angiotensin II by ACE
  • Fibrinolysis
  • Secretion of VIP (vasoactive intestinal polypeptide) which relaxes the bronchial tree.

Respiratory Tract Anatomy

• The inspired air travels to the alveoli via nasal passages, pharynx, trachea, bronchioles, and alveolar ducts.
• Airways divide approximately 23 times between the trachea and the alveolar sacs.
• The first 16 generations form the conducting zone, transporting gas to and from the body exterior, consisting of bronchi, bronchioles, and terminal bronchioles.
• The remaining seven generations form the transitional and respiratory zones, where gas exchange occurs; respiratory bronchioles, alveolar ducts, and alveoli.

Structures of Respiratory Zone/ Airway

• Alveoli are lined by two types of epithelial cells:
  • Type I cells: thin, flat cells with extensions, enabling gas exchange
  • Type II cells: thicker, containing lamellar bodies which secrete surfactant
• Total surface area of 300 million alveoli is approximately 70 m2
• Total surface area is 70-80 m2
• There are about 300 million alveoli in both lungs, providing extensive surface area for gas exchange.
• Alveolar walls and capillaries are separated by a thin layer of tissue, promoting efficient gas exchange, with a separation of 0.5µm.

Other Special Respiratory Cells

• Special types of epithelial cells like pulmonary alveolar macrophages (PAMs), lymphocytes, plasma cells, APUD cells and mast cells are present in the lungs.
  • Mast cells contain heparin, various lipids, histamine and various proteases, which are involved in allergic reactions.

Defense Mechanisms- Prevention of Dust Particles

• Dust particles entering the nostrils are prevented from reaching the lungs through filtration by hairs in the nasal mucous membrane.
• Small particles are trapped by the nasal mucous membrane, and those escaping are removed by the phagocytic action of macrophages in the alveoli.
• Particles escaping these defenses are expelled through cough or sneeze reflexes.

Explore the fundamental processes of respiration, including external and internal respiration, and understand how oxygen and carbon dioxide are exchanged in the body. This quiz covers the basic mechanics of pulmonary ventilation and gas exchange, providing insights into the functioning of the respiratory system.