The Chemical Symphony of Breathing: An Exploration of the Human Respiratory System

• The human respiratory system is essential for life, facilitating the exchange of oxygen and carbon dioxide.   

• It also helps maintain the body's acid-base balance.   

• Understanding the chemical reactions involved is crucial for comprehending respiratory physiology and conditions.

Anatomy and Physiology of the Human Respiratory System

• Functionally divided into the conducting zone (airways) and the respiratory zone (gas exchange)
  

  

  Human Respiratory system

A. Upper Respiratory Tract

• 1. Nose and Nasal Cavity 
  

  • Air entry point, responsible for filtration, warming, and humidification.   

  • Lined with mucus-producing cells and cilia to trap and remove debris.   

  
  

  2. Pharynx (Throat)  

  • Common passage for air and food, divided into nasopharynx, oropharynx, and laryngopharynx.   

  • Lining varies from respiratory epithelium to stratified squamous epithelium.   

  3. Larynx (Voice Box) 
  

  • Located at the top of the trachea, involved in respiration and phonation.   

  • Contains vocal cords for sound production.   

  • Regulates airflow into the trachea.

B. Lower Respiratory Tract

• 1. Trachea (Windpipe) 
  

  • Tube reinforced by C-shaped cartilage rings to prevent collapse.   

  • Lined with ciliated columnar epithelium and goblet cells for trapping and removing particles.   

  2. Bronchi and Bronchioles 
  

  • Trachea branches into two main bronchi, one for each lung.   

  • Bronchi further subdivide into smaller bronchi and then into bronchioles.   

  • Extensive branching increases surface area for gas exchange.   

  • Bronchi are supported by cartilage, while bronchioles have smooth muscle for airflow regulation.   

  3. Alveoli 
  

  • Tiny air sacs at the end of bronchioles, primary sites of gas exchange.   

  • Approximately 150 million alveoli in adult lungs.   

  • Thin walls surrounded by capillaries for efficient diffusion.   

  • Lined with surfactant to reduce surface tension and prevent collapse.   

  4. Lungs  

  • Main organs of respiration, located in the thoracic cavity and protected by the rib cage.   

  • Right lung has three lobes, left lung has two.   

  5. Diaphragm and Pleura 
  

  • Diaphragm is the primary muscle for breathing; contraction leads to inhalation, relaxation to exhalation.   

  • Lungs are enclosed by the pleura, a two-layered membrane with pleural fluid to minimize friction

III. The Process of Gas Exchange in the Alveoli

• Exchange of oxygen and carbon dioxide occurs via diffusion, driven by partial pressure differences.   

• Partial pressure of oxygen (PO₂) is high in alveoli, low in deoxygenated blood, causing oxygen to diffuse into the blood.   

• Partial pressure of carbon dioxide (PCO₂) is high in deoxygenated blood, low in alveoli, causing carbon dioxide to diffuse into the alveoli.   

• Efficient exchange due to vast surface area and thin respiratory membrane.   

• Partial pressure in arterial blood: PO₂ ≈ 100 mmHg, PCO₂ ≈ 40 mmHg.   

• Partial pressure in venous blood: PO₂ ≈ 40 mmHg, PCO₂ ≈ 45-48 mmHg.   

  
  

IV. Oxygen Transport in the Blood

• Majority of oxygen (98%) is transported bound to hemoglobin in red blood cells.   

• Each hemoglobin molecule can carry up to four oxygen molecules.   

• Chemical Equation for Oxygen Binding: Hb + O₂ ⇌ HbO₂    

• Detailed Equilibrium Reaction: HbH⁺(aq) + O₂(aq) ⇌ HbO₂(aq) + H⁺(aq).   

• Hemoglobin's affinity for oxygen is influenced by:

  • Partial pressure of oxygen (pO₂)    

  • Partial pressure of carbon dioxide (pCO₂)    

  • pH    

  • Temperature    

  • 2,3-bisphosphoglycerate (2,3-BPG)
    

• Bohr effect: Increased pCO₂ and decreased pH reduce hemoglobin's affinity for oxygen, promoting oxygen release in active tissues.   
  

V. Carbon Dioxide Transport in the Blood

• Carbon dioxide is transported from tissues to lungs via three main mechanisms : 

A. Dissolved CO₂

• Small portion (7-10%) transported directly in blood plasma.   

• Amount dissolved is proportional to its partial pressure (Henry's law).   

• Carbon dioxide is more soluble in blood than oxygen.   
  

B. Bicarbonate Ions

• Majority (70%) transported as bicarbonate ions (HCO₃⁻) in plasma.   

• Occurs primarily within red blood cells.   

• Carbon dioxide reacts with water to form carbonic acid (H₂CO₃), catalyzed by carbonic anhydrase.   

• Carbonic acid dissociates into hydrogen ion (H⁺) and bicarbonate ion (HCO₃⁻).   

• Bicarbonate ions move out of red blood cells into plasma in exchange for chloride ions (chloride shift).   
  

C. Carbaminohemoglobin

• 20-25% of carbon dioxide binds to hemoglobin, forming carbaminohemoglobin (HbCO₂).   

• Chemical Equation: CO₂ + Hb ⇌ HbCO₂.   

• Carbon dioxide binds to amino acid portions of globin, not heme.   

• Haldane effect: Deoxygenated hemoglobin has a greater affinity for carbon dioxide.   
  

VI. Cellular Respiration

• Respiratory system supplies oxygen for cellular respiration and removes carbon dioxide.   

• Cellular respiration converts biochemical energy from nutrients into ATP.   

• Aerobic cellular respiration breaks down glucose in the presence of oxygen.   

• Overall Balanced Chemical Equation: C₆H₁₂O₆ (glucose) + 6O₂ → 6CO₂ + 6H₂O + energy (ATP).   

• Reactants: Glucose and oxygen.   

• Products: Carbon dioxide, water, and ATP.   
  

VII. Other Relevant Chemical Reactions and Combinations

A. Lung Surfactant

• Complex mixture of phospholipids and proteins produced by type II alveolar cells.   

• Reduces surface tension within alveoli.   

• Primary lipid component: Dipalmitoyl phosphatidylcholine (DPPC).   

• Surfactant-associated proteins (SP-A, SP-B, SP-C, SP-D) contribute to function.   

• Ozone can react with surfactant lipids and proteins, potentially leading to respiratory distress.    

B. Regulation of Breathing

• Involves neural and chemical mechanisms.  

  1. Chemical Regulation

  
  

  • Chemoreceptors in brainstem (central) and carotid/aortic bodies (peripheral) monitor O₂, CO₂, and pH.   

  • Central chemoreceptors sensitive to pH changes in cerebrospinal fluid (CSF) due to blood CO₂.   

  • Reaction in CSF: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.   

  • Peripheral chemoreceptors sensitive to low O₂, changes in CO₂, and pH.   

    
    

  2. Respiratory Acidosis

  
  

  • Lungs cannot effectively remove CO₂, often due to hypoventilation.   

  • Accumulation of CO₂ lowers blood pH (< 7.35).   

  • Chemical Reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (shifted to the right).

    
    

  3. Respiratory Alkalosis

  
  

  • Hyperventilation leads to excessive removal of CO₂.   

  • Reduced CO₂ increases blood pH (> 7.45).   

  • Chemical Reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (shifted to the left).