Human Respiratory System

Human Respiratory System

1. Introduction & Respiratory Organs Across Animals

A. Basic Definitions

• Breathing (Pulmonary Ventilation): Physical process of gaseous exchange — Inhalation ($O_2$-rich air in) and Exhalation ($CO_2$-rich air out).
• Respiration: Biochemical process involving the oxidation of food (glucose) inside cells to release energy (ATP).

B. Respiratory Organs in Animal Kingdom

C. Human Respiratory System Pathway

1. External Nares (Nostrils): Paired openings at the base of the nose.
2. Nasal Cavity: Divided by nasal septum. Contains:• Vestibule: Hairs and oil glands to trap dust particles.

   • Respiratory Region: 3 nasal conchae (bony projections) for conditioning and warming air.

   • Olfactory Region: Lined by Schneiderian membrane for sense of smell.

3. Pharynx: Common passage for food and air. Divided into Nasopharynx, Oropharynx, and Laryngopharynx.
4. Larynx (Voice Box): Cartilaginous structure made of 9 cartilages:• Unpaired (3): Epiglottis (elastic), Thyroid (largest, Adam’s apple), Crico-thyroid/Cricoid (signet ring-shaped).

   • Paired (3): Arytenoid, Corniculate, Cuneiform.

   • Vocal Cords: 1 pair false (upper), 1 pair true (lower). Post-puberty testosterone makes male vocal cords thick and long $\to$ Low pitch voice.

2. Tracheobronchial Tree, Lungs & Breathing Mechanism

A. Trachea & Bronchial Tree Division

• Trachea (Windpipe): Straight tube lined by pseudostratified ciliated columnar epithelium. Supported by 16–20 incomplete C-shaped hyaline cartilaginous rings to prevent collapse during low pressure.
• Divides at the level of 5th Thoracic Vertebra ($T_5$) into Primary ($1^\circ$) Right and Left Bronchi.
• Division Branching:$$\text{Trachea} \to 1^\circ \text{Bronchi} \to 2^\circ \text{Bronchi} \to 3^\circ \text{Bronchi} \to \text{Terminal Bronchiole} \to \text{Respiratory Bronchiole} \to \text{Alveolar Duct} \to \text{Alveoli}$$
• Conducting Zone: External nostrils to Terminal bronchioles (transports air, clears dust, humidifies). C-shaped rings present up to initial bronchioles.
• Respiratory Zone: Respiratory bronchioles to Alveoli (actual site of gas exchange). Lined by simple squamous epithelium.

B. Structure of Lungs & Alveoli

• Covered by double-layered pleura: Outer Parietal Pleura (in contact with thoracic wall) and Inner Visceral Pleura (in contact with lung surface), with pleural fluid reducing friction.
• Right Lung: Larger, 3 lobes (Superior, Middle, Inferior) with horizontal and oblique fissures.
• Left Lung: Smaller, 2 lobes (Superior, Inferior) with cardiac notch.
• Alveoli (300 Million in both lungs): Functional units of gas exchange. Lined by:• Pneumocyte Type I: Simple squamous cells for gas diffusion.

  • Pneumocyte Type II: Secretes Lecithin (Phospholipid Surfactant) which lowers surface tension and prevents alveolar collapse.

  • Atalectasis: Collapse of alveoli due to surfactant deficiency.

• Diffusion Barrier Thickness: Consists of Alveolar epithelium, Basement membrane, and Capillary endothelium. Total thickness $< 0.5\ \mu\text{m}$ (less than 1 mm).

C. Mechanism of Inspiration

• Active Process (2 Seconds): Contraction of Diaphragm (Phrenic muscles) flattens it (increases antero-posterior thoracic volume) + Contraction of External Intercostal Muscles (EICM) lifts ribs and sternum upward/outward (increases dorso-ventral thoracic volume).
• Increased thoracic volume $\to$ Increased pulmonary volume $\to$ Intra-pulmonary pressure drops below atmospheric pressure $\to$ Air flows into lungs ($P \propto \frac{1}{V}$ – Boyle’s Law).

3. Expiration, Forced Breathing & Pulmonary Volumes

A. Expiration & Forced Breathing

• Normal Expiration (Passive Process – 3 Seconds): Relaxation of Diaphragm (returns to dome shape) and EICM brings ribs/sternum back to normal position $\to$ Decreased thoracic volume $\to$ Increased intra-pulmonary pressure above atmospheric level $\to$ Air expelled.
• Forced Inspiration (Active): Involves accessory muscles: Sternocleidomastoid, Scalene, and Anterior Serratus muscles. Predominant thoracic breathing seen in pregnant women.
• Forced Expiration (Active): Involves Abdominal Rectus Muscles and Internal Intercostal Muscles (IICM).

B. Pulmonary Volumes & Capacities (Measured via Spirometer)

4. Exchange of Gases & Oxygen Transport

A. Partial Pressures ($mm\ Hg$) Table

B. Transport of Oxygen ($O_2$)

• 3% in Dissolved State: Transported via blood plasma.
• 97% as Oxyhaemoglobin ($Hb_4O_8$): Reversible oxygenation with Haemoglobin in RBCs ($Hb_4 + 4O_2 \rightleftharpoons Hb_4O_8$). Each $1\text{ g}$ of $Hb$ carries $1.34\text{ mL } O_2$.
• $100\text{ mL}$ of arterial blood carries $\approx 19.4\text{ mL } O_2$; venous blood carries $\approx 14.4\text{ mL } O_2$.
• Delivery Capacity: Every $100\text{ mL}$ of oxygenated blood delivers $5\text{ mL}$ of $O_2$ to tissues under normal physiological conditions (up to $15\text{ mL}$ during strenuous exercise).

C. Oxygen-Haemoglobin Dissociation Curve & Shifts

Sigmoid curve plotting $\% \text{ saturation of } Hb$ against $pO_2$. $P_{50}$ is the partial pressure at which $Hb$ is 50% saturated ($P_{50} \propto \frac{1}{\text{Affinity}}$).

• Left Shift (Association at Alveoli): High $pO_2$, Low $pCO_2$, Low $H^+$ (High pH), Low Temperature, Low 2,3-BPG $\to$ High $Hb\text{-}O_2$ affinity.
• Right Shift / Bohr Effect (Dissociation at Tissues): Low $pO_2$, High $pCO_2$, High $H^+$ (Low pH / acidic), High Temperature, High 2,3-BPG $\to$ Low $Hb\text{-}O_2$ affinity. High $pCO_2$ facilitates $O_2$ unloading at tissues.
• Fetal Haemoglobin ($HbF$): Has 2 $\alpha$ and 2 $\gamma$ chains; higher $O_2$ affinity than adult $HbA$ (Curve lies left of adult curve).

5. Transport of $CO_2$, Chloride Shift & Regulation

A. Transport of Carbon Dioxide ($CO_2$)

• 7% in Dissolved State: As carbonic acid in plasma.
• 23% as Carbamino-haemoglobin ($Hb\cdot CO_2$): Bound directly to amine group of $Hb$. High $pCO_2$ at tissues favors binding; high $pO_2$ at alveoli favors dissociation.
• 70% as Bicarbonate Ions ($HCO_3^-$): $CO_2 + H_2O \xrightleftharpoons{\text{Carbonic Anhydrase}} H_2CO_3 \xrightleftharpoons{} H^+ + HCO_3^-$. Enzyme Carbonic Anhydrase is present in extremely high concentration inside RBCs.
• Every $100\text{ mL}$ of deoxygenated blood delivers $4\text{ mL}$ of $CO_2$ to alveoli.

B. Hamburger’s Phenomenon (Chloride Shift) & Haldane Effect

• Chloride Shift (At Tissue Level): As $HCO_3^-$ diffuses out of RBC into plasma, $Cl^-$ ions move from plasma into RBC to maintain ionic/electrical neutrality.
• Reverse Chloride Shift (At Alveolar Level): $Cl^-$ moves out of RBC into plasma, while $HCO_3^-$ enters RBC to convert back to $CO_2$ and $H_2O$.
• Haldane Effect: Binding of $O_2$ with $Hb$ at alveoli displaces $CO_2$ from carbamino-haemoglobin and releases $H^+$ ions, driving $HCO_3^-$ conversion to $CO_2$ for exhalation.

C. Neural Control of Respiration

• Normal breathing rate: 12–16 times/minute in adults (up to 44 times/minute in infants).
• Respiratory Rhythm Centre (RRC): Located in Medulla Oblongata; primary centre regulating normal involuntary rhythm.
• Pneumotaxic Centre: Located in Pons Varolii; can moderate functions of RRC. Sends neural signals to reduce duration of inspiration $\to$ alters respiratory rate (faster/shallower breathing).
• Chemosensitive Area: Located adjacent to RRC; highly sensitive to $CO_2$ and $H^+$ ion concentrations. Triggers RRC to adjust breathing rate to eliminate gases.
• Aortic Arch & Carotid Artery Receptors: Detect $CO_2$ and $H^+$ levels in arterial blood and send signals to RRC.

6. Respiratory Disorders

• 1. Asthma: Allergic disorder caused by allergen exposure $\to$ Mast cells release Histamine $\to$ Causes bronchoconstriction and inflammation of bronchi/bronchioles $\to$ Difficulty in breathing with characteristic wheezing sound.• Treatment: Antihistamines & Steroid inhalers.
• 2. Emphysema: Chronic respiratory disorder mostly caused by cigarette smoking. Alveolar walls are damaged, resulting in a significant reduction of the respiratory surface area.
• 3. Occupational Respiratory Disorders (ORD): Occurs in workers in stone-breaking, grinding, mining, or construction industries. Inhalation of fine dust leads to long-term inflammation, Fibrosis (proliferation of fibrous tissue), and lung damage.• Examples: Silicosis, Asbestosis.

  • Prevention: Wearing protective face masks.

• 4. Carbon Monoxide (CO) Poisoning: Incomplete combustion of carbon. $CO$ has $200\times$ higher affinity for Haemoglobin than $O_2$, forming irreversible Carboxyhaemoglobin ($HbCO$). Leads to tissue hypoxia, asphyxiation, suffocation, and death.
• 5. Hypoxia: Condition of low $O_2$ supply to tissues.• Hypoxic Hypoxia: Low arterial $pO_2$ at high altitudes.

  • Anaemic Hypoxia: Low RBC/Hb count.

  • Ischemic Hypoxia: Reduced blood flow to tissues due to vascular blockage.