The Respiratory System

Introduction

The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. In humans, its primary function is to transport oxygen from the atmosphere into the bloodstream and to remove carbon dioxide from the bloodstream into the atmosphere.

Overview of the Respiratory System

Nose/Mouth Trachea Lungs Diaphragm

Figure 1: Major components of the human respiratory system

Structures and Functions

1. Upper Respiratory Tract

Nasal Cavity

  • Filters, warms, and humidifies inhaled air
  • Contains olfactory receptors
  • Lined with ciliated mucous membrane

Pharynx (Throat)

  • Common passage for air and food
  • Contains tonsils (lymphoid tissue)
  • Divided into naso-, oro-, and laryngopharynx

Larynx (Voice Box)

  • Contains vocal cords for sound production
  • Epiglottis prevents food entering trachea
  • Made of cartilage (Adam's apple)
Nasal Cavity Pharynx Larynx

Figure 2a: Upper respiratory tract

2. Lower Respiratory Tract

Trachea (Windpipe)

  • 10-12 cm long tube with C-shaped cartilage rings
  • Lined with ciliated pseudostratified epithelium
  • Branches into two primary bronchi

Bronchial Tree

  • Primary bronchi → secondary bronchi → tertiary bronchi
  • Progressively smaller with less cartilage
  • Terminate in bronchioles (1mm diameter)

Lungs

  • Right lung (3 lobes), Left lung (2 lobes)
  • Contain ~300 million alveoli (total surface area 70m²)
  • Pleural membranes reduce friction
Trachea Bronchi Lungs (Alveoli shown)

Figure 2b: Lower respiratory tract

3. Alveoli and Gas Exchange

Tiny air sacs where gas exchange occurs between air and blood:

  • ~0.2mm diameter, single-cell thick walls
  • Surrounded by capillary network
  • Coated with pulmonary surfactant
  • Total surface area ~70m² (tennis court size)

Gas Exchange Process

  1. Oxygen diffuses from alveoli to capillaries
  2. Carbon dioxide diffuses from capillaries to alveoli
  3. Driven by concentration gradients
  4. Facilitated by thin respiratory membrane
Alveolus (air sac) Capillaries O₂ CO₂

Figure 3: Alveolar gas exchange

Mechanics of Breathing

1. Inhalation (Inspiration)

  • Active process requiring muscle contraction
  • Diaphragm contracts and flattens
  • External intercostal muscles lift ribs
  • Thoracic cavity volume increases
  • Intrapulmonary pressure decreases
  • Air flows into lungs (down pressure gradient)
Diaphragm Air enters

Figure 4a: Inhalation process

2. Exhalation (Expiration)

  • Normally passive process (except forced expiration)
  • Diaphragm relaxes and domes upward
  • Ribs move downward and inward
  • Thoracic cavity volume decreases
  • Intrapulmonary pressure increases
  • Air flows out of lungs
Diaphragm Air exits

Figure 4b: Exhalation process

Respiratory Volumes and Capacities

Measurement Volume Description
Tidal Volume (TV) 500 mL Air inhaled/exhaled during normal breathing
Inspiratory Reserve Volume (IRV) 3100 mL Additional air that can be inhaled forcibly
Expiratory Reserve Volume (ERV) 1200 mL Additional air that can be exhaled forcibly
Residual Volume (RV) 1200 mL Air remaining in lungs after maximum exhalation
Vital Capacity (VC) 4800 mL TV + IRV + ERV (total exchangeable air)
Total Lung Capacity (TLC) 6000 mL VC + RV (all air in lungs)

Control of Breathing

Nervous System Regulation

Medulla Oblongata

  • Contains inspiratory and expiratory centers
  • Sets basic rhythm of breathing (12-15 breaths/min)
  • Sends impulses to diaphragm and intercostals

Pons

  • Pneumotaxic center regulates breathing rate
  • Apneustic center promotes deep inhalation
  • Fine-tunes medulla's respiratory rhythm
Pons Medulla Oblongata

Figure 5: Brain stem respiratory centers

Chemical Regulation

Central Chemoreceptors

  • Located in medulla
  • Respond to CO₂ levels in cerebrospinal fluid
  • Increased CO₂ → increased ventilation

Peripheral Chemoreceptors

  • Located in carotid and aortic bodies
  • Respond to O₂, CO₂, and pH in blood
  • Low O₂ → increased ventilation
Carotid Body Aortic Body Chemical Sensors in Blood

Figure 6: Peripheral chemoreceptor locations

Common Respiratory Disorders

Disorder Description Causes/Risk Factors
Asthma Chronic airway inflammation and bronchoconstriction Allergens, exercise, stress
Chronic Bronchitis Long-term inflammation of bronchi Smoking, air pollution
Emphysema Alveolar wall destruction reducing surface area Smoking, genetic factors
Pneumonia Alveolar inflammation and fluid accumulation Bacterial/viral infection
Lung Cancer Uncontrolled cell growth in lung tissue Smoking, radon, asbestos

Glossary of Terms

Alveolus
Tiny air sac in lungs where gas exchange occurs.
Bronchus
Main passageway into the lungs (plural: bronchi).
Diaphragm
Dome-shaped muscle separating thorax from abdomen.
Hemoglobin
Oxygen-carrying protein in red blood cells.
Partial Pressure
Pressure exerted by a single gas in a mixture.
Surfactant
Lipoprotein reducing surface tension in alveoli.
Tidal Volume
Amount of air inhaled/exhaled during normal breathing.

Self-Assessment Questions

Respiratory System Assessment

1. Trace the pathway of air from the nose to the alveoli.
Nose/Mouth → Pharynx → Larynx → Trachea → Primary Bronchi → Secondary Bronchi → Tertiary Bronchi → Bronchioles → Alveolar Ducts → Alveoli
2. Explain how the diaphragm and intercostal muscles work during inhalation.
The diaphragm contracts and flattens downward, while external intercostal muscles contract to lift the rib cage upward and outward. This increases thoracic volume, decreasing lung pressure, causing air to flow in.
3. What is the role of surfactant in the alveoli?
Pulmonary surfactant reduces surface tension in alveoli, preventing their collapse during exhalation and making inflation easier during inhalation.
4. How does carbon dioxide concentration affect breathing rate?
Increased CO₂ levels (hypercapnia) lower blood pH, stimulating central and peripheral chemoreceptors to increase breathing rate and depth to eliminate excess CO₂.
5. Compare the structures of bronchi and bronchioles.
Bronchi are larger, have cartilage rings, and ciliated epithelium. Bronchioles are smaller (≤1mm), lack cartilage, and have more smooth muscle to regulate airflow.
6. Describe the process of gas exchange in the alveoli.
Oxygen diffuses from alveoli (high concentration) into capillaries (low concentration) while carbon dioxide diffuses in the opposite direction, driven by partial pressure gradients. The thin respiratory membrane (0.5μm thick) facilitates this exchange.
7. What is the function of the ciliated epithelium in the respiratory tract?
Cilia move mucus upward in a coordinated beating motion (mucociliary escalator), trapping and removing dust, pathogens, and debris from the airways to protect the lungs.
8. Explain how oxygen is transported in the blood.
98.5% binds to hemoglobin in red blood cells forming oxyhemoglobin, while 1.5% dissolves directly in plasma. Each hemoglobin molecule can carry 4 oxygen molecules.
9. Why does breathing rate increase during exercise?
Increased CO₂ production from muscle activity lowers blood pH, stimulating chemoreceptors. Additionally, proprioceptors in muscles and joints signal increased oxygen demand, while adrenaline and higher body temperature also stimulate breathing centers.
10. What are the effects of smoking on the respiratory system?
Damages cilia (reducing mucus clearance), causes chronic bronchitis, destroys alveoli (emphysema), increases lung cancer risk, narrows airways, and reduces oxygen-carrying capacity due to carbon monoxide binding to hemoglobin.

Summary of Key Points