Respiratory System

Respiratory System

Introduction

·       The respiratory system is responsible for the exchange of gases between the body and the external environment.

·        Its primary function is to supply oxygen (O₂) to tissues for cellular metabolism and remove carbon dioxide (CO₂), a metabolic waste product.

·       Oxygen is essential for cellular respiration, which generates ATP (energy).

·       Carbon dioxide must be eliminated to maintain acid-base balance and prevent acidosis.

·       The respiratory system works in close association with the circulatory system (cardiovascular system) to transport gases to and from tissues.

Anatomy of Respiratory Organs

1. Upper Respiratory Tract

• Nose and Nasal Cavity:
• First entry point for air; lined with hair and mucosa.
• Functions: filter, warm, and humidify air.
• Pharynx (Throat):
• A muscular tube shared by respiratory and digestive systems.
• Regions:
• Nasopharynx (behind nasal cavity)
• Oropharynx (behind oral cavity)
• Laryngopharynx (behind larynx)
• Larynx (Voice Box):
• Connects pharynx to trachea.
• Contains vocal cords (sound production).
• Epiglottis prevents food from entering respiratory tract.

2. Lower Respiratory Tract

• Trachea (Windpipe):
• Tube supported by C-shaped cartilage rings.
• Divides into right and left primary bronchi.
• Bronchi and Bronchioles:
• Bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Terminal bronchioles.
• Function: distribute air into lungs.
• Lungs:
• Cone-shaped organs located in thoracic cavity.
• Right lung: 3 lobes (superior, middle, inferior).
• Left lung: 2 lobes (superior, inferior) + cardiac notch.
• Covered by pleura:
• Visceral pleura (covers lung surface)
• Parietal pleura (lines thoracic wall)
• Pleural fluid reduces friction.
• Alveoli:
• Tiny sac-like structures (functional units of gas exchange).
• Surrounded by capillaries.
• Lined by Type I alveolar cells (gas exchange) and Type II alveolar cells (secrete surfactant to reduce surface tension).

Mechanism of Respiration

Respiration occurs in two phases:

1. External Respiration

• Exchange of gases between alveoli and pulmonary capillaries.
• O₂ diffuses into blood, CO₂ diffuses into alveoli.

2. Internal Respiration

• Exchange of gases between systemic blood capillaries and tissue cells.
• O₂ diffuses into tissues, CO₂ diffuses into blood.

3. Pulmonary Ventilation (Breathing)

• Process of moving air in and out of lungs.
• Two phases:

(a) Inspiration (Inhalation) – Active process

• Muscles involved:
• Diaphragm contracts → moves downward.
• External intercostal muscles contract → ribs move upward and outward.
• Result:
• Thoracic cavity volume increases.
• Intrapulmonary pressure falls below atmospheric pressure.
• Air flows into lungs.

(b) Expiration (Exhalation) – Normally passive

• Muscles involved:
• Diaphragm relaxes → moves upward.
• Intercostal muscles relax → ribs move down and in.
• Result:
• Thoracic cavity volume decreases.
• Intrapulmonary pressure increases above atmospheric pressure.
• Air flows out of lungs.
• Forced expiration involves abdominal and internal intercostal muscles.

Factors Affecting Respiration

1. Chemical Factors
• ↑CO₂ levels (hypercapnia) → strongest stimulus for respiration.
• ↓O₂ levels (hypoxia) stimulate breathing (especially in chronic lung disease).
• Blood pH (H⁺ concentration) also regulates respiration.
2. Neural Factors
• Brainstem respiratory centers (medulla and pons) regulate rhythm.
• Stretch receptors in lungs prevent overinflation (Hering-Breuer reflex).
3. Physical Factors
• Exercise, body temperature (fever increases rate), body position.
4. Psychological Factors
• Stress, anxiety, emotions influence breathing rate (hyperventilation in fear, slow in relaxation).
5. Pathological Factors
• Diseases (asthma, COPD, pneumonia) alter breathing efficiency.

Nervous Control of Respiration

Respiration is mainly controlled by centers in the brainstem:

1. Medullary Centers
• Dorsal Respiratory Group (DRG): Controls inspiration (basic rhythm).
• Ventral Respiratory Group (VRG): Active during forced breathing (expiration and inspiration).
2. Pontine Centers (Pons)
• Pneumotaxic Center: Limits inspiration, prevents lung overfilling.
• Apneustic Center: Prolongs inspiration when active.
3. Chemoreceptors
• Central chemoreceptors (in medulla) detect CO₂ and H⁺ in cerebrospinal fluid.
• Peripheral chemoreceptors (carotid and aortic bodies) sense O₂, CO₂, and pH.
4. Reflex Control
• Hering-Breuer reflex: Stretch receptors stop inspiration to prevent lung damage.
• Irritant receptors: Trigger coughing, sneezing.

Lung Volumes and Capacities

Lung Volumes

1. Tidal Volume (TV):
• Volume of air inhaled/exhaled in normal breathing. (~500 ml)
2. Inspiratory Reserve Volume (IRV):
• Extra air inhaled during forceful inspiration. (~3000 ml)
3. Expiratory Reserve Volume (ERV):
• Extra air exhaled forcefully after normal expiration. (~1100 ml)
4. Residual Volume (RV):
• Air remaining in lungs after maximal exhalation. (~1200 ml)

Lung Capacities (Combination of volumes)

1. Inspiratory Capacity (IC): TV + IRV
• Maximum air inhaled after normal expiration (~3500 ml).
2. Functional Residual Capacity (FRC): ERV + RV
• Air left in lungs after normal expiration (~2300 ml).
3. Vital Capacity (VC): TV + IRV + ERV
• Maximum air exhaled after maximum inspiration (~4600 ml).
4. Total Lung Capacity (TLC): TV + IRV + ERV + RV
• Maximum air lungs can hold (~5800 ml).

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