Introduction to Respiration

Introduction to Respiration

• Respiration is the process of acquiring oxygen and eliminating carbon dioxide.
• Three steps in the human body:
  • Pulmonary ventilation: Gas exchange between the atmosphere and lung tissues.
  • External respiration: Gas exchange between lung tissues and blood.
  • Internal respiration: Gas exchange between blood and body tissues.
• Functions of the respiratory system:
  • Exchanges gases.
  • Regulates blood pH.
  • Permits vocal sounds and the sense of smell, filters inhaled air, and excretes wastes during exhalation.
  • Oto(rhino)laryngology is the study of the respiratory system.
• Cells need oxygen for aerobic cellular respiration.

Anatomy of the Respiratory System

• Structurally divided into upper and lower respiratory systems.
  • Upper: Nose, nasal cavity, pharynx, and associated structures.
  • Lower: Larynx, trachea, bronchi, and lungs.
• Functionally divided into conducting and respiratory zones.
  • Conducting zone: Directs air, filters, warms, and humidifies air.
  • Respiratory zone: Site of gas exchange (respiratory bronchioles, alveolar ducts, alveolar sacs).

Upper Respiratory System

• Nose: Made of bone, cartilage, and connective tissues. Air enters through external nares.
• Nasal Cavity: Interior space of the nose, divided by the nasal septum. Contains:
  • Paranasal sinuses: Lined with mucous membranes.
  • Nasal conchae: Swirl inhaled air.
  • Olfactory epithelium: Sensory receptors for smells (no goblet cells).
  • Pharynx: Tube of skeletal muscle, lined with mucous membrane, subdivided into:
    • Nasopharynx (superior): Lined with ciliated pseudostratified columnar epithelium, sweeps mucus to the pharynx.
    • Oropharynx (intermediate): Common passage for air and food, lined with non-keratinized stratified squamous epithelium, contains tonsils (trap pathogens).
    • Laryngopharynx (inferior): Similar to oropharynx.

Larynx

• Tube of nine cartilages, including the thyroid cartilage (Adam's apple), epiglottis (flap of cartilage), and cricoid cartilage (landmark for tracheotomies).
• Vocal folds (true vocal cords): Form elastic ligaments, vibrate to produce sound, pitch controlled by skeletal muscles on cartilages.
• Vestibular folds (false vocal cords): Come together when holding breath.

Trachea

• 2.5 cm wide x 12 cm long tube.
• 16-20 rings of hyaline cartilage, connected by dense connective tissue, keeping trachea patent (open).
• Anterior to the esophagus. Lined with ciliated pseudostratified epithelium.

Bronchi

• Trachea splits into right and left bronchi.
• Carina: Ridge at branchpoint, sensitive mucous membrane.
• Branch into a bronchial tree, smaller tubes called terminal bronchioles, ending in lung sacs.
• Mucous membrane changes throughout the bronchial tree. The supporting cartilage and proportion of smooth muscle also change throughout the bronchial tree.

Lungs

• Wrapped in pleural membrane (two serous membranes).
• Pleural fluid reduces friction and surface tension.
• Separated by the mediastinum and its organs.
• Extend from clavicles to diaphragm.
• Base (inferior portion), apex (superior portion), concave shape conforms to the dome-shape of the diaphragm.
• Medial surfaces include:
  • Hilum: Permits passage of bronchi, blood vessels, nerves, lymphatic vessels.
  • Cardiac notch: Provides space for the heart, decreases size of the left lung.
• Fissures divide the lungs into lobes:
  • Oblique fissure separates superior and inferior lobes.
  • Horizontal fissure borders the middle lobe superiorly (right lung only).
  • Lobar bronchi named after lobes (superior, middle, inferior).
• Bronchopulmonary segments: 13 in the right lung; 8 in the left.
• Lobules: Smaller compartments containing a terminal bronchiole, arteriole & venule, lymphatic vessel, and elastic connective tissue.

Respiratory Bronchioles and Alveoli

• Microscopic branches of bronchial branches.
• Lined with simple cuboidal epithelium, branching into alveolar ducts (simple squamous epithelium).
• Alveoli: Air sacs for gas exchange.
• Two types of alveolar cells:
  • Type I: Simple squamous epithelium, facilitates gas diffusion.
  • Type II: Nonciliated cuboidal epithelium at septa between alveoli, secretes surfactant (phospholipids + lipoproteins) to prevent sticking.
• Respiratory membrane: Alveoli + associated capillaries (very thin, 0.5 µm).

Blood Supply to the Lungs

• Pulmonary arteries: Bring deoxygenated blood from the right ventricle for oxygenation.
• Bronchial arteries: Branch from the aorta, supply oxygenated blood to lung tissue.

Gas Exchange and Ventilation

• Pulmonary ventilation: Inhalation & exhalation, regulated by pressure changes in the thoracic cavity.
• Inhalation (inspiration):
  • Lungs lower pressure below atmospheric pressure.
  • Diaphragm contracts and depresses, assisted by intercostal muscles to increase thoracic cavity volume.
  • Intrapleural pressure is negative (lower than atmospheric pressure) and keeps the pleural membrane suctioned to the thoracic cavity wall.
• Exhalation (expiration):
  • Passive process.
  • Respiratory muscles relax, and elastic recoil of lung tissues increases pressure inside the lungs.
  • Gases diffuse out of the body.
• Factors affecting pulmonary ventilation:
  • Surfactant (insufficient surface tension can lead to respiratory distress syndrome).
  • Compliance (distensibility of elastic tissues, high compliance = low effort; low compliance = high effort);
  • Resistance (airway diameter, obstruction).

Lung Volumes and Capacities

• Lung volume: Measured by spirometer, output is spirogram.
• Lung capacity: Sums of specific lung volumes.

Principles of Gas Exchange

• Gases move from high to low partial pressure.
• CO2 is more soluble than O2.
• External respiration: Gas exchange between alveoli and blood (maximizes oxygenation during exercise, blood flow increases to keep up with oxygen demand)
• Internal respiration: Gas exchange between blood and tissues (CO2 constantly leaves tissues and goes into blood).

Oxygen Transport

• 98.5% of O2 carried on hemoglobin (Hb) in erythrocytes, rest is dissolved in plasma.
• Hb binds O2 reversibly.
• Affecting saturation:
  • PO2 (high PO2 favors Hb-O2 formation).
  • Hb affinity (affected by acidity, PCO2, temperature, 2,3 BPG, and type of hemoglobin).

Carbon Dioxide Transport

• Three main forms:
  • Dissolved in plasma.
  • Bound to proteins (carbamino compounds).
  • Transported as bicarbonate.
• Chloride shift maintains electrical balance.

Regulation of Breathing

• Respiratory center: Medulla oblongata and pons.
  • Medullary respiratory group (DRG: normal breathing, VRG: forceful breathing ).
  • Pontine respiratory group influences normal breathing by affecting the DRG.
• Cortical influences: Voluntary control via cerebral cortex.
• Other influences: Chemoreceptors (central and peripheral) respond to changes in PCO2 and pH.
• Hyperventilation: Response to low blood pH, increases blood pH. Hypocapnia (low PCO2) can result, and may cause hypoxia (insufficient oxygen).
• Inflation reflex: Prevents overinflation of lungs. Baroceptors in bronchi and bronchioles, inhibiting the DRG to relax respiratory muscles.
• Other factors affecting breathing: emotions, temperature, pain, airway irritation, blood pressure.
• Exercise: Increased pulmonary perfusion, ventilation, and chemoreceptor stimulation.

Homeostatic Imbalances of Respiratory System

• Smoking can cause COPD, emphysema (destruction of alveolar walls).