Respiration Overview and Anatomy

Respiration Overview

• 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 phonation (vocal sounds), sense of smell, filters inhaled air, and excretes wastes during exhalation.
  • Oto(rhino)laryngology is the study of the respiratory system.
• Why do cells need oxygen? Aerobic cellular respiration.
• Three respiratory surfaces in the body. External respiration occurs at the alveoli.

Anatomy of the Respiratory System

• Upper respiratory system: Nose, nasal cavity, pharynx, and associated structures.
• Lower respiratory system: Larynx, trachea, bronchi, and lungs.
• Conducting zone: Directs air to the respiratory zone; filters, warms, and humidifies air.
• Respiratory zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Upper Respiratory System

• Nose: Made of bone, cartilage, and connective tissue. Air enters through external nares (nostrils).

• Nasal cavity: Interior and anterior space of the nose. Bounded by nasal bones and oral cavity. Bone and cartilage keep passages unobstructed. Divided into left and right halves by nasal septum. Contains paranasal sinuses (lined with mucous membranes, aid in vibration for singing and speech), nasal conchae (swirl inhaled air), and olfactory epithelium (sensory receptors for smells, no goblet cells).

• Pharynx: Tube of skeletal muscle lined with mucous membrane. Starts at internal nares and continues to cricoid cartilage. Subdivided into:

  • Nasopharynx (superior): Lined with ciliated pseudostratified columnar epithelium.
  • Oropharynx (intermediate): Common passage for air and food. Lined with non-keratinized stratified squamous epithelium. Contains tonsils.
  • Laryngopharynx (inferior): Similar structure to oropharynx.

• Larynx (voice box): Tube comprising nine rings of cartilage.

  • Thyroid cartilage (Adam's apple): A hyaline cartilage forming the anterior surface of the larynx; larger growth in males stimulated by sex hormones.
  • Epiglottis: Flap of elastic cartilage. Covers entrance during swallowing.
  • Cricoid cartilage: Ring of hyaline cartilage at the inferior portion of the larynx; landmark for tracheotomies.
  • Vocal folds (true vocal cords): Made of non-keratinized stratified squamous epithelium; form elastic ligaments; vibrate when air moves through, producing sound; tension from skeletal muscles changes pitch.
  • 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 CT); keeps trachea patent; lined with ciliated pseudostratified epithelium. Anterior to the esophagus.

• Bronchi: Trachea splits into right and left bronchi. Carina is a ridge at the branchpoint; sensitive mucous membrane. Branch into smaller tubes in the lungs (bronchial tree) that end in terminal bronchioles. Mucous membrane lining changes throughout the bronchial tree. Supporting cartilage and smooth muscle proportion change, too.

• Lungs: Wrapped in pleural membrane (2 serous membranes); pleural fluid reduces friction; lungs separated by mediastinum. Extend from superior to clavicles to diaphragm; rest on costal surfaces of ribs. Superior portion is the apex and inferior is the base. Medial surfaces are the hilum (passage for bronchi, vessels, nerves, lymphatics) and the cardiac notch (provides space for the heart, makes left lung smaller). Fissures divide the lungs into lobes.

  • Oblique fissure separates superior and inferior lobes.
  • Horizontal fissure borders middle lobe superiorly (right lung only).
  • Lobar bronchi branch into segmental bronchi (each supports a bronchopulmonary segment).
    • 13 segmental bronchi in right lung; 8 in left lung.
    • Damaged segments can be removed without affecting surrounding tissue.
  • Lobules are smaller compartments consisting of a branch of a terminal bronchiole, an arteriole + venule, a lymphatic vessel, all wrapped in elastic connective tissue.
  • Respiratory bronchioles are microscopic branches of bronchi, lined with simple cuboidal epithelium. They divide into alveolar ducts with simple squamous epithelium.

• Alveoli: Air sacs where pulmonary and external respiration occur. Extensive surface area. Alveolar ducts terminate in alveolar sacs (like clusters of grapes); each sac = alveolus.

  • Type I alveolar cells: Simple squamous epithelium; thinness for gas diffusion.
  • Type II alveolar cells: Nonciliated cuboidal epithelium at the septa; secrete surfactant (phospholipids + lipoproteins), preventing alveolar wall sticking; patrolled by macrophages.

• Respiratory membrane: Alveoli + associated capillaries (0.5 µm thick); alveolar wall, epithelial basement membrane, capillary basement membrane, capillary endothelium (direct contact with blood).

• Blood Supply to Lungs: Two sets of arteries:

  • Pulmonary arteries: Bring deoxygenated blood from right ventricle for oxygenation. Constrict in response to hypoxia.
  • Bronchial arteries: Branch from aorta, deliver oxygenated blood to the muscular tissue of the lungs.

• Patency (airway unobstructed): Crucial for gas exchange; compromised by injuries, septum deviation, inflammation, infections, or hypersensitivity reactions.

Gas Exchange and Ventilation

• Pulmonary ventilation: Inhalation and exhalation; leads to gas exchange at alveoli. Regulated by pressure change in thoracic cavity. Requires respiratory muscle contraction.

• Inhalation (inspiration): Active process. Lungs expand, pressure decreases below atmospheric pressure. Gases move from high to low partial pressure.

  • Boyle's law: Pressure inside a container is inversely proportional to the volume of the container; If we increase volume, pressure decreases.
  • Diaphragm contraction depresses it, decreasing lung pressure. Assisted by intercostal muscles.
  • Intrapleural pressure: Negative pressure in pleural cavity (lower than atmospheric); keeps pleural membrane on the wall. When wall expands, lungs expand.

• Exhalation (expiration): Passive process. Respiratory muscles relax; elastic recoil of lung tissue generates pressure and pushes air out. Abdominal and internal intercostal muscles can assist in active exhalation during strenuous exercise.

  • Increased lung pressure, relative to outside, drives air out.

• Factors affecting pulmonary ventilation:

  • Surfactant: Essential for ventilation. Insufficient leads to respiratory distress syndrome.
  • Compliance: Distensibility of elastic tissues. High compliance means low effort required, vice versa. Issues like fibrosis, injury, etc., lead to decreased compliance.
  • Resistance: Product of airway diameter. Larger diameter = less resistance. Smooth muscle contraction/relaxation regulates. Obstruction increases resistance (COPD).

• Lung volumes and capacities: Measured using a spirometer; output is a spirogram.

Principles of Gas Exchange

• Movement of oxygen and carbon dioxide is passive diffusion from areas of high to low partial pressure. Solubility is also a factor (CO2 is more soluble in water).

• External Respiration: gas exchange between alveoli and blood across pulmonary capillaries (PO2 in alveoli is lower during exercise). Slow movement through capillaries maximizes oxygenation.

• Internal Respiration: gas exchange between blood and body tissues (tissue cells produce CO2; higher PCO2 outside systemic capillaries); Oxygen moves from blood to tissues.

• Factors affecting respiration rate:

  • Partial pressure gradient
  • Surface area
  • Diffusion distance
  • Molecular weight and solubility

Oxygen Transport

• Hemoglobin (Hb) in erythrocytes transports 98.5% of oxygen; rest is dissolved in blood plasma.
  • Hb consists of 4 protein subunits; 1 heme molecule per subunit; oxygen binds to iron atom in heme.
• Hb binds oxygen reversibly.
  • Increased PO2 favors Hb-O2 formation. Hb is saturated when nearly all Hb is Hb-O2. Factors affecting Hb saturation:
    • PO2, Hb affinity for oxygen. Affinity is affected by:
      • Acidity: Increased [H+] decreases affinity.
      • PCO2: High PCO2 lowers pH, decreasing affinity.
      • Temperature: Increased temperature decreases affinity.
      • 2,3-bisphosphoglycerate (BPG): Decreases affinity.
      • Hemoglobin type—Fetal Hb (Hb-F) has higher affinity

Carbon Dioxide Transport

• Carbon dioxide transport: Dissolved in blood plasma (7%), bound to proteins (23%), transported as bicarbonate (70%).
  • Chloride shift maintains electrical balance in erythrocytes.
  • Reverse chloride shift: At pulmonary capillaries, CO2 is eliminated.

Regulation of Breathing

• Respiratory center: Cluster of neurons in the brain (medulla oblongata and pons) that regulates respiratory muscles.

  • Medullary respiratory group (DRG/VRG). PVRG influence DRG for normal breathing.
  • Pontine respiratory group influences DRG for normal breathing.

• Cortical influences on breathing: Voluntary control.

• Chemoreceptors: Sense chemical changes in blood.

  • Central chemoreceptors (medulla oblongata): Sense PCO2 and/or H+ in cerebrospinal fluid (CSF).
  • Peripheral chemoreceptors (aortic and carotid bodies): Sense PCO2 and PO2 in blood.

• Hyperventilation (respiration rate & depth increase): Response to low blood pH.

• Hypocapnia: Can result from hyperventilation causing insufficient oxygen, (hypoxia).

• Inflation reflex: Prevents overinflation of lungs during exercise. Baroceptors detect lung stretch, inhibiting the respiratory center via the vagus nerve.

• Other influences on breathing: Emotions, temperature, pain, airway irritation, blood pressure.

• Exercise and the respiratory system: Increased pulmonary perfusion and ventilation; chemoreceptors trigger increased rate/depth of breathing.

• Homeostatic imbalances: Tobacco smoking causes COPD (e.g., emphysema) which reduces the efficiency of gas exchange.

Explore the fundamental processes of respiration, including the steps of pulmonary ventilation, external respiration, and internal respiration. Understand the anatomy of the respiratory system, distinguishing between the upper and lower respiratory tracts, as well as the roles they play in gas exchange and other vital functions.