The Respiratory System: Overview

Major Functions and Processes

The respiratory system is essential for gas exchange, supplying oxygen to cells and removing carbon dioxide produced during cellular respiration. It works closely with the cardiovascular system to ensure efficient transport and exchange of gases throughout the body.

• Gas Exchange: The primary function is to supply oxygen and remove carbon dioxide.

• Respiration: Involves four processes: pulmonary ventilation, pulmonary gas exchange, transport of respiratory gases, and tissue gas exchange.

• Other Functions: Includes olfaction (smell) and speech.

Functional Anatomy of the Respiratory System

Major Organs and Divisions

The respiratory system is divided into upper and lower regions, each with distinct anatomical structures and functions.

• Upper Respiratory System: Nose, paranasal sinuses, and pharynx.

• Lower Respiratory System: Larynx, trachea, bronchi (and their branches), lungs, and alveoli.

• Respiratory Muscles: Classified as part of the muscular system, these muscles facilitate breathing.

Upper Respiratory System

Nose and Paranasal Sinuses

The nose is the only externally visible part of the respiratory system and serves multiple functions, including filtering, warming, and moistening air.

• External Nose: Composed of bones and cartilage, with surface features such as the root, bridge, dorsum nasi, apex, and nostrils.

• Nasal Cavity: Divided by the nasal septum, lined with mucosa (olfactory and respiratory), and contains nasal conchae to increase surface area and enhance air filtration.

• Paranasal Sinuses: Located in surrounding bones, they lighten the skull and help warm and moisten air.

Pharynx

The pharynx connects the nasal cavity to the larynx and mouth to the esophagus. It is divided into three regions: nasopharynx, oropharynx, and laryngopharynx.

• Nasopharynx: Serves as an airway, lined with ciliated epithelium, and contains pharyngeal tonsils.

• Oropharynx: Passageway for food and air, lined with stratified squamous epithelium, contains palatine and lingual tonsils.

• Laryngopharynx: Passageway for food and air, continuous with the esophagus.

Lower Respiratory System

Larynx

The larynx, or voice box, provides an open airway, routes air and food, and houses the vocal folds for sound production.

• Cartilaginous Framework: Includes thyroid, cricoid, arytenoid, cuneiform, corniculate cartilages, and the epiglottis.

• Vocal Folds: True vocal cords vibrate to produce sound; vestibular folds (false vocal cords) help close the glottis during swallowing.

• Epithelium: Stratified squamous above vocal folds; ciliated columnar below.

Trachea

The trachea, or windpipe, is a flexible tube supported by C-shaped cartilage rings, lined with ciliated epithelium to move mucus upward.

• Layers: Mucosa, submucosa, and adventitia.

• Trachealis Muscle: Contracts during coughing to expel mucus.

• Carina: Last cartilage marking the division into main bronchi.

Bronchi and Subdivisions

The bronchial tree undergoes extensive branching, transitioning from conducting zone structures to respiratory zone structures.

• Main Bronchi: Right and left, entering lungs at the hilum.

• Lobar and Segmental Bronchi: Branch further into bronchioles and terminal bronchioles.

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveolar sacs.

Alveoli and Respiratory Membrane

Alveoli are the sites of gas exchange, surrounded by capillaries and elastic fibers. The respiratory membrane is thin, allowing efficient diffusion.

• Alveolar Cells: Type I (squamous) for gas exchange; Type II (cuboidal) secrete surfactant.

• Alveolar Macrophages: Remove debris and pathogens.

• Alveolar Pores: Equalize air pressure and provide alternate routes.

Lungs and Pleurae

Gross Anatomy of the Lungs

Each lung is divided into lobes and bronchopulmonary segments, surrounded by pleurae and connected to the mediastinum.

• Right Lung: Three lobes (superior, middle, inferior).

• Left Lung: Two lobes (superior, inferior) with cardiac notch.

• Bronchopulmonary Segments: Each served by its own artery, vein, and bronchus.

Pleurae

The pleurae are double-layered serous membranes that surround each lung and provide lubrication and surface tension.

• Parietal Pleura: Lines thoracic wall and diaphragm.

• Visceral Pleura: Covers external lung surface.

• Pleural Fluid: Lubricates and helps keep lungs attached to thoracic wall.

Respiratory Physiology

Pulmonary Ventilation

Pulmonary ventilation consists of inspiration and expiration, driven by changes in thoracic volume and pressure.

• Boyle's Law: Pressure and volume of a gas are inversely related:

• Inspiration: Active process involving diaphragm and external intercostals.

• Expiration: Passive during quiet breathing; active during forced expiration.

Physical Factors Influencing Ventilation

Ventilation is influenced by airway resistance, alveolar surface tension, and lung compliance.

• Airway Resistance: Friction in airways; greatest in medium-sized bronchi.

• Alveolar Surface Tension: Surfactant reduces surface tension, preventing alveolar collapse.

• Lung Compliance: Measure of lung distensibility; higher compliance means easier ventilation.

Pulmonary Volumes and Capacities

Measuring Pulmonary Function

Pulmonary volumes and capacities are measured to assess respiratory health. Spirometry is a common tool for this purpose.

• Tidal Volume (TV): Air moved in/out with each breath (~500 mL).

• Inspiratory Reserve Volume (IRV): Air forcibly inspired beyond TV.

• Expiratory Reserve Volume (ERV): Air forcibly expired beyond TV.

• Residual Volume (RV): Air remaining in lungs after forced expiration.

• Vital Capacity (VC): Total exchangeable air:

• Total Lung Capacity (TLC): Sum of all volumes:

Gas Exchange

Basic Properties of Gases

Gas exchange relies on partial pressure gradients and solubility. Dalton's law and Henry's law govern these principles.

• Dalton's Law: Total pressure is the sum of partial pressures of individual gases.

• Henry's Law: Gas dissolves in liquid in proportion to its partial pressure and solubility.

Pulmonary and Tissue Gas Exchange

Gas exchange occurs in the lungs (pulmonary) and tissues (systemic), driven by partial pressure differences.

• Oxygen: Diffuses from alveoli to blood, then from blood to tissues.

• Carbon Dioxide: Diffuses from tissues to blood, then from blood to alveoli.

Transport of Respiratory Gases

Oxygen Transport

Oxygen is transported in blood primarily bound to hemoglobin, with a small amount dissolved in plasma.

• Oxyhemoglobin: Hemoglobin bound to oxygen.

• Oxygen-Hemoglobin Dissociation Curve: Shows relationship between partial pressure of oxygen and hemoglobin saturation.

• Bohr Effect: Increased CO2 and decreased pH reduce hemoglobin's affinity for oxygen, enhancing unloading in tissues.

Carbon Dioxide Transport

Carbon dioxide is transported in three forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions.

• Bicarbonate Formation: CO2 combines with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions.

• Chloride Shift: Exchange of bicarbonate and chloride ions between RBCs and plasma.

• Haldane Effect: Deoxygenated hemoglobin increases CO2 transport.

Regulation of Breathing

Neural Mechanisms

Breathing is controlled by centers in the brain stem, including the medullary and pontine respiratory centers.

• Ventral Respiratory Group (VRG): Generates basic rhythm.

• Dorsal Respiratory Group (DRG): Integrates input from peripheral receptors.

• Pontine Centers: Smooth transitions between inspiration and expiration.

Chemical Regulation

Chemoreceptors monitor levels of CO2, O2, and pH, adjusting breathing rate and depth accordingly.

• Central Chemoreceptors: Located in the brain stem, sensitive to CO2 and pH.

• Peripheral Chemoreceptors: Located in carotid and aortic bodies, sensitive to O2 and pH.

Clinical Aspects and Disorders

Chronic Obstructive Pulmonary Disease (COPD)

COPD includes emphysema and chronic bronchitis, characterized by irreversible decreased ability to force air out of the lungs.

• Emphysema: Permanent enlargement of alveoli, reduced elasticity.

• Chronic Bronchitis: Chronic inflammation and excessive mucus production.

Other Disorders

• Asthma: Reversible airway obstruction due to inflammation and bronchospasm.

• Tuberculosis: Infectious disease caused by Mycobacterium tuberculosis.

• Lung Cancer: Leading cause of cancer deaths, often due to smoking.

• Sleep Apnea: Temporary cessation of breathing during sleep, often due to airway collapse.

• Cystic Fibrosis: Genetic disorder causing thick mucus and frequent infections.

Developmental Aspects

Embryonic Development

The respiratory system develops early in embryogenesis, with structures forming by week 4–5 and functional lungs by week 28. Respiratory efficiency decreases with age.