The Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, sound production, and maintaining homeostasis in the body. It consists of organs and structures that facilitate the movement of air and the exchange of oxygen and carbon dioxide between the atmosphere and the bloodstream.

• Gas Exchange: Exchanges oxygen (O2) and carbon dioxide (CO2) between blood and the external environment.

• Air Movement: Moves air to and from exchange surfaces in the lungs.

• Air Conditioning: Passageways purify, warm, and humidify incoming air.

• Protection: Defends against pathogenic invasion.

• Sound Production: Produces sound for speech.

• Regulation: Helps regulate blood volume, pressure, and pH.

Major Processes of Respiration

Respiration involves several processes that ensure efficient gas exchange and transport.

• Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

• External Respiration: Exchange of O2 and CO2 between lungs and blood.

• Transport: Movement of O2 and CO2 in the blood.

• Internal Respiration: Exchange of O2 and CO2 between blood and tissues.

Functional Anatomy of the Respiratory System

Upper Respiratory Tract

• Nose and Nasal Cavity: Provides airway, moistens/warms air, filters particles, serves as a resonating chamber, and houses olfactory receptors.

• Paranasal Sinuses: Located in frontal, sphenoid, ethmoid, and maxillary bones; lighten the skull, act as resonance chambers, secrete mucus, and help warm/moisten air.

• Pharynx: Muscular tube connecting nasal cavity and mouth to larynx and esophagus; divided into nasopharynx, oropharynx, and laryngopharynx.

Lower Respiratory Tract

• Larynx (Voice Box): Provides airway, routes air/food, and produces sound; composed of nine cartilages including thyroid (Adam's apple), cricoid, and epiglottis.

• Trachea (Windpipe): Extends from larynx into mediastinum; supported by C-shaped cartilage rings; lined with ciliated pseudostratified epithelium and goblet cells.

• Bronchi and Bronchioles: Trachea divides into right and left main bronchi, which branch into lobar (secondary) bronchi and further into bronchioles; changes in cartilage, epithelium, and smooth muscle occur along the conducting zone.

• Lungs and Alveoli: Lungs contain millions of alveoli, the sites of gas exchange; alveoli are surrounded by capillaries and have thin walls for diffusion.

Respiratory Membrane

The respiratory membrane is the barrier for gas exchange between alveolar air and blood.

• Structure: Consists of alveolar epithelium, capillary endothelium, and fused basement membranes.

• Cell Types: Type I alveolar cells (squamous epithelium), Type II alveolar cells (secrete surfactant), and alveolar macrophages (defense).

Blood Supply and Innervation

Pulmonary and Bronchial Circulation

• Pulmonary Circulation: Pulmonary arteries deliver deoxygenated blood to lungs; pulmonary veins return oxygenated blood to heart.

• Bronchial Circulation: Bronchial arteries supply oxygenated blood to lung tissue (except alveoli).

Innervation

• Parasympathetic Fibers: Cause bronchoconstriction.

• Sympathetic Fibers: Cause bronchodilation.

Pleurae and Pressure Relationships

Pleurae

• Parietal Pleura: Lines thoracic wall and diaphragm.

• Visceral Pleura: Covers external lung surface.

• Pleural Fluid: Lubricates and creates surface tension for lung expansion.

Pressure Relationships

• Atmospheric Pressure (Patm): Pressure exerted by air outside the body (760 mm Hg at sea level).

• Intrapulmonary Pressure (Ppul): Pressure in alveoli; fluctuates with breathing.

• Intrapleural Pressure (Pip): Pressure in pleural cavity; always negative relative to Ppul.

• Transpulmonary Pressure: ; keeps lungs inflated.

Pulmonary Ventilation

Boyle's Law

Describes the inverse relationship between pressure and volume of a gas:

Mechanics of Breathing

• Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume and decreasing pressure, causing air to flow in.

• Expiration: Muscles relax, thoracic volume decreases, pressure increases, and air flows out.

• Forced Breathing: Accessory muscles (scalenes, sternocleidomastoid, pectoralis minor) assist during vigorous activity.

Physical Factors Influencing Ventilation

• Airway Resistance: Friction in airways; normally low due to large airway diameters and branching.

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

• Lung Compliance: Measure of lung stretchability; high compliance means easier expansion.

• Compliance Equation:

Respiratory Volumes and Capacities

• Tidal Volume (TV): Amount of air inhaled/exhaled per breath (~500 mL).

• Anatomical Dead Space: Air in passageways not involved in gas exchange (~150 mL).

• Alveolar Dead Space: Nonfunctional alveoli.

• Total Dead Space: Sum of anatomical and alveolar dead space.

Gas Exchange and Transport

External and Internal Respiration

• External Respiration: O2 and CO2 exchange between lungs and blood.

• Internal Respiration: O2 and CO2 exchange between blood and tissues.

Gas Laws

• Dalton's Law: Total pressure of a gas mixture equals the sum of partial pressures of each gas.

• Henry's Law: Amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

Oxygen Transport

• Hemoglobin: 98.5% of O2 is bound to hemoglobin; 1.5% dissolved in plasma.

• Oxyhemoglobin Dissociation Curve: S-shaped curve showing relationship between O2 partial pressure and hemoglobin saturation.

• Bohr Effect: Increased CO2 and H+ lower hemoglobin's affinity for O2, enhancing unloading in tissues.

Carbon Dioxide Transport

• Dissolved in Plasma: 7-10% as CO2.

• Carbaminohemoglobin: 20% bound to hemoglobin.

• Bicarbonate Ions: 70% as HCO3- in plasma.

• Chloride Shift: Exchange of Cl- and HCO3- across RBC membrane.

• Buffer System: Carbonic acid-bicarbonate buffer helps regulate blood pH.

Control of Respiration

Neural Mechanisms

• Medullary Centers: Ventral respiratory group (VRG) sets basic rhythm; dorsal respiratory group (DRG) integrates sensory input.

• Pontine Centers: Smooth transitions between inspiration and expiration.

Chemical Factors

• Central Chemoreceptors: Located in brainstem; respond to changes in CO2 and pH.

• Peripheral Chemoreceptors: Located in aortic arch and carotid arteries; respond to O2, CO2, and pH.

• Hypercapnia: Increased CO2 stimulates increased breathing rate.

• Hypoxia: Low O2 stimulates breathing only when levels are very low.

• Acidosis: Low pH increases respiratory rate.

Respiratory Adjustments and Disorders

Exercise and High Altitude

• Exercise: Increases ventilation (hyperpnea) to meet metabolic demands; O2 and CO2 levels remain stable.

• High Altitude: Lower atmospheric pressure and O2 levels; acclimatization involves increased ventilation and erythropoiesis.

Respiratory Diseases

• COPD: Chronic obstructive pulmonary disease includes emphysema and chronic bronchitis; characterized by irreversible airflow limitation.

• Asthma: Acute episodes of airway inflammation and bronchospasm.

• Tuberculosis: Infectious disease caused by Mycobacterium tuberculosis.

• Lung Cancer: Leading cause of cancer death; types include adenocarcinoma, squamous cell carcinoma, and small cell carcinoma.

• Sleep Apnea: Temporary cessation of breathing during sleep; can be obstructive or central in origin.

Table: Comparison of Respiratory Volumes

Key Equations

• Boyle's Law:

• Transpulmonary Pressure:

• Lung Compliance:

• Dalton's Law:

Example: During exercise, increased CO2 production leads to increased ventilation to maintain blood pH and gas levels.

Additional info: Some values and definitions inferred from standard anatomy & physiology textbooks for completeness.