Respiratory System: Structure, Function, and Physiology

Respiratory System

Overview and Functions

The respiratory system is responsible for gas exchange, voice production, regulation of plasma pH, olfaction, and protection of respiratory surfaces. It enables the body to obtain oxygen, remove carbon dioxide, and maintain acid-base balance.

• Gas Exchange: Exchange of O2 and CO2 between air and blood.

• Voice Production: Air movement through the larynx produces sound.

• Regulation of Plasma pH: By controlling CO2 levels.

• Olfaction: Sense of smell.

• Protection: Prevents pathogen invasion, dehydration, and temperature changes.

Types of Respiration

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

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

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

• Cellular Respiration: Breakdown of nutrients in mitochondria to produce ATP.

Anatomy of the Respiratory System

Divisions

• Upper Respiratory Tract: Nose, nasal cavity, paranasal sinuses, pharynx, larynx. Functions: filter, warm, humidify incoming air.

• Lower Respiratory Tract: Trachea, bronchi, bronchioles, alveoli. Functions: conduct air to gas exchange sites.

Zones

• Conducting Zone: Transports, filters, humidifies, and warms air. Includes nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles (except respiratory bronchioles).

• Respiratory Zone: Site of gas exchange. Includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Respiratory Structures

• Nose: Entry point for air; contains nasal hairs for filtration.

• Nasal Cavities: Separated by nasal septum; contain conchae and meatuses for air turbulence and conditioning.

• Pharynx: Shared by respiratory and digestive systems; divided into nasopharynx, oropharynx, laryngopharynx.

• Larynx: Maintains airway, prevents food entry, produces sound. Contains 9 cartilages (3 paired, 3 unpaired).

• Trachea: Windpipe; reinforced by C-shaped cartilage rings; divides into primary bronchi at the carina.

• Bronchial Tree: Primary bronchi → secondary bronchi (right: 3, left: 2) → tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.

• Lungs: Right lung (3 lobes, 2 fissures), left lung (2 lobes, 1 fissure); covered by pleura (parietal and visceral); pleural cavity contains serous fluid.

Histology Review

• Epithelium changes from pseudostratified columnar (upper airways) to simple cuboidal and then simple squamous (alveoli).

• Cartilage decreases and disappears in bronchioles.

• Goblet cells and cilia decrease as airways branch.

• Smooth muscle increases in bronchioles.

Alveoli

• Type I Cells: Simple squamous epithelium for gas exchange (97% of surface).

• Type II Cells: Cuboidal cells that secrete surfactant to reduce surface tension.

• Macrophages: Remove particles by phagocytosis.

Pathway of Air

Nose/Mouth → Pharynx → Larynx → Trachea → Carina → Primary Bronchi → Secondary Bronchi → Tertiary Bronchi → Quaternary Bronchi → Bronchioles → Terminal Bronchioles → Respiratory Bronchioles → Alveolar Ducts → Alveolar Sacs → Alveoli

Pulmonary Circulation

• Pulmonary Arteries: Carry deoxygenated blood to lungs.

• Pulmonary Veins: Carry oxygenated blood to heart.

• Low resistance: Pulmonary circulation has lower resistance and pressure than systemic circulation.

• Anatomic Shunt: Blood bypasses oxygenation regions, decreasing arterial PO2.

Respiratory Muscles

• Diaphragm: Primary muscle for inspiration.

• Sternocleidomastoid, External/Internal Intercostals, Abdominal Muscles: Assist in forced breathing.

Respiratory Physiology

Respiratory Mechanics

Breathing involves inspiration and expiration, driven by pressure gradients between alveoli and atmosphere. The relationship between pressure and volume is described by Boyle's Law.

• Inspiration: Active process; diaphragm contracts, thoracic volume increases, alveolar pressure decreases, air flows in.

• Expiration: Passive process; diaphragm relaxes, thoracic volume decreases, alveolar pressure increases, air flows out.

• Forced Breathing: Accessory muscles increase thoracic volume (inspiration) or decrease it (expiration).

Gas Laws

• Boyle's Law: $P V = \text{constant}$ (Pressure inversely proportional to volume at constant temperature)

• Dalton's Law: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$ (Total pressure is sum of partial pressures)

• Henry's Law: $C_g = k P_g$ (Amount of gas dissolved in solution is proportional to partial pressure)

• Fick's Law: $\Delta M / \Delta t = D A (C_2 - C_1) / L$ (Diffusion rate depends on area, concentration gradient, and distance)

Definitions and Key Terms

• Respiratory Rate: Breaths per minute.

• Eupnea: Quiet, normal breathing (12-18 breaths/min).

• Apnea: Absence of breathing.

• Hypoventilation: Low respiration rate; CO2 accumulates.

• Hyperventilation: Increased rate/depth; CO2 decreases.

• Diaphragmatic Breathing: Deep breathing via diaphragm.

• Costal Breathing: Shallow breathing via rib cage.

• Forced Breathing (Hyperpnea): Active inspiration and expiration.

• Hypocapnia: Low CO2 (compensatory hypoventilation).

• Hypercapnia: High CO2 (compensatory hyperventilation).

• Dyspnea: Difficulty breathing.

• Hypoxia: Low tissue O2 levels.

• Anoxia: Absence of oxygen.

• Diffusion: Movement from high to low concentration (Fick's Law).

• Atmospheric Pressure: 760 mmHg.

• Partial Pressure: Pressure contributed by each gas.

• Conducting Zone: Airway not involved in gas exchange.

• Respiratory Zone: Airway involved in gas exchange.

• Anatomical Dead Space (VD): Air volume not participating in gas exchange (150 ml).

• Pulmonary Ventilation: Movement of air in/out of lungs.

• Minute Ventilation (VM): $VM = VT \times \text{rate}$ (Total air per minute; ~6L/min)

• Alveolar Ventilation (VA): $VA = f(V_T - VD)$ (Air reaching alveoli per minute; ~4.2L/min)

• Alveolar-Arterial O2 Gradient (A-a gradient): Difference between alveolar and arterial O2 partial pressures.

• Ventilation/Perfusion Ratio (Va/Q): Ratio of alveolar ventilation to pulmonary perfusion; normal is 1.0.

Factors Affecting Pulmonary Ventilation

• Airway Resistance: Normally low; increased by bronchoconstriction, mucus, or infection.

• Surface Tension: Reduced by surfactant; prevents alveolar collapse.

• Compliance: Ease of lung expansion; low compliance requires more force.

• Elastance: Reciprocal of compliance; measures lung recoil.

Compliance Formula:

• $C_L = \frac{\Delta V_L}{\Delta P_L}$

• Normal value: 20 ml/cmH2O

Elastance Formula:

• $E = \frac{1}{C} = \frac{\Delta P}{\Delta V}$

Lung Volumes and Capacities

• Tidal Volume (TV, VT): Air per breath (~500 ml).

• Inspiratory Reserve Volume (IRV): Extra air inspired above TV (~3000 ml).

• Expiratory Reserve Volume (ERV): Extra air expired beyond normal (~1200 ml).

• Residual Volume (RV): Air remaining after forceful expiration (~1200 ml).

• Total Lung Capacity (TLC): Maximum lung volume (~6000 ml); $TLC = VC + RV$

• Vital Capacity (VC): Max air expired after max inspiration (~4800 ml); $VC = IRV + TV + ERV$

• Functional Residual Capacity (FRC): Air left after normal expiration (~2300 ml); $FRC = ERV + RV$

• Inspiratory Capacity (IC): Max air inspired after normal expiration (~3500 ml); $IC = TV + IRV$

Gas Transport in Blood

Oxygen Transport

• Hemoglobin (Hb): Protein with 4 heme groups; each binds 1 O2.

• 98.5% of O2 is bound to Hb; 1.5% dissolved in plasma.

• Oxyhemoglobin: Hb with O2 bound.

• Reduced Hemoglobin: Hb without O2.

• Loading/Unloading Equation: $HHb + O_2 \leftrightarrow HbO_2 + H^+$

• Hb saturation depends on PO2; in lungs (PO2 = 104 mmHg), Hb is fully saturated; in tissues (PO2 = 40 mmHg), Hb is 75% saturated.

• Factors Affecting Hb-O2 Binding: Temperature, pH, 2,3-DPG.

• Oxygen-Hemoglobin Dissociation Curve: Shows relationship between Hb saturation and PO2.

Carbon Dioxide Transport

• 10% dissolved in plasma.

• 20% bound to Hb as carbaminohemoglobin ($Hb + CO_2 \leftrightarrow HbCO_2$).

• 70% as bicarbonate ion in plasma ($CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$), catalyzed by carbonic anhydrase.

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

Respiratory Control

Respiratory Centers

• Medulla: Sets basic rhythm.

• Apneustic Center (Lower Pons): Prolongs inspiration.

• Pneumotaxic Center (Upper Pons): Inhibits apneustic center; shortens inspiration.

Neural Mechanisms

• Respiratory muscles are skeletal; require somatic nerve impulses.

• Diaphragm innervated by phrenic nerve.

• Dual voluntary and involuntary control.

• Central Chemoreceptors: Respond to CO2 levels (not O2); regulate respiratory activity.

• Pontine Centers: Modify medullary activity.

Respiratory Reflexes

• Peripheral Chemoreceptors: In carotid and aortic bodies; respond to PO2, PCO2, pH.

• Baroreceptors: Respond to blood pressure changes.

• Stretch Receptors: Initiate Hering-Breuer reflex to prevent overinflation.

Acid-Base Balance and Compensation

Effect of Respiratory Rate and Depth on Plasma pH

• Hyperventilation: Decreases CO2, decreases H+, increases pH (respiratory alkalosis).

• Hypoventilation: Increases CO2, increases H+, decreases pH (respiratory acidosis).

Compensation Pathways

• Respiratory Acidosis: Kidneys increase HCO3- reabsorption and H+ excretion.

• Respiratory Alkalosis: Kidneys excrete HCO3-; compensation takes 2-3 days.

Clinical and Pathological Considerations

• Physiologic Changes: High altitude, exercise.

• Pathological Changes: Edema, pulmonary diseases, SIDS, ARDS, pneumonia, pneumothorax, hemothorax.

Muscles of the Larynx

MUSCLE	ACTION/FUNCTION
Posterior cricoarytenoid	Abducts vocal fold
Lateral cricoarytenoid	Adducts vocal fold
Cricothyroid	Tenses vocal fold
Thyroarytenoid	Relaxes vocal fold
Thyroepiglotticus	Opens laryngeal inlet
Aryepiglotticus	Closes laryngeal inlet
Oblique and transverse arytenoids	Closes laryngeal inlet

Summary Table: Lung Volumes and Capacities

Volume/Capacity	Definition	Normal Value (ml)	Formula
Tidal Volume (TV)	Air per breath	500	-
Inspiratory Reserve Volume (IRV)	Extra air inspired above TV	3000	-
Expiratory Reserve Volume (ERV)	Extra air expired beyond normal	1200	-
Residual Volume (RV)	Air remaining after forceful expiration	1200	-
Total Lung Capacity (TLC)	Maximum lung volume	6000	TLC = VC + RV
Vital Capacity (VC)	Max air expired after max inspiration	4800	VC = IRV + TV + ERV
Functional Residual Capacity (FRC)	Air left after normal expiration	2300	FRC = ERV + RV
Inspiratory Capacity (IC)	Max air inspired after normal expiration	3500	IC = TV + IRV

Additional info:

• CO2 diffuses faster than O2 due to higher solubility and molecular weight.

• Normal alveolar ventilation is less than minute ventilation due to anatomical dead space.

• Chronic hypoxia leads to physiological adaptations such as increased TLC and diffusing capacity.

• Respiratory control is primarily regulated by CO2 levels, not O2.