Comprehensive Study Notes: The Respiratory System

Respiratory System

Overview and Functions

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It also plays roles in voice production, regulation of plasma pH, olfaction, and protection of respiratory surfaces.

• 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, the respiratory system helps maintain acid-base balance.

• Olfaction: Sense of smell via olfactory receptors in the nasal cavity.

• 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: Metabolic processes in mitochondria producing ATP from nutrients.

Anatomy of the Respiratory System

Divisions

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

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

Zones

• Conducting Zone: Transports, filters, humidifies, and warms air. Includes nasal cavity to terminal bronchioles. Epithelium: pseudostratified ciliated columnar with goblet cells.

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

Major Structures

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

• Nasal Cavities: Separated by septum; contain conchae and meatuses to increase surface area and turbulence for air processing.

• Pharynx: Shared by respiratory and digestive systems; divided into nasopharynx (pseudostratified ciliated), oropharynx, and laryngopharynx (both stratified squamous non-keratinized).

• Larynx: Voice box; contains 9 cartilages (3 paired, 3 unpaired). Functions in airway maintenance, sound production, and protection during swallowing.

• Trachea: Windpipe; reinforced by C-shaped cartilage rings; lined with respiratory epithelium.

• Bronchi and Bronchial Tree: Trachea divides into right and left primary bronchi, then into secondary (lobar) and tertiary (segmental) bronchi, and further into bronchioles.

• Bronchioles: Less than 1 mm in diameter, lack cartilage, dominated by smooth muscle.

• Alveoli: Terminal air sacs for gas exchange; lined by Type I (simple squamous) and Type II (surfactant-secreting) cells, and contain macrophages.

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

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 (gas exchange)

Lungs and Pleura

• Lobes: Right lung (3 lobes), left lung (2 lobes).

• Pleura: Parietal (lines thoracic cavity), visceral (covers lungs), pleural cavity (contains serous fluid).

• Blood Supply: Pulmonary arteries (deoxygenated blood), pulmonary veins (oxygenated blood), bronchial arteries (systemic supply).

• Nerve Supply: Sympathetic (bronchodilation), parasympathetic (bronchoconstriction).

Respiratory Physiology

Mechanics of Breathing

• Inspiration: Active process; diaphragm and external intercostals contract, thoracic volume increases, alveolar pressure decreases, air flows in.

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

• Forced Breathing: Accessory muscles (e.g., sternocleidomastoid, scalenes, abdominal muscles) involved.

Key Pressures

• Atmospheric Pressure: Pressure of air outside the body.

• Intrapulmonary Pressure: Pressure within alveoli.

• Intrapleural Pressure: Pressure within pleural cavity (always lower than alveolar pressure).

Boyle's Law

At constant temperature, pressure and volume of a gas are inversely related:

$P V = \text{constant}$

Gas Laws

• Boyle's Law: $P V = \text{constant}$

• Dalton's Law: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$

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

• Fick's Law: $\text{Diffusion rate} = \Delta M / \Delta t = D A (C_2 - C_1) / L$

Definitions and Key Terms

• Respiratory Rate: Breaths per minute (normal: 12-18).

• Eupnea: Quiet, normal breathing.

• Apnea: Absence of breathing.

• Hypoventilation: Low respiratory rate; CO2 accumulates.

• Hyperventilation: High respiratory rate; CO2 decreases.

• Diaphragmatic Breathing: Deep breathing using diaphragm.

• Costal Breathing: Shallow breathing using rib cage muscles.

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

• Hypocapnia: Low CO2 in blood.

• Hypercapnia: High CO2 in blood.

• Dyspnea: Difficulty breathing.

• Hypoxia: Low tissue O2 levels.

• Anoxia: Absence of O2.

• Diffusion: Movement of molecules from high to low concentration.

• Partial Pressure: Pressure contributed by each gas in a mixture.

• Conducting Zone: Airways not involved in gas exchange.

• Respiratory Zone: Airways involved in gas exchange.

• Anatomical Dead Space (VD): Volume of air not participating in gas exchange (about 150 ml).

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

• Minute Ventilation (VM): $V_M = V_T \times \text{rate}$

• Alveolar Ventilation (VA): $V_A = f (V_T - V_D)$

• Ventilation/Perfusion Ratio (VA/Q): Ratio of air reaching alveoli to blood flow in pulmonary capillaries (normal ≈ 1.0).

Lung Volumes and Capacities

• Tidal Volume (TV): Volume of 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 expiration (≈ 1200 ml).

• Residual Volume (RV): Air remaining after maximal expiration (≈ 1200 ml).

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

• Vital Capacity (VC): Maximal air expired after maximal inspiration (≈ 4800 ml). $VC = IRV + TV + ERV$

• Functional Residual Capacity (FRC): Volume after normal expiration (≈ 2300 ml). $FRC = ERV + RV$

• Inspiratory Capacity (IC): Maximal inspired volume after normal expiration (≈ 3500 ml). $IC = TV + IRV$

Gas Transport in Blood

Oxygen Transport

• Hemoglobin (Hb): Protein in RBCs; each molecule binds up to 4 O2 molecules.

• Oxyhemoglobin: Hb with O2 bound.

• Reduced Hemoglobin: Hb without O2.

• O2 Transport: 1.5% dissolved in plasma, 98.5% bound to Hb.

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

• Oxygen-Hemoglobin Dissociation Curve: Shows relationship between PO2 and Hb saturation. Shifted by pH, temperature, and 2,3-BPG.

Carbon Dioxide Transport

• 10% Dissolved in Plasma

• 20% Bound to Hemoglobin: $Hb + CO_2 \leftrightarrow HbCO_2$ (carbaminohemoglobin)

• 70% as Bicarbonate: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$ (catalyzed by carbonic anhydrase)

• Chloride Shift: Exchange of HCO3- and Cl- between RBCs and plasma to maintain charge balance.

Control of Respiration

Respiratory Centers

• Medulla: Sets basic rhythm of breathing.

• Apneustic Center (Lower Pons): Prolongs inspiration.

• Pneumotaxic Center (Upper Pons): Inhibits inspiration, regulates rate and depth.

Neural Mechanisms

• Respiratory muscles are skeletal and require somatic nerve impulses.

• Diaphragm innervated by phrenic nerve.

• Voluntary and involuntary control pathways exist.

Chemoreceptors and Reflexes

• Central Chemoreceptors: Located in medulla; respond to CO2 (via H+ in CSF).

• Peripheral Chemoreceptors: Located in carotid and aortic bodies; respond to low O2, high CO2, low pH.

• Baroreceptors: Sense blood pressure changes.

• Stretch Receptors: Hering-Breuer reflex prevents over-inflation of lungs.

Acid-Base Balance and Respiratory Disorders

Effect of Respiration on Plasma pH

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

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

Compensation Mechanisms

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

• Respiratory Alkalosis: Kidneys excrete more HCO3-.

Pathological Conditions

• Edema: Fluid accumulation in tissues.

• Pneumonia: Infection of the lungs.

• Pneumothorax: Air in pleural cavity causing lung collapse.

• Hemothorax: Blood in pleural cavity.

• ARDS: Acute respiratory distress syndrome.

• SIDS: Sudden infant death syndrome.

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 inspired above TV	3000	-
Expiratory Reserve Volume (ERV)	Extra expired beyond normal	1200	-
Residual Volume (RV)	Air remaining after maximal expiration	1200	-
Total Lung Capacity (TLC)	Maximal lung volume	6000	TLC = VC + RV
Vital Capacity (VC)	Maximal expired after maximal inspiration	4800	VC = IRV + TV + ERV
Functional Residual Capacity (FRC)	Volume after normal expiration	2300	FRC = ERV + RV
Inspiratory Capacity (IC)	Maximal inspired after normal expiration	3500	IC = TV + IRV

Key Equations

• Minute Ventilation: $V_M = V_T \times f$

• Alveolar Ventilation: $V_A = f (V_T - V_D)$

• Boyle's Law: $P V = \text{constant}$

• Dalton's Law: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$

• Henry's Law: $C_g = k P_g$

• Fick's Law: $\text{Diffusion rate} = \Delta M / \Delta t = D A (C_2 - C_1) / L$

• Compliance: $C_L = \Delta V_L / \Delta P_L$

• Elastance: $E = 1/C = \Delta P / \Delta V$

• O2 Transport: $HHb + O_2 \leftrightarrow HbO_2 + H^+$

• CO2 Transport: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$

Additional info:

• CO2 diffuses more rapidly than O2 due to higher solubility.

• Regional differences in ventilation: Lower lung regions ventilate better due to higher compliance.

• Ventilation/Perfusion mismatch can lead to hypoxemia or inefficient gas exchange.

• Chronic hypoxia leads to physiological adaptations such as increased ventilation, red blood cell count, and lung capacity.