Respiratory System Physiology (MBBS 1st Year)

The respiratory system is one of physiology's four fundamental systems. Its complex, conceptual nature makes it critical for understanding clinical applications and a frequent source of long questions in medical exams. Respiratory System Physiology MBBS 1st Year explores its intricate mechanisms, from breathing mechanics to gas exchange and regulation.

Respiratory System Physiology (MBBS 1st Year)

The Respiratory System is paramount in Physiology, alongside the Cardiovascular, Central Nervous, and Kidney systems. Mastering these is crucial due to their significant clinical applications and complex concepts. Unlike "comfort zone" systems, the Respiratory System is highly conceptual, requiring thorough study as clinical cases and long questions often originate from it.

Divisions of the Respiratory System

The Respiratory System is studied through major sections: Mechanics of Breathing (lung distension, pressures), Pulmonary Circulation and V/Q Ratio, Gas Transport (oxygen and carbon dioxide), Regulation of Breathing (neural, chemical), and Applied Respiratory Physiology (hypoxia, high altitude, deep-sea diving). For last-minute revision, prioritize Mechanics of Breathing and Gas Transport as five-star topics for clinical relevance and long questions.

Chapter 1: Mechanics of Breathing

This chapter is a common source of long questions. The terms "Breathing" and "Ventilation" are used interchangeably by physiologists. Key topics include:

• Long Question: Describe the Mechanics of Breathing / Mechanics of Ventilation (Introduction, Muscles, Pressures, Applied Aspects).

• Long Question: Lung Volumes and Capacities (including Timed Vital Capacity).

• Short Notes: Surfactant, Compliance, Dead Space, Non-respiratory Functions of the Lungs.

Mechanics of Breathing / Mechanics of Ventilation

This section details how the lungs and thoracic cage distend during breathing, along with associated mechanisms and pressures. A comprehensive answer covers:

1. Introduction

2. Muscles of Breathing

3. Pressures Involved in the Process of Breathing

4. Applied Aspects

1. Introduction

Breathing involves two processes: Inspiration (distension, air in) and Expiration (deflation, air out).

In normal quiet breathing: Inspiration is an active process (muscle contraction), while Expiration is a passive process.

• Inspiration: Requires active muscle contraction to distend the thoracic cage and lungs.

• Expiration: Is passive as the system passively recoils due to elastic properties after active distension ceases. Although muscle relaxation requires ATP, the system's return to its original state is passive in this context.

2. Muscles of Breathing

Two main groups: Diaphragm (contracts, descends, increasing vertical diameter; contributes to abdominal breathing) and External Intercostals (elevate rib cage, increasing anterior-posterior diameter; contribute to thoracic breathing).

Clinical Applications:

• Pure Thoracic Breathing: Occurs in conditions like Peritonitis (abdominal guarding).

• Pure Abdominal Breathing: Occurs in conditions like Pleurisy or Pleural Effusion (pain with thoracic movement).

Muscles of Forceful Breathing

• Accessory Muscles of Inspiration: Alae nasi, Sternocleidomastoid, Scalene muscles, Serratus anterior.

• Muscles of Forceful Expiration: Internal intercostals and Abdominal muscles.

Thoracic Cage Movements

• Bucket Handle Movement: Middle to lower ribs.

• Pump Handle Movement: Sternum and ribs.

3. Pressures Involved in the Process of Breathing

• Definition: Pressure between the two layers of the pleura.

• Characteristic: Mostly negative, except during forceful expiration (becomes positive).

• Purpose: To keep the lung distended, counteracting the lung's natural collapsing tendency.

• Generation: Due to the elastic recoil of the lung, specifically its inward recoil tendency.

• Role of Pleural Fluid: Prevents pleura from separating during inspiration due to Hydraulic Traction (similar to two wet glass slides).

• Mechanism of Inspiration: Diaphragm/intercostals contract, parietal pleura moves out, visceral pleura follows (due to fluid), causing lungs to distend.

Short Note: Surfactant

Definition and Function:

Surfactant is a surface-active agent that reduces surface tension in the fluid lining the alveoli. Surface tension causes alveoli to collapse. By decreasing it, surfactant:

• Decreases the collapsibility of alveoli.

• Increases the distensibility (stretchability) and compliance of the lungs.

Synthesis and Cell Types:

• Type II pneumocytes (5% of alveolar surface area) synthesize and secrete surfactant from tubular myelin (found in lamellar inclusion bodies).

• Type I pneumocytes (93%) are for gas exchange. Type III pneumocytes (2%) are chemoreceptors.

Composition:

1. Dipalmitoyl Phosphatidylcholine (DPPC): Main constituent.

2. Surface Apoproteins (SPA, SPB, SPC, SPD): Regulate surfactant turnover.

3. Calcium Ions (Ca²⁺): Important for fast spread of surfactant.

Functions of Surfactant:

1. Decreases surface tension, reducing collapsibility and increasing distensibility/compliance.

2. Helps keep alveoli dry (oily nature prevents fluid accumulation).

3. Stabilizes the alveolar system of interdependence (crucial for extra marks).

Alveolar Stability (Absence vs. Presence of Surfactant):

• Concept: Alveoli are hollow spheres obeying LaPlace's Law: P = 2T/R.

• Absence of Surfactant: Small alveoli have higher pressure, causing air to flow to large alveoli, leading to collapse of small alveoli and overdistension of large alveoli (instability).

• Presence of Surfactant: In small alveoli, surfactant is concentrated, dramatically reducing T, thus reducing P. In large alveoli, surfactant is scattered, making it less efficient, so T and P are relatively higher. This causes air to flow from larger to smaller alveoli, leading to stabilization.

Clinical Applications (Surfactant Deficiency):

• Acute Respiratory Distress Syndrome (ARDS) (Adults): Destruction of Type II pneumocytes leads to absence of surfactant, high surface tension, alveolar collapse, and low compliance.

• Hyaline Membrane Disease (HMD) / Infant Respiratory Distress Syndrome (IRDS) (Neonates): Surfactant deficiency in premature infants. Treatment: Corticosteroids to mother stimulate fetal surfactant production.

• Physio Plus Plus: Fetal Lung Maturation Indicator: Lecithin-to-Sphingomyelin (L/S) ratio in amniotic fluid. An L/S ratio of 2:1 or higher indicates fetal lung maturity.

Short Note: Lung Compliance

Definition and Properties:

Compliance is a measure of the distensibility or stretchability of the lung. Like a nylon stocking, individual fibers are not stretchable, but the woven tissue is distensible. It is inversely related to surface tension.

Mathematical Relationship: Compliance = ΔV/ΔP (change in lung volume per unit change of pressure).

Types of Compliance:

1. Static Compliance: Lung distensibility as a stationary tissue. Value: 200 ml/cm H2O. A 2.5 cm H2O pressure change (quiet inspiration) results in 500 ml volume change (Tidal Volume).

2. Specific Compliance: Adjusts for lung size (Compliance / Functional Residual Capacity (FRC)).

3. Dynamic Compliance: Compliance during breathing stages.

Dynamic Compliance during Breathing Cycle:

• Start of Inspiration (Lower Lung Volumes): Alveoli are small, surfactant is concentrated, surface tension is low, so compliance is high.

• End of Inspiration (Higher Lung Volumes): Alveoli are large, surfactant is scattered (less efficient), surface tension is high, so compliance is low.

Compliance at Different Lung Volumes

• Towards lower lung volumes, compliance is better.

• Towards higher lung volumes, compliance is low. At the end of inspiration, alveoli are full and large, scattering surfactant, leading to high surface tension and low compliance. More pressure yields small distension (poor ΔV/ΔP).

Hysteresis

The compliance diagram is called Hysteresis (a graph or system that follows two different paths).

• Diagram Axes: X-axis: Pressure (cm H2O); Y-axis: Volume distension.

• Curve Interpretation: Inspiration rises, expiration descends.

• Phases of Compliance:

• Start of Inspiration: Steep portion, good compliance.

• End of Inspiration: Flat portion, poor compliance.

• End of Expiration: Steepest portion, greatest/maximum compliance.

• When illustrating the compliance diagram, explicitly show ΔV/ΔP as good at the start of inspiration, poor by the end of inspiration, and best by the end of expiration.

Applied Aspects of Compliance

1. Normal Compliance Values:

• Lungs alone: 200 ml/cm H2O.

• Lungs + Thorax together: 100 to 110 ml/cm H2O. Reduction is due to opposing tendencies of lungs (inward recoil) and thoracic cage (outward movement).

2. Decreased Compliance (Restrictive Lung Diseases):

• Conditions: Restrictive Lung Disease, Pulmonary Edema, Interstitial Lung Disease, Thoracic Cage Abnormalities.

• Graphical Representation: Curve shifts downward and rightward.

• Breathing Pattern: Shallow and rapid, breathing near lower lung volumes (most economical).

3. Increased Compliance (Emphysema):

• Condition: Emphysema (early stages).

• Mechanism: Destruction of alveolar septa makes lungs easier to distend.

• Graphical Representation: Curve becomes almost vertical.

Work of Breathing

• Normal quiet breathing: Work done during inspiration (active); no work during expiration (passive).

• Work Done (W) = ΔV * ΔP.

• Three Fractions of Work Done:

1. Compliance Work (65%): Distending lung parenchyma.

2. Airway Resistance Work (28%): Overcoming airway resistance. Fraction increases in COPD.

3. Tissue Resistance Work (7%): Stretching surrounding tissues. Fraction increases in Interstitial Lung Disease.

Chronic Obstructive Pulmonary Diseases (COPDs)

• Core Feature: Airway obstruction (narrowing, edematous, inflamed airways).

• Examples: Bronchial Asthma, Chronic Bronchitis, Emphysema.

• Breathing Mechanics:

• Inspiration is relatively easy.

• Expiration is more difficult (positive pressure and lung recoil compress airways).

• Consequences: Early closure of airways, increased Residual Volume (air trapping), progressive increase in Total Lung Capacity.

• Reasoning Question: Why is there a barrel-shaped chest in emphysema? Due to chronically increased Total Lung Capacity from air trapping and over-inflation.

• Classification:

• Chronic Bronchitis: "Blue Bloaters" (cyanosis, edema).

• Emphysema: "Pink Puffers" (maintain oxygenation, pursed lips).

• Breathing Pattern: Slow and deep breathing, near the higher lung volumes (prevents early airway closure).

• Pulmonary Function Tests (PFTs): Primarily check expiratory flow. Key tests: FEV1, PEFR, MMFR.

Comparison of FEV1/VC Ratio:

• COPD: FEV1/VC ratio is decreased.

• Restrictive Disease: FEV1/VC ratio is increased (or normal).

Lung Volumes and Capacities

Pulmonary Function Testing (PFT):

• Instrument: Hutchinson's Spirometer.

• Graphical Record: Spirogram.

• Procedure: Spirometry.
  PFTs measure four lung volumes and four capacities.

Four Lung Volumes

1. Tidal Volume (TV): Air inspired/expired with each quiet breath. Value: 500 ml.

2. Inspiratory Reserve Volume (IRV): Forcefully inspired air above TV. Value: 3000 ml.

3. Expiratory Reserve Volume (ERV): Forcefully exhaled air beyond quiet expiration. Value: 1100 ml.

4. Residual Volume (RV): Air remaining after maximal forceful expiration. Value: 1200 ml.

Four Lung Capacities

Capacities are combinations of volumes.

1. Inspiratory Capacity (IC): Maximum air inspired. Formula: IC = TV + IRV.

2. Functional Residual Capacity (FRC): Air in lungs at end of quiet expiration. Formula: FRC = ERV + RV. Value: 2300 ml. At FRC, opposing tendencies of thoracic cage (outward) and lungs (inward) are balanced (equilibrium position).

3. Vital Capacity (VC): Maximum air forcefully exhaled after maximal forceful inspiration. Formula: VC = IRV + TV + ERV. VC decreases in Restrictive Diseases.

4. Total Lung Capacity (TLC): Total air lungs can hold after maximal forceful inspiration. Formula: TLC = IRV + TV + ERV + RV or TLC = VC + RV.

• Maximal inspiration: Lungs at TLC.

• Maximal expiration: Lungs at RV.

• End of quiet expiration: Lungs at FRC.
  General Note: All lung volumes/capacities are 5-10% lower in females.

Timed Vital Capacity (TVC) and Other PFT Measures

TVC is also known as Forced Expiratory Volume (FEV).

• FEV1: Forced Expiratory Volume in 1 second.

• Normal: 80-84% of Vital Capacity.

• Clinical Significance: FEV1 < 70% of VC indicates obstruction (COPD).

• FEV2: 90-94% of VC. FEV3: 97% of VC.

FEV1/VC Ratio:

• Decreased FEV1/VC ratio: Indicates COPD.

• Increased FEV1/VC ratio: Indicates Restrictive Disease.

Other Expiratory Flow Tests:

• Peak Expiratory Flow Rate (PEFR): Velocity of expiratory airflow. Normal: 10-12 L/sec.

• Maximum Mid-Expiratory Flow Rate (MMFR): Airflow velocity during middle expiration. Normal: 3-3.25 L/sec. Decreased MMFR indicates small airway obstruction.

Ventilation-Perfusion (V/Q) Ratio

• Respiratory Minute Volume (RMV): Tidal Volume x Respiratory Rate = 6-8 L/min.

• Alveolar Ventilation (VA): (Tidal Volume - Dead Space Volume) x Respiratory Rate ≈ 4 L/min.

• V/Q Ratio: Alveolar Ventilation (VA) / Perfusion (Q). Normal Average Value: 0.8 (4 L/min VA / 5 L/min Q).

Gas Transport

This chapter focuses primarily on oxygen transport.

Oxygen Transport

If time is limited, O2 transport and Lung Volumes and Capacities are critical. Oxygen transport involves two stages: 1. From Atmospheric Air up to Blood. 2. In Blood up to Tissues.

1. Oxygen Transport from Atmospheric Air up to Blood

1. PO2 in Atmospheric Air at Sea Level: 159 mmHg (20% of 760 mmHg).

2. PO2 in Inspired Air (Dead Space): Humidification (PH2O = 47 mmHg) reduces PO2 to 149 mmHg.

3. PO2 in Alveolar Air: O2 diffuses out, CO2 diffuses in (PCO2 ≈ 46 mmHg), replacing O2. PO2 decreases to ≈ 104 mmHg.

• Alveolar Air Equation: PAO2 = [FiO2 * (PBarometric - PH2O)] - (PACO2 / R)

• R (Respiratory Quotient): 0.8 (CO2 produced / O2 consumed).

4. PO2 in Arterial Blood: Equilibration at 104 mmHg. Physiological shunt (deoxygenated bronchial venous blood mixing) slightly lowers arterial PO2 to 95 mmHg.

• Alveolo-Arterial Difference in PO2 (A-a DO2): PAO2 - PaO2 = 9-11 mmHg (normal).

• ADO2 increases in: Right-to-Left Shunt, Interstitial Lung Disease.

• ADO2 remains normal in: Hypoventilation.

Oxygen-Hemoglobin Dissociation Curve

Physiologists are obsessed with this concept.

1. Why is it S-shaped (Sigmoid)? Due to conformational changes in hemoglobin and positive cooperativity. Initially, Hb is in T-state (tense, low affinity); the first O2 binds slowly. Binding of first O2 induces R-state (relaxed, high affinity); subsequent O2 molecules bind very rapidly. Principle of Positive Cooperativity: Binding of one O2 molecule facilitates subsequent O2 binding.

2. Advantage of the S-shape?

• P50: PO2 at 50% Hb saturation (25-28 mmHg).

• Flat Upper Portion (PO2 100 to 60 mmHg): Hb "holds onto" its oxygen (high saturation), beneficial at lower ambient PO2.

• Steep Lower Portion (PO2 60 to 30 mmHg): Hb rapidly liberates large amount of oxygen to tissues for small PO2 drop.

3. What causes shifts of the curve to the right or left?

• A. Shift to the Right (Downwards Shift): Easy O2 liberation (Hb affinity for O2 decreases). Conditions: Increased H+ (acidosis, decreased pH), Increased PCO2, Increased Temperature, Increased 2,3-Bisphosphoglycerate (2,3-BPG). Context: Found in active tissues. Bohr Effect: Increased PCO2 (and H+) shifts curve right.

• B. Shift to the Left (Upwards Shift): Hb holds O2 more tightly (Hb affinity for O2 increases). Conditions: Decreased H+ (alkalosis, increased pH), Decreased PCO2, Decreased Temperature, Decreased 2,3-BPG, Fetal Hemoglobin (HbF), Carbon Monoxide (CO) Poisoning.

Regulation of Breathing

Regulated by both neural and chemical mechanisms.

1. Neural Regulation of Breathing

• Control Mechanisms: Voluntary (motor cortex) and Involuntary/Automatic (brainstem respiratory center).

• Ondine's Curse: Loss of automatic breathing (e.g., CO2 Narcosis, Bulbar Poliomyelitis).

• Respiratory Center: Four groups (2 medullary, 2 pontine).

• Medullary Centers: Dorsal Respiratory Group (DRG) (basic inspiratory rhythm, Inspiratory Ramp Signal), Pre-Bötzinger Complex (pacemaker), Ventral Respiratory Group (VRG) (controls forceful expiration muscles).

• Pontine Centers: Modulate medullary centers. Apneustic Center (promotes slow, deep breathing), Pneumotaxic Center (promotes rapid, shallow breathing).

• Effects of Lesions: Vagotomy: Prolonged inspiration. Transection at Lower Medulla: Spontaneous breathing stops. Transection in Upper Medulla: Irregular, jerky breathing. Mid-Pontine Transection: Apneustic Breathing (slow, deep, prolonged inspiration with spasm). Damage to Lower Pons (Pneumotaxic intact): Rapid and shallow breathing.

• Abnormal Breathing Patterns: Cheyne-Stokes, Kussmaul's (Diabetic Ketoacidosis), Biot's (meningitis).

2. Chemical Regulation of Breathing

Involves chemoreceptors monitoring blood and CSF chemistry.

Central Chemoreceptor Function

• Responds to H+ in cerebrospinal fluid (CSF) or brain interstitial fluid.

• H+ is formed from arterial CO2, which easily crosses the Blood-Brain Barrier. In CSF, CO2 + H2O -> H2CO3 -> H+.

• Effect: Increased CO2 and H+ stimulates ventilation; Decreased CO2 and H+ depresses ventilation.

Role of CO2 vs. O2 in Ventilation Drive

• CO2 is the primary chemical driver of ventilation, not oxygen.

• Reasoning: Hypoxia stimulates peripheral chemoreceptors, increasing breathing. But this causes CO2 and H+ wash-out, which then depresses central chemoreceptors, creating a paradoxical situation where hypoxia is present but ventilation is depressed.

Hypoxia: Definition and Types

Hypoxia is reduced oxygen content of the blood and reduced oxygen supply to the tissues.

Four Types:

1. Hypoxic Hypoxia: Reduced atmospheric O2 (e.g., high altitude) or O2 intake problems (heart/lung disease).

2. Anemic Hypoxia: Normal O2 intake but reduced O2-carrying capacity (anemia, carbon monoxide poisoning).

3. Stagnant Hypoxia (Ischemic/Hypokinetic): Normal O2 intake/carrying capacity, but reduced blood flow to tissues (heart failure).

4. Histotoxic Hypoxia: O2 delivery/transport normal, but tissues cannot utilize O2 due to poisoning (cyanide poisoning).

Hypoxia: PO2 Levels, Signs, Symptoms, and Treatment

• PO2 Levels: Normal in Anemic and Histotoxic hypoxia. Decreases in Hypoxic and Stagnant.

• Signs/Symptoms: Tiredness, weakness, easy fatigability, nausea, vomiting, breathlessness, dizziness, unconsciousness.

• Treatment: Underlying cause, Oxygen therapy (100% O2, Hyperbaric O2).

High Altitude: Problems and Initial Physiological Responses

• Primary Problem: Low barometric pressure, leading to hypoxic hypoxia.

• Initial Responses: Ventilation stimulated via peripheral chemoreceptors (breathlessness). O2 dissociation curve shifts to the left (initial ascent), then shifts to the right (during acclimatization). Pulmonary vasoconstriction.

• Severe Conditions: Acute Cerebral Edema, Acute Pulmonary Edema.

High Altitude Acclimatization Mechanisms

Key Changes:

1. Increased 2,3-Bisphosphoglycerate (BPG): Shifts O2 dissociation curve to the right, facilitating O2 delivery.

2. Increased sensitivity of peripheral chemoreceptors: Enhances hypoxia detection.

3. Decreased sensitivity of central chemoreceptors: Prevents CO2/H+ changes from affecting breathing.

Deep Sea Physiology: Problems of High Barometric Pressure

Problems from high barometric pressure at depth. Two conditions:

1. Nitrogen Narcosis: While at depth.

2. Caisson's Disease (Decompression Sickness / Dysbarism): Upon sudden ascent.

Nitrogen Narcosis: Mechanism and Symptoms

• Mechanism: High pressure dissolves nitrogen (high affinity for lipids) in body fluids and neuronal membranes, affecting CNS neurons.

• Symptoms: Similar to alcohol intoxication (confusion, memory loss, affected motor skills).

Caisson's Disease (Decompression Sickness): Mechanism and Symptoms

• Mechanism: Sudden decrease in pressure during rapid ascent causes dissolved nitrogen to expand and form bubbles in blood/tissues.

• Symptoms:

• Bends: Severe pain in joints/muscles.

• Chokes: Choking sensation (pulmonary embolism).

Caisson's Disease: Prevention and Treatment

• Prevention: Slow, controlled ascent (US Navy decompression tables).

• Curative Treatment: Decompression chambers (pressure increased then slowly decreased).