Lecture 9 ExPhys

Core Functions and Processes of the Respiratory System

The respiratory and cardiovascular systems function together as an integrated delivery system to supply oxygen ( $O_2$ ) to and remove carbon dioxide ( $CO_2$ ) from all body tissues.
This transport involves four distinct processes: - Pulmonary Ventilation (Breathing): The physical movement of air into and out of the lungs. - Pulmonary Diffusion: The exchange of $O_2$ and $CO_2$ specifically between the lungs and the blood. - Transport of $O_2$ and $CO_2$ : The movement of these gases via the bloodstream. - Capillary Diffusion: The exchange of $O_2$ and $CO_2$ between the capillary blood and metabolically active tissues.

Anatomy and Mechanics of Pulmonary Ventilation

Pulmonary Ventilation (Breeding): The process of moving air in and out of the lungs.
Air Pathway and Preparation: - Air enters through the nose (and occasionally the mouth). - The nasal passage allows air to be warmed, humidified, and filtered.
Functional Zones: - Transport Structures: The nasal cavity, pharynx, larynx, trachea, primary bronchi, and bronchioles serve primarily as conduits for air. - Respiratory Units: Gas exchange occurs only once air reaches the smallest units: the respiratory bronchioles and the alveoli.
Visual Components of the System: - Nasal cavity - Pharynx - Larynx - Trachea - Primary bronchi - Bronchiole - Alveoli (surrounded by a capillary network) - Lung

The Mechanics of Inspiration

Inspiration: An active process requiring muscle contraction.
Muscular Action: - Diaphragm: Contracts and flattens downward toward the abdomen. - External Intercostal Muscles: These move the ribs and sternum. The ribs swing upward and outward, while the sternum swings upward and forward.
Purpose: To expand the thoracic cavity as much as possible to increase lung volume and draw air in.
Boyle’s Law: States that there is an inverse relationship between pressure and volume at a constant temperature ( $P \propto \frac{1}{V}$ ).
Pressure Dynamics: - As lung volume increases, the intrapulmonary pressure (pressure inside the lungs) becomes lower than the atmospheric air pressure outside the body. - Because the respiratory tract is open to the outside, air rushes in to equalize this pressure difference.
Rest vs. Exercise: - At rest, the pressure change required for adequate ventilation is small. - During maximal respiratory effort, intrapulmonary pressure can decrease by $80-100\,mmHg$ . - During forced or labored breathing (as seen in heavy exercise), inspiration is assisted by accessory muscles: the scalenes, sternocleidomastoid, and pectorals.

The Mechanics of Expiration

Expiration at Rest: - A passive process involving the relaxation of inspiratory muscles and the elastic recoil of lung tissue. - The diaphragm returns to its normal position; the external intercostals relax, and the ribs/sternum return to resting positions. - Thoracic volume decreases, which increases lung pressure, forcing air out.
Forced Breathing (Active Expiration): - Becomes an active process during intense activity. - Internal Intercostal Muscles: Actively pull the ribs down and inward. - Respiratory Pump: Changes in intra-abdominal and intrathoracic pressure during forced breathing assist in returning venous blood back to the heart.

Pulmonary Volumes and Capacities

Spirometry: A clinical technique used to measure the volumes of air inspired and expired.
Clinical Utility: Used to diagnose respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD), and emphysema.
Standard Lung Volume Definitions: - Tidal Volume ( $TV$ or $V_T$ ): Volume of air inspired or expired with each breath during quiet breathing. - Residual Volume ( $RV$ ): Volume of air remaining in the lungs after a maximal expiration. - Expiratory Reserve Volume ( $ERV$ ): Maximal volume of air expired from the resting end-expiratory level. - Inspiratory Reserve Volume ( $IRV$ ): Maximal volume of air inspired from the resting end-inspiratory level.
Standard Lung Capacity Definitions: - Inspiratory Capacity ( $IC$ ): Maximal volume of air inspired from the end-expiratory level ( $IRV + TV$ ). - Vital Capacity ( $VC$ ): Maximal volume of air expired from the maximal inspiratory level. - Inspiratory Vital Capacity ( $IVC$ ): Maximal volume of air inspired from the maximal expiratory level. - Functional Residual Capacity ( $FRC$ ): Volume of air remaining in the lungs at the end-expiratory level ( $RV + ERV$ ). - Total Lung Capacity ( $TLC$ ): Volume of air in the lungs after maximal inspiration; the sum of all volume compartments.

Pulmonary Diffusion and Blood Flow

Functions of Pulmonary Diffusion: 1. Replenishes the oxygen supply in the blood (depleted at the tissue level during oxidative energy production). 2. Removes carbon dioxide from venous blood returning from systemic tissues.
Blood Flow Dynamics: - At rest, the lungs receive approximately $4-6\,L/min$ of blood flow, which matches the Cardiac Output ( $CO$ ). - Pulmonary circulation blood flow matches systemic circulation blood flow.
Pressure Gradations: - Mean pressure in the pulmonary artery: $\sim 15\,mmHg$ . - Mean pressure in the aorta: $\sim 95\,mmHg$ . - Pressure in the left atrium: $\sim 5\,mmHg$ .
Resistance Equation: $Pressure = \text{flow} \times \text{resistance}$ . - Resistance in the pulmonary vascular system is proportionally much lower than in systemic circulation because the pressure change across the system is lower.

Gas Exchange Physics

Partial Pressures of Gases: Each gas in a mixture exerts a pressure in proportion to its concentration.
Dalton’s Law: The total pressure of a gas mixture is the sum of the partial pressures ( $PP$ ) of the individual gases.
Composition of Atmospheric Air: - Nitrogen: $79.04\%$ - Oxygen: $20.93\%$ - Carbon Dioxide: $0.03\%$
Sea Level Atmospheric Pressure: $760\,mmHg$ .
Henry’s Law: Gases dissolve in liquids in proportion to their partial pressures (the pressure gradient between the blood and alveoli).
Fick’s Law: The rate of diffusion through tissue (like the respiratory membrane) is proportional to the surface area and the difference in partial pressure, while being inversely proportional to the tissue thickness.
Diffusion Constants: Each gas has a unique constant. $CO_2$ has a much lower diffusion constant than $O_2$ , allowing it to diffuse easily even with a smaller pressure gradient.

Gas Exchange Specifics at Alveoli and Muscles

Alveolar Gas Exchange: - Oxygen diffusion capacity: The rate at which $O_2$ diffuses from alveoli into the blood. - Resting rate: $\sim 21\,ml\,min^{-1}\,mmHg^{-1}$ . - Maximal exercise rate: May increase up to $3\times$ the resting rate (approximately $80\,ml/min$ ). - Exercise increases the gradient because venous blood returns severely desaturated and perfusion (blood flow) in the lungs is significantly higher.
Muscular Gas Exchange: - Oxygen is unloaded because the $PO_2$ in active muscle is substantially lower than in arterial blood. - Myoglobin: Transports oxygen within the muscle to the mitochondria for oxidative metabolism. - $CO_2$ exits cells via simple diffusion following the pressure gradient from muscle to capillaries.

Transport of Oxygen and Carbon Dioxide in the Blood

Oxygen Transport: - Primarily bound to hemoglobin ( $Hb$ ) in red blood cells; a small amount is dissolved in plasma. - Each molecule of hemoglobin can carry $4$ molecules of oxygen.
Oxyhemoglobin Dissociation Curve: - Flat Upper Portion: Large changes in $PO_2$ result in only small changes in hemoglobin saturation (occurs in lungs). - Steep Middle Portion: Small changes in $PO_2$ cause drastic changes in saturation (unloading phase in tissues).
Factors Affecting Unloading: - pH Level: Lower (more acidic) pH increases $O_2$ unloading (Bohr effect). - Temperature: Higher blood temperature increases $O_2$ unloading.
Carbon Dioxide Transport (3 Forms): 1. Bicarbonate Ions: The most common form; involves a reaction releasing $H^+$ , leading to the Bohr effect. 2. Dissolved in Plasma. 3. Bound to Hemoglobin: Does not compete with oxygen for binding sites.

Blood Oxygen-Carrying Capacity

Depends primarily on the blood hemoglobin content.
Hemoglobin Averages: - Men: $14-18\,g/100\,ml$ of blood. - Women: $12-16\,g/100\,ml$ of blood.
Oxygen Capacity: Each gram of $Hb$ combines with $\sim 1.34\,ml$ of oxygen.
Total Capacity: $\sim 16\text{ to }24\,ml$ of oxygen per $100\,ml$ of blood when fully saturated.
Saturation: At rest, blood is $98-99\%$ saturated. During high-intensity exercise, reduced contact time can lower this saturation, limiting performance.

Regulation of Pulmonary Ventilation

Involuntary Regulation: Motor neurons for respiratory muscles are regulated by centers in the brain stem (medulla oblongata and pons).
Voluntary Control: The cortex can override involuntary regulation.
Chemical Regulation: - Brain areas respond to changes in $CO_2$ and $H^+$ levels. - $PCO_2$ is the strongest stimulus for breathing regulation because $CO_2$ crosses the blood-brain barrier. - Chemoreceptors in the aortic arch and carotid arteries monitor $PO_2$ , $PCO_2$ , and $H^+$ concentrations.

Respiratory Responses to Acute Exercise

Onset of Exercise: Ventilation increases immediately, often before muscle contraction begins (anticipatory response).
Phase 1 (Initial): Mediated by neural factors from the brain's respiratory control centers.
Phase 2 (Gradual): Controlled by chemical changes in arterial blood (increased $CO_2$ and $H^+$ from metabolism).
Exercise-Ventilation Link: Ventilation increases in direct proportion to metabolic needs.
Post-Exercise: Ventilation remains elevated longer than metabolic demand to restore acid-base balance and address blood temperature and $PCO_2$ levels.

Breathing Irregularities during Exercise

Dyspnea: Shortness of breath. common in individuals with poor aerobic fitness who cannot adjust breathing to manage high arterial $PCO_2$ and $H^+$ ; often due to weak respiratory muscles fatiguing easily.
Exercise-Induced Asthma: Lower airway obstruction (coughing, wheezing, dyspnea) caused by exercise.
Exercise-Induced Bronchospasm: Measured by a reduction in FEV1 (forced expiratory volume in $1\,second$ ). Often caused by hyperventilation leading to airway dehydration or cold/polluted air.
Hyperventilation: Ventilation in excess of metabolic need (often due to anxiety). Results in decreased $PCO_2$ and increased blood pH, which can reduce ventilatory drive.
Valsalva Maneuver: A dangerous procedure involving closing the glottis while forcibly contracting abdominal and respiratory muscles. This increases intra-abdominal and intrathoracic pressure, trapping air in the lungs. It restricts venous return and can significantly decrease Cardiac Output ( $CO$ ) and arterial blood pressure if held.

Ventilation and Energy Metabolism

Terms: - $VE$ : Volume of air expired or ventilated. - $VO_2$ : Amount of oxygen consumed by tissues. - $VO_2max$ : Maximum oxygen utilization during intense effort.
Ventilatory Equivalent for Oxygen ( $VE/VO_2$ ): - Ratio between ventilated air and $O_2$ consumed. - Resting: $23-28\,L$ of air per liter of $O_2$ . - Intense Exercise: Can exceed $30\,L$ of air per liter of $O_2$ consumed.
Ventilatory Threshold: - The point where ventilation increases disproportionately to $O_2$ consumption; typically occurs at $55-70\%VO_2max$ . - Corresponds with the threshold where lactate begins to accumulate, leading to increased $H^+$ and $CO_2$ , which triggers chemoreceptors to spike ventilation.

Acid-Base Balance and Respiratory Regulation

Respiratory Limitations: At rest, respiratory muscles use $\sim 2\%$ of total oxygen uptake. This cost increases with exercise. However, maximal voluntary ventilation is usually much higher than ventilation at maximal exercise, meaning the system is rarely the sole limiting factor.
Acidosis: High-intensity exercise produces lactic acid and carbonic acid, which release $H^+$ ions, lowering pH.
Chemical Buffers: Basic substances that neutralize $H^+$ : 1. Bicarbonate ( $HCO_3^-$ ) 2. Inorganic Phosphates 3. Proteins
Bicarbonate Mechanism: $H^+ + HCO_3^- \rightarrow H_2CO_3 \text{ (carbonic acid)} \rightarrow H_2O + CO_2$ . - The $CO_2$ is exhaled; the $H^+$ is effectively removed.
pH and Lactate Levels: - Sprinting can lower muscle pH from $7.1\text{ to } < 6.7$ . - Resting lactate: $1.2\,mmol/kg$ .
Recovery: - By-products reach equilibrium in $5-10\,minutes$ post-exercise. - Reestablishing resting lactate levels can take hours; active recovery speeds this up. - $H^+$ levels return to normal within $40\,minutes$ via chemical buffering and respiration.