Respiratory Physiology: Regulation, Mechanics, and Gas Exchange

Respiratory Physiology

Homeostatic Regulation of Ventilation

The body maintains stable levels of oxygen (O2) and carbon dioxide (CO2) in the blood through a series of reflexes and feedback mechanisms. These processes involve specialized receptors, neural control centers, and effector pathways that adjust ventilation as needed.

• Peripheral Chemoreceptors: Located in the carotid and aortic bodies, these receptors detect changes in arterial PO2, PCO2, and pH. They primarily respond to low O2 (hypoxemia) and increased CO2 (hypercapnia).

• Central Chemoreceptors: Found in the medulla oblongata, these receptors are sensitive to changes in the pH of cerebrospinal fluid, which reflects arterial CO2 levels.

• Brainstem Control Centers: The medullary respiratory centers (dorsal and ventral respiratory groups) and the pontine centers regulate the rhythm and depth of breathing.

• Efferent Pathways: Motor neurons in the spinal cord send signals via the phrenic and intercostal nerves to the diaphragm and intercostal muscles, controlling inspiration and expiration.

• Negative Feedback Reflex: Increased arterial CO2 or decreased O2 stimulates chemoreceptors, which signal the respiratory centers to increase ventilation, restoring normal gas levels.

• Ventilation-Perfusion Ratio (V/Q): The ratio of alveolar ventilation to pulmonary blood flow; optimal gas exchange occurs when ventilation and perfusion are matched.

• Local Feedback in Airways:

  • Bronchiole Smooth Muscle: Local CO2 levels cause bronchodilation (high CO2) or bronchoconstriction (low CO2), optimizing airflow to alveoli.

  • Pulmonary Arteriole Smooth Muscle: Low alveolar O2 causes vasoconstriction, redirecting blood to better-ventilated alveoli.

Pressure Changes During Ventilation

Ventilation is driven by pressure gradients created by changes in lung volume. Several physical laws and physiological properties govern these changes.

• Boyle's Law: The pressure of a gas is inversely proportional to its volume at constant temperature: $P_1 V_1 = P_2 V_2$

• Pressure Changes:

  • Alveolar (Intrapulmonary) Pressure: Pressure within the alveoli; falls below atmospheric pressure during inspiration, rises during expiration.

  • Thoracic (Intrapleural) Pressure: Pressure within the pleural cavity; always negative relative to alveolar pressure, helping keep lungs inflated.

  • Transpulmonary Pressure: Difference between alveolar and intrapleural pressure; determines lung expansion.

• Airflow Equation: Airflow (F) through the airways is determined by the pressure gradient (ΔP) and resistance (R): $F = \frac{\Delta P}{R}$

• Lung Properties:

  • Compliance: The ease with which lungs expand; high compliance means lungs expand easily.

  • Elasticity: The tendency of lungs to return to their original size after stretching.

  • Surface Tension: The force exerted by fluid lining the alveoli; surfactant reduces surface tension, preventing alveolar collapse.

• Restrictive vs. Obstructive Disorders:

  • Restrictive: Reduced lung compliance (e.g., fibrosis); decreased lung volumes.

  • Obstructive: Increased airway resistance (e.g., asthma, COPD); difficulty exhaling.

• Airway Resistance: Controlled by airway diameter, smooth muscle tone, and mucus; increased resistance reduces airflow.

• Lung Volumes and Capacities:

  • Tidal Volume (TV): Air moved per breath (~500 mL).

  • Inspiratory Reserve Volume (IRV): Extra air inhaled after normal inspiration.

  • Expiratory Reserve Volume (ERV): Extra air exhaled after normal expiration.

  • Residual Volume (RV): Air remaining after maximal exhalation.

  • Capacities: Combinations of volumes (e.g., Vital Capacity = TV + IRV + ERV).

• Minute Ventilation vs. Alveolar Ventilation:

  • Minute Ventilation (VE): Total air entering/leaving lungs per minute: $VE = TV \times f$ (f = breaths/min).

  • Alveolar Ventilation (VA): Air reaching alveoli per minute: $VA = (TV - dead\ space) \times f$

• Calculations: Use the above formulas to determine ventilation rates.

Pulmonary and Systemic Gas Exchange

Gas exchange occurs across the respiratory membrane in the lungs and between blood and tissues in the systemic circuit. The process is governed by partial pressure gradients and membrane properties.

• Respiratory Membrane: Composed of alveolar epithelium, capillary endothelium, and their fused basement membranes; thin for efficient diffusion.

• Factors Affecting Diffusion: Surface area, membrane thickness, partial pressure gradients, and gas solubility.

• Dalton’s Law: The total pressure of a gas mixture equals the sum of the partial pressures of each component: $P_{total} = P_1 + P_2 + ... + P_n$

• Partial Pressures (approximate values):

  • Alveolar Air: PO2 ≈ 104 mmHg, PCO2 ≈ 40 mmHg

  • Arterial Blood: PO2 ≈ 95-100 mmHg, PCO2 ≈ 40 mmHg

  • Tissues: PO2 ≈ 40 mmHg, PCO2 ≈ 45 mmHg

  • Venous Blood: PO2 ≈ 40 mmHg, PCO2 ≈ 45 mmHg

• Pulmonary Exchange: O2 diffuses from alveoli to blood; CO2 diffuses from blood to alveoli.

• Systemic Exchange: O2 diffuses from blood to tissues; CO2 diffuses from tissues to blood.

Oxygen and Carbon Dioxide Transport in Blood

Oxygen and carbon dioxide are transported in the blood by different mechanisms, involving both physical and chemical processes.

• Hemoglobin (Hb): A protein in red blood cells with four heme groups, each binding one O2 molecule; facilitates O2 transport.

• Hemoglobin-Oxygen Dissociation Curve: Shows the relationship between PO2 and hemoglobin saturation; sigmoidal shape due to cooperative binding.

• Key Terms:

  • Affinity: Strength of Hb for O2.

  • Loading: O2 binding to Hb (in lungs).

  • Unloading: O2 release from Hb (in tissues).

  • Leftward Shift: Increased affinity (e.g., low temperature, high pH).

  • Rightward Shift: Decreased affinity (e.g., high temperature, low pH, high CO2).

• O2 Saturation: Percentage of Hb binding sites occupied by O2; measured by pulse oximetry.

• CO2 Transport Forms:

  1. Dissolved in plasma (~7%)

  2. Bound to hemoglobin as carbaminohemoglobin (~23%)

  3. As bicarbonate ion (HCO3-) (~70%)

• CO2 Transport Equation:

  $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

  • CO2: Carbon dioxide

  • H2O: Water

  • H2CO3: Carbonic acid

  • H+: Hydrogen ion

  • HCO3-: Bicarbonate ion

• Role of Carbonic Anhydrase: Enzyme in red blood cells that catalyzes the conversion of CO2 and H2O to carbonic acid, speeding up the reaction.

• CO2 Exchange in Systemic Capillaries and Veins: CO2 enters blood from tissues, converted to HCO3- in RBCs, transported in plasma.

• CO2 Exchange in Pulmonary Capillaries and Arteries: HCO3- re-enters RBCs, converted back to CO2, which diffuses into alveoli for exhalation.

• Exercise Effects on Hb-O2 Curve: Strenuous exercise increases temperature, CO2, and acidity, causing a rightward shift and promoting O2 unloading to tissues.

Summary Table: Forms of Gas Transport in Blood

Gas	Transport Form	Percentage
Oxygen (O2)	Bound to hemoglobin	~98.5%
Oxygen (O2)	Dissolved in plasma	~1.5%
Carbon Dioxide (CO2)	Dissolved in plasma	~7%
Carbon Dioxide (CO2)	Bound to hemoglobin (carbaminohemoglobin)	~23%
Carbon Dioxide (CO2)	As bicarbonate ion (HCO3-)	~70%

Example: During exercise, increased CO2 production and temperature in muscles cause a rightward shift in the hemoglobin-oxygen dissociation curve, enhancing O2 delivery to active tissues.

Additional info: The above notes expand on the provided learning objectives with academic context, definitions, and examples to ensure a comprehensive understanding suitable for college-level Anatomy & Physiology students.