Respiratory Physiology: Homeostatic Control, Ventilation, Gas Exchange, and Gas Transport

Respiratory Physiology

Homeostatic Control 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 chemoreceptors, control centers in the brainstem, and efferent pathways to respiratory muscles.

• 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 and high CO2 levels, sending signals to the brainstem to adjust ventilation.

• Central Chemoreceptors: Found in the medulla oblongata, these receptors are sensitive to changes in the pH of cerebrospinal fluid, which reflects arterial CO2 levels. They play a major role in regulating ventilation in response to hypercapnia (elevated CO2).

• Control Centers in the Brainstem: The medullary respiratory centers (dorsal and ventral respiratory groups) and the pontine respiratory group coordinate 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 the mechanics of ventilation.

• Negative Feedback Reflex: Changes in arterial PO2 and PCO2 are detected by chemoreceptors, which adjust ventilation to restore homeostasis.

• 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 Control:

  • Bronchiole Smooth Muscle: Local increases in CO2 cause bronchodilation, improving airflow to under-ventilated areas.

  • 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, as described by Boyle's Law. Several pressures are important in this process.

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

• Alveolar (Intrapulmonary) Pressure: The pressure within the alveoli; changes during inspiration and expiration to drive airflow.

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

• Transpulmonary Pressure: The difference between alveolar and intrapleural pressure; determines lung expansion. Equation: $P_{tp} = P_{alv} - P_{ip}$

• Factors Affecting Airflow: Airflow through the airways is determined by pressure differences and resistance. Equation: $F = \frac{\Delta P}{R}$ Where F = airflow, ΔP = pressure gradient, R = resistance.

• Compliance: The ease with which the lungs and thoracic wall can expand. High compliance means easy expansion; low compliance indicates stiffness.

• Elasticity: The tendency of the lungs to return to their original size after being stretched.

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

• Restrictive vs. Obstructive Lung 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 production.

• Lung Volumes and Capacities:

  • Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath.

  • Inspiratory Reserve Volume (IRV): Additional air that can be inhaled after a normal inspiration.

  • Expiratory Reserve Volume (ERV): Additional air that can be exhaled after a normal expiration.

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

  • Vital Capacity (VC): TV + IRV + ERV.

  • Total Lung Capacity (TLC): VC + RV.

• Minute Ventilation vs. Alveolar Ventilation:

  • Minute Ventilation (VE): Total volume of air entering or leaving the lungs per minute. Equation: $V_E = \text{Tidal Volume} \times \text{Respiratory Rate}$

  • Alveolar Ventilation (VA): Volume of air reaching alveoli per minute (subtracting dead space). Equation: $V_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$

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; site of gas exchange.

• 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 gas. Equation: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$ Partial pressure of a gas: $P_{\text{gas}} = \text{Fraction of gas} \times \text{Total pressure}$

• Partial Pressures in Different Compartments:

  Location	PO2 (mmHg)	PCO2 (mmHg)
  Alveolar Air	~104	~40
  Arterial Blood	~95	~40
  Tissues	<40	>45
  Venous Blood	~40	~45

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

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

Oxygen and Carbon Dioxide Transport in Blood

O2 and CO2 are transported in the blood by different mechanisms, involving hemoglobin and chemical reactions.

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

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

• Key Terms:

  • Affinity: Strength of O2 binding to Hb.

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

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

  • Leftward Shift: Increased affinity (e.g., low temperature, low CO2).

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

• 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: $\mathrm{CO_2} + \mathrm{H_2O} \leftrightarrow \mathrm{H_2CO_3} \leftrightarrow \mathrm{H^+} + \mathrm{HCO_3^-}$

• Role of Carbonic Anhydrase: Enzyme in red blood cells that catalyzes the conversion of CO2 and H2O to carbonic acid (H2CO3).

• CO2 Exchange in Systemic Capillaries and Veins: CO2 enters blood, forms HCO3-, and is transported to lungs.

• CO2 Exchange in Pulmonary Capillaries and Systemic Arteries: HCO3- is converted back to CO2 for exhalation.

• Effects of Strenuous Exercise: Increased temperature, CO2, and decreased pH shift the Hb-O2 curve rightward, enhancing O2 unloading to tissues.

Additional info: Academic context and equations have been expanded for clarity and completeness. Table values for partial pressures are standard physiological values.