Mechanics of Breathing Physiology | Pulmonary Ventilation

Gas diffusion across the respiratory membrane is affected by partial pressure gradient, concentration gradient, gas solubility, and membrane thickness. Partial pressure is the pressure exerted by an individual gas in a mixture. In the atmosphere, oxygen partial pressure is approximately 159 mmHg, while in alveolar air it is approximately 104 mmHg. In deoxygenated blood, oxygen partial pressure is approximately 40 mmHg. These gradients drive oxygen diffusion from atmosphere to alveoli to blood to tissues. Carbon dioxide diffuses from tissues (45 mmHg) to blood (40 mmHg) to alveoli (40 mmHg) to atmosphere (0 mmHg).

Gas diffusion depends on the partial pressure gradient across the respiratory membrane. For oxygen: alveolar PO2 is approximately 104 mmHg, while blood PO2 is about 40 mmHg, creating an average gradient of 11 mmHg. For carbon dioxide: blood PCO2 is about 45 mmHg, while alveolar PCO2 is about 40 mmHg, creating a gradient of 5 mmHg. Despite the smaller CO2 gradient, CO2 diffuses efficiently because it is 20 times more diffusible than oxygen.

Gas exchange across the respiratory membrane is affected by membrane thickness, partial pressure gradients, and gas solubility. Carbon dioxide is 20-25 times more soluble in water than oxygen, allowing efficient diffusion despite smaller partial pressure gradients. Partial pressures: alveolar O2 (104 mmHg), pulmonary capillary O2 (40 mmHg), systemic arterial O2 (95 mmHg), tissue O2 (40 mmHg); alveolar CO2 (40 mmHg), pulmonary capillary CO2 (45 mmHg), systemic arterial CO2 (40 mmHg), tissue CO2 (45 mmHg).

Gas exchange follows partial pressure gradients: oxygen moves from alveoli (104 mmHg) to blood (40 mmHg), while carbon dioxide moves from blood (46 mmHg) to alveoli (40 mmHg). The respiratory membrane consists of alveolar epithelium, fused basement membranes, and capillary endothelium, creating an extremely thin barrier (0.5 micrometers) for efficient diffusion. Each alveolus is surrounded by dense capillary networks maximizing exchange surface area.

Gas diffusion across the alveolar-capillary membrane occurs according to partial pressure gradients. Oxygen partial pressure in the alveoli is approximately 104 mmHg, while in pulmonary capillary blood it is approximately 40 mmHg. This gradient drives oxygen diffusion from alveoli into the blood. Carbon dioxide partial pressure in pulmonary capillary blood is approximately 46 mmHg, while in alveoli it is approximately 40 mmHg, driving carbon dioxide diffusion from blood into alveoli.

Oxygen transport occurs primarily through hemoglobin binding (~97%) with small amounts dissolved in plasma (~3%). Carbon dioxide transport involves three mechanisms: dissolved in plasma (~7%), bound to hemoglobin as carbaminohemoglobin (~23%), and converted to bicarbonate ions (~70%). The conversion CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- is catalyzed by carbonic anhydrase in red blood cells. Hemoglobin also acts as a buffer by binding hydrogen ions, maintaining blood pH around 7.4 despite CO2 fluctuations.

Hemoglobin, a protein in red blood cells containing iron atoms, binds oxygen molecules to form oxyhemoglobin for transport from lungs to tissues. Carbon dioxide is transported in three forms: dissolved in plasma (7%), bound to hemoglobin as carbaminohemoglobin (23%), and converted to bicarbonate ions (70%). The bicarbonate form involves carbonic anhydrase enzyme catalyzing CO2 + H2O → H2CO3 → HCO3- + H+. Sodium bicarbonate maintains blood pH through the bicarbonate buffer system. The Hamburger shift (Bohr effect) describes how hemoglobin's oxygen affinity decreases with increased CO2 or decreased pH, ensuring oxygen release in metabolically active tissues.

Oxygen transport relies on hemoglobin binding in red blood cells, with oxygen dissociating from hemoglobin in tissues as partial pressure drops. Carbon dioxide transport involves three mechanisms: carbaminohemoglobin formation, bicarbonate ion production via carbonic anhydrase, and dissolved gas. The bicarbonate system creates carbonic acid, which dissociates into hydrogen ions and bicarbonate. Hydrogen ions bind to hemoglobin, acting as a buffer to prevent blood acidification. In the lungs, oxygen displaces hydrogen from hemoglobin, allowing carbon dioxide to be exhaled. This integrated system maintains blood pH and gas exchange efficiency.

Oxygen is transported in blood: 97% as oxyhemoglobin, 3% dissolved in plasma. Carbon dioxide is transported: 70% as bicarbonate ions (via carbonic anhydrase enzyme in RBCs), 23% as carbaminohemoglobin, 7% dissolved in plasma. Oxygen-hemoglobin association occurs in alveoli where O2 concentration is high. Dissociation occurs in tissues where O2 concentration is low. The oxygen-hemoglobin dissociation curve is sigmoid-shaped, showing relationship between O2 partial pressure and hemoglobin saturation. Association occurs with high O2 partial pressure, low CO2 partial pressure, low temperature, and low H+ concentration (high pH). Dissociation occurs with low O2 partial pressure, high CO2 partial pressure, high temperature, and high H+ concentration (low pH/acidic condition).

Ventilatory disorders are classified into obstructive and restrictive categories with distinct pathophysiological mechanisms. Obstructive diseases involve increased airway resistance from excessive secretions (chronic bronchitis), reduced radial traction from destroyed elastic tissue (emphysema), or airway hyperreactivity (asthma). Restrictive diseases involve parenchymal problems including inflammation, fibrosis, and collagen deposition causing stiff, less compliant lungs. Pulmonary function testing uses spirometry to measure tidal volume, inspiratory reserve, expiratory reserve, residual volume, vital capacity, and total lung capacity. In restrictive disease, all volumes decrease due to stiff lungs operating at smaller sizes. In obstructive disease, total lung capacity and functional residual capacity increase due to air trapping, while vital capacity decreases due to airway obstruction.

Obstructive lung diseases (e.g., asthma, COPD, emphysema) involve airflow resistance during exhalation, characterized by decreased FEV1/FVC ratio, elevated total lung capacity, residual volume, and peak flow; while restrictive lung diseases (e.g., pulmonary fibrosis, obesity, kyphoscoliosis) involve impaired lung expansion during inhalation, characterized by decreased total lung capacity, residual volume, and peak flow with normal or elevated FEV1/FVC ratio.

This video explains the fundamental differences between obstructive and restrictive lung diseases, covering pathophysiology, clinical presentation, and treatment approaches. Obstructive diseases (COPD, asthma, bronchitis) trap air in the lungs, causing barrel chest and reduced FEV1, while restrictive diseases (ARDS, pulmonary fibrosis, pneumonia) limit lung expansion and decrease compliance. ARDS progresses through exudative, proliferative, and fibrotic stages, with severity classified by P/F ratio. Treatment approaches differ significantly, with COPD requiring cautious oxygen administration due to CO2 retention, while ARDS requires mechanical ventilation with lung-protective strategies.

Obstructive lung diseases (asthma, chronic bronchitis, emphysema, and bronchiectasis collectively called COPD) involve difficulty getting air out of the lungs despite being able to get air in. Restrictive lung diseases (such as pulmonary fibrosis) involve difficulty getting air in (restricted from filling) but air can be expelled normally. Emphysema specifically shows increased compliance (easy to expand) but decreased elastic recoil (cannot expel air effectively), making it an obstructive disease.

Obstructive lung diseases (such as asthma, COPD, and bronchitis) obstruct airways, causing incomplete expiration and a decreased FEV1/FVC ratio on flow-volume loops, while restrictive lung diseases (such as fibrosis, sarcoidosis, and lobectomy) limit lung expansion, resulting in proportionally reduced volumes but maintaining the same FEV1/FVC ratio; mixed syndromes combine both patterns with decreased FEV1, decreased FVC, increased residual volume, and increased FEV1/FVC ratio.

Neural regulation of respiration involves respiratory centers in the brainstem: the respiratory rhythm center in the medulla oblongata generates the basic breathing rhythm, and the pneumotaxic center in the pons modulates this function by limiting inspiration duration. Chemical regulation involves chemoreceptors: central chemoreceptors in the medulla are sensitive to CO2 and H+ in cerebrospinal fluid, while peripheral chemoreceptors in carotid and aortic bodies detect changes in blood gases and pH.

The respiratory system is regulated by chemoreceptors that monitor blood gas levels: central chemoreceptors in the medulla oblongata detect changes in arterial CO2 and pH by responding to hydrogen ion concentration (formed when CO2 crosses the blood-brain barrier and reacts with water), while peripheral chemoreceptors in the carotid and aortic bodies detect decreases in arterial oxygen partial pressure (PaO2) through oxygen-sensitive potassium channels; both types of receptors send signals to respiratory centers in the brainstem, which then increase ventilation to maintain optimal blood gas concentrations.

Respiration is regulated by neural and chemical mechanisms. Neural regulation involves two brain centers: the respiratory rhythm center (RRC) in the medulla oblongata, which generates the basic breathing rhythm controlling inspiration and expiration, and the pneumotaxic center in the pons, which moderates RRC function and controls breathing rate. Chemical regulation involves chemoreceptors detecting blood gases: central chemoreceptors in the medulla detect CO2 levels in cerebrospinal fluid, while peripheral chemoreceptors in carotid bodies, aortic bodies, and the medulla detect increased CO2, decreased O2, and decreased pH, signaling respiratory centers to increase breathing rate.

Respiration is regulated by two mechanisms: (1) Neural regulation - involves the respiratory rhythm center in medulla oblongata and pneumotaxic center in pons, and (2) Chemical regulation - involves chemoreceptors (carotid body, carotid artery, aortic body) that detect changes in pH and CO2 levels.

Respiration is controlled by respiratory centers in the brainstem: medulla oblongata (inspiratory and expiratory centers) and pons (pneumotaxic and apneustic centers). Chemoreceptors monitor blood gases: central chemoreceptors respond to CSF pH changes from CO2, peripheral chemoreceptors in carotid and aortic bodies respond to arterial PO2, PCO2, and pH. Increased CO2 and decreased pH stimulate ventilation more sensitively than hypoxia. The respiratory rhythm is generated by alternating firing of inspiratory and expiratory neurons.