Human Respiratory System

• Breathing involves the exchange of oxygen (O2) and carbon dioxide (CO2) between an organism and its environment.

Respiratory Organs

• Various organisms use different structures for gas exchange, depending on their habitat and complexity.
• Invertebrates like earthworms use moist skin for cutaneous respiration.
• Insects have a tracheal system with spiracles for direct gas exchange with tissues.
• Aquatic arthropods and mollusks employ gills.
• Vertebrates, including humans, primarily use lungs.

Human Respiratory System

• The human respiratory system includes the nasal passages, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
• Air enters through the nostrils, is filtered by hairs, and warmed and moistened in the nasal passages.
• The pharynx is a common passage for air and food.
• The larynx (voice box) contains the vocal cords.
• The trachea divides into the right and left bronchi, which enter the lungs.
• Bronchi further divide into smaller bronchioles.
• The bronchioles terminate in alveoli, which are air-filled sacs.

Alveoli

• Alveoli are the primary sites of gas exchange in the lungs.
• They are thin-walled and highly vascularized to facilitate diffusion of O2 and CO2.
• The alveolar surface area is very large, maximizing gas exchange.
• Type I alveolar cells form the structure of the alveolar wall.
• Type II alveolar cells secrete surfactant, which reduces surface tension and prevents alveolar collapse.

Mechanism of Breathing

• Breathing involves two phases: inspiration (inhalation) and expiration (exhalation).
• Inspiration is an active process driven by the contraction of the diaphragm and external intercostal muscles.
• Contraction of the diaphragm increases the volume of the thoracic cavity in the vertical axis.
• Contraction of the external intercostal muscles lifts the ribs and sternum, increasing the volume of the thoracic cavity in the horizontal axis.
• The increase in thoracic volume reduces the intra-pulmonary pressure, causing air to flow into the lungs.
• Expiration is typically a passive process.
• The diaphragm and intercostal muscles relax, decreasing the thoracic volume.
• The intra-pulmonary pressure increases, forcing air out of the lungs.
• Forceful expiration involves the abdominal and internal intercostal muscles.

Lung Volumes and Capacities

• Tidal Volume (TV): The volume of air inhaled or exhaled during normal breathing (approximately 500 mL).
• Inspiratory Reserve Volume (IRV): The additional volume of air that can be inhaled after a normal inspiration.
• Expiratory Reserve Volume (ERV): The additional volume of air that can be exhaled after a normal expiration.
• Residual Volume (RV): The volume of air remaining in the lungs after a maximal expiration.
• Inspiratory Capacity (IC): The total volume of air a person can inhale after a normal expiration (TV + IRV).
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal expiration (ERV + RV).
• Vital Capacity (VC): The maximum volume of air a person can exhale after a maximal inspiration (TV + IRV + ERV).
• Total Lung Capacity (TLC): The total volume of air the lungs can hold (TV + IRV + ERV + RV).

Exchange of Gases

• Gas exchange occurs primarily in the alveoli and involves the diffusion of O2 and CO2 across the respiratory membrane.
• The respiratory membrane consists of the alveolar epithelium, the capillary endothelium, and their fused basement membranes.
• Diffusion is driven by partial pressure gradients.
• The partial pressure of O2 (PO2) is higher in the alveoli than in the blood, so O2 diffuses into the blood.
• The partial pressure of CO2 (PCO2) is higher in the blood than in the alveoli, so CO2 diffuses into the alveoli.
• Solubility of the gases also influences diffusion rates.

Transport of Gases

• Oxygen is transported in the blood in two forms: dissolved in plasma and bound to hemoglobin.
• Most oxygen (97%) is transported bound to hemoglobin in red blood cells.
• Hemoglobin is a protein that contains four heme groups, each of which can bind one molecule of O2.
• The binding of O2 to hemoglobin is influenced by PO2, temperature, PCO2, and pH.
• Carbon dioxide is transported in the blood in three forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions.
• About 70% of CO2 is transported as bicarbonate ions (HCO3-) in the plasma.
• CO2 reacts with water to form carbonic acid (H2CO3), which dissociates into H+ and HCO3-.
• This reaction is catalyzed by the enzyme carbonic anhydrase in red blood cells.
• About 20-25% of CO2 is transported bound to hemoglobin as carbaminohemoglobin.
• The remaining CO2 is dissolved in plasma.

Regulation of Breathing

• Breathing is regulated by neural and chemical mechanisms.
• The respiratory center in the brainstem (medulla oblongata and pons) controls the rate and depth of breathing.
• The medulla oblongata contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG).
• The DRG primarily controls inspiration, while the VRG controls both inspiration and expiration, especially during forced breathing.
• The pons contains the pneumotaxic center and the apneustic center, which modulate the activity of the medullary centers.
• Chemical regulation involves chemoreceptors that detect changes in blood PCO2, pH, and PO2.
• Central chemoreceptors in the medulla oblongata are sensitive to changes in pH and PCO2 in the cerebrospinal fluid.
• Peripheral chemoreceptors in the carotid bodies and aortic bodies are sensitive to changes in PCO2, pH, and PO2 in the blood.
• Increased PCO2 or decreased pH stimulates the respiratory center, increasing the rate and depth of breathing, conversely decreased PCO2 or increased pH inhibits the respiratory center, decreasing the rate and depth of breathing.
• Hypoxia (low PO2) also stimulates breathing, but to a lesser extent than hypercapnia (high PCO2).

Disorders of the Respiratory System

• Asthma: Chronic inflammatory disease of the airways, causing bronchospasm and difficulty breathing.
• Emphysema: Destruction of the alveoli, reducing the surface area for gas exchange.
• Bronchitis: Inflammation of the bronchi, causing coughing and mucus production.
• Pneumonia: Infection of the lungs, causing inflammation and fluid accumulation.
• Cystic Fibrosis: Genetic disorder causing build-up of thick sticky mucus in the lungs, making it hard to breathe.
• Lung Cancer: Uncontrolled growth of abnormal cells in the lungs.