Respiration is the process by which organisms exchange gases with their environment, primarily involving the intake of oxygen (O2) and the release of carbon dioxide (CO2). In humans, respiration consists of two main processes: external respiration, which occurs in the lungs, and internal respiration, which occurs at the cellular level in tissues throughout the body. The mechanism of respiration involves several key steps:
1. Pulmonary Ventilation (Breathing):
– Inspiration (Inhalation):
– Inspiration is the process of inhaling air into the lungs.
– During inspiration, the diaphragm and external intercostal muscles contract.
– The diaphragm moves downward, and the rib cage expands, increasing the volume of the thoracic cavity.
– This increase in volume decreases the pressure inside the lungs, creating a pressure gradient that causes air to flow into the lungs from the atmosphere.
  – The alveolar pressure becomes lower than atmospheric pressure, causing air to rush into the lungs until equilibrium is reached.
– Expiration (Exhalation):
– Expiration is the process of exhaling air out of the lungs.
– During expiration, the diaphragm and external intercostal muscles relax.
– The diaphragm moves upward, and the rib cage recoils, decreasing the volume of the thoracic cavity.
– This decrease in volume increases the pressure inside the lungs, creating a pressure gradient that causes air to flow out of the lungs into the atmosphere.
– The alveolar pressure becomes higher than atmospheric pressure, causing air to be expelled from the lungs until equilibrium is reached.
2. Gas Exchange in the Lungs (External Respiration):
– Alveolar Gas Exchange:
– Gas exchange occurs between the air in the alveoli and the pulmonary capillaries surrounding the alveoli.
– Oxygen (O2) from the inhaled air diffuses across the thin alveolar membrane into the bloodstream, where it binds to hemoglobin in red blood cells.
– Carbon dioxide (CO2), a waste product of cellular metabolism, diffuses from the bloodstream into the alveoli to be exhaled.
– Transport of Gases:
– Oxygen-rich blood is carried from the lungs to the heart by the pulmonary veins and distributed to tissues throughout the body via the systemic circulation.
– Carbon dioxide-rich blood returns to the lungs via the pulmonary arteries to be exhaled during expiration.
3. Cellular Respiration (Internal Respiration):
– Tissue Gas Exchange:
– Oxygen-rich blood is delivered to tissues via systemic arteries.
– At the cellular level, oxygen diffuses from the capillaries into the tissue cells, where it is used in cellular respiration to produce energy (ATP) through aerobic metabolism.
– Carbon dioxide, a byproduct of cellular metabolism, diffuses from the tissue cells into the capillaries to be transported back to the lungs for exhalation.
4. Regulation of Respiration:
– Central Control:
– Respiration is regulated by the respiratory center in the brainstem, particularly the medulla oblongata and pons.
– The medullary respiratory center sets the basic rhythm of breathing by generating rhythmic impulses to the respiratory muscles.
– The pons helps regulate the depth and rate of breathing by modifying the activity of the medullary respiratory center.
– Chemoreceptors:
– Chemoreceptors located in the carotid bodies and aortic bodies sense changes in blood pH, oxygen levels (partial pressure of oxygen, or PO2), and carbon dioxide levels (partial pressure of carbon dioxide, or PCO2).
– Increased levels of carbon dioxide or decreased levels of oxygen or pH stimulate chemoreceptors to send signals to the respiratory center to adjust breathing rate and depth accordingly.
5. Other Factors Affecting Respiration:
– Lung Compliance and Elasticity:
– The Lung compliance refers to the ease with which the lungs can expand and contract.
– Lung elasticity refers to the tendency of the lungs to return to their resting volume after being stretched during inspiration.
– Changes in lung compliance and elasticity can affect respiratory function, such as in conditions like pulmonary fibrosis or emphysema.
– Airway Resistance:
– Airway resistance refers to the resistance encountered by airflow in the air passages.
– Factors affecting airway resistance include the diameter of the airways, smooth muscle tone, and the presence of obstructions.
– Increased airway resistance can impede airflow and affect breathing efficiency.
Understanding the mechanism of respiration is essential for comprehending how oxygen is delivered to tissues and carbon dioxide is removed from the body, vital processes for sustaining life. Disruption or dysfunction in any part of the respiratory system can impair gas exchange and lead to respiratory disorders or respiratory failure.
