Respiration is the process by which the respiratory system facilitates the exchange of gases, primarily oxygen and carbon dioxide, between the body and the environment.
The mechanism of respiration can be divided into four main processes: ventilation, external respiration, internal respiration, and cellular respiration.
1. Ventilation:
Ventilation refers to the mechanical act of breathing, which involves inhalation (breathing in) and exhalation (breathing out).
a. Inhalation:
During inhalation, the diaphragm contracts and moves downward, while the external intercostal muscles contract and lift the ribcage up and outward.
This increases the volume of the thoracic cavity, creating a negative pressure that draws air into the lungs through the nose or mouth, pharynx, larynx, trachea, bronchi, and bronchioles, ultimately reaching the alveoli.
b. Exhalation:
During exhalation, the diaphragm and external intercostal muscles relax, causing the thoracic cavity to decrease in volume.
This creates a positive pressure within the lungs, forcing air out through the same path it entered.
2. External respiration:
This refers to the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries.
Oxygen from the air diffuses across the thin alveolar walls and capillary endothelium, binding to haemoglobin in red blood cells.
Simultaneously, carbon dioxide from the blood diffuses into the alveoli, where it can be expelled during exhalation.
3. Internal respiration:
This is the exchange of gases between the systemic capillaries and the body's tissues.
Oxygen-rich blood is transported from the lungs via the pulmonary veins to the left side of the heart, which pumps it through the systemic circulation.
Oxygen diffuses from the systemic capillaries into the tissues, while carbon dioxide diffuses from the tissues into the capillaries.
The carbon dioxide-rich blood is then returned to the right side of the heart and pumped back to the lungs through the pulmonary arteries.
4. Cellular respiration:
This is the process by which cells use oxygen to produce energy in the form of adenosine triphosphate (ATP) from nutrients, primarily glucose.
This process takes place in the mitochondria and has three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle or TCA cycle), and the electron transport chain.
As a by-product of cellular respiration, carbon dioxide is produced and must be removed from the cells and transported back to the lungs for elimination.