Alveoli are the tiny air sacs where the exchange of oxygen and carbon dioxide takes place. Their thin walls and extensive capillary network facilitate efficient diffusion of gases.

The diaphragm is the main muscle that contracts during inhalation, moving downward to increase the volume of the thoracic cavity. This action creates a negative pressure that draws air into the lungs.

The respiratory system is primarily responsible for exchanging oxygen and carbon dioxide between the environment and the bloodstream. This gas exchange is crucial for cellular respiration and overall metabolism.

Gas exchange takes place in the alveolar sacs, where the thin walls allow oxygen to diffuse into the blood and carbon dioxide to be expelled. This process is essential for maintaining the body's oxygen supply.

External respiration refers to the exchange of gases between the alveoli and the pulmonary capillaries. This process is distinct from internal respiration, where oxygen is exchanged between blood and body cells.

During inhalation, the diaphragm contracts and moves downward, which increases the volume of the thoracic cavity. This expansion generates a negative pressure that draws air into the lungs.

Under normal conditions, the partial pressure of oxygen in the alveoli is around 100 mm Hg. This gradient is necessary to drive the diffusion of oxygen into the blood.

Nitrogen constitutes about 78% of the air we breathe, making it the dominant gas in the atmosphere. While it does not participate directly in gas exchange, its presence helps establish the partial pressures of other gases.

The intercostal muscles play a key role by assisting in the expansion of the rib cage during inhalation. Their contraction helps increase the thoracic volume, which facilitates the entry of air into the lungs.

An increase in alveolar carbon dioxide lowers blood pH, which in turn stimulates the respiratory center to increase the rate and depth of breathing. This is a reflex response aimed at expelling excess CO2 and restoring homeostasis.

The pleural cavity is lined by two membranes with a thin layer of fluid between them, which creates a negative pressure. This negative pressure keeps the lungs adhered to the chest wall and prevents collapse during the breathing cycle.

The nasal cavity conditions the air as it is inhaled, warming and humidifying it and filtering out particulates. This process protects the respiratory tract and ensures that the air reaching the lungs is adequately prepared for gas exchange.

Approximately 70% of carbon dioxide is transported in the blood as bicarbonate ions after reacting with water under the catalysis of carbonic anhydrase. This conversion facilitates the efficient removal of CO2 from tissues.

The trachea is lined with pseudostratified ciliated columnar epithelium, which traps inhaled particles and moves mucus upward toward the throat. This ciliary action helps keep the airways clear of debris and pathogens.

Gas exchange in the alveoli occurs by diffusion, which is driven by the concentration gradients of oxygen and carbon dioxide between the alveolar air and the blood. This passive process requires no energy and ensures efficient transfer of gases.

At high altitudes, the reduced atmospheric pressure lowers the partial pressure of oxygen in the alveoli. This decrease triggers hyperventilation as the body attempts to maintain adequate oxygenation.

Surfactant is a substance that reduces surface tension within the alveoli, preventing their collapse during exhalation. Without adequate surfactant, the alveoli can collapse, leading to atelectasis and reduced efficiency in gas exchange.

Pulmonary fibrosis involves the scarring and stiffening of lung tissues, which leads to decreased lung compliance. This reduction in elasticity makes it harder for the lungs to expand during inhalation, impairing overall respiratory function.

The Bohr effect describes how increases in carbon dioxide and decreases in pH lead to a reduction in hemoglobin's affinity for oxygen. This facilitates the release of oxygen in tissues where it is needed for cellular metabolism.

Internal respiration is the process in which oxygen diffuses from the capillaries into body tissues, while carbon dioxide moves from the tissues into the blood. This critical exchange supports cellular metabolism and energy production.