The process of breathing, or respiration, is a fundamental physiological function essential for the survival of most living organisms. It facilitates the continuous supply of oxygen from the atmosphere to the body’s cells, where it is utilized for metabolic processes to generate energy. Simultaneously, it enables the removal of carbon dioxide, a harmful byproduct of these metabolic reactions. While simple organisms can exchange gases directly with their environment through diffusion, more complex animals have evolved specialized respiratory systems to efficiently manage this vital exchange.

The human respiratory system is a highly sophisticated apparatus designed for this purpose. The pathway of air begins at the external nostrils, leading into the nasal passage and pharynx, a common passage for both air and food. Air then travels through the larynx (the voice box) into the trachea, a tube supported by cartilaginous rings that prevent its collapse. The trachea bifurcates into two primary bronchi, which enter the right and left lungs, respectively. Inside the lungs, the bronchi further divide into a network of smaller bronchioles, culminating in tiny, thin-walled, sac-like structures called alveoli. It is within these millions of alveoli that the actual exchange of gases with the blood occurs.

The mechanism of breathing involves two distinct phases: inspiration (inhalation) and expiration (exhalation). These actions are driven by the creation of a pressure gradient between the lungs and the atmosphere, achieved through the coordinated movement of the diaphragm and the intercostal muscles situated between the ribs. During inspiration, an active process, the diaphragm contracts and flattens, while the external intercostal muscles lift the ribs upwards and outwards. This increases the volume of the thoracic cavity, causing the lungs to expand. The resulting decrease in intra-pulmonary pressure relative to atmospheric pressure forces air to rush into the lungs. Expiration is typically a passive process where the diaphragm and intercostal muscles relax, decreasing the thoracic volume, increasing the intra-pulmonary pressure, and forcing air out of the lungs.

The volume of air involved in these movements can be quantified by several respiratory volumes and capacities. Tidal Volume (TV) is the amount of air inhaled or exhaled during normal breathing. Inspiratory Reserve Volume (IRV) and Expiratory Reserve Volume (ERV) represent the additional volumes of air that can be forcibly inhaled and exhaled, respectively. Even after a forceful exhalation, some air, known as the Residual Volume (RV), remains in the lungs.

Gas exchange in the alveoli and the body tissues occurs through simple diffusion, driven by differences in the partial pressures of oxygen (pO₂) and carbon dioxide (pCO₂). In the alveoli, the pO₂ is high and the pCO₂ is low compared to the deoxygenated blood arriving from the tissues. This gradient facilitates the diffusion of oxygen from the alveoli into the blood and of carbon dioxide from the blood into the alveoli. At the tissue level, the opposite occurs: pO₂ is low and pCO₂ is high, causing oxygen to diffuse from the blood into the tissues and carbon dioxide to move from the tissues into the blood.

Once in the blood, these gases are transported throughout the body. Approximately 97% of oxygen is transported by binding to hemoglobin, a protein found in red blood cells, forming oxyhemoglobin. The binding and dissociation of oxygen from hemoglobin are influenced by factors like pO₂, pCO₂, and pH, a relationship described by the oxygen-hemoglobin dissociation curve. Carbon dioxide is transported in three main forms: about 70% is converted to bicarbonate ions, approximately 20-25% binds to hemoglobin to form carbaminohemoglobin, and a small fraction is dissolved directly in the blood plasma.

The rhythmic process of breathing is regulated by the nervous system. The primary control center is the respiratory rhythm center located in the medulla of the brain. Another center in the pons, the pneumotaxic center, can moderate the signals from the medulla to alter the breathing rate. Additionally, a chemosensitive area near the rhythm center and receptors in the aorta and carotid artery are highly sensitive to changes in blood pCO₂ and hydrogen ion concentration, adjusting the respiratory rate to maintain homeostasis.