The primary function of the respiratory system is to supply oxygen to the body tissues and remove carbon dioxide. The upper respiratory tract filters, warms, and humidifies incoming air before it reaches the delicate lower structures.
The lower respiratory tract extends from the trachea down into the dense tissue of the lungs. Air travels down the trachea, a rigid tube reinforced by C-shaped rings of hyaline cartilage that keep the airway open.
The trachea branches into the left and right main bronchi, which enter the lungs. Inside the lungs, these bronchi branch further into smaller secondary and tertiary bronchi, eventually becoming microscopic bronchioles. Unlike the trachea, bronchioles lack cartilage and rely on smooth muscle walls to regulate airflow resistance.
Gas exchange occurs within millions of tiny air sacs called alveoli. These clusters form the functional units of the lung parenchyma.
The respiratory membrane consists of the fused alveolar and capillary walls, measuring less than a micrometer thick to allow effortless diffusion.
Pulmonary ventilation is the physical movement of air into and out of the lungs, governed by Boyle's Law, which states that pressure and volume are inversely related.
During inspiration, the diaphragm contracts and flattens while external intercostal muscles pull the ribs upward and outward. This increases thoracic cavity volume, lowering intrapulmonary pressure below atmospheric pressure, causing air to rush in. Expiration is normally a passive process where the inspiratory muscles relax, thoracic volume decreases, and internal pressure rises above atmospheric pressure, forcing air out.
Gas exchange occurs across the respiratory membrane via simple diffusion, driven by partial pressure gradients. Oxygen moves from the alveoli into the deoxygenated pulmonary capillary blood, while carbon dioxide moves in the opposite direction.
Once in the blood, about 98.5% of oxygen binds reversibly to hemoglobin inside red blood cells, forming oxyhemoglobin. The remaining tiny fraction dissolves directly into plasma. Carbon dioxide is transported in three ways: dissolved in plasma, attached to hemoglobin as carbaminohemoglobin, and primarily as bicarbonate ions dissolved in the blood plasma.
The rhythmic rate and depth of breathing are controlled by respiratory centers located in the brainstem, specifically the medulla oblongata and the pons.
An increase in arterial carbon dioxide (hypercapnia) is the most powerful chemical stimulus driving an increased ventilation rate to clear the excess acid.