Hemoglobin is a protein in red blood cells that binds to oxygen in the lungs and transports it to tissues, while also carrying carbon dioxide back to the lungs for exhalation.

Exhalation occurs when the diaphragm and intercostal muscles relax, reducing the volume of the thoracic cavity and increasing pressure in the lungs, forcing air out.

Breathing in humans is a mechanical process controlled by the respiratory center in the brain. The diaphragm and intercostal muscles contract to increase lung volume, drawing air in (inhalation), and then relax to expel air (exhalation). This process is regulated by changes in pressure within the lungs relative to the outside environment.

Gaseous exchange in the alveoli occurs through diffusion. Oxygen from the inhaled air diffuses through the thin alveolar walls into the bloodstream, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.

Oxygen is transported in the blood primarily by binding to hemoglobin in red blood cells. Carbon dioxide is carried as bicarbonate ions in the plasma, with a small portion bound to hemoglobin.