During which phase of respiration does oxygen enter the bloodstream?

During inhalation, the diaphragm contracts and the thoracic cavity expands, creating a negative pressure that allows air to flow into the lungs. This influx of air includes oxygen, which diffuses across the alveolar membranes into the pulmonary capillaries. Once in the bloodstream, oxygen binds to hemoglobin molecules in red blood cells, allowing it to be transported throughout the body.

The other phases mentioned do not involve the active intake of oxygen into the bloodstream. Exhalation primarily involves the expulsion of carbon dioxide, a waste product of cellular respiration, rather than the uptake of oxygen. The resting phase generally refers to normal breathing patterns at rest, which encompasses both inhalation and exhalation but does not specifically highlight the phase where oxygen enters the bloodstream. Carbon dioxide diffusion describes the movement of this gas from the blood into the alveoli for exhalation, further emphasizing that oxygen uptake occurs during the inhalation phase.