Oxygen moving from the bloodstream into a cell down a concentration gradient is a form of
passive transport called diffusion.
Diffusion is a process by which molecules or ions move from an area of higher concentration to an area of lower concentration. This movement occurs down the concentration gradient, which is the gradual change in concentration from one area to another.
In the case of oxygen moving from the bloodstream into a cell, the concentration of oxygen is generally higher in the bloodstream than inside the cell. Therefore, oxygen molecules will naturally diffuse across the cell membrane to equalize the concentration on both sides. This process does not require any energy expenditure from the cell and is thus considered a form of passive transport.
To understand this concept better, imagine a densely packed room filled with a strong scent. When someone opens the door, the scent molecules start to diffuse out of the room and spread throughout the larger area. This diffusion happens because the concentration of the scent inside the room is higher compared to the outside. Similarly, when oxygen-rich blood reaches cells, the oxygen molecules will diffuse from the higher concentration in the bloodstream to the lower concentration inside the cell.