How does oxygen enter the body?

Breathe in, you can feel your chest expanding and your body lifting up. When you breathe in, oxygen in the air around us moves into our lungs. The air moves through tubes called bronchi which branch, like a tree into tiny sacs called alveoli. The alveoli are only one cell thick, but because there are millions of them, all together they have a large surface area. This means molecules can pass easily through their walls as they have a smaller distance to travel and that plenty of oxygen can get through into the blood. The alveoli are surrounded by tiny blood vessels called capillaries which give them a good supply of blood . On the inside of the alveoli, it is moist, so that molecules of oxygen can dissolve into the water.
This means that diffusion happens faster, which is the process by which there is a movement of a molecule from a high to low concentration. Oxygen then passes into the capillary and finds a red blood cell. A red blood cell is a has a biconcave shape and unlike other cells, has no nucleus, this is so that it can carry even more oxygen. These red blood cells are moving throughout the blood vessels carrying oxygen away to the rest of the body. This maintains the concentration difference meaning that oxygen concentration is high in the alveoli and low in the blood vessel. The red blood cells then travel through the body to find tissues and organs that need oxygen. Oxygen will diffuse from a region of high to low concentration in the body cells too.