### Steps of the Oxygen Cascade

$1.**$Breathing $($Ventilation$)**$

$-**$What Happens$**$: When you breathe in$,$ oxygen from the air enters your lungs.

$-**$Pressure Change$**$: The oxygen starts with a partial pressure of about $**0\mathrm{.}2$ atm$**$ in the air. When it reaches the alveoli $($tiny air sacs in the lungs$),$ the pressure drops to about $**0\mathrm{.}13$ atm$**.$ This happens because some oxygen mixes with carbon dioxide in the lungs.

$2.**$Moving to Blood $($Diffusion$)**$

$-**$What Happens$**$: Oxygen moves from the alveoli into the bloodstream.

$-**$Pressure Change$**$: In the blood, the oxygen pressure rises to about $**0\mathrm{.}12$ atm$**$ after diffusion. This pressure is lower than in the alveoli because some oxygen has been used by the body.

$3.**$Flowing to Tissues $($Perfusion$)**$

$-**$What Happens$**$: The oxygen$-$rich blood travels from the heart to all parts of the body.

$-**$Pressure Change$**$: As the blood reaches the small blood vessels $($capillaries$)$ that supply tissues, the oxygen pressure drops to about $**0\mathrm{.}09$ atm$**.$ This drop occurs because oxygen is being used by the tissues for energy.

$4.**$Entering Cells $($Diffusion to Mitochondria$)**$

$-**$What Happens$**$: Oxygen moves from the capillaries into the cells, reaching the mitochondria where it$$s used to produce energy.

$-**$Pressure Change$**$: In the mitochondria, the oxygen pressure must be kept above $**0\mathrm{.}001$ atm$**$ for it to work efficiently. The pressure in the mitochondria is lower than in the capillaries because the cells are constantly using oxygen.

### Summary

As oxygen travels from the air to your cells, its partial pressure decreases at each step. This process is essential for delivering enough oxygen to your body so that your cells can produce energy.