A little bit of biology, I promise to keep it simple:

Oxygen gets carried in your blood by red blood cells, which contain a molecule called Hemoglobin (which you see on your blood test as HgB).

In healthy people, this is what happens:

1. More Oxygen in the air leads to more Oxygen into the lungs;

2. This leads to more Oxygen diffusing into the blood;

3. This leads to more Oxygen attached to HbG (up to a point).

"more Oxygen in the blood" is called a higher "partial pressure of Oxygen" (PO2), remember that for a second.

The ability of your HbG to "bind" (attach itself) to Oxygen and carry it DECREASES in 3 conditions (not disease related):

1. When temperature increases (which happens when you exercise)

2. When your blood becomes more acidic (which happens when you exercise)

3. When there is more CO2 in your blood (which happens when you exercise).

Remember what we said earlier: to increase how much Oxygen gets carried by your HbG, you need a higher amount of Oxygen in the blood (PO2).

So HOW do you increase PO2 if training indoors?

1. Take deeper, slower breaths (this slows down the speed of your blood through the lungs, allowing more Oxygen to diffuse in and CO2 out).

2. Ventilate the room! The amount of Oxygen indoors is LOWER than outdoors naturally, and when you exercise hard, you're using up some of it (especially if there are 2-3 more people working out).

3. Cool yourself down (drink cold water and use a fan/AC): remember, the ability of HbG to carry Oxygen is negatively impacted by a rise in blood temperature (don't be fooled by thinking sweating more = more calories burned, it's the opposite).

4. If doing high intensity exercise (which builds up acidity in the blood), rest a little bit longer compared to outdoors, so you give more time for your body to reduce the acidity.

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