I was talking to my friends a lot about this. Some of them claim that, when smoking (let's say weed), you don't need to hold your breath in. They say it's basically the same if you hold your breath in for 15 seconds or for just 3 seconds.

I didn't agree, I said the smoke needs some time to get absorbed into blood. So I did some more research. I grabbed my biology book and learned that:
Diffusion is the only way for oxygen (and carbon dioxide) to get from our lungs into our blood (and vice versa). Now I also understand that all of the nitrogen (78% of the air) we inhale gets out of our body, because our body doesn't need it. How come the other chemicals get in our blood anyway?

Anyway the question is: How quickly do the chemicals we smoke get in our blood? I'm 100% sure that time is relevant, but I have no idea how important that is.

One friend recently told me a theory about smoking weed. He said that after inhaling the smoke, I should inhale as much air as I could, so that the smoke is being pushed towards the lungs, and that way the process of absorbing should be faster.

Does anyone know anything about this stuff?

Chemicals smoked into the respiratory tract enter the blood almost immediately. The longer you hold in the gas, the more diffusion activity will be exhibited.

the longer you hold the hit , the higher you'll get

however there is no need to go over 15 seconds

With Weed I almost never HOLD hits for more then 5 seconds or so, with DMT I almost always hold as long as I can 15 seconds like above poster said, and I do in fact intake air after the hit, not because I feel it helps it kick in faster, but because I feel it pushes all the vapor in my mouth/esophagus into my lungs, for better absorption and less waste, also it helps with the urge to cough if you get hot vapor irritating your throat nothing is more soothing than an immediate intake of nice fresh air after it.