Well... pumping isn't supposed to draw blood in your penis, but to actually stretch the tissue of the penis, pulling the penis that is inside of the body into the tube as well. Some fluid will be drawn into the tissues of the penis, but it is not blood but rather lymph fluid and interstitial water.
I will repost this 'cause you need to read it:
The Logic Behind Pumping
Well... the penis is surrounded by fascia tissue which is a hard as nails rubber-like membrane that covers the corpus spongeosum and cavernosum. There are three factors which can govern penis size, the most important one being the elasticity of the fascia tissue that surrounds the internal parts of the penis. It is this elasticity that allows the penis to expand like a balloon when the smooth muscle cells relax and fill up with blood. The second factor is the size and elasticity of the surface area of the skin surrounding the penis. the more skin you've got the larger your erections can expand and the more elasticity your skin has can affect the size of your erections also. The third is the obvious one... larger and more proliferate smooth muscle cells and the actual size of the chambers of the penis.
Pumping can have a positive effect on expanding the fascia tissue which surrounds the penis... vacuum pressure is the ONLY method which can actually stretch that stubborn fascia tissue. Pumping can also have an effect on expanding the surface area of the skin surrounding the penis. Pumping also enlarges the smooth muscle cells and the actual chambers which contain them. The third effect is pulling fluid into the skin cells and the area between the chambers of the penis and it's fascia tissue, actually expanding the stubborn fascia through hydraulic pressure.
This all results in a temporarily larger penis both in the flaccid and the erect state. What you do, is pump again the next day or every other day and kind of let the enlargement effect build and multiply. The skin will keep expanding and the fascia tissue keep stretching as long as you continue to pump. It's just like lifting weights... if you stop... bye bye muscles. If you don't stop, like weight lifting... you will have a larger and more elastic fascia tissue and a larger and more elastic skin surface are at the very least. What you will also have, is larger smooth muscle cells and an expanded corpus cavernosum and spongeosum through hydraulic pressure. You will have a progressively expanding penis if you DO IT RIGHT... pumping is no joke and requires SERIOUS research and time to spend on the craft.
I love my big bloated cock... I do it everyday if I can and only take breaks from pumping when necessary. It has taken me YEARS to get to 8 inches in girth because I have the type-2 diabetic gene and that makes your skin susceptible to injury from the vacuum pressure and I also have very little foreskin which offers the vulnerable inner foreskin no protection from the vacuum pressure offered by the pump. So, it has been hit or miss and getting a routine together that worked for me has been a challenge, but one that was well worth it, although there have been MANY bumps along the road to pumping success.
The Hazards of Pumping
1).You can wreck havoc on the delicate capillaries that line the tissue of the inner foreskin actually bursting and destroying these tiny blood vessels. The previous post mentioned oxygen deprivation which can also destroy these capillaries by starving them of their rich blood supply.
2). The skin can be damaged by the vacuum pressure the same way that people in hospitals can suffer from bed sores. Pressure sores are crippling to a vacuum pumping routine because they take a while to heal, interrupting the incremental growth process.
3). Water blisters can form on the glans and (God forbid) the inner foreskin... these little buggers are EXTREMELY difficult to avoid once you get a few.
Cold applied to the sight where the water blister usually appears during breaks in the routine, is the only remedy for this pugnacious and pernicious pitfall of pumping. The other hazards can be avoided by using a pump with a gauge and watching the clock. There are many different methods for maintaining circulation during your stay in the tube, find one that works best for you 'cause it is important to maintaining the integrity of your capillaries.
Keep the tube warm at all times and do not ever enter a cold cylinder.
Drink plenty of fluids (an extra glass of fluid per hour of pumping should do the trick)
Slather the penis with lotion or the moisturizer of your choice. A pumped penis is MUCH more prone to drying out than normal skin... if the skin of the penis gets dry, you are screwed, no pumping will take place that day or the next, until the skin is re-hydrated and it's integrity is once again maintained.
Pump with a semi... not flaccid, but not hard... if you become hard in the tube, that is okay, but raging boners will detract from your ability to pump huge. | 
