I have a soft spot for games that have a scientific aspect to them. It would be cool to see character/enemy/scenery bios that describe what these game elements are, their biological functions they perform, and how they interact with each other.

The whole idea of being inside an organism is awesome, but the game gets a bit monotonous after playing it a few times. It would be nice if the levels/enemies were randomized. Missions, gifts incentives, power ups, and recognizing high scores would spice up the gameplay and give players an incentive to keep playing.

Directing the little macrophage into a vein/artery gave me quite a surprise–as I thought it was an obstacle that would kill me–but I ended up traveling at high speeds through the bloodstream while surrounded by blood cells that have a propensity to clot. More areas like this would add variety to the gameplay (brain, intestines, pumping heart, eyes, etc). That being said, I keep dying after a minute or more, so I don't know if the developers have more surprises waiting later in the game. Randomization would help players experience these variations without having to repeatedly overcome the same succession of enemies and obstacles.

I'd like to add the intro music is awesome. Bringing some of that goofy humor into the gameplay (via sound effects, power ups, missions) could add some variety too.

Overall, good job. The controls and mechanics work well. Some of the toughest parts are when you have to make the character swim high, then follow the ceiling down while avoiding an obstacle on to the ground. There is a lot potential for adding fun level variations (similar to how you take a detour through the bloodstream), education, and humor.