The complexity of human physiology is hardly a reason to think humans were intelligently designed. In fact, I believe it is the exact opposite. Machines designed by humans, i.e., by intelligence, are simple. Consider the wheel, the catapult, rope, chariot, spear, gun, all the way to more "complex" machines like the internal combustion engine, the hot air balloon, refrigeration, electricity, or even the semi-conductor which utilizes a system of electric 1's and 0's. Are these "complex"?
If you were God, why in the hell would you have created humans this way? Why would He have created a liquid like blood to convey oxygen to different cells and organs pumped by a heart all governed by an intricate web of electric nerves? Why don't we have 20 arms, eyes in the back of our heads, or wheels for feet? Instead, we are an upright, slow, plodding hunk of fragile flesh and bones that must be constantly nourished by air, water, and food.