Gravity is the force of attraction between objects. When you are in orbit, there is still that force of attraction (gravity) pulling you and the earth together. The reason you don't crash into the earth is not because you have "zero gravity", but because you are moving. In fact, you are falling. Gravity is pulling you straight down towards the center of the earth. The thing is, you're moving so fast that you are always missing the earth. If it weren't for the earth's gravitational pull, you would continue off into space in a straight line. Gravity keeps you in orbit, thus you are not in "zero" gravity. And if it weren't for your forward motion, you'd fall directly towards the earth.
The effect of gravity is always there. Think of it this way - the moon feels the earth's gravity, right? That's why it orbits the earth. It, too, is just continually missing the earth because of its motion. And the earth feels the moon's gravity. That is why we have tides. The water on the earth, and in fact everything on earth, feels the pull of the moon's gravity. It's so small (microgravity) that you don't notice it on your body, but it's strong enough to cause the tides.
And similarly, when you are in orbit in the space station, everything in your body (your blood, skin, hair, etc.) are still pulled towards the earth - sort of a tidal force affecting your bodily fluids. Thus you are not in "zero" gravity - your blood is not truly floating wherever it wants to go - it is constantly being pulled towards the earth. So being oriented with your head nearer the earth than your toes is like standing on your head on the surface of the earth, it's just that the forces are much weaker. In true zero gravity - out in deep space so far away from any galaxies that gravitational pull is at a minimum (it's never truly zero), your blood would act differently than it does in low earth orbit where it feels the earth's gravitational pull.