To help visualize how fluid gets from the blood to the outside of your body through sweat glands, [here is a diagram.](http://images.wisegeek.com/human-skin-diagram.jpg) The cells in the sweat gland take fluid out of the blood and then secrete it on the other side of them as sweat.
All bodily secretions work like this, with the cells that produce the secretion taking the raw materials from the blood.
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