A horse with a sunken or shrunken flank or belly is known by horse people as being "drawn up".

Normal healthy horses maintain a balance of water movement between intestinal contents and the bloodstream, depending upon their hydration. A huge large colon full of heavy, wet feed material is what provides much of the contour of the normal horse's lower abdomen and flank. If that wet feed material dries out and shrinks, then the abdomen appears drawn up.

The intestinal contents are in intimate contact with the bloodstream. The colon is intimately involved in the horse's water balance. Imagine the wet-hay sludge content of the 20 gallon intestine coming into contact with a drier than normal "sponge", which is the blood within the vessels of the circulation.

Water flows out of the wet colon contents back into the drier circulation. This dehydrates the mass of sludge in the colon. The result of dehydration then is shrinkage of this mass and a drawn-up appearance. In a healthy horse, there is some fluctuation in the fullness of the flank from this process. After intense exercise,and the associated water and electrolyte loss from sweat, horses will naturally be somewhat drawn up.