Set the letter's value in the Refers To box. You can create a value from data in a cell in the worksheet or you can add your own number. By default, Excel puts a reference to the currently selected cell in the box. If this contains your value, keep the default.
Gravely (the adverb) comes from the adjective, which comes from Latin (gravis), where it meant heavy or serious (this is where we get gravity--heaviness). Gravely ill means seriously ill, not soon to be in the grave. (and proof [1]). Bonus tidbit: grave (the noun) comes from words that originally meant to dig then a place dug, as for burial or, a grave. This is the same root as engrave.
How dangerous is a tortuous aorta is a question that can only be answered by a physician and on an individual basis, though there are certain parameters which make this condition more dangerous in certain individuals. The Aorta. The aorta is the blood vessel attached to the heart, carrying freshly oxygenated blood to the rest of the body. In healthy individuals, the aorta is generally straight, following a short path away from the heart. Those with a tortuous aorta, however, may have twists and other complications within the blood vessel.