How to select text by lines? Follow

Hi,

I want to be able to start a selection of text and move the caret to the end of the next line with the selection.  I can't find any shortcuts or ways to do this via CMD SHIFT A or Google Search.  I can do this easily with my mouse--I just select the text and drag my mouse down until I selected the lines I wanted.  How can this be done using a keyboard shortcut?

As an example, CMD SHIFT LEFT/RIGHT-ARROW moves the selection left/right.  I want an equivalent which moves the cursor to the line above or below, continuing selection.  This should be CMD SHIFT UP/DOWN-ARROW, but that doesn't work.