Which is the following is an example of compartmentation at the intracellular level? a. regions of the brain are specialized for different functions. b. the walls of arteries consist of an outer connective tissue layer, a middle layer of smooth muscle and elastic fibers, and an inner endothelial lining. c. the heart is divided into four chambers, with two upper chambers that receive incoming blood and two lower, more muscular chambers that send blood out into the circulation. d. skeletal muscle cells have organelles called sarcoplasmic reticulum (sr) that contain large stores of calcium ions. e. the calcium is released during muscle contraction and then pumped back into the sr as the muscle relaxes.