Final answer:
The location not involved when regulatory systems act to raise low blood calcium is the stomach. The regulatory system, primarily involving parathyroid hormone (PTH), targets the bones, kidneys, and small intestine to maintain calcium homeostasis.
Step-by-step explanation:
When blood calcium falls too low, a regulatory system acts in three locations to raise it: the bones, small intestine, and kidneys. Therefore, the location that is not one of these where the regulatory system acts is option a. stomach. Parathyroid hormone (PTH) is critical in this regulatory system, specifically targeting the bones, where it stimulates osteoclasts to release calcium into the blood and inhibits osteoblasts to prevent calcium deposition. In the kidneys, PTH stimulates the reabsorption of calcium to prevent its excretion. PTH also indirectly affects the small intestine by triggering the formation of calcitriol, which in turn increases the absorption of dietary calcium.
It is important to note that, when calcium levels are too high or too low, the primarily affected body system is the skeletal system. Additionally, calcium homeostasis is a vital process for many body functions, and it involves the interactions of the skeletal, endocrine, urinary (kidneys), and digestive (intestines) systems.