Final answer:
The pancreas regulates blood glucose levels through the secretion of insulin and glucagon by its islets of Langerhans, which maintain blood sugar homeostasis.
Step-by-step explanation:
The endocrine gland responsible for regulating the increases and decreases of blood glucose concentration is the pancreas. The pancreas contains specialized cells known as islets of Langerhans, which include alpha cells that produce glucagon and beta cells that produce insulin. When blood glucose levels are high, insulin is released to facilitate the uptake of glucose by the body's cells, thereby reducing blood sugar concentrations. In contrast, when blood glucose levels are low, glucagon is secreted to promote the conversion of glycogen to glucose in the liver, increasing blood sugar concentrations. These hormones function in a feedback loop to maintain blood glucose homeostasis.