Glucagon acts in a negative feedback loop with insulin to regulate blood sugar concentration. Both hormones are secreted from the Islets of Langerhans in the pancreas. Alpha cells in the pancreas have high levels of glucagon synthase, allowing them to synthesize the hormone.

Essentially, glucagon synthase is located in alpha cells, which are found in Islets of Langerhans within the pancreas; thus, all of these answers describe the site of glucagon synthesis.

There is a key difference between exocrine and endocrine glands. Exocrine glands release their product to the external environment, while endocrine glands release their product directly into bodily fluids. The answer is the structure that accomplishes both of these actions. The pancreas does both: it releases digestive enzymes into the small intestine (external environment), and releases hormones into the bloodstream (body fluid). Remember that the entire digestive tract is considered part of the outside environment and is lined by epithelium, rather than endothelium.

The adrenal cortex and thyroid perform only endocrine functions, while the gall bladder is only exocrine.

Insulin is secreted from the beta cells of the pancreas. The alpha cells secrete glucagon, which acts in a negative feedback loop with insulin to regulate blood glucose levels.

The acinar cells of the pancreas secrete digestive enzymes, and serve the exocrine (rather than the endocrine) function of the pancreas. The adrenal medulla secretes epinephrine and norepinephrine, while the adrenal cortex releases aldosterone.

Somatostatin is synthesized in the delta cells in the islets of Langerhans in the pancreas, and serves to inhibit insulin and glucagon secretion from beta and alpha cells, respectively. Somatostatin serves a number of other purposes, including inhibition of growth hormone and inhibition of the release of pancreatic digestive enzyme release.

Amylin, along with insulin, is released from the beta cells of the pancreas and acts as a synergist to insulin. Ghrelin is released from epsilon cells of the pancreas and has an array of different functions, including inducing appetite. Thyroxine refers to the thyroid hormone T4, or tetraiiodothyronine, which plays a role in metabolism.

Endocrine organs are capable of secreting hormones into the bloodstream. Each of the given answer options performs some sort of endocrine function.

The testes secrete testosterone, and are responsible for the development of most secondary sex characteristics in males.

The thyroid gland secretes calcitonin and thyroid hormones, responsible for elevating calcium levels and increasing metabolism, respectively.

The pancreas and liver are both endocrine AND exocrine organs. As an endocrine organ, the pancreas secretes the hormones insulin and glucagon. As an exocrine organ, it secretes several enzymes that are essential for digestion in the small intestine. The liver secretes insulin-like growth factors in response to stimulation by growth hormone. As an exocrine organ, it is also responsible for producing bile to aid in digestion.

The endocrine system involves secretions by ductless glands into the bloodstream. The pancreas, pituitary gland, thyroid, and hypothalamus are all endocrine organs that secrete hormone signals to the body. The pancreas, however, also functions as an exocrine gland. When the pancreas secretes insulin and glucagon into the blood stream, it serves as an endocrine organ. When it secretes digestive enzymes through the pancreatic duct into the small intestine, it serves an exocrine function.

The question states that both hormone A and hormone B act on the collecting ducts of the kidneys. There are two major hormones that act on the collecting ducts: aldosterone and antidiuretic hormone (ADH). Aldosterone is a steroid hormone that functions to increase sodium and chlorine reabsorption from urine in the collecting ducts, whereas ADH is a peptide hormone that functions to increase water reabsorption from urine in the collecting ducts (by increasing the permeability to water). This means that hormone A is ADH and hormone B is aldosterone.

Recall thatsteroid hormones, such as aldosterone, are nonpolar molecules. On the other hand, peptide hormones are made up of protein subunits and consist of many polar groups; therefore, ADH is a polar molecule. Since water is a polar molecule, only ADH (hormone A) will dissolve in water.

Insulin is released into the blood from the pancreas and functions to decrease the glucose concentration in the blood. Elevated levels of blood glucose induces insulin release. Once released, insulin transports glucose back into the cells or facilitates the formation of glycogen. Glycogen consists of multiple glucose subunits and is stored in the liver for later use. Calcitonin, on the other hand, functions to decrease the calcium concentration in the blood. Osteoclasts are cells in bones that break down bones and release bone calcium into the blood. Calcitonin inhibits the activity of osteoclasts and decreases the calcium concentration in the blood. Statement I is false.

Antagonistic hormones are a pair of hormones that have the opposite effects. For example, insulin and glucagon are antagonistic hormones because insulin functions to decrease blood glucose levels, whereas glucagon functions to increase blood glucose levels. Similarly, calcitonin and parathyroid hormone (PTH) are antagonistic hormones because calcitonin functions to decrease blood calcium levels whereas PTH functions to increase blood calcium levels. Insulin and calcitonin are not antagonistic hormones because they do not have opposite effects. Statement II is false.

Recall that insulin and calcitonin are part of the endocrine system. Hormones in the endocrine system are characterized by their ability to travel through blood and act on cells that are far away from the glands that released them; therefore, insulin and calcitonin do not act on cells surrounding their respective glands alone. Statement III is false.

Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas in response to high blood glucose levels. Alpha cells in the islets of Langerhans secrete glucagon, the antagonist of insulin, in response to low blood glucose levels.

The thyroid secretes thyroxine (also known as tetraiodothyronine or T4), which controls the body's metabolism. The thyroid also secretes calcitonin, which promotes bone formation. Cortisol is secreted by the adrenal cortex and works to regulate blood glucose levels and has an anti-inflammatory effect. Cortisol is a corticosteroid that is secreted in response to adrenocorticotropic hormone.

Seeing as type I diabetes results from the destruction of the -cells in the pancreas, this is the only feasible answer. These cells are responsible for producing insulin.