Many important organs of the endocrine system are found in the abdominal and pelvic regions of the body, including the adrenal glands and the pancreas. These glands produce hormones that play vital roles in the body’s regulation of minerals, glucose, and energy allocation. The digestive and excretory organs of this region also produce hormones to manage digestion, blood cell production, and mineral homeostasis as well.
The adrenal glands are a pair of triangular glands that are located along the superior edge of the kidneys. Each gland consists of two regions - an outer cortex and an inner medulla - that have distinct structures and functions within the gland. The medulla produces the stress hormones epinephrine (adrenaline) and norepinephrine (noradrenaline) under stimulation by the sympathetic division of the autonomic nervous system (ANS). These hormones help to augment the body’s “fight or flight” response to stress and exertion by increasing the heart’s blood output to the muscles; increasing the efficiency of the lungs; and releasing energy for the muscles to use.
The cortex produces three main classes of hormones: glucocorticoids, mineralocorticoids, and gonadocorticoids.
- Glucocorticoids, including cortisol, help the body to raise blood glucose levels by breaking down triglycerides stored in adipose tissue and converting it to glucose. These hormones also help to reduce inflammation, making them clinically useful for treatment of rashes and other inflamed tissues.
- Mineralocorticoids, including aldosterone, help to balance the concentration of ions in the body. Sodium, potassium, and hydrogen ions are selectively excreted through the kidneys or retained in the body under the control of these hormones.
- Finally, the gonadocorticoids are androgens, which act as the precursors for testosterone in males and estrogen in females. Androgens also act as weak substitutes for testosterone in females and prepubescent boys.
Also assisting in the body’s sugar and energy homeostasis is the pancreas. The pancreas is a long, tapered gland that extends horizontally inferior to the stomach. While the vast majority of the pancreas is devoted to the production of digestive enzymes, about 1% of its structure consists of endocrine cells that produce hormones.
- Alpha cells release the hormone glucagon in response to stimulation by the sympathetic division of the ANS. Glucagon travels through the bloodstream to the liver to trigger the breakdown of glycogen molecules stored in liver cells. Glycogen is broken down into glucose molecules, which are released into the blood to raise the body’s blood glucose levels and provide energy for the body’s cells.
- Beta cells produce insulin in response to stimulation by the parasympathetic division of the ANS. Insulin reduces elevated blood glucose levels by stimulating the liver and muscles to absorb glucose and form glycogen molecules. It also stimulates adipose tissue to absorb glucose molecules from the blood and convert them into triglycerides for long-term storage.
The testes and ovaries, known collectively as the gonads, also produce important sex hormones as organs of the endocrine system.
- In females, the ovaries produce estrogens (the female sex hormones) as well as progesterone. Estrogens play many roles in the body, including the regulation of the female reproductive cycle, the development of ova, and the female secondary sex characteristics. Progesterone is produced during ovulation and pregnancy to maintain a proper environment in the uterus for fetal development.
- In males, the testes produce testosterone, the primary male sex hormone. Testosterone is responsible for the production of sperm along with the male secondary sex characteristics.
Other Hormone-producing Organs
In addition to the true glands of the abdomen and pelvis, several hormones are produced by the digestive organs and kidneys. To effectively control the digestion of diverse foods, the stomach and duodenum monitor their contents and produce hormones to influence the behavior of other organs.
- The stomach produces the hormone gastrin to increase its own secretion of hydrochloric acid and reduce the emptying of the stomach into the duodenum.
- In the duodenum of the small intestine, the hormones secretin and cholecystokinin are produced to aid its digestion of fats and proteins. Secretin is released in response to acidic chyme entering the duodenum from the stomach, signaling the pancreas to release bicarbonate ions to neutralize the acid. Cholecystokinin (CCK) is released in response to the presence of fats and proteins in food and triggers the pancreas to release many digestive enzymes. CCK also has receptors in the hypothalamus that produce the feeling of satiety, or fullness, in response to CCK. The satiety-inducing property of CCK explains why foods that are rich in fats and proteins are more filling than those consisting of only carbohydrates.
The kidneys produce several important hormones that are related to the composition of the blood.
- In response to low blood calcium levels, the kidneys produce calcitriol, the active form of vitamin D that helps the body to absorb calcium from food.
- In response to decreases in the number of red blood cells or platelets, the kidneys can produce the hormone erythropoietin (EPO) or thrombopoietin to boost the production of those blood cells or platelets by red bone marrow.