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Endocrine Glands

Role of Other Organs and Tissues in the Endocrine System

Several organs, including the kidneys, liver, thymus, heart, and skin, are not classified as primary endocrine organs but play important roles in endocrine function of the body.

There are organs of the body that are not a part of the endocrine system yet still produce hormones that affect body functions. The heart is part of the cardiovascular system, but the natriuretic peptide hormone it secretes increases intake of sodium chloride and therefore increases extracellular fluid volume. The skin is part of the integumentary system, but when exposed to UV light it produces an inactive form of vitamin D that upon activation is vital to the health and growth of bones.

The kidneys, liver, and heart all play a role in the regulation of blood pressure and blood volume. The liver produces angiotensinogen, a precursor to the hormone angiotensin. Kidneys secrete the hormone renin in response to low blood pressure. Once in the bloodstream, renin hydrolyzes angiotensinogen to produce angiotensin I. This is then converted to the much more powerful hormone angiotensin II via the work of angiotensin-converting enzyme. Angiotensin II causes blood vessels to constrict, which increases blood pressure. It also causes a decrease in excretion in the kidneys, which increases blood volume. The heart also plays a role in the regulation of blood pressure and blood volume. Cardiac muscle cells in the wall of the atria detect increases in atrial blood volume through increased stretching of the atrial wall. These cells then secrete atrial natriuretic peptide (ANP), which targets the kidneys and increases excretion of water and sodium, rapidly reducing blood volume.

Organs with Endocrine Function

The kidneys, liver, thymus, heart, and skin are not classically a part of the endocrine system, yet they produce hormones that affect other organs and processes within the human body.
The liver and kidneys, along with the skin, are also involved in calcium homeostasis. Following UVB (the rays that cause sunburn) exposure, the skin produces a type of vitamin D3 called cholecalciferol. This form of vitamin D is inactive; it is converted to calcifediol by the liver. The kidneys then convert calcifediol into calcitriol, the active form of vitamin D. Calcitriol helps grow and maintain healthy bones. Calcitriol, along with the parathyroid hormone (PTH), increases blood calcium levels by stimulating osteoclasts and increasing dietary calcium uptake. Thus, the skin, liver, and kidneys all act with the thyroid and parathyroid to regulate calcium levels.

Vitamin D Metabolism

Sometimes several organs work together to produce and maintain hormone balance in the body. The production of vitamin D starts with exposure to UVB rays from sunlight and involves the work of the skin, liver, and kidneys. Vitamin D is vital for bone growth and calcium homeostasis in the body.
The kidneys and liver also play a role in the production of red blood cells in bone marrow. Both organs produce and secrete the hormone erythropoietin (EPO), which stimulates the production of red blood cells. Finally, the liver has several other endocrine functions. Insulin-like growth factor 1 (IGF-1) is another hormone produced and secreted by the liver. It plays an important role in cell proliferation and the inhibition of cell death in a variety of tissues throughout the body. Hepcidin is another hormone primarily synthesized and secreted by the liver. It acts on the gut and the liver to regulate iron metabolism and reduce iron levels in the body.

The thymus is a soft, almost triangular lymphoid tissue located in the lower part of the neck, just above the heart and behind the sternum, or breastbone. During childhood, the thymus produces the hormones thymosin and T-cell growth factors that aid in the maturation of a particular type of white blood cell, the T cell. Immature T cells from the bone marrow enter the thymus, where they are presented with antigens, or foreign material that induces an immune response in the body. These T cells then mature and "learn" to recognize this foreign material to keep the body safe from infection. The thymus performs this vital role in immune preparation throughout childhood up until puberty, after which point the organ shrinks and changes into a fibrous, fatty tissue.

Stomach, Small Intestine, and Adipose Tissue

The stomach, small intestine, and adipose tissue have endocrine functions related to digestion.

The stomach and small intestine have a significant role to play in the endocrine system. These organs secrete several hormones that aid in digestive function. These hormones are synthesized and secreted by enteroendocrine cells, specialized cells within the gastrointestinal system that produce gastrointestinal hormones or peptides. For example, the hormone cholecystokinin (CCK) is produced by enteroendocrine cells in the duodenum, the first segment of the small intestine. In response to a meal, CCK is released, and it causes the pancreas and gallbladder to release digestive enzymes and bile. Cholecystokinin also acts as a hunger suppressant.

Gastrin is another digestive hormone. It is released by enteroendocrine cells in the stomach, duodenum, and pancreas. These cells release gastrin following distension of the stomach (when food enters the stomach) and the presence of partially digested amino acids. Gastrin stimulates the secretion of hydrochloric acid by parietal cells of the stomach. The presence of too much acid in the stomach inhibits gastrin release, providing negative feedback.

Some digestive-system hormones are also involved in the regulation of appetite. The hormone ghrelin is sometimes called the "hunger hormone." It is produced by cells in the gastrointestinal tract when the stomach is empty, but when the stomach is stretched with food, ghrelin secretion stops. Ghrelin acts on hypothalamic cells to increase hunger. Ghrelin increases gastric acid secretion in the stomach and promotes gastrointestinal motility, actions that prepare the system for a meal. The cells in the brain that contain ghrelin receptors also contain receptors for the hormone leptin. Leptin works in opposition to ghrelin and is synthesized by adipose, or fat, tissue in the body. Leptin serves as a satiety signal and inhibits hunger. The main function of adipose tissue is energy storage in the form of fat, but it also produces a variety of hormones and factors in addition to leptin. Another important product of adipose is resistin, which is associated with inflammation and obesity in the body. It also plays a role in type II diabetes mellitus and insulin resistance, although the mechanism is not completely understood at this time. Further, adipose tissue serves to metabolize glucocorticoids and sex steroids (estrogen, testosterone) in the body.

Peptide YY (PYY) is another hormone produced by enteroendocrine cells in the gastrointestinal tract. Peptide YY increases following a meal and decreases with fasting. It acts as a satiety signal, reducing appetite. In addition, PYY also slows motility in the ileum and colon, increasing nutrient absorption and digestive efficiency.

Testes, Ovaries, and Placenta

The testes, ovaries, and placenta produce hormones necessary for reproduction.

The male and female reproductive glands, the testes and ovaries, play a significant role in the endocrine system. In addition to producing sperm, Leydig cells (located between the sperm-producing seminiferous tubules in the testes) produce androgen hormones including the hormone testosterone. Luteinizing hormone (LH) is secreted from the anterior pituitary. In women it triggers ovulation and corpus luteal development; in men it stimulates testicular interstitial cells to produce and secrete testosterone. Follicle-stimulating hormone (FSH), secreted from the anterior pituitary, stimulates the growth of ovarian follicles in women; in men, it acts on testicular Sertoli cells to stimulate sperm production (spermatogenesis). Sertoli cells, located in the seminiferous tubules of the testes, produce the hormone inhibin. Androgen hormones stimulate the production of inhibin, which inhibits FSH production in the anterior pituitary, providing negative feedback to regulate sperm production.

Androgen hormones, including testosterone, have both anabolic (from anabolism, the building of complex molecules) and androgenic effects throughout the body. The anabolic effects of androgen hormones include the promotion of tissue growth such as an increase in muscle mass and bone density. The androgenic effects of androgen hormones include the maturation of reproductive organs, the promotion of sperm production, and the development and maintenance of secondary sex characteristics such as body hair.

The female reproductive glands, the ovaries, also secrete hormones that play a critical role in the regulation of the menstrual cycle and fertility. The hormones secreted by the ovaries include estrogen, inhibin, and progesterone. Low levels of estrogen and progesterone cause the anterior pituitary to release FSH, which initiates follicle maturation in the ovary. The follicle, a fluid-filled sac containing an egg, produces estrogen, which acts on the uterus to prepare it for pregnancy. Estrogen also induces a positive feedback loop by triggering the hypothalamus to stimulate LH release from the anterior pituitary. This LH surge, in turn, triggers ovulation. During ovulation the egg is released from the follicle. The ruptured follicle, now called the corpus luteum, continues to secrete estrogen, along with progesterone, and these hormones continue to prepare the body for pregnancy. If the egg is not fertilized, estrogen and progesterone levels decrease. The body sheds the uterine lining as a result of this drop in progesterone. If the egg is fertilized, human chorionic gonadotropin (hCG) maintains the corpus luteum, which keeps progesterone levels high until the placenta is developed. Once developed, the placenta produces estrogen and progesterone, which maintain the pregnancy and cause significant changes throughout the body to support the developing fetus. As in the testes, inhibin is also produced by the ovaries. Follicle-stimulating hormone stimulates inhibin production by the ovaries; this in turn suppresses FSH production in the anterior pituitary.

In addition to its role in the regulation of the menstrual cycle, estrogen also causes the maturation and maintenance of female reproductive organs and secondary sex characteristics. Progesterone (and other progestins) have other wide-ranging effects in the body besides the maintenance of pregnancy. Progestins moderate the effects of other hormones, play a role in breast development, and, along with estrogens, are involved in skin health.

The Reproductive Hormones

Testosterone and estrogen/progesterone levels are tightly controlled through a negative feedback system. Detecting an increase in the circulating levels of these hormones, the anterior pituitary stops secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).