Role of Other Organs and Tissues in the Endocrine System

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.
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. 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.

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.