A differentiated part of an organism, such as an eye, wing, or leaf, that performs a specific function. It is composed of a tissue or tissues which function together.

Pituitary Gland consists of a stalk linking the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes.

The anterior pituitary is a glandular structure under contorl of the hypothalamus. Together, the pituitary gland and hypothalamus control many endocrine functions. They secrete many hormones, some of which are crucial for the female menstrual cycle, preganncy, birth, and lactation. These important hormones include follicle-stimulating hormone (FSH), which stimulates development and maturation of a follicle in one of a woman’s ovaries, and leutinizing hormone (LH), which causes the bursting of that follicle (= ovulation) and the formation of a corpus luteum from the remains of the follicle.

The posterior pituitary stores and releases hormones into the blood. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary where they are dumped into the blood. ADH controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth.

The hypothalamus produces releasing hormone TRH (thyrotropin releasing hormone) that causes the anterior pituitary to release TSH (thyroid stimulating hormone) that cause the thyroid gland to release thyroid hormones. Thyroid hormones (thyroxine and triiodothyroxonine) regulate carbohydrate metabolism by cells, and early in life, are involved in growth. Low circulating levels increase TRH and TSH output; high levels reduce their output. Goiter.

Posterior Pituitary Gland

Cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus send axons via the pituitary stalk to terminals in the posterior pituitary that release two hormones into the bloodstream. Vasopressin, a.k.a. antidiuretic hormone (ADH), acts on the kidney to inhibit formation of urine, to conserve water and increase blood pressure.

Oxytocin stimulates contraction of the uterus during childbirth, triggers the milk letdown reflex during nursing, and may be the “love” hormone, released by terminals outside the hypothalamus.

Although suckling alone causes oxytocin release at first, the mother may become conditioned to release oxytocin upon hearing the baby’s cry. Thus, higher faculties can control hypothalamic and thus hormonal output.

Anterior Pituitary Gland (tropic hormones)

Hypothalamic cells produce specific releasing hormones and inhibiting hormones, which they secrete from axon terminals in the median eminence into a portal system of blood vessels to flood the anterior pituitary, causing the latter cells to release their specific tropic hormones. Tropic hormones are pituitary hormones that affect other endocrine glands.

The anterior pituitary releases six tropic hormones:

Hypothalamus releases gonadotropin Releasing hormone (GnRH), causing pituitary to release LH (stiumalting ovulaiton) The hypothalamus contains neurons that control releases from the anterior pituitary. Seven hypothalamic hormones are released into a portal system connecting the hypothalamus and pituitary, and cause targets in the pituitary to release eight hormones. Hypothalamus receptors monitor blood levels of thyroid hormones. Low blood levels of Thyroid-stimulating hormone (TSH) cause the release of TSH-releasing hormone from the hypothalamus, which in turn causes the release of TSH from the anterior pituitary. TSH travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures.

Sex Organs, which not only produce gametes but also secrete sex hormones. Sex hormone secretion is controlled by the pituitary gland hormones such as FSH and LH. While both sexes make some of each of the hormones, typically male testes secrete primarily androgens including testosterone. Female ovaries make estrogen and progesterone in varying amounts depending on where in her cycle a woman is. In a pregnant woman, the baby’s placenta also secretes hormones to maintain the pregnancy.

Adrenal Glands are located one above each kidney. The adrenal gland is divided into an inner medula and an outer cortex. The medulla synthesizes 2 hormones, epinephrine (adrenaline) and norepinephrine. When the sympathetic nervous system stimulates the cortex causes hormones related to fight-or-flight to get secreted into the blood. The adrenal cortex produces several steroid hormones in 3 classes: mineralocorticoids (maintain electrolyte balance), glucocorticoids (long-term, slow response to stress by raising blood glucose levels by breaking down fats and proteins, and also suppresss immune system and inhibit inflammatory response), and corticosteroids such as cortisone (which are anti-inflammatory hormones suppressing secretion, and suppress immune system.

In response to ACTH from the pituitary gland, the adrenal cortex makes adrenocorticoids. These include

1. Glucocorticoids, like cortisol, increase blood glucose, cause protein breakdown, and are anti-inflammatory.
2. Mineralocorticoids, specifically aldosterone, which acts on kidneys to retain sodium and conserve water.
3. Sex steroids, specifically androstenedione.

The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine in response to sympathetic activity from the hypothalamus.

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The pancreas contains exocrine cells and endocrine cell clusters. Exocrine cells secrete digestive enzymes into the small intestines.

Endocrine cell clusters (aka pancreatic islets or islets of Langerhans) secrete insulin and glucagon. Insulin and glucagon regulate blood glucose levels. After a meal, when blood glucose levels rise, insulin is released so that cells can uptake the elevated glucose levels. Liver and skeletal muscles form glycogen, a carbohydrate. As glucose levels fall, insulin production is inhibited. Glucagon causes breakdown of glycogen into glycose, which is in turn released into the blood to maintain homeostatic glucose levels. Glucagon production is stimulated by low blood glucose levels, and inhibited when they rise.

In this sense, the pancreas serves as a ductless gland by regulating blood glucose levels:

• When blood glucose levels rise, insulin is released so that cells uptake the glucose and it is removed from the bloodstream. Also, the liver and skeletal muscles store glucose as a carbohydrate named glycogen.
• When blood glucose levels drop, glucagon is released so that the glycogen in liver and skeletal muscles is released as glycose. This maintains homeostatic blood glucose levels.

When blood glucose levels are not properly regulated, diabetes results. For example, if not enough insulin is produced (or the liver does not properly respond) then blood sugar can rise out of control and the result is diabetes mellitus. Diabetes causes impairment in functioning of eyes, circulation, CNS & PNS, and kidneys; it is the 2^nd leading cause of death. The two major types of diabetes are shown below:

• Type I Diabetes involves inadequate levels of insulin secretion, usually from a genetic cause. This results in blood glucose levels to rise, possibly out of control.
• Type II diabetes develops in adults form both genetic and environmental causes. Loss of response of insulin targets rather than lack of insulin causes this.

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The pineal Gland secretes melatonin and is one of the brain’s only unitary structures. The pineal gland is located near the center of the human brain, and is stimulated by nerves from the eyes. In some other animals, the pineal gland is close to the surface of the skin and is directly stimulated by light. The pineal gland secretes melatonin at night (dark) and, as a result, secretes more melatonin in winter when the nights are longer. Melatonin promotes sleep and decreases activity of the gonads. In addition, melatonin affects thyroid and adrenal cortex functions and (in some animals) skin pigmentation.

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Although not a gland, it is important to note that the stomach lining secretes gastrin, a pepide, into the stomach. Gastrin promotes digestion by stimulating release of digestive juices and stimulates stomach movements that mix food and digestive fluids.

The stomach lining is part of the exocrine system, which operates via ducts like sweat and the digestive enzyme described above.

The average adult contains 2-3 liters of bone marrow, a primary lymphoid organ located in the core of long bones. Bone marrow is composed of myeloid tissue (tissue capable of hematopoiesis) and has three major functions: production and support of stem cells; B cell maturation; and antibody production.

Bone marrow has a definite architecture, with environments containing unique factors to guide differentiation of each cell type. The myeloid tissue promotes differentiation into different cell types based on signals from outside the barrow marrow. Bone marrow also contains memory B cells, similar to lymph node germinal centers.