The Endocrine System

Human Anatomy

Chapter 25

• This system includes all the glands of the body that secrete hormones directly into the blood stream. They do not contain ducts, some glands may be both endocrine and exocrine (ex: pancreas). The hormones are chemical stimulants that control changes in the body. Endocrine glands may be stimulated by the nervous system or chemical changes in the body.

I. The endrocrine system: an overview

• Endocrine glands are studied by endocrinologist. These scientists have determined that despite the fact that hormones travel all over the body through the blood stream and that all tissues are exposed to them not all are affected. Hormones affect specific tissues called target tissues. The cells of these organs have receptors that bind the specific hormone and are stimulated by that reaction.

• A. Endocrine organs- some organs are “pure” in that the entire structure is composed of endocrine cells, these include: the pituitary gland, the pineal gland, the thryroid and parathryroid gland, and the adrenal glands. The following perform functions in other system asides from the endocrine: pancreas, thymus, testes, ovaries, and hypothalamus. Endocrine glands have a high supply of blood and lymph vessels.

• B. Hormones- each hormone type had its own chemical structure and thus performs different actions.

• 1. Classes of hormones- amino acid-based hormones contain amino acids or small proteins as part of their structure. Steroids are hormones that contain a cholesterol group as part of its structure.

• 2. Basic hormone action- Cells have receptors on their surface that bind only specific types of hormones. When receptor binding occurs a program response/reaction takes place. Hormones do not carry information but are only released to stimulate a chemical reaction. Tropic hormones are secreted by an endocrine to stimulate the activity of another endocrine gland.

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• 3. Control of hormone secretion- the following list condition under which an endocrine gland releases hormones:

• a. Humoral stimuli: gland responds to chemical changes in the body such as changing levels or ions or nutrients in the blood.

• b. Neural stimuli: some glands are signal by nervous system (sympathetic nerves) to release hormones in order to induce physiological changes.

• c. Hormonal stimuli: some endocrine glands are controlled by other endocrine glands and thus respond to hormone.

• d. Feedback loops: hormone levels are also controlled by feedback loops that either stimulate or halt the production of a hormone

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II. Major endocrine organs Be familiar with the location of each gland, the hormones they release and

its function.A. The pituitary gland: is also called the hypophysis and is located within the

cranium inferior to the brain. It sits on the hypophyseal fossa of the sella turcica (sphenoid bone). This gland connects to the hypothalums through a stalk called the infundibulum, the infundibulum connect to the tuber cinereum of the hypothalamus.

Pituitary Gland and visible leukocytes

• 1. Adenohypophysis: also known as the anterior pituitary gland. The majority of the hormones produced are amino acid based hormones. Different cell types produce different hormones including:a. Growth hormone (GH)- signals growth of the body

b. Somatotropic hormone(SH) or somatotropin- signals growth of the bodyc. Prolactin- stimulates the breast tissue to produce milkd. Thyroid-Stimulating hormone- stimulates the thyroid gland to secrete hormones that influence our metabolism.e. Adrenocorticotropic hormone (ACTH)- stimulates adrenal gland to signal its hormones that help us deal with stress.f. Melanocyte-stimulating hormone (MSH)- stimulates melanocytes of the epidermis to produce more melanin, thus darkening the skin.g. Follicle stimulating hormone (FSH)- act on testes or ovaries an stimulate the maturation of egg/sperm and induce secretion of sex hormonesh. Leutinizing hormone (LH)- During the menstrual cycle it signals ovulation

• 2. Hypothalamic control of hormone secretion from the adenohypophysis: the hypothalamus had neurons that produce hormones and release them much like neurotransmitters are released. These travel through the hypophyseal portal system into the anterior pituitary and stimulate its hormone secretion. The hypophysial portal system involves two beds of capillaries connected by a vein. It allows a high level of horomone concentration within a small region and it is designed so that the hormones released by the hypothalamus travel directly to the anterior pituitary and it in turn release hormones into systemic circulation.

• 3. Neurohypophysis: also known as the posterior pituitary gland, it is technically part of the brain and it composed of neurons. Rather than making hormones it stores those produced by the hypothalamus and releases them when it receives a neural impuls. The hormones produced by the hypothalamus but released by the neurohypophysis are:a. Antidiuretic hormone (ADH)- stimulates the collecting tubules of the kidney to concentrate the urine in order to reduce water loss.b. Oxytocin- induces contractions of smooth muscle of the sexual organs, such when a mother experiences contractions of the uterus during labor.

• B. The thyroid gland: located just inferior to the larynx, it has two lobes that give it the appearance of butterfly wings. It is the largest endocrine gland and it releases thyroid hormones and calcitonin.

• a. Thyroid hormone: T4 and T3, amino acid based hormones that contain iodine, main function is to increase metabolic rate.

• b. Calcitonin- reduces excessive levels of calcium ion in the blood by slowing down osteoclast activity, actively secreted during childhood.

Histology of Thryroid and Parathyroid Gland

• C. The parathyroid gland: yellowish in color, lie posterior or the thyroid gland. There are at least two pairs of glands but some may have as many as four pairs. The hormone it releases is parathyroid hormone (PTH) which has the opposite effect if calcitonin. It increase calcium ion levels in the blood by either activating the osteoclast, stimulating the kidney to reabsorb more calcium, or activation vitamin D production for calcium to be absorbed from food.

• D. The adrenal (suprarenal) gland: These are two glands located superiorly to the kidney and are actually two different glands within one structure.

• 1. adrenal medulla: considered part of the autonomic nervous system, it releases adrenaline or epinephrine to assist in the “fight or flight” response.

• 2. adrenal cortex: secretes steroids called corticosteroids. Aldosterone (mineralocorticoid) is secreted in response to a decrease in blood volume or blood pressure stimulating the kidneys to reabsorb more water or sodium. Another group of hormones is called the glucocorticoids which include cortisol. These hormones keep blood glucose levels high to maintain brain activity during stressful situations. They can direct lymphocytes, or decrease inflammation.

• E. The pineal gland: also located in the brain and controlled by the hypothalamus, it is composed of endocrine cells that secrete melatonin which regulates the internal body clocks: it induces sleep.

• F. The pancreas: located in the abdominal cavity it contains endocrine cells called the islets of Langerhans that produce amino acid based hormones. The cells release insulin which stimulates absorption of glucose by tissue cells when blood glucose levels are high, and also releases glucagon that stimulates the liver to release sugar into the blood when blood glucose levels are low.

Histology of the pancreas

• G. The thymus: located in lower neck, anterior thorax, and posterior to the sternum. It secretes amino acid based hormones called thymic hormones like thymopoeitin and thymosin that stimulate T-lymphocytes to become immunocompetent.

• H. The gonads: testes and ovaries are organs that secret steroid sex hormones. Androgens released by these glands are changed into testosterone, estrogen, and progesterone. They maintain primary and secondary sex characteristics and cells involved in reproduction.

III. Other endocrine structures:• A. The heart has sensory cells in the atria that stimulate the kidneys

to produce more urine containing salt. Getting rid of the excess fluid and salt reduces excess blood volume and salt blood levels, and reduces blood pressure.

• B. The GI tract has scattered cells that release amino acid/peptide hormones chemically similar to neurotransmitters. They affect functions related to regulating digestion, blood chemistry, and blood flow.

• C. The placenta is produced only when conception occurs, it secrets hormones that prevent the uterus from getting rid of the nutrient layer to which it is attached. It also produces other steroid hormones: estrogen, progesterone, corticotropin-releasing hormone, and human chorionic gonadotropin.

• D. The kidneys release rennin which regulates blood pressure and erythropoietin which stimulates erythrocyte production.

• E. When exposed to UV rays, the skin produces a steroid hormone precursor to vitamin D. Vitamin D helps in the absorption of calcium.

A woman with Cushins Disease