The Endocrine Glands

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 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.

II. Major endocrine organs Be familiar with the location of each gland, the hormones they release and its function. The pituitary gland: It sits on the sella turcica (sphenoid bone). This gland connects to the hypothalums Regulates the activity of the other glands in the body. Divided into an anterior and posterior section

1. Adenohypophysis: also known as the anterior pituitary gland 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 body c. Prolactin- stimulates the breast tissue to produce milk d. 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 hormones h. 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 on 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 of 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 gland: These are two glands located superiorly to the kidney and are actually two different glands within one structure. 1. adrenal medulla: inner layer, considered part of the autonomic nervous system, it releases adrenaline or epinephrine to assist in the “fight or flight” response.

2. adrenal cortex: outer layer, 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.

F. The pancreas: located in the abdominal cavity, under the stomach, it contains endocrine cells and exocrine digestive enzyme cells.

Histology of the pancreas

G. The thymus: located in lower neck, superior to the heart, and posterior to the sternum. It secretes amino acid based hormones called thymic hormones. 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.

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 Digestive tract has scattered cells that release 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. D. The kidneys release hormones which regulates blood pressure and stimulates erythrocyte production.