The Endocrine System. Endocrine System: Overview. Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus
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Endocrine System: Overview • Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones • Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus • The pancreas and gonads produce both hormones and exocrine products
The hypothalamus has both neural functions and releases hormones Other tissues and organs that produce hormones – adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart Endocrine System: Overview
Hormones are substances secreted by cells that act to regulate the activity of other cells in the body. • Hormones affect all cells in the body and are made and secreted by endocrine glands. • Endocrine glands are ductless organs that secret hormones either into the bloodstream or the fluid around cells.
The endocrine glands can be found through out the body and are collectively known as the endocrine system. • Endocrine glands, such as the pancreas, can also be exocrine glands. • Exocrine glands secrete substances through ducts to specific locations inside and outside the body.
Types of Hormones • Hormones can be grouped into two types based on their structure. Hormones can either be amino acid-based hormones or steroid hormones. • Amino acid based-hormones are made of amino acids, either a single modified amino acid or a protein made of 3-200 amino acids, and are water soluble. • Steroid hormones are lipid hormones that the body makes from cholesterol and are fat soluble. • Similar to steroid hormones are thyroid hormones.
Regardless of which type of hormone is being activated, all hormones affect only their target cells. • Target cells are specific cells to which a hormone travels to produce a specific effect. • On the target cells are receptors. Receptors are proteins that bind to specific signal molecules, such as hormones, that cause a cell to respond.
Amino acid-based hormones • Amino acid-based hormones bind to receptor proteins on the cell membrane and are called first messengers. • Second messengers are then activated. A second messenger is a molecule that initiates changes inside a cell in response to the binding of a specific substance to a receptor on the outside of a cell. • Changes to the cell through enzyme action then occur in a cascade fashion.
Steroid and Thyroid Hormones • Steroid and thyroid hormones are similar because they are both fat soluble. • Because these hormones are fat soluble they can pass through the cell membrane. • Thus, these hormones can enter their target cells and bind directly to receptor sites in the cytoplasm or nucleus and directly activate enzymes.
Two other types of chemical messengers that are classified as hormones are neuropeptides and prostaglandins. • Neuropeptides are hormones secreted by the nervous system and tend to affect many cells near the nerve cells that release them. • Prostaglandins are modified fatty acids that are secreted by most cells and tend to accumulate in areas where tissues are disturbed or injured.
Hypothalamus and Pituitary Gland • Two organs, the hypothalamus and the pituitary gland, control the initial release of many hormones for the endocrine system. • The hypothalamus is the area of the brain that coordinates many activities of the nervous and endocrine systems.
The hypothalamus secretes: • 1. ADH = Antidiuretic Hormone=stimulates reabsorption of water in the body. • 2. Oxytocin- Used during childbirth to bring on labor contractions.
The nerve cells in the hypothalamus that secrete hormones are calledneurosecretory cells. • These cells secrete two types of hormones to the pituitary gland. • Releasing hormonesstimulate the anterior pituitary to make and secrete hormones. • Release-inhibiting hormonesinhibit production and secretion of anterior-pituitary hormones.
1. Known as the master gland. Controls all other glands. 2. It is stimulated by hormones from the hypothalamus to target other glands. The Pituitary Gland
The six hormones of the adenohypophysis: • Are abbreviated as GH, TSH, ACTH, FSH, LH, and PRL • Regulate the activity of other endocrine glands • In addition, pro-opiomelanocortin (POMC): • Has been isolated from the pituitary • Is enzymatically split into ACTH, opiates, and MSH
Produced by somatotropic cells of the anterior lobe that: Stimulate most cells, but target bone and skeletal muscle Promote protein synthesis and encourage the use of fats for fuel Most effects are mediated indirectly by somatomedins Growth Hormone (GH)
Acromegaly • Widening of bones
Thyroid Stimulating Hormone (Thyrotropin) • Tropic hormone that stimulates the normal development and secretory activity of the thyroid gland • Triggered by hypothalamic peptide thyrotropin-releasing hormone (TRH) • Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of TSH
Adrenocorticotropic Hormone (Corticotropin) • Stimulates the adrenal cortex to release corticosteroids • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in a daily rhythm • Internal and external factors such as fever, hypoglycemia, and stressors can trigger the release of CRH
Gonadotropins • Gonadotropins – follicle-stimulating hormone (FSH) and luteinizing hormone (LH) • Regulate the function of the ovaries and testes • FSH stimulates gamete (egg or sperm) production • Absent from the blood in prepubertal boys and girls • Triggered by the hypothalamic gonadotropin-releasing hormone (GnRH) during and after puberty
Functions of GonadotropinsIn females • LH works with FSH to cause maturation of the ovarian follicle • LH works alone to trigger ovulation (expulsion of the egg from the follicle) • LH promotes synthesis and release of estrogens and progesterone
Functions of GonadotropinsIn males • LH stimulates interstitial cells of the testes to produce testosterone • LH is also referred to as interstitial cell-stimulating hormone (ICSH)
Prolactin (PRL) • In females, stimulates milk production by the breasts • Triggered by the hypothalamic prolactin-releasing hormone (PRH) • Inhibited by prolactin-inhibiting hormone (PIH) • Blood levels rise toward the end of pregnancy • Suckling stimulates PRH release and encourages continued milk production
The Posterior Pituitary and Hypothalamic Hormones • Posterior pituitary – made of axons of hypothalamic neurons, stores antidiuretic hormone (ADH) and oxytocin • ADH and oxytocin are synthesized in the hypothalamus • ADH influences water balance • Oxytocin stimulates smooth muscle contraction in breasts and uterus • Both use PIP-calcium second-messenger mechanism
Oxytocin • Oxytocin is a strong stimulant of uterine contraction • Regulated by a positive feedback mechanism to oxytocin in the blood • This leads to increased intensity of uterine contractions, ending in birth • Oxytocin triggers milk ejection (“letdown” reflex) in women producing milk
Antidiuretic Hormone (ADH) • ADH helps to avoid dehydration or water overload • Prevents urine formation • Osmoreceptors monitor the solute concentration of the blood • With high solutes, ADH is synthesized and released, thus preserving water • With low solutes, ADH is not released, thus causing water loss from the body • Alcohol inhibits ADH release and causes copious urine output
The tropic hormones that are released are: • Thyroid-stimulating hormone (TSH) • Adrenocorticotropic hormone (ACTH) • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH)
3. It secretes: • A. GH= Growth Hormone • B. ACTH= Adrenocorticotrophic Hormone • C. FSH= Follicle Stimulating Hormone • D. LH=Luteinizing Hormone • E. TSH=Thyroid Stimulating Hormone
The Thyroid Gland • The thyroid gland is located near the larynx and helps maintain a normal heart rate, blood pressure, and body temperature by increasing or decreasing cellular metabolic rates. • It secretes thyroxine. • The thyroid gland is also important for development.
The largest endocrine gland, located in the anterior neck, consists of two lateral lobes connected by a median tissue mass called the isthmus Composed of follicles that produce the glycoprotein thyroglobulin Colloid (thyroglobulin + iodine) fills the lumen of the follicles and is the precursor of thyroid hormone Other endocrine cells, the parafollicular cells, produce the hormone calcitonin Thyroid Gland
Thyroid Hormone • Thyroid hormone – the body’s major metabolic hormone • Consists of two closely related iodine-containing compounds • T4 – thyroxine; has two tyrosine molecules plus four bound iodine atoms • T3 – triiodothyronine; has two tyrosines with three bound iodine atoms
Effects of Thyroid Hormone • TH is concerned with: • Glucose oxidation • Increasing metabolic rate • Heat production • TH plays a role in: • Maintaining blood pressure • Regulating tissue growth • Developing skeletal and nervous systems • Maturation and reproductive capabilities
Abnormal thyroid activity can result in hypothyroidism or hyperthyroidism. • Overproduction of thyroid hormones is calledhyperthyroidismand can have symptoms that vary from overactivity to high body temperature. • A deficiency in a thyroid hormone is known ashypothyroidismand can have symptoms that vary from weight gain to retardation.
Tiny glands embedded in the posterior aspect of the thyroid Cells are arranged in cords containing oxyphil and chief cells Chief (principal) cells secrete PTH PTH (parathormone) regulates calcium balance in the blood Parathyroid Glands
PTH release increases Ca2+ in the blood as it: Stimulates osteoclasts to digest bone matrix Enhances the reabsorption of Ca2+ and the secretion of phosphate by the kidneys Increases absorption of Ca2+ by intestinal mucosal cells Rising Ca2+ in the blood inhibits PTH release Effects of Parathyroid Hormone
Parathyroid Gland • The parathyroid glands is made up of four glands embedded in the two thyroid glands. • These glands secrete theparathyroid hormone, (calcitonin) which stimulates the transfer of calcium ions from the bones to the blood.