Anatomy & Physiology

1. Anatomy & Physiology Lesson 8

2. THE ENDOCRINE SYSTEM • Two body systems are responsible for sending and receiving sensory information and coordinating body responses. These are the nervous system and the endocrine system. Together, they are sometimes referred to as the neuro-endocrine system. • The endocrine system regulates body activities by releasing hormones (chemical messengers) into the bloodstream, where they are carried throughout the entire body. • Hormonal responses may be almost instantaneous, or may occur days later. • There is a wide variety of hormonal effects.

3. HORMONE FUNCTIONS • Regulate the chemical composition and volume of the internal environment (extracellular fluid). • Help regulate metabolism and energy balance. • Help regulate contraction of smooth and cardiac muscle fibers and secretion by glands. • Help maintain homeostasis, despite disruptions, such as infection, trauma, emotional stress, dehydration, starvation, hemorrhage, and temperature extremes.

4. HORMONE FUNCTIONS • Regulate certain activities of the immune system. • Play a role in the smooth, sequential integration of growth and development. • Contribute to the basic processes of reproduction, including gamete production, fertilization, nourishment of the embryo and fetus, delivery, and nourishment of the newborn.

5. EXOCRINE & ENDOCRINE GLANDS • The body contains two kinds of glands: • Exocrine glands secrete their products into body ducts, which carry the products into body cavities, the lumen of an organ, or the outer surface of the body. • Sudoriferous glands, sebaceous glands, mucous glands, and digestive glands. • Endocrine glands secrete their products (hormones) into the extracellular space around the secretory cells. The secretions diffuse into capillaries and are carried throughout the body by the circulatory system.

6. ENDOCRINE SYSTEM • The endocrine system is composed of the body’s endocrine glands. These include: • The pituitary, thyroid, parathyroid, adrenal, and pineal glands • There are also many organs that have cells which secrete hormones, but are not exclusively endocrine organs. These include: • The hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, and placenta.

7. ENDOCRINE SYSTEM

8. HORMONES • Hormones can have very powerful effects, even when present in very low concentrations. • There are approximately 50 different hormones produced in the human body. Most of these only affect a few types of cells. • The specific cells which are affected by a hormone are called target cells. • Hormones influence their target cells by binding to proteins or glycoproteins in the cell membrane called receptors. Only the target cells for a certain hormone have receptors that will recognize and bind to that hormone.

9. HORMONES • Hormones that pass into the blood and act on distant target cells are called circulating hormones or endocrines. • Circulating hormones may linger in the blood for minutes to hours, exerting their effects for a prolonged period of time. • Eventually, circulating hormones are inactivated by the liver and excreted by the kidneys. • Hormones that act locally without first entering the bloodstream are called local hormones. • Local hormones that act on neighboring cells are called paracrines. • Local hormones that act on the same cell that secreted them are called autocrines. • Local hormones are usually inactivated very quickly.

10. HORMONES • Chemically, there are four principal classes of hormones: • Steroids are lipids derived from cholesterol. • Produced in the adrenal cortex, kidneys, testes, and ovaries. • Include aldosterone, cortisol, androgens, calcitriol, testosterone, extrogens, and progesterone. • Biogenic amines are structurally simple molecules, often synthesized by modifying tyrosine (an amino acid). • Produced in the thyroid gland, adrenal medulla, mast cells, blood platelets, and pineal gland. • Include thyroid hormones (T3 and T4), epinephrine, norepinephrine, histamine, serotonin, and melatonin.

11. HORMONES • Peptides and proteins consist of chains of 3-200 amino acids. Some also have carbohydrate groups attached, making them glycoprotiens. • Produced in the hypothalamus, anterior pituitary gland, pancreas, parathyroid glands, thyroid gland, stomach, small intestine, and kidneys. • Include hypothalamic releasing and inhibiting hormones, oxytocin, antidiuretic hormone, anterior pituitary gland hormones, insulin, glucagon, somatostatin, pancreatic polypeptide, parathyroid hormone, calcitonin, digestion-regulating hormones, and erythropoietin. • Eicosanoids are derived from arachidonic acid, a 20-carbon fatty acid. They act as local hormones, and sometimes also as circulating hormones. • Produced by all cells except red blood cells. • Include prostaglandins and leukotrienes.

12. MECHANISMS OF HORMONE ACTION • The response to a hormone is dependent upon both the hormone and the target cell. The same hormone can have very different effects on different target cells. • Possible hormone effects include synthesis of new molecules, changing the permeability of the plasma membrane, stimulating transport of a substance into or out of the target cell, altering the rate of a metabolic reaction, or causing contraction of smooth or cardiac muscles.

13. MECHANISMS OF HORMONE ACTION • Lipid-soluble hormones (including steroids and thyroid hormones) are able to pass through the plasma membrane of the target cell, so their receptors are located within the cell. • These hormones affect cell function by altering gene expression.

14. MECHANISMS OF HORMONE ACTION • Water-soluble hormones (including catecholamine, peptide, and protein hormones) cannot pass through a plasma membrane, so must attach themselves to integral protein receptors that extend into the interstitial fluid. Since these hormones can only deliver their messages to the plasma membrane, they are called first messengers. A second messenger is required to relay the message into the interior of the cell. • These hormones affect cell function by activating plasma membrane receptors, initiating a cascade of intracellular events.

15. CONTROL OF HORMONE SECRETION • Secretion of hormones is regulated to prevent oversecretion or undersecretion. • Hormone secretion is controlled by signals from the nervous system, chemical changes in the blood, or other hormones. • Usually, negative feedback systems regulate hormonal secretions.

16. THE PITUITARY GLAND • The pituitary gland or hypophysis is attached to the hypothalamus at the base of the brain. • The hypothalamus is the major integrating link between the nervous and endocrine systems. • Although the pituitary gland is sometimes called the “master” endocrine gland, it is actually subject to control by the hypothalamus, which receives input from other regions of the brain. • Together, the hypothalamus and pituitary gland regulate virtually all aspects of growth, development, metabolism, and homeostasis. • The pituitary gland can be divided into the posterior pituitary and anterior pituitary.

17. THE POSTERIOR PITUITARY • The posterior pituitary works as a unit with the hypothalamus. • Although the posterior pituitary does not synthesize its own hormones, it does store and release oxytocin (OT) and antidiuretic hormone (ADH) produced in the hypothalamus. • OT controls uterine contractions during delivery and milk ejection during breastfeeeding. • ADH causes retention of body water, controlling the body’s water-balancing mechanism

18. THE ANTERIOR PITUITARY • The anterior pituitary secretes hormones that control a wide range of bodily activities. • The hypothalamus regulates the anterior pituitary by producing releasing hormones that stimulate release of anterior pituitary gland hormones and inhibiting hormones that suppress release of anterior pituitary gland hormones. • The Anterior pituitary has five princple types of cells which secrete seven major hormones.

19. ANTERIOR PITUITARY CELLS • Somatotrophs produce human growth hormone (hCG) or somatotropin, which stimulates general body growth and regulates certain aspects of metabolism. • Thyrotrophs produce thyroid stimulating hormone (TSH), which controls secretions and other activities of the thyroid gland. • Gonadoptrophs produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Together FSH and LH stimulate the secretion of estrogen and progesterone and the maturation of oocytes in the ovaries and the secretion of testosterone and sperm production in the testes. • Lactotrophs procude prolactin (PRL), which initiates milk production in the mammary glands. • Corticotrophs produce adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH). ACTH stimulates the adrenal cortex to secrete glucocorticoids. MSH affects skin pigmentation.

20. THE THYROID GLAND • The thyroid gland is shaped like a butterfly, located in the front center of the neck, just inferior to the larynx. • The thyroid is responsible for regulating metabolism, growth, and development (including bone, nerve, and muscle tissues). • The thyroid is the only endocrine gland that stores large quantities of its secretory product (usually about a 100 day supply). • Disorders associated with malfunctioning of the thyroid gland include cretinism, myxedema, Graves’ disease, and goiter.

21. THE THYROID GLAND • The thyroid gland produces three hormones: Thyroxine or tetraiodothyronine (T4), triiodothyronine (T3) (which are called thyroid hormones) and calcitonin. • Calcitonin helps to regulate calcium homeostasis. • T3 is the more potent of the thyroid hormones, but T4 is more abundant. • Both T3 and T4 are synthesized from iodine and the amino acid tyrosine, under the influence of a negative feedback loop controlled by the hypothalamus. • Most thyroid hormones becomes bound to blood proteins, making them unavailable for the body to use. Only about 1% of thyroid hormones remain in the free, unbound form which can be utilized by the body.

22. THE PARATHYROID GLANDS • The four parathyroid glands are located on the posterior surface of the thyroid gland. • The parathyroid glands produce three forms of parathyroid hormone (PTH) or parathormone. • PTH responds to Ca2+ levels in the blood to regulate the homeostasis of calcium and phosphate ions. When Ca2+ levels fall in the blood, PTH prompts the release of calcium from the bones. • Too little Ca2+ results in tetany.

23. THE ADRENAL GLANDS • The paired adrenal (suprarenal) glands lie superior to the kidneys. • The adrenal glands are composed of the outer adrenal cortex and the inner adrenal medulla. • The adrenal cortex produces a total of about 40 different hormones which are collectively known as corticosteroids. • The complete loss of adrenocorticol hormones leads to death within a few days to a week, due to dehydration and electrolyte imballances.

24. THE ADRENAL GLANDS • The adrenal cortex produces three classes of hormones: mineralocorticoids (affect mineral homeostasis, help control water and mineral balance), glucocorticoids (affect glucose homeostasis, help regulate normal metabolism and stress resistance), and androgens (male sex hormones). • Aldosterone accounts for about 95% of mineralocorticoid activity. It regulates salt excretion by the kidneys and is involved in the use of carbohydrates. • Cortisol (hydrocortisone), corticosterone, and cortisone are the glucocorticoids. They aid in the production and storage of glucose, reduce fat inflammation following trauma, and are essential to immune function. Cortisol accounts for about 95% of glucocorticoid acctivity.

25. THE ADRENAL GLANDS • The adrenal medulla contains hormone-producing cells called chromaffin cells. • Chromaffin cells are under direct control of the ANS (autonomic nervous system), so hormone release can occur very quickly. • Chromaffin cells produce epinephrine (adrenaline) and norepinephrine (noradrenalin or NE), which are involved in the “fight or flight” response, help resist stress, and help regulate blood pressure.

26. THE PANCREAS • Since the pancreas is classified as both and endocrine organ and an exocrine organ, it will also be discussed again with the digestive system. • The pancreas is a flat organ located posterior and slightly inferior to the stomach. • About 99% of pancreatic cells are arranged in clusters called acini. These exocrine cells produce digestive enzymes. • Located between the acini are tiny groups of endocrine tissue called pancreatic islets or islets of Langerhans.

27. THE PANCREAS • Each pancreatic islet contains four types of hormone-secreting cells: Alpha or A cells, beta or B cells, delta or D cells, and F cells. • Alpha cells secrete glucagon, which increases blood glucose levels. • Beta cells secrete insulin, which decreases blood glucose levels. • Delta cells secrete somatostatin (identical to growth hormone inhibiting hormone secreted by the hypothalamus), which inhibits insulin release and may slow the absorption of nutrients from the GI tract. • F cells secrete pancreatic polypeptide, which inhibits secretion of somatostatin, contraction of the gallbladder, and secretion of pancreatic digestive enzymes.

28. THE OVARIES AND TESTES • The ovaries and testes are paired oval organs referred to as gonads. • The ovaries are the female gonads, located in the pelvic cavity. They secrete estrogens and progesterone, which are responsible for the development and maintenance of female sexual characteristics, as well regulating the female reproductive system [in conjunction with gonadotropic hormones from the pituitary gland]). The ovaries also produce relaxin, which soften connective tissues in preparation for childbirth. • The testes are the male gonads, located in the scrotum. They secrete testosterone, which is responsible for male sexual characteristics, and inhibin, which controls sperm production by inhibiting follicle stimulating hormone.

29. THE PINEAL GLAND • The pineal gland is attached to the roof of the third ventricle of the brain. It is covered by a capsule formed by the pia mater and consists of masses of neuroglia and secretory cells called pinealocytes. • The physiological role of the pineal gland remains unclear. • The pineal gland secretes melatonin, which is thought to promote sleepiness and help regulate the body’s biological clock. In animals that breed during specific seasons, melatonin apparently alters their capacity for reproduction, but it has not been shown to have a similar effect on humans.

30. THE THYMUS • The thymusgland was previously discussed in the lymphatic system lesson. • Hormones produced by the thymus gland are thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. These hormones promote the proliferation and maturation of T cells (white blood cells which destroy microbes and foreign substances). Thymic hormones may also help to retard the aging process.

31. OTHER ENDOCRINE TISSUES • The GI tract synthesizes several digestive hormones, including gastrin, gastric inhibitory peptide(GIP), secretin, and cholecystokinin (CCK). • The placenta produces the pregnancy-related hormones human chorionic gonadotropin (hCG), estrogens, progesterone, relaxin, and human chorionic somatomammotropin. • When deprived of oxygen, the kidneys release erythropoietin, which stimulates red blood cell production. The kidneys also produce calcitriol, the active form of vitamin D. • The atria of the heart produce atrial natriuretic peptide (ANP), which helps regulate blood pressure.