Endocrine System Chemical Control
Endocrine System - the system that secretes hormones in the body - hormones can last for minutes or for hours - a major gland, once called the master gland, is the pituitary - things under this system include regulation of blood sugar levels and your metabolism
Endocrine Functions: Hormones Hormones: Come from endocrine glands Circulate in the blood stream Act on specific cells in the body
Hormones can be classified as fast acting or slow acting. It is also interesting to note that some hormones travel bound to plasma proteins Alterations in plasma protein concentrations (as can result from say liver failure) can lead to abnormal endocrine function. A malfunctioning liver may fail to break down hormones which will also manifest itself as an endocrine disorder.
Notice that glandular tissue receives 'input' from a number of sources including signals (via hormones) from other glands, nervous input, and importantly by monitoring the extracellular environment in which it sits. Those tissues which are stimulated by hormones often require a low level of that hormone just to remain healthy; larger concentrations causing the tissue to secrete. The influence of the low levels of hormone is known as the trophic effect of a hormone.
Next to the nervous system the endocrine system is the system that controls the body. However, the endocrine system uses hormones to stimulate the metabolic activities of the cells. These hormones are released into the blood stream. Tissue response to hormones usually occur after a lag time of seconds or days. Once started the responses tend to be much more prolonged than those that are induced by the nervous system.
Control in The Endocrine System The endocrine system comprises (like the nervous system) of relatively simple components, which interact and work together in a complex fashion. With respect to the endocrine system, there are two roles for negative feedback. The first is the control of the internal environment - regulating some internal 'variable'; the second is control of the release of hormones.
Example of a Negative Feedback Loop: Homeostasis
Location of Major Endrocrine Organs Figure 9.3 Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 9.14
Endocrine System Characteristics Access to every cell Each hormone acts only on specific cells (target cells) Only specific cells have receptors for specific hormones Endocrine control slower than nervous system Endocrine and nervous systems interact
Classification of Hormones: Steroid Steroid Hormones: Lipid soluble, chemically derived from cholesterol Enter target cells Activate specific genes to produce specific proteins Slower acting than nonsteroid hormones, minutes to hours
Steroid Hormone Action on a Target Cell Figure 13.2
Steroid Hormone Action Video
Classification of Hormones: Nonsteroid Nonsteroid hormones: Water soluble Bind to receptors on target cell membranes Work through intermediate mechanisms to activate existing enzymes Faster action than steroid hormones, seconds to minutes
Nonsteroid Hormone Action on a Target Cell Figure 13.3
Non Steroid Hormone Action Video
The Pituitary Gland The pituitary gland is a small oval-shaped gland found at the base of the brain below the optic nerve. It secretes hormones which control and regulate the other glands in the body. These glands release hormones which are responsible for the body's growth and metabolism.
Hormones of the Pituitary Gland Table 13.1
Hypothalamus and the Anterior Pituitary Gland Figure 13.7
Hypothalamus and the Anterior Pituitary Gland Anterior pituitary Connection to hypothalamus: releasing and inhibiting hormones from hypothalamus travel to pituitary through pituitary portal system Hormones ACTH: stimulates adrenal cortex TSH: acts on thyroid gland FSH, LH, Prolactin: related to control of reproductive cycles and lactation Growth Hormone: widespread effects on body
Hypothalamus and the Posterior Pituitary Gland Posterior pituitary Connection to hypothalamus: hormones made in hypothalamus, stored in posterior pituitary Hormones: nonsteroidal Antidiuretic hormone (ADH): conserves water in kidneys, regulates water balance in body Oxytocin: causes uterine contractions during labor and milk ejection through neuroendocrine reflex
Pituitary Disorders Gigantism: hypersecretion of growth hormone Pituitary Dwarfism: hyposecretion of growth hormone
Pituitary Gigantism
Various Hormones Growth hormone - controls growth Prolactin - stimulates production of milk after childbirth ACTH (adrenocorticotrophic hormone) - stimulates production of hormones from the adrenal glands TSH (thyroid stimulating hormone) - stimulates production of hormones from thyroid gland FSH (follicle stimulating hormone) and LSH (luteinizing stimulating hormone) - stimulate activity in the ovaries of women and the testes in men. ADH (anti-diuretic hormone) - controls the concentration of urine Oxytocin - stimulates the contraction of the womb during childbirth and the secretion of milk for breast feeding.
The organs of the endocrine system are small and unimpressive. The endocrine glands of the body include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus. Hormones are chemical compounds that regulate metabolic function of cells in the body. There are two types of hormones: amino acid-based hormones and steroids. Most hormones are amino acid-based. Steroid hormones are synthesized from cholesterol. Only the gonadal hormones and adrenocortical hormones are steroids.
A hormone is a chemical that is synthesized and secreted by a group of specialized cells that are called glands. Glands are classified in two ways: exocrine or endocrine. Exocrine glands use ducts to transport their secretions. Endocrine glands, on the other hand, don t use ducts to transport their hormones. Instead they secrete into the blood stream for distribution.
Another key component of the endocrine system is the receptors for the hormones on the surface of the cell membrane. Receptors are composed of proteins and imbedded in the cell membrane just like other proteins. There are over 50 hormones in the human body, and they can be grouped together by their chemical structure. Steroids are produced from cholesterol. Peptides are chains of amino acids. Other hormones are derived from amino acids. Several glands comprise the endocrine system. The hypothalamus integrates the endocrine system and the nervous system. It receives signals from the brain and the peripheral nerves. It then initiates the endocrine system. The cells that comprise the hypothalamus are called nuerosecretory cells. The cells are able to receive a nerve impulse just like a nerve cell would, but instead of passing it on to another cell they trigger the release of hormones in the blood stream.
The pituitary is sometimes referred to as the master gland. However, most of the time the pituitary receives orders from the hypothalamus. There are two lobes on the pituitary. The posterior lobe, called the neurhypophysis, stores hormones manufactured by the hypothalamus and secretes them on command. The anterior lobe, called the adenohypophysis, is able to produce it s own hormones which act on other glands.
Endocrine Glands The thyroid gland is located on the front surface of the trachea. It has two lobes, and it produces two hormones: Triiodothyronine (T3) and Thyroxine (T4). Both T3 and T4 have the same effect on the target cell, but T3 is usually most active. The thyroid also regulates metabolism. The thyroidal secretions are controlled by both the hypothalamus and the pituitary.
The pancreas secretes insulin, a hormone that regulates glucose in take of the cells. A deficiency in this hormone results in diabetes mellitus. Insulin regulates the blood sugar levels by stimulating cells to take in glucose. It also stimulates the synthesis of protein and fat storage. The adrenal medulla is the gland behind the fight-orflight reaction. It secretes two hormones: epinephrine and norepinephrine. (Epinephrine is also known as adrenaline.) Epinephrine secretion is triggered by stress, both positive and negative stress. It increases the rate and the stroke volume of the heart.
Pancreas: Endocrine Functions Structures: islets of Langerhans Hormones: nonsteroidal Glucagon: made by alpha cells, raises blood sugar Insulin: made by beta cells, lowers blood sugar
Roles of Insulin and Glucagon in Regulating Blood Glucose Figure 13.9
Adrenal Glands: Cortex Adrenal cortex Secretion: mediated through hypothalamuspituitary secretions Hormones: steroidal Glucocorticoids: cortisol blood glucose regulation Mineralocorticoids: aldosterone sodium/fluid/urine
Secretion of Cortisol Figure 13.10
Adrenal Glands: Medulla Adrenal medulla: neuroendocrine organ Secretion: sympathetic nervous system Hormones: nonsteroidal Epinephrine and Norepinephrine: enhance function of sympathetic nervous system (Fight or flight)
Roles of the Hypothalamus and Adrenal Glands in the Stress Response Figure 9.12 Slide 9.31
Thyroid Gland Secretion: mediated through hypothalamuspituitary secretions Steroidal hormones: Thyroxine (T 4 ) and Triiodothyronine (T 3 ): both regulate production of ATP from glucose, affects metabolic rate Calcitonin: lowers blood calcium levels
Negative Feedback Control of Thyroxine Secretion Figure 13.13
Homeostatic Regulation of Blood Calcium Concentration
Homeostatic Regulation of Blood Calcium concentration (cont.)
Parathyroid Glands Secretion: response to lowered blood calcium levels Hormone: Parathyroid hormone (PTH), nonsteroidal Removes calcium and phosphate from bone Increases absorption of calcium by the digestive tract Increases renal retention of calcium and excretion of phosphate
The testis and ovaries secrete hormones that are grouped into three major categories: androgens, estrogens, and progestins. All three are found in both males and females, but each in varying amounts. Androgens are primarily produces by the testis. Testosterone is the primary hormone in this group. Androgens stimulate the maintenance and development of the male reproductive system. Estrogens are the female equivalent of Androgens. They maintain the female reproductive system and trigger the development of female second sex characteristics. Progestins maintain and prepare the uterus for growth and development of an embryo.
Testes Hormone: testosterone (steroidal) Functions: Regulates development and normal functioning of sperm, male reproductive organs, male sex drive Development of male secondary sex characteristics
Ovaries Hormones (steroidal) and functions: Estrogen: initiates development of secondary sex characteristics, regulates menstrual cycle Progesterone: regulates menstrual cycle
Ovarian Cycle Figure 16.6
Disorders of the Endocrine System Thyroid Hypothyroidism: Children: cretinism Adults: myxedema Hyperthyroidism: Graves Disease Adrenal glands Addison s Disease: failure of adrenal cortex, cortisol deficiency Cushing s Syndrome: excessive cortisol
Grave s Disease overactive thyroid