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ENDOCRINE SYSTEM
Functions of the Endocrine System o Water equilibrium: Controls water equilibrium Regulating solute concentration of the blood o Growth, metabolism, and tissue maturation: Controls the growth of many tissues ( bone and muscle ) Degree of metabolism of various tissues which aids Maintenance of the normal body temperature Normal mental functions Maturation of tissues appears in the development of adult features and adult behavior Determined by the endocrine system o Heart rate and blood pressure management: Assist in managing the heart rate and blood pressure Aids in preparing the body for physical motion o Immune system control: Helps regulate the production and functions of immune cells o Reproductive function controls: Regulates the development and the functions of the reproductive systems in males and females o Uterine contractions and milk release: Controls urine contractions throughout the delivery of the newborn Stimulates milk release from the breast in lactating females o Ion management: Regulates 𝑵𝒂+, 𝑲+, 𝒂𝒏𝒅 𝑪𝒂𝟐+^ concentrations in the blood. o Blood glucose regulator: Controls blood glucose levels and other nutrient levels in the blood o Direct gene activation: Steroid hormones can diffuse through plasma membranes of their target cells Steroid hormone enters the nucleus and binds to a specific receptor Hormone-receptor complex binds to specific sites on the cell’s DNA Activating certain genes to transcribe messenger RNA mRNA then is translated in the cytoplasm = synthesis of new proteins o Second messenger system: water-soluble , nonsteroidal hormones-protein , and peptide hormones are unable to enter the target cells bind to receptors situated on the target cell’s plasma membrane utilize a second messenger system Anatomy of the Endocrine System o Organs of the endocrine system are small and unimpressive o Very impressive o Role in maintaining body homeostasis o True GIANTS
Hypothalamus o Major endocrine organs of the body include: The pituitary Pineal Thyroid Thymus glands Parathyroid Pancreas Adrenal Gonads o Hypothalamus: Part of the nervous system Considered a major endocrine organ because it produces several hormones Important autonomic nervous system Endocrine control center of the brain located inferior to the thalamus o Mixed functions: Function of some hormone producing glands is purely endocrine Function of others Pancreas and gonads is mixed both endocrine and exocine Pituitary Gland o Approximately the size of a pea Location : Hangs by a stalk from the inferior surface of the hypothalamus of the brain Snugly surrounded by the “ Turk’s saddle ” of the sphenoid bone Lobes: 2 functional lobes Anterior pituitary ( glandular tissue ) Posterior pituitary ( nervous tissue ) Hormones of the Anterior Pituitary Growth hormone (GH): General metabolic hormone Major effects are directed to the growth of skeletal muscles and long bones of the body Protein-sparing and anabolic hormone Causes amino acids to be built into proteins Stimulates most target cells to grow in size and divide Prolactin (PRL): Protein hormone structurally similar to growth hormone Known target in humans is the breast Stimulates and maintains milk production by the mother’s breast Adrenocorticotropic hormone (ACTH): Regulates the endocrine activity of the cortex Portion of the adrenal gland Thyroid-stimulating hormone (TSH): Also called Thyrotropin hormone influences the growth and activity of the thyroid gland
Location: Located at the base of the throat ( just inferior to the Adam’s apple ) Easily palpated during a physical examination Lobes: ^ Fairly gland consisting of two lobes joined by a central mass ( Isthmus ) Composition: Composed of hollow structures called follicles Store a sticky colloidal material Types of thyroid hormones: Often referred to as the body’s major metabolic hormone Actually has 2 active iodine-containing hormones: Thyroxine ( T4 ) Triidothyronine ( T3 ) Thyroxine: Major hormone secreted by the thyroid follicles Triidothyroni ne: Mostly formed at the target tissue by conversion of the thyroxine to triidothyronine Function: Controls the rate at which glucose is “ burned ” Oxidized and converted to body heat and chemical energy Important for normal tissue growth and development Calcitonon: ↓ blood calcium levels by causing calcium to be deposited in the bones Calcitonin is made by the so-called “ Parafollicular cells ” found in the connective tissues between the follicles Parathyroid Glands o Mostly tiny masses of glandular tissue Location: Located on the posterior surface of the thyroid gland Parathormone: Secrete parathyroid hormone ( PTH ) which is the most important regulator of calcium ion homeostasis of the blood PTH is a hypercalcemic hormone Acts to ↑ blood levels of calcium Calcitonin is a hypocalcemic hormone PTH stimulates the kidneys and intestines to absorb more calcium Adrenal Glands o May looks like a single organ nut it is structurally and functionally two endocrine organs in one
Hormones of the Adrenal Cortex o Produces three major groups of steroids hormone collectively called corticosteroids: Mineralcorticoids Glucocorticoids Sex hormones o Mineralocorticoids: ^ Primarily^ aldosterone^ produced by the outermost adrenal cortex cell layer Important in regulating the mineral ( or salt ) content of blood Concentrations of sodium and potassium ions and they also help in regulating the water and electrolyte balance in the body o Renin: An enzyme produced by the kidneys when the blood pressure drops Cause the release of aldosterone by triggering a series of reactions that form angiotension II A potent stimulator of aldosterone release o Atrial natriuretic peptide (ANP): Prevents aldosterone release Goal is to reduce blood volume and blood pressure o Glucocorticoids: The middle cortical layer mainly produces glucocorticoids Include cortisone and cortisol Promote normal cell metabolism and help the body to resist long-term stressors Primarily by ↑ blood glucose levels said to be a Hyperglycemic hormone Also reduce pain and inflammation by inhibiting some pain- causing molecules called prostaglandins o Sex hormone: ^ Both male and female sex hormones are produced by^ adrenal cortex Bulk of sex hormones produced by the innermost cortex layer are: Androgens (male sex hormones) Estrogens (female sex hormones) Hormones of the Adrenal Medulla o Like the posterior pituitary develops from a knot of nervous tissue Catecholamines: When medulla is stimulated by sympathetic nervous system neurons, it cells release two similar hormones Epinephrine (adrenaline) Norepinephrine (noradrenaline) In the bloodstream these hormones are referred to as cathecholamines Function: ^ ↑^ heart rate, blood pressure and blood glucose levels Dilate the small passageways of the lungs Prepare the body to cope with a brief or short-term stressful situation The so-called “ Alarm Stage ” of the stress response
Thymus Gland o Large in infants and children o ↓ in size through adulthood Location: ^ Upper thorax posterior to the sternum Thymosin : Essential for normal development of a special group of white blood cells ( T-lymphocytes or T cells ) Essential for immune response Gonads o Female and male gonads produce sex hormones o Sex hormones are identical to those produced by adrenal cortex cells o Major difference are the source and relative amount produced Hormones of the Ovaries o Ovaries are like a pair of almond-sized organs Location: Pelvic cavity Steroid hormones: 2 groups of steroid hormones Estrogen Progesterone Estrogen: Responsible for the development of sex characteristics at puberty Acting with progesterone Promote breast development Promote cyclic changes in the uterine lining ( menstrual cycle ) Progesterone : Acts with estrogen to bring about menstrual cycle Quiets the muscles of the uterus so that an implanted embryo will not be aborted during pregnancy Helps prepare breast tissue for lactation Hormones of the Testes o Paired oval organs in a sac Location: The testes are suspended in a sac ( scrotum ) outside the pelvic cavity Male sex hormones: Sperm Male sex hormones ( androgens ) Testosterone is the most important Testosterone: Promotes growth and maturation Causes the male’s secondary sex characteristics to appear and stimulates male sex drive Necessary for the continuous production of sperm Placenta o Remarkable organ formed temporarily in the uterus of pregnant women Function: Respiratory, excretory and nutrition delivery systems for the fetus Produces several proteins and steroid hormones Human chorionic gonadotropin: hCG produced by the developing embryo and then by the fetal part of the placenta stimulates the ovaries to continue producing estrogen and progesterone helps the lining of the uterus to not sloughed off in the menses Human placental lactogen: hPL works cooperatively with estrogen and progesterone in preparing the breasts for lactation Relaxin: ^ Another placenta hormone
Causes the pelvic ligaments and pubic symphysis to relax and become more flexible Eases birth passage Physiology of the Endocrine System o Major processes of endocrine system that they control are: Reproduction, growth and development Mobilizing body’s defenses against stressors Maintaining electrolyte, water and nutrient balance of blood Regulating cellular metabolism Energy balance The Chemistry of Hormones o The key to the incredible power of endocrine glands is the hormones they produce and secrete. Hormones: Chemical substances that are secreted by endocrine cells into the extracellular fluids Regulate metabolic activity Classification: Amino acid-based molecules ( steroids ) Proteins Peptides Amines Steroid hormones: Made from cholesterol Include sex hormones ( gonads ) Hormones ( adrenal cortex ) Amino acid-based hormones: All the others are nonsteroid amino acid derivatives Mechanism of Hormone Action o Blood-borne hormones circulate to all organs of the body o Given hormone affects only certain tissue cells or organs Target cells: Specific protein receptors must be present on its plasma membrane Only when binding occurs can the hormone influence the working of cells The function of hormones: By altering cellular activity ↑ or ↓ the rate of a normal or usual metabolic process rather than stimulating a new one Changes in hormone binding:
- Changes in plasma membrane permeability ( electrical state )
- Synthesis of protein ( such as enzymes ) in the cell.
- Activation or inactivation of enzymes
- Stimulation of mitosis
- Promotion of secretory activity Control of Hormone Release Negative feedback mechanisms: The chief means of regulating blood levels of nearly all hormones Endocrine gland stimuli: Activate the endocrine organs fall into 3 major categories : Hormonal Humoral Neural Hormonal stimuli: Most common stimulus Endocrine organs are prodded into action by other hormones Humoral stimuli: Changes blood levels of certain ions Nutrients may also stimulate hormone release
