The body contains two kinds of glands: endocrine and exocrine
Exocrine Glands: (sudoriferous, sebaceous, and digestive) secrete their products through ducts into body cavities or onto body surfaces.
Endocrine Glands: Secrete their products (hormones) into the extracellular spaces around the secretory cells, rather than into ducts. The secretion then diffuses into capillaries and is carried away by the blood.
Certain parts of the nervous system causes muscles to contract and glands to secrete; the endocrine system. Hormones, in turn, may promote or inhibit the generation of nerve impulses.
The nervous system causes muscles to contract and glands to secrete the endocrine system affects virtually all body tissues, altering metabolic activities, regulating growth and development and guiding reproductive processes.
Nerve impulses are generally much more rapid in producing their effects than are hormones; the effects of the nervous system are also quite brief compared with those of the endocrine system.
Water-soluble hormones alter cell function by activating plasma membrane receptors, which initiate a cascade of events inside the cell.
After a water-soluble hormone is released from an endocrine gland, it circulates in the blood, reaches a target cell, and brings a specific message to that cell; since such a hormone can deliver its message only to the plasma membrane, it is a called the “first messenger.”
A second messenger is needed to relay the message inside the cell where hormone-stimulated responses can take place; the best known second messenger is cyclic AMP (cAMP) but other substances are known second messengers.
G-Proteins are a common feature of the most second messenger systems; the symptoms of cholera are a direct result of the cholera toxin on G-Proteins in the intestinal lining.
Most water-soluble hormones circulate in plasma in a free, unattached form.
Most lipid-soluble hormones bind to transport proteins to be carried in blood.
The transport proteins improve the transportability of lipid-soluble hormones by making them temporarily water-soluble, retard passage of the small hormone molecules through the kidney filter thus slowing the rate of hormone loss in urine, and provide a ready reserve of hormone already present in blood.
Protein and peptide hormones, such as insulin, will be destroyed by digestive enzymes and must be given by injection.
The responsiveness of a target cell to a hormone depends on several factors: the hormone’s concentration, the abundance of the target cell’s hormone receptors, and the influences exerted by the other hormones.
A target cell responds more vigorously when the level of a hormone rises or when it has more receptors (up-regulation) .
In addition, the actions of some hormones on target cells require a simultaneous or recent exposure to a second hormone. The second hormone in this case is said to have a Permissive Effect.
Examples of Permissive Effect: Epinephrine alone only weakly stimulates lipolysis (the breakdown of triglycerides) but when small amounts of thyroid hormones (T3 and T4) are present, the same amount of epinephrine stimulates lipolysis much more powerfully.
Sometimes the permissive hormone increases the number of receptors for the other hormone and sometimes it promotes the synthesis of an enzyme required for the expression of the other hormone’s effects.
Most hormones are released in short bursts, with little or no release between bursts. Regulation of hormone secretion normally maintains homeostasis and prevents overproduction or underproduction of a particular hormone; when these regulating mechanisms do not operate properly, disorders result.
Hormone secretion is controlled by signals from the nervous system, by chemical changes in the blood, and by other hormones.
Most often, negative feedback systems regulate hormonal secretions.
The hypothalamus is the major integrating link between the nervous and endocrine systems.
The hypothalamus and the pituitary gland (hypophysis) regulate virtually all aspects of growth, development, metabolism, and homeostasis.
The pituitary gland is located in the sella turcica of the sphenoid bone and is differentiated into the anterior pituitary ( adenohypophysis ), the posterior pituitary ( neurohypophysis ), and pars intermedia (avascular zone in between).
The thyroid gland is located just below the larynx and has right and left lateral lobes (Figure 18.10a).
Histologically, the thyroid consists of the thyroid follicles composed of follicular cells , which secrete the thyroid hormones thyroxine ( T4 ) and triiodothyronine ( T3 ), and parafollicular cells , which secrete calcitonin ( CT ).
Formation, Storage, and Release of Thyroid Hormones
Thyroid hormones are synthesized from iodine and tyrosine within a large glycoprotein molecule called thyroglobulin ( TGB ) and are transported in the blood by plasma proteins, mostly thyroxine-binding globulin (TBG).
The formation, storage, and release steps include iodide trapping, synthesis of thyroglobulin, oxidation of iodide, iodination of tyrosine, coupling of T1 and T2, pinocytosis and digestion of colloid, secretion of thyroid hormones, and transport in blood.
Thyroid hormones regulate oxygen use and basal metabolic rate, cellular metabolism, and growth and development.
Secretion of thyroid hormone is controlled by the level of iodine in the thyroid gland and by negative feedback systems involving both the hypothalamus and the anterior pituitary gland.
Calcitonin lowers the blood level of calcium. Secretion is controlled by calcium levels in the blood.
The parathyroid glands are embedded on the posterior surfaces of the lateral lobes of the thyroid and contain principal cells , which produce parathyroid hormone , and oxyphil cells , whose function is unknown.
Parathyroid hormone ( PTH ) regulates the homeostasis of calcium and phosphate by increasing blood calcium level and decreasing blood phosphate level.
PTH increases the number and activity of osteoclasts, increases the rate of Ca+2 and Mg+2 from reabsorption from urine and inhibits the reabsorption of HPO4-2 so more is secreted in the urine, and promotes formation of calcitriol, which increases the absorption of Ca+2, Mg+2,and HPO4-2 from the GI tract.
Blood calcium level directly controls the secretion of calcitonin and parathyroid hormone via negative feedback loops that do not involve the pituitary gland.
Adrenal Medulla--The adrenal medulla consists of hormone-producing cells, called chromaffin cells, which surround large blood-filled sinuses.
Medullary secretions are epinephrine and norepinephrine (NE), which produce effects similar to sympathetic responses.
They are released under stress by direct innervation from the autonomic nervous system. Like the glucocorticoids of the adrenal cortex, these hormones help the body resist stress. However, unlike the cortical hormones, the medullary hormones are not essential for life.
Ovaries are located in the pelvic cavity and produce sex hormones ( estrogens and progesterone ) related to development and maintenance of female sexual characteristics, reproductive cycle, pregnancy, lactation, and normal reproductive functions. The ovaries also produce inhibin and relaxin.
Testes lie inside the scrotum and produce sex hormones (primarily testosterone ) related to the development and maintenance of male sexual characteristics and normal reproductive functions. The testes also produce inhibin .
The pineal gland ( epiphysis cerebri ) is attached to the roof of the third ventricle, inside the brain.
Histologically, it consists of secretory parenchymal cells called pinealocytes , neuroglia cells, and scattered postganglionic sympathetic fibers. The pineal secrets melatonin in a diurnal rhythm linked to the dark-light cycle.
Seasonal affective disorder ( SAD ), a type of depression that arises during the winter months when day length is short, is thought to be due, in part, to over-production of melatonin. Bright light therapy, repeated doses of several hours exposure to artificial light as bright as sunlight, may provide relief for this disorder and for jet lag.
Several body tissues other than those usually classified as endocrine glands also contain endocrine tissue and thus secrete hormones.
Eicosanoids--Eicosanoids, ( prostaglandins [ PGs ] and leukotrienes [ LTs ]) act as paracrines and autocrines in most body tissues by altering the production of second messengers, such as cyclic AMP.
Prostaglandins have a wide range of biological activity in normal physiology and pathology.
Aspirin and related nonsteroidal anti-inflammatory drugs ( NSAIDS ), such as ibuprofen and acetaminophen, inhibit a key enzyme in prostaglandin synthesis and are used to treat a wide variety of inflammatory disorders.
Growth Factors-- Growth factors are hormones that stimulate cell growth and division. Examples include epidermal growth factor (EGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), nerve growth factor (NGF), tumor angiogenesis factors (TAFs), insulinlike growth factor (IFG), and cytokines.
Homeostatic mechanisms attempt to counteract the everyday stresses of living. If successful, the internal environment maintains normal physiological limits of chemistry, temperature, and pressure. If a stress is extreme, unusual, or long-lasting, however, the normal mechanisms may not be sufficient, triggering a wide-ranging set of bodily changes called the stress response or general adaptation syndrome ( GAS ).
Unlike the homeostatic mechanisms, this syndrome does not maintain a constant internal environment. It does just the opposite to prepare the body to meet an emergency.
Productive stress is termed eustress ; whereas, harmful stress is termed distress .
The stimuli that produce the general adaptation syndrome are called stressors .
Stressors include almost any disturbance: heat or cold, surgical operations, poisons, infections, fever, and strong emotional responses.