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118 Cards in this Set

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What is Direct Communication?
Occurs between two cells of the same type, and the cells must be in extensive physical contact. Transmission occurs through gap juctions. These chemicals include ions, small solutes, and lipid soluble materials.
What is paracrine communication?
Is the use of chemical messengers to transfer information from cell to cell within a single tissue. The chemicals involve are paracrine factors(cytokines, or local hormones). Transmission is through extracellular fluid.
What is endocrine communication?
Is the use of hormones to corrdinate cellular activities in tissues in distant portions of the body. Transmission is through the cirulatory system. Each hormone has a target cell with appropriate receptors.
What is synaptic communication?
The release of a neurotransmitter at a synaptic cleft very close to the target cells that bear the appropriate receptors. Then an action potential is propagated along the length of the axon. Is ideal for crisis management.
What are hormones?
Chemical messengers that are released in one tissue and transported in the bloodstream to reach specific cells in other tissues. Each hormone has target cells that has receptors needed to bind and read the hormonal message.
What is the difference between paracrine factors and hormones?
Paracrine factors can diffuse out of their tissue orgin and have widespread effects, and hormones can affect their tissue of origin as well as distant targets.
What can hormones do?
1. Stimulate the synthesis of an enzyme or a structural protein not already present in the cytoplasm by activating appropriate genes in the cell nucleus
2. Increase or decrease the rate of synthesis of a particular enzyme or other protein by changing the rate of transcription or translation or
3. Turn an existing enzyme on or off by changing its shape or structure.
What is the endocrine system?
Includes all the endocrine cells and tissues of the body which produce hormones or paracrine factors that have effects beyond their tissues of origin.
What is the difference between endocrine and exocrine cells?
Endocrine cells are glandular secretory cells that release their secretions into the extracellular fluid. Exocrine cells secrete their products onto epithelial surfaces, generally by way of ducts.
What are the three groups of hormones?
1. Amino acid derivatives
2. Peptide hormones
3. Lipid derivatives
What are Amino Acid derivatives?
Are small molecules that are structurally related to amino acids, the building blocks or proteins. Include epinephrine, norepinephrine, dopamine, the thyroid hormones, and melatonin.
What are peptide Hormones and name the two groups.
Are chains of amino acids. Divided into two groups. One large and diverse group includes homones that range from short polypeptide cahins, such as antidiuretic hormone (ADH) and oxytocin, to small proteins, such as growth hormone and prolactin. This group includes all the hormones secreted by the hypothalamus, heart, thymus, digestive tract, pancreas, and posterior lobe of the pituitary gland, As well as most of the hormones secreted by the anterior lobe of the pituitary gland.
The second group consists or glycoproteins. Have more than 200 amino acids long and have carbohydrate side chains. Include thyroid-stimulating hormone(TSH), luteinizing hormone(LH), and follicle-stimulating hormone(FSH) for the anterior lobe of the pituitary gland and other organs.
What are lipid derivatives?
1. steroid hormones, derived from cholesterol
2. eicsanoids, derived from arachidonic acid
What are steroid hormones?
Are released by male and female reproductuve organs (androgens by the testes, estrogens and progestins by the ovaries), the adrenal glands(corticosteroids), and kidneys(calcitriol). They are lipid soluble, can pass through the cell membrane but they are not water soluble.
What are eicosanoids?
Are small molecules with a five-carbon ring at one end. Are important paracrine factors that coordinate cellular activitiesand affect enzymatic processes(such as blood clotting) that occur in extracellular fluids.
Name some important eicosanoids hormones?
Leukotrienes are released by activate white blood cells, or leukocytes. Leukotrienes are important in coordinating tissue responses to injury or disease.
Prostaglandins are produced in most tissues of the body. Are a family of hormones. Involved primarily in coordinating local cellular activities.
Where does hormone release typically occur?
Where capillaries are abundant, and the hormones quickly enter the bloodstream for distribution throughout the body.
Freely hormones are inactivated when?
1. it diffuses out of the bloodstream and binds to receptors in target tissues is absorded and broken down by cells of the liver or kidneys or
3. it is boken down by enzymes in the plasma or interstitial fluid.
They are replaced by the release of bound hormones.
What are the two places hormone receptors are located?
1. On the cell membrane or
2. Inside the cell
Does hormones that bind to receptors in the cell membrane have a direct effect?
Any hormone that binds to receptors in the cell membrane does not have direct effects on the target cell.
What do first and second messengers do?
A hormone acts as a first messenger, a substance that causes the apperance of a second messenger in the cytoplasm. The second messenger functions inside the cell as an enzyme activator, inhibitor, or cofactor, with the net result being a change in the rates of various metabolic reactions.
Name some second messengers.
1. cyclic-AMP, a derivative of ATP
2. cyclic-GMP, a derivative of GTP
3. calcium ions
What is a receptor cascade?
The arrival of a single hormone may promote the release of more than one type of second messenger, or the production of a linked sequence of messengers known as a receptor cascade. Through this the hormone can alter many aspects of cell function at the same time.
What is Down-regulation?
Is the process in which the presence of a hormone triggers a decrease in the number of hormone receptors. When levels of a particular hormone are high, cells become less sensitive to it.
What is up-regulation?
Is a process in which the absence of a hormone triggers an increase in the number of hormone receptors. When levels of a particular hormone are low, cells become more sensitive to it.
What is generally involved in linking the first and second messenger.
G protein, an enzyme complex coupled to a membrance receptor. The name G protein refers to the fact that these proteins bind GTP. It is activated when a hormone binds to its receptor at the membrane surface.
Peptide hormones, catecholamines, and eicosanoids bind to membrane receptors and activate G proteins. Each affecting the concentration of second messengers in the cytoplasm. This activation can lead to?
1. Activation of adenylate cyclase and subsequent formation of cyclic-AMP
2. Opening of calcium ion channels and calcium release in the cytoplasm
3. Inhibition of cellular activities by reduction of second messenger concentrations
(They do not have to depolarize)
What does Adenylate cyclase do?
Converts ATP into cyclic-AMP.
What does Phosphodiesterase do?
Inactivates cyclic-AMP by converting it to AMP.
Many G proteins, when activated, exert their effects by?
Many G proteins, when activated, exert their effects by changing the concentration of the second messenger cyclic-AMP(cAMP) within the cell.
What does an increase in cAMP do?
In most cases, the result is an increase in cAMP levels, and this accelerates metabolic activity within the cell.
What does an decrease in cAMP levels do?
Inbibition of cellular activity
What happens when steroid hormones diffuse across the lipid part of the cell membrane? Then what can happen?
They bind to receptors in the cytoplasm or nucleus. Then the hormone-receptor complexes activate or deactivate specific genes.
Steroid hormones alter what?
Steroid hormones can alter the rate of DNA transcription in the nucleus, and thus change the pattern of protein synthesis. Alteration in the synthesis of enzymes or structural proteins will directly affect both the metabolic activity and the structure or the target cell.
How do Thyroid hormones cross the cell membrane?
By diffusion or a carrier mechanism
Thyroid hormones bind receptors located where and what changes do they cause?
Once thyroid hormones enter the cytosol they bind to receptors within the nucleus and on mitochondria. The hormone-receptor complexes in the nucleus activate specific genes or change the rate of transcription. The change in rate affects the metabolic activities of the cell by increasing or decreasing the concentration of specific enzymes. Thyroid hormones bound to mitochondria increase the mitochondrial rate of ATP production. (Aerobic respiration)
What is a negative feedback mechanism?
Control endocrine reflexes. A stimulus triggers the production of a hormone whose direct or indirect effects reduce the intensity of the stimulus.
What are the three methods of hypothalamic control over endocrine function?
1. Secretion of regulatory hormones to control activity of anterior lobe of pituitary gland
2. Production of ADH and oxytocin
3. Control of sympathetic output to adrenal medulla
Where does the pituitary gland lie?
In the sella turcica, a depression in the sphenoid bone. It hangs inferior to the hypothalamus.
What connects the pituitary gland and hypothalamus?
What is the pituitary gland divded into?
The pituitary gland can be divided into the posterior (neurohypophysis) and anterior lobes(adenohypophysis). There are seven peptide hormones in the anterior lobe and two in the posterior lobe. All nine use cAMP as a second messenger.
What happens at the median eminence of the hypothalamus?
The hypothalamus controls the production of hormones in the anterior lobe. At the median eminence neurons release regulatory factors either relaeasing hormones RH or inhibiting hormones IH into the surrounding interstitial fluids through fenestrated capillaries.
What are fenestrated capillaries?
Allow relatively large molecules to enter or leave the circulatory system.
What is the hypopyseal portal system?
Provides an efficient means of chemical communication by ensuring that all the blood entering the portal vessels will reach the intended target cells before it returns to general circulation.
What links two capillary networks together?
Blood vessels that link two capillary networks are called portal vessels.
What are the two classes of hypothalamic regulatory hormones?
1. realeasing hormone(RH)-stimulates the synthesis and secretion of one or more hormones at the anterior lobe
2. inhibiting hormone(IH)-prevents the synthesis and secretion of hormones from the anterior lobe
The regulatory hormones released at the hypothalamus are transported directly to the anterior lobe by the?
The regulatory hormones released at the hypothalamus are transported directly to the anterior lobe by the hypophyseal portal system.
What is negative feedback?
The rate at which the hypothalamus secretes regulatory hormones is controlled by negative feedback.
What are the hormones of the anterior lobe?
1. thyroid-stimulating hormone
2. adrenocorticotropic
3. follicle-stimulating hormone
4. luteinizing
5. prolactin
6. growth hormone
7. melanocyte-stimulating hormone
Thyroid-stimulating hormone(TSH)
Triggers the release of thyroid hormones. Thyrotropin-releasing hormone(TRH) promotes the secretion of TSH. As the circulating concentration of thyroid hormones rise, the rates of TRH and TSH production decline.
Adrenocorticotropic hormone(ACTH)
also known as corticotropic, stimulates the release of steroid hormones by the adrenal cortex. ACTH specifically targets cells that produce glucocorticoids, hormones that affect glucose metabolism. ACTH release occurs under the stimulation of corticotropin-releasing hormone(CRH) from the hypothalamus. As glucocorticoid level increase, the rate of CRH release and ACTH release decline.
What do follicle-stimulating hormones and luteinizing hormones do?
regulate activities of the gonads
Follicle-stimulating hormone(FSH)
stimulates follicle development in females and in combination with lueinizing hormones, stimulates estrogen secretion in females and sperm production in males.
Luteinizing hormone(LH)
Induces ovulation, the production of reproductive cells in females. It also promotes the secretion, by the ovaries, of estrogens and the progestins, which prepare the body for pregnancy. Causes androgen production in males. Gonadotropin-releasing hormone(GnRH) promotes the secretion of both FSH and LH.
Works with other hormones to stimulate mammary gland development. Also stimulates milk production by the mammary glands. The prolactin production is inbibited by prolactin-inhibiting hormone(PIH)-the neurotransmitter dopamine. The hypothalamus also secretes a prolactin-releasing hormone, prolactin-releasing factor(PRF. Circulating PRL stimulates PIH release and inhibits the secretion of PRF.
Growth Hormone or somatotropin
stimulates cell growth and replication through the release of somatomedins or IGFs from the liver cells. The production of GH is regulated by growth hormone-releasing hormone(GH-RH) and growth hormone-inhibiting hormone(GH-IH).
Growth hormone
Stimulates cell growth and replication by accelerating the rate of protein synthesis. Skeletal muscle cells and chondrocytes(cartilage cells) are paticularly sensitive to GH Levels.
The primary mechnaism, which is indirect.
Liver cells respond to the presence of GH by synthesizing and releasing somatomedins, or insulinlike growth factors which are peptide hormones that bind to receptor sites on a variety of cell membranes. In skeletal muscle fibers, cartilage cells, and other targe cells, somatomedins increase the rate of uptake of amino acids and their incorporation into new proteins. These effects develop almost immediately after GH is released; they are particulary important after a meal, when the blood contains high concentrations of glucose and amimo acids. In functional terms, cells can now obtain ATP eaily through the aerobic metabolism of glucose, and amino acids are readily available for protein synthesis. Under these conditions, GH, acting through the somatomedins, stimulates protein synthesis and cell growth.
Direct mechanism
The direct action of GH are more selective and tend not to appear until after blood glucose and amino acid concentrations have returned to normal levels. In adipose tissue, GH stimulates the breakdown of stored triglycerides by adipocytes(fat cells), which then release fatty acids into the blood. In the liver, GH stimulates the breakdown of glycogen reserves by liver cells, which then release glucose into the bloodstream.
Melanocyte-stimulating hormone(MSH)
Stimulates the melanocytes of the skin, increasing their production of melanin, a brown, black, or yellow-brown pigment. The release of MSH is inhibited by dopamine.
Posterior lobe
Antidiuretic hormone and oxytocin move along axons in the infundibulum to the basement membranes of capillaries in the posterior lobe by means of axoplamic transport.
Antidiuretic Hormone(ADH)
Decrease amount of water lost at the kidneys. Elevates blood volume and pressure.
stimulates smooth muscel tissue in the wall of the uterus, promoting labor and delivery. Also emission-ejection of secretions of the prostate gland, sperm, and other glands into the male reproductive tract before ejaculation.
What are thyroid follicles and follicle cells?
Thyroid glands contains large numbers of thyroid follicles. The follicle cells surround a follicle cavity that holds a viscous colloid, a fluid containing large quantities of suspended proteins. A network of capillaries surrounds each follicle, delivering nutrients and regulatory hormones to the glandular cells and accepting their secretory products and metabolic wastes.
What is thyroglobulin?
A globular protein synthesized by follicle cells and secrete it into the colloid of the thyroid follicles. Each thyroglobulin molecule contains the amino acid tyrosine, the building block of thyroid hormones.
The first step in the formation of thyroid hormones:
1. Iodide ions are absorbed from the diet at the digestive tract and are delivered to the thyroid by the bloodstream. Carrier proteins in the basal membrane of the follicle cells transport iodide ions into the cytoplasm.
The second step in the formation of thyroid hormones:
2. Iodide ions diffuse to the apical surface of each follicle cell, where they are converted to an activated form of iodide by the enzyme thyroid peroxidase. This reaction sequence also attaches one or two iodide ions to the tyrosine molecules of thyroglobulin.
The third step in the formation of thyroid hormones:
3. Tyrosine molecules to which iodide ions have been attached are paired, forming molecules of thyroid hormones that remain incorporated into thyroglobulin. The pairing process is probably performed by thyroid peroxidase. The hormone thyroxine also known as T4 contains four iodide ions. Triiodothyronine or T3 is a related molecule containing three iodide ions. Eventually, each molecule of thyroglobulin contains four to eight molecules of T3 or T4 hormone or both.
Thyroid hormones
Inside a cell they bind to receptors in the cytoplasm and receptors on the surfaces of mitochondria, and receptors in the nucleus.
T3 versus T4
Thyroid gland produces large amounts of T4, but T3 is primarily responsible for the observed effects of thyroid hormones: a strong, immediate, and short-lived increase in the rate of cellular metabolism. Peripheral tissues have two sources of T3.
What are the two sources of T3?
1. Release by the thyroid gland.
2. The conversion of T4 to T3
Iodine and Thyroid hormones
Iodine in the diet is absorbed at the digestive tract. The iodide ions are actively transported into the thyroid follicle cellls, so the concentration of iodide inside the thyroid follicle cells is generally aobut 30 times higher than that in the blood plasma. The active transport mechanism for iodide is stimulated by TSH. The resulting increase in the rate of iodide movement into the cytoplasm accelerates the formation of thyroid hormones.
C cells and calcitonin
A second population of endocrine cells lies sandwiched between the cuboidal follicle cells and their basement membrane. These cells, which are larger than those of the follicular epithelium and do not stain as clearly, are C(clear)cells. C cells produce the hormone calcitoin(CT), which aid in the regulation of CA2+ concentrations in body fluids.
The net effect of calcitonin release is a drop in the Ca concentration in body fluids, accomplished by?
1. the inhibition of osteoclasts, which slows the rate of Ca release from bone
2. the stimulation and then fall off of Ca excretion at the kidneys.
Is the control of calcitonin a direct endocrine regulation?
The c cells respond directly to elevations in the Ca concentration of blood. When those concentrations rise, calcitonin secretion increases. The Ca concentration then drop, eliminating the stimulus and "turning off" the c cells.
Calcium ions concentrations also affect the sodium permeabilities of excitable membranes.How?
At high Ca concentrations, sodium permeability decreases and membranes become less responsive. Such problems are relatively rare. Problems caused by lower than normal Ca concentration are equally dangerous and are much more common. When calcium ion concentrations decline, sodium permeabilities increase and cells become extremely excitable.
Where are parathyroid glands located?
There are normally two pairs of parathyroid glands embedded in the posterior surfaces of the thyroid gland.
The parathyroid glands have at least two cell populations:
The chief cells produce parathyroid hormone; the functions of the other cells, called oxyphils, are unknown.
What do the Chief cells do?
Like the C cells of the thyroid gland, the chief cells monitor the circulating concentration of calcium ions. When the Ca concentration of the blood falls below normal the chief cells secrete parathyroid hormone(PTH). The net result of PTH secretions is an increase in Ca concentration in body fluids.
What are the effect of the parathyroid hormone?
1. Stimulates osteoclasts, accelerating mineral turnover and the release of Ca from the bone.
2. It inhibits osteoblasts, reducing the rate of calcium deposition in bone.
3. It enhances the reabsorption of Ca at the kidneys, reducing urinary loss.
4. It stimulates the formation and secretion of calcitriol at the kidneys; enhance Ca and PO absorption by the digestive tract.
Adrenal Glands are divided into the?
Divided into the adrenal cortex and the adrenal medulla. Highly vascularized.
Adrenal cortex contains
Mineralocorticoids, Glucocorticoids, and Androgens.
Mineralocorticoids primarily aldosterone
Targets: the kidneys. Hormonal Effects: increase renal reabsorption of Na and water(especially in the presence of ADH) and accelerate urinary loss of K. Regulatory Control: Stimulated by angiotensin II; inhibited by natriuretic peptides(ANP and BNP).
What is Mineralocorticoids and Aldosterone?
Mineralocorticoids steroid hormones that affect the electrolyte composition of body fluids. Aldosterone is the principal mineralocorticoid produced by the adrenal cortex. Aldosteron secretion stimulates the conservation of sodium ions and the elimination of potassium ions. It causes the retention of sodium ions at the kidneys, sweat glands, salivary glands, and pancreas, preventing sodium los s in urine, sweat, saliva, and digestive secretions. The retention of sodium is accompanied by a loss of potassium. As a secondary effect, the reabsorption of sodium enhances the osmotic reabsorption of water at the kidneys, sweat glands, salivary glands, and pancreas.
Aldosterone secretion occurs in response to?
Aldosterone secretion occurs in response to a drop in blood sodium content, blood volume, or blood pressure or to a rise in blood potassium concentration.
Glucocorticoids(starches and sugars)
When stimulated by ACTH from the anterior lobe of the pituitary, secretes primarily cortisol, along with smaller amounts of the related steroid corticosterone. The liver converts some of the circulating cortisol to cortisone, another active glucocorticoid.
Glucocorticoids targets, hormonal effects and regulatory control?
Targets: most cells. Hormonal Effects: release amino acids from skeletal muscles, and lipids from adipose tissues; promote liver formation of glucose and glycogen; promote peripheral utilization of lipids; anti-inflammatory effects. Regulatory Control: stimulated by ACTH from the anterior lobe of the pituitary gland.
Androgens hormonal effects and regulatory control?
Hormonal Effects: Uncertain sigificance under normal conditions. Regulatory Control: stimulated by ACTH; significance uncertain.
The secretory activities of the adrenal medulla are controlled by?
the sympathetic division of the autonomic nervous system.
Hormones: epinephrine and norepinephrine. Targets: most cells. Hormonal Effects: Increases cardiac activity, blood pressure, glycogen breakdown, blood glucose levels; releases lipids by adipose tissue. Regulatory Control: Stimulated during sympathetic activation by sympathetic preganglionic fibers.
Epinephrine and Norepinephrine Effects?
The peripheral effects of these hormones, which result from interaction with alpha and beta receptors on cell membranes. Stimulation of alpha and beta receptors, the most common types, accelerates the utilization of cellular energy and the mobilization of enery reserves.
What does the Pineal Gland contain and what do those cells synthesize?
contains neurons, neuroglia, and special secretory cells called pinealocytes. These cells synthesize the hormone melatonin from the molecules of the neurotransmitter serotonin. Collaterals from the visual pathways enter the pineal gland and affect the rate of melatonin production, which is lowest during the daylight hours nad highest at night.
Melatonin functions:
1. Inhibiting reproductive functions.
2. Protecting against damage by free raicals. Melatonin is a very effective antioxidant.
3. Setting Circadian Rhythms.
Pancreas is divided into the exocrine pancreas and the endocrine pancreas
Exocrine pancreas consists of clusters of gland cells, called pancreatic acini, and their atttached ducts. Together, the gland and duct cells secrete large quantities of an alkaline, enzyme-rich fluid that reaches the lumen of the digestive tract by traveling along a network of secretory ducts.
The endocrine pancreas consists of small groups of cells scatterd among the exocrine cells. The endocrine clusters are known as pancreatic islets(no ducts).
What are the pancreatic islets are surrounded by?
An extensive, fenestrated capillary network that carries its hormones into the bloodstream.
What are the four types of cells the islet contains?
1. Alpha cells
2. Beta cells
3. Delta cells
4. F cells
What do Alpha cells do?
Produce the hormone glucagon. Glucagon raises blood glucose levels by increasing the rates of glycogen breakdown and glucose release by the liver.
What do Beta cells do?
Produce the hormone insulin. Insulin lowers blood glucose levels by increasing the rate of glucose uptake and utilization by most body cells and increasing glycogen synthesis in skeletal muscles and the liver
What do Delta cells do?
Produce a peptide hormone identical to growth hormone-inhibiting hormone, a hypothalamic regulatory hormone. It suppresses the release of gluccagon and insulin by other islet cells and slows the rates of food absoption and enzyme secretion along the digestive tract.
What do F cells do?
Produce the hormone pancreatic polypeptid(PP). Inhibits gallbladder contractions and regulates the production of some pancreatic enzymes, and may also help control the rate of nutrient absorption by the digestive tract.
What are glucagon and insulin responsible for?
Regulation of blood glucose concentrations. These hormones interact to control blood glucose levels. When blood glucose levels rise, beta cells secrete insulin, which then stimulates the transport of glucose across cell membranes. When blood glucose levels decline, alpha cells secrete glucagon, which stimulates glucose release by the liver.
When is insulin released and by what?
Insulin is a peptide hormone release by beta cells when glucose levels exceed normal levels
Effects of insulin on its target cells:
1. Accelerataion of Glucose Uptake(All Target Cells). This effect results from an increase in the number of glucose transport proteins in the cell membrane. These proteins transport glucose into the cell by facilitated diffusion, a movement that follows the concentration gradient for glucose and for whcih ATP is not required.
Effects of insulin on it target cells:
2. The Acceleration of Glucose Utilization(All Target Cells) and Enhanced ATP Production. This effect occurs form two reasons: 1. The rate of glucose use is proportional to it availability-when more glucose enters the cell, more is used. 2. second messengers activate a key enzyme involved in the intial step of glycolysis.
Effects of insulin on its target cells:
3. The stimulation of Glycogen Formation(Skeletal Muscles and Liver Cells). When excess glucose enter these cells, it is stored in the form of glycogen.
Effects of insulin on its target cells:
4. The Stimulation of Amino Acid Absorption and Protein Synthesis.
Effects of insulin on it target cells:
5. Stimulation of Triglyceride Formation in Adipose Tissues. Insulin stimulates the absorption of fatty acids and glycerol by adipocytes, which store these components as triglycerides. Adipocytes also increase their absorption of glucose; excess glucose is used in the synthesis of addtional triglycerides.
When glucose concentrations fall below normal, alpha cells release what?
When glucose concentrations fall below normal, alpha cells release glucagon and energy reserves are mobilized.
What happens when glucagon binds to a receptor in the cell membrane?
When glucagon binds to a receptor in the cell membrane, the hormone activates adenylate cyclase. cAMP acts as a second messenger that activates cytoplasmic enzymes.
What are the effects of glucagon?
1. Stimulating the Breakdown of Triglycerides in Adipose Tissues. The adipocytes then release the fatty acids into the bloodstream for use by other tissues.
2. Stimulating the Production of Glucose in the Liver. Liver cells absorb amino acids form the bloodstream, convert them to glucose, and release the glucose into the circulation.
3. Stimulating the Breakdown of Glycogen in Skeletal Muscle and Liver Cells.
What are the results of glucagon?
The results are a reduction in glucose use and the release of more glucose into the bloodstream. Blood glucose concentrations soon rise toward normal levels.
What is the autonomic activity of insulin?
The parasympathetic stimulation enhances insulin relase and sympathetic stimulation inhibits it.
Release the steroid hormone calcitriol, the peptide hormone erythropoietin and the enzyme renin. Calcitriol is important for calcium ion homeostasis; erythropoietin and renin are involved in the regulation of blood volume and blood pressure.
Erythropoietin (EPO)
Is a peptide hormone released by the kidneys in response to low oxygen levels in kidney tissues. EPO stimulates the production of red blood cells by bone marrow. The increase in the number of red blood cells elevates blood volume. Because these cells transport oxygen, the increase in their number improves oxygen delivery to peripheral tissues.
Renin is released in response to what?
Is released by specialized kidney cells in response to (1) sympathetic stimulation (2) a decline in renal blood flow.
Steps involved in renin-angiotensin system:
First, renin converts angiotensinogen, a plasma protein produced by the liver, to angiotensin I. In the capillaries of the lungs, angiotensin I is then modified to the hormone angiotensin II, which stimulates the secretion of aldosterone by the adrenal cortex and ADH at the posterior lobe of the pituitary gland. The combination of aldosterone and ADH restricts salt and water losses at the kidneys. Angiotensin II also stimulates thirst and elevates blood pressure.
If blood volume becomes too great, these cells are stretched excessively, to the point where they begin to secrete natriuretic peptides. Natriuretic peptides promote the loss of sodium and water at the kidneys and inhibit renin release and the secretion of ADH and aldosterone.
Thymus produces several hormones that are important to the development and maintenace of immune defenses. Thymosin is the name orignally given to an extract from the thymus that promotes the development and maturation of lymphocytes, the white blood cells responsible for immunity.