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Overview of the endocrine system
Hormones -messenger molecules, are synthesized and secreted by endocrine glands. Endocrine cells are ductless: They secretes hormones, into the nearby extracellular space and subsequently diffuse into the blood stream
The circulatory system delivers hormones to distant target cells/tissues. (targets cells is a general term referring to the cells that the hormone acts upon.)
Target cells/ tissue have receptors that are specific for certain hormones. Hormone- receptor response.
Hormones are classified as either steroids (proteins , glycoproteins , peptids, amines)
- derivatives of cholesterol.
- derivatives of tyrosine (EX: thyroid hormones, epinephrine, norepinephrine)
- Protein & peptide HormonesDNA↓
- Amino acids↓
- Peo-hormones & pre-hormones
- (precursor Molecules)↓
- Cleavage of prohormones↓
- Protein or peptide hormone
Regulation of Hormone Secretion
Regulation via negative feedback
- Hormone secretion is precisely regulated via feedback loops
- → Stimulus ↓
- Control center↓
negative feedback systems negate or oppose deviations away from the set point
- Hormone secretion/release is self limiting
- (stops at homeostasis)
Regulation via positive feedback
Positive feedback systems Enhance or exaggerate the effects of the stimulus.
- The hormone stimulates it own secretion.
- (enhance stimulus moves farther aways from homeostasis so the event can happen. ex: childbirth
Regulation of hormone receptors
A hormone delivers it message to a cell by uniting with the binding site of its receptor.
Hormones determine the sensitivity of the target tissue by regulating the number ( or sensitivity) of their receptors
A) Down-regulation of receptors: a hormone decreases the number of receptors for itself ( or other hormones ). Down regulation occurs in response to a prolonged increase in hormone levels.
B) Up regulation of receptors: A hormone increases the number of receptors for itself ( or other hormone) upregulation often occurs in response to a prolonged decrease hormone levels.
Actions of Hormones
Hormones exert their effects by altering metabolic processes: Synthesize proteins, alter enzyme activity , alter the rate of transport through the cell membranes. etc
Steroid and thyroid hormones
- The more receptors the hormone binds on its target cells, the greater the response.
- A. Steroid hormones and thyroid hormones
→ enter the target cell → Bind with receptors to form hormone-receptor complexes. → activate specific genes →synthesize specific proteins.
The degree of cellular response is proportional to the number of hormone -receptorcomplexes formed.
B. Nonsteroid Hormones
bind with cell surface receptors →stimulate membrane proteins to induce the formation of secondary messenger molecules→ Activate protein kinases →Activate other protein substrates → change cellular processes.
A small number of hormone- receptor complexes can catalyze formation of a large number of secondary messenger ,molecules: thus , cellular response is amplified.
- C. PROSTAGLANDINS (Paracrine substance)
- Prostaglandins are paracrine substances that have powerful hormone- like effects , even in small amounts.
Prostaglandin modulate hormones that regulate formation of cyclic AMP.
Pituitary Gland (Hypophysis)-
The hypothalamus is a homeostatic control center. It is an important link between the nervous system and the endocrine system. It receives neural input about certain internal environmental conditions, such as water and solute balance, temp, and carbohydrate metabolism.
The pituitary gland is sometimes called the "master gland" because it secretes eight different hormones that, in turn, regulate many other endocrine glands. It is located at the base of the brain and housed in the sella turcica . It consists of two lobes: a posterior lobe that appears to be a bulging extensions of the connecting stalk (infundibulum), and a larger more distant anterior lobe.
Hypothalamic- pituitary relationships
1. Hypothalamic-anterior pituitary Axis:
The hypothalamus secretes releasing hormones
, which stimulate the anterior pituitary
to release its hormones, which, in turn, stimulate peripheral endocrine glands to release its hormones.
Hypothalamus→ anterior pituitary→ peripheral gland→ target cells→ response/action
- Anterior Pituitary hormones
- a. Growth hormone (GH)
- b. Thyroid- stimulating hormones (TSH)
- c. Adrenocorticotropic hormone (ACTH)
- d. Follicle- stimulating Hormone (FSH)
- e. Luteinizing hormone (LH)
- d. Prolactin (PRL)
Hypothalamic- posterior pituitary Axis
Antidiuretic hormone (ADH) and oxytocin are are synthesized in cells bodies of hypothalamic neurons: packaged in secretory granules; and transported down axons for storage with nerve terminals in the posterior pituitary gland.
Nerve impulses from the hypothalamus then stimulate nerve terminals in the posterior pituitary to release these hormones.
Hypothalamus↠ Posterior pituitary ⇢target cells⇢ response/action
Hormones of the pituitary Gland
Growth Hormone (GH)
- Growth Hormone (GH) Stimulates increase in size and rate of division of body cells; enhances movement of amino acids through membranes; promotes growth of long bones
Sources of control: Secretion stimulated by growth hormone-releasing hormone (GHRH) from the hypothalamus and inhibited by somatostatin (SS)
Hormones of the pituitary Gland
Sustains milk production after birth, amplifies effect of LH in males
Secretion inhibited by prolactin -release hormones inhibiting inhibiting hormone (PIH) and may be stimulated by yet to be identified prolactin releasing factor (PRF) from the hypothalamus.
Thyroid- stimulating hormone (TSH)
Controls secretions of hormones from thyroid gland
- Sources of control:
- Thyrotropin - releasing hormones (TRH) from the hypothalamus
Adrenocorticotropic hormone (ACTH)
Controls secretion of certain hormones from the adrenal cortex
Sources of control: corticotropin - releasing hormone (CRH) from the hypothalamus
Follicle - stimulating hormone (FSH)
Development of egg- containing follicles in ovaries; stimulates follicular cells to secrete estrogen; in males, stimulates production of sperm in cells.
Sources of control: Gonadotropin- releasing hormone (GnRH) from the hypothalamus
Luteinizing hormones (LH)
Promotes secretion of sex hormones ; releases egg cell in females
Sources of control: Gonadotropin- releasing hormone (GnRH) from the hypothalamus
Antidiuretic Hormone (ADH)
Causes kidneys to reduce water excretion; in high concentration, raises BP
sOURCES of control; Hypothalamus in response to changes in blood plasma concentration and blood volume
Contract muscles in uterine wall and those associated with milk- secreting glands.
Sources of control: Hypothalamus in response to stretch in uterine and vaginal walls and stimulation of breasts.
Thyroid gland review
The thyroid gland lies just below the larynx on either side and anterior to the trachea . it is a highly vascular structure that consists of two lateral lobes connected by a broad isthmus. it is specialized to remove iodine from the blood.
Structure: Thyroid gland is made of follicles, which contain open spaces lined by simple cuboidal epithelial cells , known as follicular cells. the open spaces are filled with a clear viscous colloid, which consists primarily of thyroglobulin ( a glycoprotein)
Follicular cells produce and secrete thyroxine (T4) and triiodothyronine (T3) that may either be stored in the colloid or released into nearby capillaries.
C-cells (extrafollicular cells) which lie outside the follicles, produce and secrete calcitonin
Hormones Of the thyroid gland
increases rate of energy release from carbs ; increase rate of protein synthesis; accelerates growth. stimulates activity in the nervous system.
Source of control:
TSH from the anterior pituitary gland
Triiodothyronine ( T3)
same action as thyroxine , but five time more potent
- Source of control:
- TSH from the anterior pituitary gland
Lowers blood calcium and phosphate ion concentrations by inhibiting release of calcium and phosphate ions from bones by increasing the rate at which calcium and phosphate ions are deposited in deposited in bones. increases excretion of calcium by the kidneys.
source of control; elevated blood Ca , digestive hormones
Parathyroid gland review
parathyroid glands are located on the posterior surface of the thyroid gland. there are four in number.
Parathyroid hormone (PTH) increases blood ca+ and decreases blood PO4 through actions in the bones , kidneys and intestines
Adrenal glands (suprarenal glands)
adrenal gland are closely associated with the kidneys . A gland sits atop each kidney and is embedded in the perirenal fat. The gland is divided into two distinct regions:
Adrenal medulla (inner region)is intimately connected with the sympathetic nervous system and secretes epinephrine (80%) and norepinephrine (20%)
Adrenal cortex( outer region) is composed of closely packed massed of epithelial that form an outer, a middle , and an inner zone of the cortex - zona glomerulosa, zona fasciculata, and zona reticularis, respectively
Hormones of the adrenal cortex
Aldosterone- helps regulate the concentration of extracellular electrolytes, by conserving Na+ and excreting K+
Source of control; electrolyte concentrations in bodily fluids and renin-angiotensin mechanism
Cortisol- decreases protein synthesis , increases fatty acid release. and stimulate glucose synthesis from non carbohydrates.
Source of control; CRH from the hypothalamus and ACTH from the anterior pituitary gland
Androgens - supplement the sex hormones produced by the gonads ; may be converted into estrogens
The endocrine portion of the pancreas consists of pancreatic islets (islets of langerhans
), which are formed by three distinct types of hormone-secreting cells:
- 1. Alpha cells - secrete glucagon
- 2. Beta cells- secrete insulin
- 3. Delta cells- secrete somatostatin
Hormones of pancreatic islets
Glucagon- stimulates the liver to break down glycogen and convert non carbs into glucose; stimulates breakdown of fats.
Sources of control: blood glucose concentration
Promotes formation of glycogen from glucose, inhibits conversion of non carbs into glucose , and enhances movement of glucose through adipose tissue and muscle cell membranes, decreasing blood glucose concentration; promotes transport of amino acids into cells; enhances synthesis of proteins and fats.
Source of control: blood glucose
Somatostatin-inhibits insulin and glucagon release
Source of control is undetermined
diabetes mellitus is a group of metabolic disorders sharing the common underlying feature of hyperglycemia. Hyperglycemia results from defects in insulin secretion. insulin action or both.
DM is the leading cause of end stage renal disease, adult onset blindness , and nontraumatic amputations in the US . It greatly increase the risk for developing coronary artery disease and cerebrovascular disease.
Type 1 DM
- Type 1 diabetes accounts for apx 10% of all dm causes
- Type 1 diabetes is a autoimmune disease in which islet destruction i caused by primarily by T- lymphocyte reacting against B cell antigens, resulting in B- cell mass.
It is characterized as an absolute deficiency of insulin secretion
and typically manifest in childhood.
Most individuals with type-1 DM depend of exogenous insulin supplementation for survival.
Without insulin, type 1 diabetic pt develop serious metabolic complications such as acute ketoacidosis and coma.
Type 2 DM
Roughly 90% of diabetic pt have type 2 DM
type 2 dm is caused by a combination of:
1. insulin resistance:decrease ability of peripheral tissues to respond to insulin (due to hyperproduction)
2. Relative insulin deficiency: inadequate compensatory response of insulin secretion by pancreatic B -CELLS (not enough)
Insulin resistance is a characteristic feature of most individuals with type 2 dm and is an almost universal finding in diabetic individuals who are obese.
(can be controlled by diet)
Pineal gland/ thymus. and reproductive glands review
Pineal gland- is attached to the thalamus near the roof of the third ventricle
, part of the regulation of circadian rhythms
The thymus gland lies posterior to the sternum and between the lungs. It shrinks with age.
Secrets thymosin, which affects the production of certain lymphocytes that , in turn, provide immunity.
Testes secrete testosterone
- ovaries secrete estrogen and progesterone
- Placenta secrete estrogen, progesterone, and a gonadotropin.