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exocrine glands

release enzymes to the external environment through ducts. Include sweat, oil, mucous, and digestive glands.

endocrine glands

release hormones directly into body fluids. For example, the pancreas acts as both an exocrine gland, releasing digestive enzymes through the pancreatic duct, and an endocrine gland releasing insulin and glucagon directly into the blood.

Endocrine hormones

may take anywhere from seconds to days to produce their effects. They do no not move directly to their target tissue, but are released into the general circulation. All hormones act by binding to proteins called receptors. Each receptor is highly specific for its hormone.

endocrine system

effects are to alter metabolic activities, regulate growth and development, and guide reproduction. The endocrine system works in conjunction with the nervous system. Many endocrine glands are stimulated by neurons to secrete their hormones. Hormones exist in three basic chemistry types: 1) peptide hormones 2) steroid hormones and 3) tyrosine derivatives.

peptide hormones

derived from peptides. All peptide hormones are manufactured in the rough ER, cleaved in the ER lumen, and transported to the Golgi apparatus. The Golgi packages the hormone into secretory vesicles, and upon stimulation by another hormone or a nervous signal, the cell releases the vesicles via exocytosis. Peptide hormones are water soluble, and thus move freely through the blood, but have difficulty diffusing through the cell membrane of the effector.

effector

the target cell of the hormone; the cell that the hormone is meant to affect. Instead of diffusing through the membrane, peptide hormones attach to a membrane bound receptor. Once bound by a hormone, the receptor may act in several ways. Another effect of the hormone binding to the receptor may be to activate an intracellular second messenger such as cAMP, cGMP, or calmodulin.

second messenger

activates or deactivates enzymes and/or ion channels and often creates a 'cascade' of chemical reactions that amplifies the effect of the hormone. A cascade is one way that a small concentration of hormone can have a significant effect.

steroid hormones

derived from and are chemically similar to cholesterol. They are formed in a series of steps taking place mainly in the smooth endoplasmic reticulum and the mitochondria. Since they are lipids, steroids typically require a protein transport molecule in order to dissolve into the blood stream. Being lipid soluble, steroids diffuse through the cell membrane of their effector. Once inside the cell, they combine with a receptor in the cytosol. The receptor transports the steroid into the nulceus, and the steroid acts at the transcription level. The typical effect of a steroid hormone is to increase certain membrane or cellular proteins within the effector

Important steroid hormones for the MCAT

1. the glucorticoids and mineral corticoids of the adrenal cortex: cortisol and aldosterone;
2. the gonadal hormones: estrogen, progesterone, testosterone. (estrogen and progesterone are also produced by the placenta).

Thyroid Hormones

lipid soluble and must be carried in the blood by plasma protein carriers. They are slowly released to their target tissues and bind to receptors inside the nucleus. The tyrosine derivatives are: the thyroid hormones: T₃ ( triiodothyronine contains 3 iodine atoma) and T₄ (thyroxine contains 4 iodine atoms). All tyrosine derivative hormones are formed by enzymes in the cytosol or in the rough ER.

catecholamines

adrenal medulla hormones. epinephrine and norepinephrine. They bind to receptors on the target tissue and act mainly through the second messenger cAMP.

negative feedback

the correct answer is the hormone that is responding to the condition; not creating it.

specific hormones and their function

Hormones should be grouped according to the gland that secretes them. A given gland produces one of either steroids, peptides, or tyrosine derivatives, but not two. (The adrenal glands are really two glands. The cortex produces steroids; the medulla produces catecholamines. The thyroid secretes T3 and T4, which are tyrosine derivatives, and calcitonin, which is a peptide).

anterior pituitary gland

located in the brain beneath the hypothalmus. The hypothalmus controls the release of the anterior pituitary hormones w/ reeasing and inhibitory hormones of its own. The ap releases 6 major hormones and several minor hormones. ALL ARE PEPTIDE HORMONES. 1. hGH 2. adrenocorticotropin (ACTH) 3. TSH 4. FSH 5. LH and 6. prolactin.

human growth hormone (hGH)

stimulates growth in almost all the cells of the body. It stimulates growth by increasing episodes mitosis, increasing cell size, increasing the rate of protein synthesis, mobilizing fat stores, increasing the use of fatty acids for energy, and decreasing the use of glucose. the effect of proteins by hGH is accomplished by increasing amino acid transport across the cell membrane, increasing translation and transcription, and decreasing the breakdown of proteins and amino acids.

adrenocortictropic hormone (ACTH)

a peptide, stimulates the adrenal cortex to release glucocorticoids via the second messenger system using cAMP. Release of ACTH is stimulated by many types of stress Glucorticoids are stress hormones.

Thyroid-stimulating hormone (TSH)

a peptide, stimulates the thyroid to release T₃ and T₄ via the second messenger system using cAMP. T₃ and T₄ concentrations have a negative feedback effect on TSH release, both at the anterior pituitary and the hypothalmus.

Prolactin

a peptide, promotes lactation (milk production) by the breasts. The reason that milk is not normally produced before birth is due to the inhibitory effects of milk production by progesterone and estrogen. The act of suckling, which stimulates the hypothalmus to stimulate the ap to release prolactin, inhibits the menstrual cycle.

Posterior pituitary

composed of mainly of support tissue for nerve endings extending from the hypothalmus. The hormones oxytocin and ADH are synthesized in the neural cell bodies of the hypothalmus, and transported down axons to the posterior pituitary where they are released into the blood. Both oxytocin and ADH are small polypeptides.

oxytocin

small peptide hormone that increase uterine contractions during pregnancy and causes milk to be ejected from the breasts.

antidiuretic hormone (ADH) ( aka vasopressin)

small peptide hormone which causes the collecting ducts of the kidney to become permeable to water reducing the amount of urine and concentrating the urine. Since fluid is reabsorbed, ADH also increases blood pressure. Coffee and beer are ADH blockers that increase urine volume.

adrenal glands

located on top of kidneys. They are generally separated into the adrenal cortex and the adrenal medulla.

adrenal cortex

the outside portion of the gland. The cortex only secretes only steroid hormones. There are two types of steroids secreted by the cortex: mineral corticoids and glucocorticoids. (The cortex also secretes a small amount of sex hormones, significant in the female but not the male).

mineral corticoids

affect the electrolyte balance in the blood stream. The major mineral corticoid is aldosterone

glucocorticoids

increase blood glucose concentration and have an even greater effect on fat and protein metabolism. The major glucocorticoid is cortisol.

Aldosterone

a steroid, is a mineral corticoid that acts in the distal convoluted tubule and the collecting duct to increase Na⁺ and Cl⁻re-absorption and K⁺ and H⁺ secretion. It creates a net gain in particles in the plasma, which results in an eventual increase in blood pressure.

cortisol

a steroid, is a glucocortoid that increases blood glucose levels by stimulating gluconeogenesis in the liver. Diminishes the capacity of the immune system to fight infection. Cortisol also degrades adipose tissue to fatty acids to be used for cellular energy. Additionally, cortisol causes a moderate decrease in the use of glucose by the cells. Cortisol also causes the degradation of nonhepatic proteins, a decrease of nonhepatic amino acids and a corresponding increase in liver and plasma proteins and amino acids.

catecholamines

are the tyrosine derivatives synthesized in the adrenal medulla: epinephrine and norepinephrine (also called adrenaline and noradrenaline). They are vasoconstrictors (they constrict blood vessels) of most internal organs and skin, but are vasodilaters of skeletal muscle (they increase blood flow); this is consistent with the 'flight-or-flight' response of these hormones.

Thyroid

located along the trachea just in front of the larynx. The thyroid hormones are triiodothyronine (T₃), thyroxine (T₄), and calcitonin.

T₃ and T₄

very similar in effect. Both hormones are lipid soluble tyrosine derivatives that diffuse through the lipid bilayer and act in the nucleus of the cells of their effector. Their general effect is to increase the basal metabolic rate (the resting metabolic rate). Thyroid secretion is regulated by TSH.

Calcitonin

a large peptide hormone released by the thyroid gland. Slight decreases blood calcium by decreasing osteoclast activity and number. Calcium levels can be effectively controlled in humans in the absence of calcitonin.

Pancreas (Islets of Langerhans)

acts as an endocrine and exocrine gland. The pancreases releases the peptide hormones insulin and glucagon.

insulin

a peptide hormone, is released by the β-cell of the pancreas. Insulin is released when blood levels of carbohydrates or proteins are high. Insulin binds to a membrane receptor beginning a cascade of reactions inside the cell. In the presence of insulin, carbohydrates are stored as glycogen in the liver and muscles, fat is stored in adipose tissue, and amino acids are taken up by the cells of the body and made into proteins. Except for the neurons in the brain and a few other cells, the cells of the body become highly permeable to glucose.

glucagon

a peptide hormone, is released by the α-cells of the pancreas. The effects glucagon are nearly opposites to those of insulin. Glucagon stimulates glycogenolysis (the breakdown of glycogen), and gluconeogenesis in the liver. It acts via second messenger system of cAMP. In higher concentrations, glucagon breaks down adipose tissue increasing the fatty acid level in the blood. The net effect of glucagon is to raise blood glucose levels.

Parathroid glands

attached to the back of the thyroid. The parathyroid glands release parathyroid hormone.

Parathyroid membrane (PTH)

a peptide, increases blood calcium. It increases osteocyte absorption of calcium and phosphate from the bone and stimulates profileration of osteoclasts. PTH increases renal calcium reabsorption and renal phosphate excretion.

male gonads

testes. Production of sperm occurs in the seminiferous tubules of the testes.

Spermatogonoia

located in the seminiferous tubules arise from epithelial tissue to become spermatocytes, spermatids, and then spermatoza.

FSH and LH

Sertoli cells stimulated by FSH surround and nurture the spermatocyte and spermatids. Leydig cells, located in the interstitum between the tubules, release testosterone when stimulated by LH.

testosterone

the primary androgen (male sex hormone), and stimulates the germ cells to sperm. testosterone is also responsible for the development of secondary sex characteristics. While testosterone helps to initiate the growth spurt at puberty, it also stimulates closure of the epiphyses of the long bones, ending growth in stature. Sertoli cells secrete inhibin, a peptide hormone (actually a glycoprotein) which acts on the pituitary gland to inhibit FSH secretion.

epididymus

once freed into the tubule lumen, the spermatozoon is carried to the epididymus to mature. Upon ejaculation, spermatozoa are propelled through the vas deferens into the urethra and out of the penis.

semen

complete mixture of spermatozoa and fluid that that leaves the penis upon ejaculation. Semen is composed of fluid from the seminal vesicles, the prostate, and the bulbourethral glands (aka Cowper's gland). Spermatozoa become activated for fertilization in a process called capacitation, which takes place in the vagina.

the female reproductive system

OOgenesis begins in the ovaries of the fetus. All the eggs of the female are arrested as primary oocytes at birth. At puberty, FSH stimulates the growth of granulosa cells around the primary oocyte. The granulosa cells secrete a viscous substance around the follicle to form a secondary follicle. Upon stimulation by LH, theca cells secrete androgen, which is converted to estradiol (a type of estrogen) by the granulosa cells in the presence of FSH and secreted into the blood.

estradiol

steroid hormone that prepares the uterine wall for pregnancy. The follicle grows and bulges from the ovary. Typically, estradiol inhibits LH secretion by the anterior pituitary. However just before ovulation (the bursting of the follicle), the estradiol level rises rapidly, actually causing a dramatic increase in LH secretion. This increase is called the luteal surge.

luteal surge

results from a positive feedback loop of rising estrogen levels which increase LH levels, which increase estrogen. The luteal surge causes the follicle to burst, releasing the egg into the body cavity. The egg is is swept into the Fallopian (uterine) tube or oviduct by the fimbriae. The remaining portion of the follicle is left behid to become the corpus luteum.

corpus luteum

secretes estradiol and progesterone throughout pregnancy or, in the case of no pregnancy, for about 2 weeks until the corpus lutem degrades into the corpus albicans.

ovulation

1. primary oocyte w/i follicle
2. growing foliicle consists of theca cells surrounding granulosa cells which surround the zona pellucida and the oocyte.
3. secondary (graffian) follicle
4. Ovulation: Follicle ruptures releasing secondary oocyte
5. corpus luteum develops from remnants of follicle
6. corpus luteum degenerates to corpus albicans if no fertilization of egg.

menstrual cycle

repeats itself every 28 days after puberty unless pregnancy occurs. With each menstrual cycle, several primordial oocytes may begin the process, but, normally only one completes the development to ovulation.

fertilization and Embryology

Once in the Fallopian tube, the egg is swept toward the uterus by cilia. Fertilization normally takes places in the Fallopian tubes. The cell membranes of the sperm head and the oocyte fuse upon contact, and the sperm nucleus enters the cytoplasm of the oocyte. Now the oocyte goes through the second meiotic division to become an ovum and releases a second polar body

Fertilization

occurs when the nuclei of the ovum and sperm fuse to form the zygote.

Cleavage

begins while the zygote is still in the Fallopian tube. The zygote goes through many cycles of mitosis. The embryo at this stage does not grow during cleavage.

morula

when the zygote is comprised of 8 or more cells. Any one of these eight cells at this stage could produce a complete individual. The cells of the morula continue to divide for four days forming a hollow ball filled with fluid. This fluid filled ball is called a blastocyst

implantation

when the blastocyst lodges in the uterus on about the 5th to 7th day after ovulation. Upon implantation, the female is said to be pregnant. Upon implantation the egg begins secreting a peptide hormone called human chorionic gonadotropin (HCG).

HCG

prevents the degeneration of the corpus luteum, and maintains its secretion of estrogen and progesterone. HCG in the blood and urine of the mother is the first outward sign of pregnancy.

placenta

formed from the tissue of the egg and the mother, and takes over the job of hormones secretion. The placenta reaches full development by the end of the first trimester, and begins secreting its own estrogen and progesterone while lowering its secretion of HCG.

determination

As the embryo develops past the eight cell stage, the cells become different from each other due to cell-cell interactions. Cells become determined to give rise a particular tissue early on. The specialization that occurs at the end of the development forming a specialized tissue cell is called differentiation. The fate of a cell is typically determined early on, but that same cell usually doesn't differentiate into a specialized tissue cell until much later at the of the developmental process.

gastrula

occurs in the second week after fertilization in a process called gastrulation. Cells begin to move slowly about the embryo for the first time. During grastulation, the three primary germ layers are formed. 1. the ectoderm 2. the mesoderm and 3. the endoderm.

ectodermemal cells

develop into the outer coverings of the body, such as the outer layers of skin, nails, and tooth enamel, and into the cells of the of the nervous system and sense organs. These are just guidelines and not absolute rules

endodermal cells

develop into the lining of the digestive tract, and into much of the liver and the pancreas. These are just guidelines and not absolute rules

mesoderm

stuff that lies between the inner and outer covering of the body, the muscle bone, and the rest. These are just guidelines and not absolute rules

neurulation

In the 3rd week when the gastrula develops into a neurula. In neuralation , the notochord (made from mesoderm) induces the overlying ectoderm to thicken and form the neural plate. The notochord eventually degenarates to become the spinal cord, brain, and most of the nervous system.

apoptosis

programmed cell death

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