Introduction (p. 281)
- The endocrine system is made up of the cells, tissues, and organs
that secrete hormones into body fluids.
- The body has two kinds of glands, exocrine (secretes products into
ducts) and endocrine (secrete products into body fluids to affect target
cells).
General Characteristics of the Endocrine
System 11.1
- The endocrine system's function is to communicate with cells using
chemicals called hormones.
- Endocrine glands and their hormones regulate a number of metabolic
processes within cells, and the whole body.
- Their actions are precise, they only affect specific target cells.
- Endocrine glands include the pituitary gland, thyroid gland, parathyroid
glands, adrenal glands, pancreas, and other hormone-secreting glands
and tissues.
Hormone Action
- Hormones are steroids, amines, peptides, proteins, or glycoproteins;
they can influence target cells even if they are present only in minute
concentrations.
- Steroid Hormones 11.2
- Steroid hormones are lipid-soluble and can pass through cell
membranes.
- Receptors for steroid hormones are located in the target cell's
nucleus.
- The hormone-receptor complex binds with the DNA and activates
specific genes that, in turn, direct the synthesis of specific proteins.
- Nonsteroid Hormones 11.3
- Nonsteroid hormones combine with receptors in target cell membranes;
the receptors have a binding site and an activity site.
- The hormone-receptor complex (as first messenger) triggers
a cascade of biological activity.
- The hormone-receptor complex generally activates a G protein,
which then activates the enzyme adenylate cyclase that is bound
to the inner cell membrane.
- This enzyme removes two phosphates from ATP to produce cyclic
AMP (the second messenger), which in turn activates protein enzymes
that activate proteins.
- These activated proteins induce changes in the cell.
- Not all nonsteroid hormones use cAMP; others use diacylglycerol
(DAG) or inositol triphosphate.
- Prostaglandins
- Prostaglandins are locally-produced lipids that affect the
organ in which they are produced.
- Prostaglandins produce a variety of effects: some relax smooth
muscle, others contract smooth muscle, some stimulate secretion
of other hormones, and others influence blood pressure and inflammation.
Control of Hormonal Secretions
- Hormone levels are very precisely regulated.
- Control Mechanisms 11-4
- Release of tropic hormones from the hypothalamus controls
secretions of the anterior pituitary.
- The nervous system influences certain endocrine glands directly.
- Other glands respond directly to changes in the internal fluid
composition.
- Negative Feedback Systems 11.5
- Commonly, negative feedback mechanisms control hormonal releases.
- In a negative feedback system, a gland is sensitive to the
concentration of the substance it regulates or which regulates it.
- When the concentration of the regulated substance reaches
a certain level (high or low), it inhibits the gland from secreting
more hormone until the concentration returns to normal.
Pituitary Gland
- The pituitary gland is attached to the base of the brain and has
an anterior lobe (anterior pituitary) and a posterior lobe (posterior
pituitary).
- The brain controls the activity of the pituitary gland.
- Releasing hormones from the hypothalamus control the secretions
of the anterior pituitary.
- The releasing hormones are carried in the bloodstream
directly to the anterior pituitary by hypophyseal portal veins.
- The posterior pituitary releases hormones into the bloodstream
in response to nerve impulses from the hypothalamus.
- Anterior Pituitary Hormones
- The anterior pituitary consists mostly of epithelial tissue
arranged around blood vessels and enclosed in a capsule of collagenous
connective tissue.
- Growth hormone (GH) stimulates body cells to grow and reproduce;
it also speeds the rate at which cells use carbohydrates and fats.
- Growth hormone-releasing hormone from the hypothalamus
increases the amount of GH released, GH release-inhibiting hormone
inhibits its release.
- Nutritional status affects the release of GH; more is released
when nutrients are insufficient.
- Prolactin (PRL) promotes milk production following the birth
of an infant.
- The effect of PRL in males is less-well understood, although
it may cause a deficiency of male sex hormones.
- Thyroid-stimulating hormone (TSH) controls the secretion of
hormones from the thyroid gland (11.9).
- Thyrotropin-releasing hormone (TRH) from the hypothalamus
regulates the release of TSH.
- As blood concentrations of thyroid hormones increases,
secretions of TRH and TSH decrease.
- Adrenocorticotropic hormone (ACTH) controls the secretion
of hormones from the adrenal cortex.
- It is regulated by corticotropin-releasing hormone from
the hypothalamus, and stress can also increase its release.
- Follicle-stimulating hormone (FSH) and luteinizing hormone
(LH) are gonadotropins affecting the male and female sex organs.
- Posterior Pituitary Hormones
- The posterior lobe consists of nerve fibers and neuroglial
cells that support nerve fibers arising in the hypothalamus.
- Neurons in the hypothalamus produce antidiuretic hormone and
oxytocin, which are stored in the posterior pituitary.
- Antidiuretic hormone (ADH) produces its effect by causing
the kidneys to conserve water.
- The hypothalamus regulates the secretion of ADH based
on the amount of water in body fluids.
- Oxytocin plays a role in childbirth by contracting muscles
in the uterine wall, and in milk-letdown by forcing milk into ducts
from the milk glands.
- Stretching of the uterus in the latter stages of pregnancy
stimulates release of oxytocin.
- Suckling of an infant at the breast stimulates release
of oxytocin after childbirth.
Thyroid Gland 11.10
- The thyroid gland is located below the larynx and consists of
two broad lobes connected by an isthmus.
- Structure of the Gland (p. 289)
- 1. The thyroid consists of secretory parts called follicles
filled with hormone-storing colloid.
- Thyroid Hormones
- The follicular cells produce two iodine-containing hormones,
thyroxine (T3) (tetraiodothyronine) and triiodothyronine (T3), that
together regulate energy metabolism.
- These two hormones increase the rate at which cells release
energy from carbohydrates, enhance protein synthesis, and stimulate
the breakdown and mobilization of lipids.
- These hormones are essential for normal growth and development.
- The hypothalamus and pituitary gland control release of
thyroid hormones.
- Extrafollicular cells of the thyroid secrete calcitonin, which
lowers blood levels of calcium and phosphate ions when they are
too high.
- Calcitonin increases the rate at which calcium is stored
in bones and excreted in the urine.
- Calcitonin secretion is regulated by negative feedback
involving blood concentrations of calcium.
Parathyroid Glands 11.11
- The four, tiny parathyroids are located on the posterior of the
thyroid.
- Structure of the Glands
- Parathyroid glands consist of tightly packed secretory cells
covered by a thin capsule of connective tissue.
- Parathyroid Hormone
- Parathyroid hormone (PTH) increases blood calcium ion concentration
and decreases phosphate ion concentration.
- PTH stimulates bone resorption by osteoclasts, which releases
calcium into the blood.
- PTH also influences the kidneys to conserve calcium and causes
increased absorption of calcium in the intestines.
- A negative feedback mechanism involving blood calcium levels
regulates release of PTH.
- Calcitonin and PTH exert opposite effects in regulating calcium
ion levels in the blood.
Adrenal Glands
- The adrenal glands sit atop the kidneys enclosed in a layer of
fat.
- Structure of the Glands
- The pyramid-shaped glands consist of an inner adrenal medulla
and an outer adrenal cortex.
- The adrenal medulla is made up of modified postganglionic
neurons that are connected to the sympathetic nervous system.
- The adrenal cortex makes up most of the adrenal glands and
consists of epithelial cells in three layers--an outer, middle,
and an inner zone.
- Hormones of the Adrenal Medulla
- The adrenal medulla secretes epinephrine and norepinephrine
into the blood stream.
- The effects of these hormones resemble those of the sympathetic
division neurotransmitters of the same name, except that they last
up to 10 times longer when they are secreted as hormones.
- The are used in times of stress and for "fight or flight."
- Release of medullary hormones is regulated by nervous impulses
from the central nervous system.
- Hormones of the Adrenal Cortex
- The cells of the adrenal cortex produce over 30 different
steroids, some of which are vital to survival, the most important
of which are aldosterone, cortisol, and the sex hormones.
- Aldosterone
- Aldosterone, a mineralocorticoid, causes the kidneys to
conserve sodium ions and thus water, and to excrete potassium
ions.
- Aldosterone is secreted in response to decreasing blood
volume and blood pressure as a result of changes in the kidney.
- Cortisol
- Cortisol, a glucocorticoid, influences the metabolism of glucose,
protein, and fat in response to conditions that stress the body
and require a greater supply of energy in the bloodstream.
- A negative feedback mechanism involving CRH from the hypothalamus
and ACTH from the anterior pituitary controls the release of cortisol.
- Stress, injury, or disease can also trigger increased release
of cortisol.
- Adrenal Sex Hormones
- Sex hormones, produced in the inner zone, are mostly of
the male type, but can be converted to female hormones in the
skin, liver, and adipose tissues.
- These hormones supplement those released by the gonads and
may stimulate early development of reproductive organs.
Pancreas11.15
- The pancreas secretes hormones as an endocrine gland, and digestive
juices to the digestive tract as an exocrine gland.
- Structure of the Gland
- The pancreas is an elongated organ posterior to the stomach.
- Its endocrine portions are the islets of Langerhans that include
two cell types--alpha cells that secrete glucagon, and beta cells
that secrete insulin.
- Hormones of the Islets of Langerhans
- Glucagon increases the blood levels of glucose by stimulating
the breakdown of glycogen and the conversion of noncarbohydrates
into glucose.
- The release of glucagon is controlled by a negative feedback
system involving low blood glucose levels.
- Insulin decreases the blood levels of glucose by stimulating
the liver to form glycogen, increasing protein synthesis, and stimulating
adipose cells to store fat.
- a. The release of insulin is controlled by a negative feedback
system involving high blood glucose levels.
- Insulin and glucagon coordinate to maintain a relatively stable
blood glucose concentration.
Other Endocrine Glands
- Pineal Gland
- The pineal gland, near the upper portion of the thalamus,
secretes melatonin, which is involved in the regulation of circadian
rhythms of the body.
- Thymus Gland
- The thymus gland, lying between the lungs under the sternum,
secretes thymosins that affect production and differentiation of
T lymphocytes that are important in immunity.
- Reproductive Glands
- The ovaries produce estrogen and progesterone.
- The placenta produces estrogen, progesterone, and a gonadotropin.
- The testes produce testosterone.
- Digestive Glands
- The digestive glands secrete hormones associated with the
processes of digestion.
- Other Hormone Producing Organs
- The heart secretes atrial natriuretic peptide affecting sodium
and the kidneys secrete erythropoietin for blood cell production.
Stress and Health
- Factors that serve as stressors to the body produce stress and
threaten homeostasis.
- Types of Stress
- Stress may be physical, psychological, or some combination
of the two.
- Physical stress threatens the survival of tissues, such as
extreme cold, prolonged exercise, or infections.
- Psychological stress results from real or perceived dangers,
and includes feelings of anger, depression, fear, and grief; sometimes
even pleasant stimuli cause stress.
- Response to Stress
- Responses to stress are designed to maintain homeostasis.
- The hypothalamus controls the general stress syndrome, which
involves increased sympathetic activity and increased secretion
of cortisol, glucagon, growth hormone, and antidiuretic hormone.
|
|
|
