Download Chapter 41 Endocrine System

General Biology Lecture
BIO 211 Regis University
Spg 1992
B. Rife
1/3
Chapter 41 Endocrine System
I
II.
III
IV.
V.
VI.
VII.
VII.
Endocrine Glands
Hypothalamus and the Pituitary Gland
Thyroid Gland
Parathyroid Glands
Adrenal Glands
Pancreas
Other Endocrine Glands
Chemical Messengers
I
Endocrine Glands
Both the nervous and endocrine systems assume a role maintaining homeostasis.
A hormone is defined as a chemical produced by one set of cells that affects a different set.
Exocrine glands (sebaceous, mucous, and digestive) secrete their products through ducts into body cavities, into the lumens of various
body organs, or onto the body’s surface.
Endocrine glands secrete hormones into the extracellular spaces around the secretory cells, rather than into ducts.
The secretion then passes into capillaries to be transported in the blood.
A. Mechanism of Hormonal Action
Chemically, hormones are classified as amines, proteins and peptides, and steroids.
1. Peptide and Protein Hormones consist of chains of amino acids (oxytocin, insulin) and
activate existing enzymes in the cell.
2. Steroid Hormones are derived from cholesterol (aldosterone, testosterone, estrogen) and
bring about the synthesis of new proteins.
3. Amines are the simplest hormones structurally (thyroid hormones, epinephrine, norepinephrine).
B. Physiology of Hormonal Action
1. The amount of hormone release is determined by the body’s need for the hormone at any given time.
2. Cells that respond to the effects of hormones are called target cells. All cells are target
cells for one or more hormones.
3. Specific receptors are found in the plasma membrane, cytoplasm, and nucleus of target
cells. Only cells that have receptors to certain hormones can respond.
II.
Hypothalamus and the Pituitary Gland
The hypothalamus is a portion of the brain that regulates the internal environment. For example, it helps to control the heart rate, body
temperature, and water balance, as well as the glandular secretions of the stomach and of the pituitary gland.
The pituitary, a small gland of about 1 cm in diameter, lies just below the hypothalamus and is divided into two portions called the posterior
pituitary and anterior pituitary.
A. Posterior Pituitary In a strict sense, the posterior lobe is not an endocrine gland, since it does not synthesis hormones. Instead, it
stores and releases two hormones.
1. Antidiuretic hormone (ADH), sometimes called vasopressin, promotes water reabsorption by the kidneys and
arteriole constriction.
The hypothalamus contains other nerve cells that are sensitive to the osmolarity of the blood. When theses cells determine that
the blood is too concentrated, ADH is released into the bloodstream. When blood reaches the correct osmolarity and becomes
dilute, the hormone is no longer released.
This is an example of control by negative feedback because the effect of the hormone (diluted blood) acts to
shut down the release of the hormone.
Negative feedback mechanisms regulate the activities of most endocrine glands.
General Biology Lecture
BIO 211 Regis University
Spg 1992
B. Rife
2/3
Oxytocin is the other hormone that is made in the hypothalamus and stored in the posterior pituitary.
Oxytocin causes the uterus to contract and is used to artificially induce labor. It also stimulates
the release of milk from the mother’s mammary glands when a baby is nursing.
B. Anterior Pituitary
1. Human growth hormone (hGH) stimulates body growth by promoting cell division, protein synthesis, and bone growth.
Disorders associated with improper levels of hGH are pituitary dwarfism, giantism, and acromegaly.
2. Lactogenic hormone (LTH) or Prolactin (PRL) causes the mammary glands to produce milk.
It also plays a role in carbohydrate and fat metabolism.
3. Melanocyte-stimulating hormone (MSH) increases skin pigmentation.
Regulation of the Anterior Pituitary
The anterior pituitary is called the master gland because it secretes the following hormones, which have an effect on other
glands.
4. Thyroid-stimulating hormone (TSH) regulates thyroid gland activities.
5. Adrenocorticotropic hormone (ACTH) regulates the activities of the adrenal cortex.
6. Follicle-stimulating hormone (FSH) regulates the activities of the ovaries or
testes depending on the sex of the individual.
7. Luteinizing hormone (LH) also regulates the gonads.
III
Thyroid Gland
The thyroid gland is located just below the larynx. The thyroid is composed of follicular cells, which secrete
thyroxine (T4) and triiodothyronine (T3) and parafollicular cells, which secrete calcitonin (CT). Thyroid
hormones are synthesized from iodine.
Thyroid hormones regulate organic metabolism and energy balance, growth and development, and activity
of the nervous system.
Cretinism, Grave’s disease and simple goiter are disorders associated with the thyroid gland.
IV.
Parathyroid Glands
The parathyroids are embedded on the posterior surfaces of the thyroid.
They produce parathyroid hormone (PTH) which regulates the homeostasis of calcium and phosphate by
increasing blood calcium level and decreasing blood phosphate level. Tetany, a disorder, results
from a deficiency of calcium. In tetany, the body shakes from continuous muscle contraction.
V.
Adrenal Glands
The adrenal glands lie atop of the kidneys.
A. Adrenal Medulla
The adrenal medulla secretes epinephrine and norepinephrine under conditions of stress. They bring about all those responses we
associate with the “fight or flight” reaction: the blood glucose level and the metabolic rate increases, as do breathing and heart rate. The
blood vessels in the intestine constrict, and those in the muscles dilate. This increased circulation to the muscles causes them to have
more stamina than usual.
B. Adrenal Cortex
The two major classes of hormones made by the adrenal cortex are glucocorticoids and the mineralocorticoids.
The glucocorticoid hormone responsible for the greatest amount of activity is cortisol. Cortisol promotes the hydrolysis of muscle protein
to amino acids which enter the blood. Cortisol also counteracts the inflammatory response.
The most significant mineralocoritcoids is aldosterone. Aldosterone regulates the level of sodium (Na+) and potassium (K+) ions in the
blood, its primary target organ is the kidney where it promotes renal absorption of sodium and renal excretion of potassium.
When blood level of sodium is low, the kidneys secrete renin. Renin is an enzyme that converts the plasma protein angiotensinogen to
angiotensin, and this molecule stimulates the adrenal cortex to release aldosterone. This is called the renin-angiotensin-aldosterone
system. The effect of this system raises the blood pressure in two ways. First, angiotensin constricts the arteries directly and secondly,
when aldosterone causes the kidneys to reabsorb sodium, blood volume is raise as water is reabsorbed.
General Biology Lecture
BIO 211 Regis University
Spg 1992
B. Rife
3/3
VI.
Pancreas
The pancreas is a flattened organ located posterior to the stomach and can be classified as both an endocrine and an exocrine gland, or
heterocrine gland. It is composed of two types of tissues: one of these produces and secretes digestive juices that go by way of the
pancreatic duct to the small intestine; the other type, called the islets of Langerhans, produces and secretes the hormones insulin and
glucagon directly into the blood.
Insulin is secreted when there is a high level of glucose in the blood, which usually occurs just after eating. Insulin has the three different
actions: 1) It stimulates fat, liver, and muscle cells to take up and metabolize glucose; 2) It stimulates the liver and muscles to store
glucose as glycogen; and 3) It promotes the build up of fats and proteins and inhibits their use as an energy source. Therefore, insulin is a
hormone that promotes storage of nutrients so that they will be on hand during leaner times. It also helps to lower the blood glucose level.
Glucagon is secreted from the pancreas in between eating, and its effects are opposite to those of insulin; glucagon stimulates the
breakdown of stored nutrients and causes the blood sugar level to rise.
Diabetes Mellitus is a condition in which the liver is not storing glucose as glycogen and the cells are using proteins and fats as an energy
source. The breakdown of proteins and fat also leads to rapid weight loss, to the buildup of acids in the blood (acidosis), and to respiratory
distress. It is acidosis that can eventually cause coma and death if the diabetic does not receive treatment.
In type I diabetes, the pancreas does not produce insulin. This type of diabetes frequently follows a viral infection.
In type II diabetes, the most common type, the pancreas is producing insulin, but the cells of the body do not respond to it.
VII.
Other Endocrine Glands
A. Gonads
The gonads are the endocrine glands that produce the hormones that determine sexual characteristics. The testes produce testosterone.
The ovaries produce estrogen and progesterone
B. Thymus
The thymus is a gland that extends below the thyroid. The thymus gland secretes several hormones related to immunity.
C. Pineal Gland
The eyes send nerve impulses to a cervical ganglion that communicates with the pineal gland, which produces melatonin at night. At
present, melatonin seems to inhibit reproductive activities by inhibiting gonadotropic hormones.
VII.
Chemical Messengers
The concept of chemical messengers has been broadened to include the following three different categories of messengers:
1. Chemical messengers that act at a distance between individuals - many organisms release these chemical messengers called
pheromones into the air or in externally deposited body fluids. These are intended as messages for other members of the same species.
(ants, bark beetles, dogs)
2. Chemical messengers that act at a distance between body parts - this category includes the endocrine secretions, which traditionally
have been called hormones. (ADH, oxytocin)
3. Chemical messengers that act locally between adjacent cells - neurotransmitters belong in this category, as do substances that are
sometimes called local hormones. For example, when skin is cut, histamine is released by mast cells and promotes the inflammatory
response.
Definition of a Hormone
In recent years, some scientists have broadened the definition of a hormone to include all types of chemical messengers.