UNIT 4: Homeostasis
Chapter 10: The Endocrine System
pg. 466
10.2: The Endocrine Glands
pg. 473 – 482
Hormones are produced by the endocrine system. These chemicals maintain
homeostasis within the body. The endocrine system is made up of many
glands located throughout the body. The glands produce hormones and
secrete them into the circulatory system to be delivered to their target cells.
Some hormones are secrete from organs, such as; the heart, liver and the
kidneys.
Figure 1: The major endocrine glands and cells in humans.
The Vertebrate Endocrine System
Hypothalamus – is the region of the brain that releases hormones to control
the pituitary gland, which, in turn, controls other endocrine glands.
Neurohormone – is a hormone produced by neurons, such as in the
hypothalamus that controls the production of other hormones in the pituitary
gland.
Pituitary gland – is a two lobed gland within the cranial cavity that produces
hormones that control the other endocrine glands.
The hypothalamus gland is found at the base of the brain and is part of the
nervous systems. The hypothalamus receives nerve impulses and produces
special type of hormones, called a neurohormone. The hypothalamus
coordinates actions between the brain and the endocrine system.
Neurohormone leaves the hypothalamus and travels to the pituitary gland.
The hypothalamus is also in direct contact with some sensory neurons to
monitor changes in body chemistry and temperature.
Pituitary gland is located in the cranium, below the brain, and has two lobes,
anterior and posterior. Neurohormones from the hypothalamus travel to the
pituitary gland via the portal vein.
Two types of neurohormones secreted from the hypothalamus are releasing
and inhibiting hormones. These hormones will affect the anterior portion of
the pituitary gland, either stimulating or inhibiting hormones secretion. The
hormones secreted from the pituitary gland control many other endocrine
glands, therefore the pituitary is known as the master gland.
See Table 1: Hypothalamic and Pituitary Hormones, pg. 474
Figure 2: The hypothalamus and the pituitary gland link the endocrine system and the
nervous system. Neurohormones produced by the hypothalamus control the anterior and
posterior pituitary hormones, which control the endocrine glands.
Anterior Pituitary Gland
Anterior Pituitary Gland produces six major hormones: prolactin, growth
hormone, thyroid-stimulating hormone, adrenocorticotropic hormone,
follicle-stimulating hormone, and luteinizing hormone.
Prolactin (PRL) – is responsible for reproductive activities and parental care
in vertebrates. PRL stimulates secretory cells in the mammary glands during
late pregnancies and milk synthesis.
Growth Hormone (GH) – stimulates cell division, protein synthesis and
bone growth in children and adolescents. GH binds to muscles and other
target tissues, causing them to release insulin-like growth factor (IGF),
which directly stimulates growth. GH also controls the conversion of
glycogen to glucose and fats to fatty acids to maintain levels in blood. GH
also regulates the intake fatty acids and amino acids by cells and limiting the
uptake of glucose by cells. Lack of GH can lead to dwarfism or the over
secretion can lead to gigantism. (2.7 m)
Thyroid-stimulating hormone (TSH) is responsible for stimulating the
thyroid gland to increase in size and release thyroid hormones.
Adrenocorticotropic hormone (ACTH) initiates the release of hormones
from the adrenal cortex.
Follicle stimulating hormone (FSH) affects egg development and female
hormones in females and sperm production in males.
Luteinizing hormone (LH) regulates part of the menstrual cycle in females
and sex hormones in males. FSH and LH are known as Gonadotropins
because they regulate the activities of the gonads (ovaries and testes)
Melanocyte-stimulating hormone (MSH) and endorphins are secreted
from the anterior pituitary gland. MSH is responsible for the secretion of
melanin that makes your skin go darker in the sun.
Endorphins (natural painkillers) act as neurotransmitters in the pathways
that control pain, inhibiting pain perception.
Posterior Pituitary Gland
The posterior pituitary gland stores and releases two important hormones
produced in the hypothalamus; antidiuretic hormone and oxytocin into the
circulatory system.
Antidiuretic Hormone (ADH) is responsible for the stimulating of the
kidney cells to absorb water from the urine, to increase blood volume.
Sensory receptors in the hypothalamus are stimulates by the high
concentration of Na+. The kidneys are able to maintain a homeostatic
balance of extracellular fluid in the body. Approximately 85% of the water
that passes through the nephron is reabsorbed from the proximal convoluted
tubule the rest is absorbed from the distal convoluted tubule only if ADH is
present.
Oxytocin stimulates the production of milk from the mammary glands and it
also plays a key role in the stimulation of contractions during child birth.
Thyroid and Parathyroid Hormones
Thyroid gland – is an endocrine gland located in the throat that is regulated
by the hypothalamus-pituitary system.
Parathyroid hormone – is a hormone secreted by the parathyroid glands hat
controls calcium and phosphate levels in the blood.
The thyroid is located in the front of the throat, and it secretes hormones that
have a wide range of effects. Thyroxine (T4) contains four iodine atoms, is
responsible for increases the metabolic rate of cells and is essential for body
growth. The thyroid is vital for growth, development, maturation, and
metabolism is all mammals.
A goiter is a condition created when the thyroid is over stimulated, the neck
begins to swell. Iodine supplements are added to food to prevent goiters in
many countries.
Figure 6: the negative feedback control of parathyroid hormone (PTH) and calcitonin by
the Ca2+ level in the blood.
Adrenal Hormones
The adrenal medulla secretes two adrenal hormones, epinephrine and
norepinephrine that act as neurotransmitters. Epinephrine prepares your
body for handling stress.
The adrenal cortex secretes several hormones. Aldosterone is involved in
osmoregulation and control of blood pressure.
Pineal Gland
Pineal gland – is an endocrine gland located in or on the brain of vertebrates
that secretes the hormone melatonin to regulate biological rhythms.