The Endocrine System

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The Endocrine System

• Human Anatomy
• Chapter 25

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• This system includes all the glands of the body
  that secrete hormones directly into the blood
  stream. They do not contain ducts, some glands
  may be both endocrine and exocrine (ex
  pancreas). The hormones are chemical stimulants
  that control changes in the body. Endocrine
  glands may be stimulated by the nervous system or
  chemical changes in the body.

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• I. The endrocrine system an overview
• Endocrine glands are studied by endocrinologist.
  These scientists have determined that despite the
  fact that hormones travel all over the body
  through the blood stream and that all tissues are
  exposed to them not all are affected. Hormones
  affect specific tissues called target tissues.
  The cells of these organs have receptors that
  bind the specific hormone and are stimulated by
  that reaction.

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• A. Endocrine organs- some organs are pure in
  that the entire structure is composed of
  endocrine cells, these include the pituitary
  gland, the pineal gland, the thryroid and
  parathryroid gland, and the adrenal glands. The
  following perform functions in other system
  asides from the endocrine pancreas, thymus,
  testes, ovaries, and hypothalamus. Endocrine
  glands have a high supply of blood and lymph
  vessels.

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• B. Hormones- each hormone type had its own
  chemical structure and thus performs different
  actions.
• 1. Classes of hormones- amino acid-based hormones
  contain amino acids or small proteins as part of
  their structure. Steroids are hormones that
  contain a cholesterol group as part of its
  structure.
• 2. Basic hormone action- Cells have receptors on
  their surface that bind only specific types of
  hormones. When receptor binding occurs a program
  response/reaction takes place. Hormones do not
  carry information but are only released to
  stimulate a chemical reaction. Tropic hormones
  are secreted by an endocrine to stimulate the
  activity of another endocrine gland.
• .

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• 3. Control of hormone secretion- the following
  list condition under which an endocrine gland
  releases hormones
• a. Humoral stimuli gland responds to chemical
  changes in the body such as changing levels or
  ions or nutrients in the blood.
• b. Neural stimuli some glands are signal by
  nervous system (sympathetic nerves) to release
  hormones in order to induce physiological
  changes.
• c. Hormonal stimuli some endocrine glands are
  controlled by other endocrine glands and thus
  respond to hormone.
• d. Feedback loops hormone levels are also
  controlled by feedback loops that either
  stimulate or halt the production of a hormone

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• II. Major endocrine organs
• Be familiar with the location of each gland,
  the hormones they release and its function.
• A. The pituitary gland is also called the
  hypophysis and is located within the cranium
  inferior to the brain. It sits on the hypophyseal
  fossa of the sella turcica (sphenoid bone). This
  gland connects to the hypothalums through a stalk
  called the infundibulum, the infundibulum connect
  to the tuber cinereum of the hypothalamus.

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Pituitary Gland and visible leukocytes
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• 1. Adenohypophysis also known as the anterior
  pituitary gland. The majority of the hormones
  produced are amino acid based hormones. Different
  cell types produce different hormones including
• a. Growth hormone (GH)- signals growth of the
  body
• b. Somatotropic hormone(SH) or somatotropin-
  signals growth of the body
• c. Prolactin- stimulates the breast tissue to
  produce milk
• d. Thyroid-Stimulating hormone- stimulates the
  thyroid gland to secrete hormones that influence
  our metabolism.
• e. Adrenocorticotropic hormone (ACTH)-
  stimulates adrenal gland to signal its hormones
  that help us deal with stress.
• f. Melanocyte-stimulating hormone (MSH)-
  stimulates melanocytes of the epidermis to
  produce more melanin, thus darkening the skin.
• g. Follicle stimulating hormone (FSH)- act on
  testes or ovaries an stimulate the maturation of
  egg/sperm and induce secretion of sex hormones
• h. Leutinizing hormone (LH)- During the
  menstrual cycle it signals ovulation

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• 2. Hypothalamic control of hormone secretion from
  the adenohypophysis the hypothalamus had neurons
  that produce hormones and release them much like
  neurotransmitters are released. These travel
  through the hypophyseal portal system into the
  anterior pituitary and stimulate its hormone
  secretion. The hypophysial portal system
  involves two beds of capillaries connected by a
  vein. It allows a high level of horomone
  concentration within a small region and it is
  designed so that the hormones released by the
  hypothalamus travel directly to the anterior
  pituitary and it in turn release hormones into
  systemic circulation.

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• 3. Neurohypophysis also known as the posterior
  pituitary gland, it is technically part of the
  brain and it composed of neurons. Rather than
  making hormones it stores those produced by the
  hypothalamus and releases them when it receives a
  neural impuls. The hormones produced by the
  hypothalamus but released by the neurohypophysis
  are
• a. Antidiuretic hormone (ADH)- stimulates the
  collecting tubules of the kidney to concentrate
  the urine in order to reduce water loss.
• b. Oxytocin- induces contractions of smooth
  muscle of the sexual organs, such when a mother
  experiences contractions of the uterus during
  labor.

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• B. The thyroid gland located just inferior to
  the larynx, it has two lobes that give it the
  appearance of butterfly wings. It is the largest
  endocrine gland and it releases thyroid hormones
  and calcitonin.
• a. Thyroid hormone T4 and T3, amino acid based
  hormones that contain iodine, main function is to
  increase metabolic rate.
• b. Calcitonin- reduces excessive levels of
  calcium ion in the blood by slowing down
  osteoclast activity, actively secreted during
  childhood.

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Histology of Thryroid and Parathyroid Gland
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• C. The parathyroid gland yellowish in color, lie
  posterior or the thyroid gland. There are at
  least two pairs of glands but some may have as
  many as four pairs. The hormone it releases is
  parathyroid hormone (PTH) which has the opposite
  effect if calcitonin. It increase calcium ion
  levels in the blood by either activating the
  osteoclast, stimulating the kidney to reabsorb
  more calcium, or activation vitamin D production
  for calcium to be absorbed from food.

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• D. The adrenal (suprarenal) gland These are two
  glands located superiorly to the kidney and are
  actually two different glands within one
  structure.
• 1. adrenal medulla considered part of the
  autonomic nervous system, it releases adrenaline
  or epinephrine to assist in the fight or flight
  response.

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• 2. adrenal cortex secretes steroids called
  corticosteroids. Aldosterone (mineralocorticoid)
  is secreted in response to a decrease in blood
  volume or blood pressure stimulating the kidneys
  to reabsorb more water or sodium. Another group
  of hormones is called the glucocorticoids which
  include cortisol. These hormones keep blood
  glucose levels high to maintain brain activity
  during stressful situations. They can direct
  lymphocytes, or decrease inflammation.

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• E. The pineal gland also located in the brain
  and controlled by the hypothalamus, it is
  composed of endocrine cells that secrete
  melatonin which regulates the internal body
  clocks it induces sleep.

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• F. The pancreas located in the abdominal cavity
  it contains endocrine cells called the islets of
  Langerhans that produce amino acid based
  hormones. The cells release insulin which
  stimulates absorption of glucose by tissue cells
  when blood glucose levels are high, and also
  releases glucagon that stimulates the liver to
  release sugar into the blood when blood glucose
  levels are low.

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Histology of the pancreas
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• G. The thymus located in lower neck, anterior
  thorax, and posterior to the sternum. It
  secretes amino acid based hormones called thymic
  hormones like thymopoeitin and thymosin that
  stimulate T-lymphocytes to become
  immunocompetent.
• H. The gonads testes and ovaries are organs that
  secret steroid sex hormones. Androgens released
  by these glands are changed into testosterone,
  estrogen, and progesterone. They maintain
  primary and secondary sex characteristics and
  cells involved in reproduction.

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• III. Other endocrine structures
• A. The heart has sensory cells in the atria that
  stimulate the kidneys to produce more urine
  containing salt. Getting rid of the excess fluid
  and salt reduces excess blood volume and salt
  blood levels, and reduces blood pressure.
• B. The GI tract has scattered cells that release
  amino acid/peptide hormones chemically similar to
  neurotransmitters. They affect functions related
  to regulating digestion, blood chemistry, and
  blood flow.
• C. The placenta is produced only when conception
  occurs, it secrets hormones that prevent the
  uterus from getting rid of the nutrient layer to
  which it is attached. It also produces other
  steroid hormones estrogen, progesterone,
  corticotropin-releasing hormone, and human
  chorionic gonadotropin.
• D. The kidneys release rennin which regulates
  blood pressure and erythropoietin which
  stimulates erythrocyte production.
• E. When exposed to UV rays, the skin produces a
  steroid hormone precursor to vitamin D. Vitamin
  D helps in the absorption of calcium.

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A woman with Cushins Disease