The Endocrine System

Endo- “internal”

-crine “secretion”

The endocrine system involves glands that secrete hormones (chemical messages) directly into the bloodstream. You can compare them to exocrine glands, which secrete outside of the body (ex: sweat glands).

It’s more useful to compare the endocrine system to the nervous system. Both systems carry out similar functions.

  • They both receive information
  • They both integrate information
  • They both generate a response to a stimulus

However, the nervous system is much faster. Neurons are able to send and receive messages much more quickly than glands. This is because neurons are using fast, electrical signals and glands have to secrete their product into the bloodstream and wait for the blood to make its way through the body. In general, the effects of hormones are going to last much longer as well.

The endocrine system is really easy to learn and really difficult at the same time. It works in a straight-forward, logical way, but there is a ton to memorize! It’s a good idea to start by understanding negative and positive feedback loops, the hypothalamic-pituitary axis, and then start making a table or chart to memorize the hormones, their target cells/tissues, and the effects that we see.

Let’s start with the hypothalamic-pituitary axis.  Basically, many hormones are going to involve a system where:

  • The hypothalamus secretes a “releasing hormone”
    • The hormone travels through the blood (through a short series of blood vessels called the hypothalamic-hypophyseal portal system)
      • The hormone reaches the Anterior Pituitary gland.
      • Cells in the anterior pituitary are stimulated to release a “tropic/trophic hormone”
        • The tropic/trophic hormone travels through the bloodstream and reaches the target tissue

For example, the Hypothalamus releases Thyrotropin Releasing Hormone, which travels through the blood in the hypothalamic-hypophyseal portal system. This stimulates cells in the anterior pituitary gland to release Thyroid Stimulating Hormone (TSH), which travels through the bloodstream to the thyroid. The thyroid is stimulated to release Thyroxine (a.k.a. tetraiodothyronine or T4) and Triiodothyronine (a.k.a. T3).

When studying, it may be helpful to make a list that looks something like this:

  • Hypothalamus secretes TRH  -> Anterior Pituitary
    • Anterior Pituitary secretes TSH  ->  Thyroid Gland
      • Thyroid releases T3 and T4  -> Most tissues throughout the body

To remember Anterior Pituitary Hormones, I use the mnemonic “Pro ATHletes GOt To GROW”

  • Prolactin
    • Prolactin is unique in the Hypothalamic-Pituitary axis, because it is always suppressed by Prolactin Inhibiting Factor (which is believed to really be dopamine)
    • Prolactin is released when suppression stops
  • ACTH
  • Gonadotropins (FSH and LH)
  • TSH
  • Growth Hormone

For the Posterior Pituitary, there are only 2 hormones.  These hormones are produced in the Hypothalamus, but stored in the Posterior Pituitary.  They are released when cells in the Posterior Pituitary are stimulated by neurons coming from the Hypothalamus:

  • Anti-diuretic Hormone (ADH) is also known as Vasopressin because:
    • It has anti-diuretic effects, causing you to retain water (which increases blood volume and therefore, blood pressure)
    • ADH also directly stimulates blood vessels to contract, increasing blood pressure
  • Oxytocin
    • Oxytocin is somewhat unique – almost all hormones are regulated by negative feedback. Normally, when the concentration of a hormone rises in the blood, the cells that produce it will stop producing it.
    • Not so, with oxytocin.  It is regulated by positive feedback, where the presence of the hormone causes more to be released.  With all positive feedback loops, you need some event to break the cycle.
      • For example, a baby’s head pressing on the cervix will cause oxytocin release, which causes the uterus to contract, which puts pressure on the cervix, which causes more oxytocin release – until eventually the baby is born and the cycle stops.
      • It’s the same thing with breastfeeding.  The baby suckles, which stimulates oxytocin release, which causes myoepithelial cells in the mammary gland to contract, pushing milk through ducts and out of the nipple. The baby drinks as it continues suckling, causing more oxytocin to be released – until finally the baby is full or asleep and stops suckling.

Other Glandular Structures include (but are not limited to):

  • Gonads (Ovaries and Testes)
    • LH -> Leydig cells of testes -> Testosterone
    • FSH -> Sustentacular cells of testes -> Sparmatid development
  • Thymus -> Thymosins
  • Kidneys
    • Erythropoietin (EPO) -> Bone marrow -> Red blood cell production
  • Adrenal Cortex
    • ACTH -> Cortisol
    • Low sodium levels -> Aldosterone
    • Testosterone (Production of testosterone in the Adrenal Cortex accounts for a very small portion of the male’s testosterone level, but it accounts for 100% of the testosterone in female blood plasma)
  • Adrenal Medulla
    • Sympathetic nervous stimulation -> Epinephrine and Norepinephrine  (also known as Adrenaline and Noradrenaline)

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