The Endocrine System

The endocrine system and nervous system work together to coordinate and control the body.  These two systems meet at the hypothalamus and pituitary gland.  The hypothalamus controls the pituitary gland and, in turn, is controlled by the feedback of hormones in the blood.

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Hormones are chemical messengers that control the growth, differentiation, and metabolism of specific target cells.  There are two classes of hormones:

  1. Amino acid compounds (which include proteins, peptides, and amines)
  2. Steroid hormones (derived from lipids)

Many tissues produce hormones, but the glands of the endocrine system are grouped together by having the production of hormones as a main function.  The endocrine glands respond to specific signals by synchronizing and releasing hormones into the circulation.  Even though there is a wide variety  of hormones which function within the body, they share certain general characteristics:

  1. Hormones have specific rates and patterns of secretion.  These basic secretion patterns are
    bulletdiurnal,
    bulletpulsatile, and
    bulletdependent on levels of circulatory substrates (i.e. calcium, sodium, potassium or hormones themselves).
  2. Hormones operate within feedback systems (which are either positive or negative) to maintain an optimal internal environment.
  3. Hormones affect only cells with appropriate receptors and then act on these cells to initiate cell functions or activities.
  4. The kidneys secrete hormones, and the liver metabolizes them, inactivating them and then rendering them more water soluble for renal excretion.

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The release of hormones occurs either in response to an alteration in the cellular environment or in the process of maintaining a regulated level of certain hormones or certain substances. Release of hormones is regulated by chemical factors, other hormonal factors and/or is under neural control. 

Negative feedback is the main mechanism for controlling blood levels of hormones.  In a negative feedback system, plasma levels of one type of hormone influence the level of other types of hormones.  An example of negative feedback follows:

  1. An increase in the anterior pituitary release of follicle-stimulating hormone (FSH) and leutinizing hormone (LH) stimulates the maturation and release of the ovum (egg) into the oviduct.
  2. Mature follicular cells secrete estrogen, and the corpus luteum secretes progesterone.
  3. Once estrogen levels reach a certain plasma peak, feedback into the anterior pituitary inhibits further secretion of FSH.  Estrogen also inhibits the release of gonadotropin-releasing hormone (GRH) from the hypothalamus.
  4. Negative feedback loops are extremely important in maintaining hormones within physiologic ranges.

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Once hormones are released into the circulatory system by the endocrine glands, they are distributed throughout the body.  The protein hormones (insulin, pituitary, hypothalamic, parathyroid) are water soluble and circulate in free form.  Because of their composition, lipid soluble hormones are transported in the blood primarily bound to proteins designated as carrier or binding proteins.

When a hormone is released into the circulatory system, it is distributed throughout the body, but only those cells with appropriate receptors for the hormone are affected.   The target cell receptors have two main functions:

  1. to recognize and bind with their particular hormones
  2. to initiate a signal to their appropriate intracellular effectors.

 

GLAND   HORMONES
Anterior Pituitary TSH (Thyroid stimulation)
ACTH (adrenocorticotropic)
FSH (follicle stimulating hormone)
LH (leutinizing hormone)
MSH (melanocyte stimulating hormone)                                               
Growth hormone
Prolactin
Posterior Pituitary ADH (Antidiuretic hormone)
Oxytocin
Hypothalamus Releasing hormones
Inhibiting hormones
Thyroid Thyroxine (Ty) & triiodothyronine (T3)
Calcitonin
Parathyroid PTH (parathyroid hormone)
Adrenal Cortex    Mineralocorticoids : aldosterone
Adrenal Medulla Epinephrine & norepinephrine
Pancreatic Islets Glucagon & Insulin
Ovary    Estrogen      
Progesterone
Testes                                    Testosterone
Thymus                                 Thymus
Placenta                                                                    Chorionic gonadotropin
Estrogen
Progesterone
Pineal          Melatonin
Heart (Atria) Atrial natriuretic hormone

                     

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