Human Physiology, 14th edition (2016)

Endocrine Glands 355

Summary

11. 3 Pituitary Gland 331
    A. The pituitary gland secretes eight hormones.
    
    
    1. The anterior pituitary secretes growth hormone, thyroid-
       stimulating hormone, adrenocorticotropic hormone, follicle-
       stimulating hormone, luteinizing hormone, and prolactin.
    
    
    2. The posterior pituitary releases antidiuretic hormone
       (also called vasopressin) and oxytocin, both of which
       are produced in the hypothalamus and transported to the
       posterior pituitary by the hypothalamo-hypophyseal tract.
       B. The release of posterior pituitary hormones is controlled by
       neuroendocrine reflexes.
       C. Secretions of the anterior pituitary are controlled by
       hypothalamic hormones that stimulate or inhibit these
       secretions.
    
    
    
    


12. Hypothalamic hormones include TRH, CRH, GnRH,
    PIH, somatostatin, and a growth hormone–releasing
    hormone (GHRH).


13. These hormones are carried to the anterior pituitary by
    the hypothalamo-hypophyseal portal system.
    D. Secretions of the anterior pituitary are also regulated by the
    feedback (usually negative feedback) exerted by target gland
    hormones.
    E. Higher brain centers, acting through the hypothalamus, can
    influence pituitary secretion.


14. 4 Adrenal Glands 337
    A. The adrenal cortex secretes mineralocorticoids (mainly
    aldosterone), glucocorticoids (mainly cortisol), and sex
    steroids (primarily weak androgens).


15. The glucocorticoids help to regulate energy balance.
    They also can inhibit inflammation and suppress
    immune function.


16. The pituitary-adrenal axis is stimulated by stress as part
    of the general adaptation syndrome.
    B. The adrenal medulla secretes epinephrine and lesser amounts
    of norepinephrine. These hormones complement the action
    of the sympathetic nervous system.


17. 5 Thyroid and Parathyroid Glands 341
    A. The thyroid follicles secrete tetraiodothyronine (T 4 , or
    thyroxine) and lesser amounts of triiodothyronine (T 3 ).


18. These hormones are formed within the colloid of the
    thyroid follicles.


19. The parafollicular cells of the thyroid secrete the hormone
    calcitonin, which may act to lower blood calcium levels.
    B. The parathyroids are small structures embedded in the
    thyroid gland. They secrete parathyroid hormone (PTH),
    which promotes a rise in blood calcium levels.

11.6 Pancreas and Other Endocrine Glands 345

A. Beta cells in the islets secrete insulin, and alpha cells secrete

glucagon.

1. Insulin lowers blood glucose and stimulates the

production of glycogen, fat, and protein.

11.1 Endocrine Glands and Hormones 317

A. Hormones are chemicals that are secreted into the blood by
endocrine glands.

1. 
   

   The chemical classes of hormones include amines,
   polypeptides, glycoproteins, and steroids.

   
   


2. 
   

   Nonpolar hormones, which can pass through the plasma
   membrane of their target cells, are called lipophilic
   hormones.
   B. Precursors of active hormones may be classified as either
   prohormones or prehormones.

   
   


3. 
   

   Prohormones are relatively inactive precursor molecules
   made in the endocrine cells.

   
   


4. 
   

   Prehormones are the normal secretions of an endocrine
   gland that must be converted to other derivatives by
   target cells in order to be active.
   C. Hormones can interact in permissive, synergistic, or
   antagonistic ways.
   D. The effects of a hormone in the body depend on its
   concentration.

   
   


5. 
   

   Abnormally high amounts of a hormone can result in
   atypical effects.

   
   


6. 
   

   Target tissues can become desensitized by high hormone
   concentrations.

   
   


7. 
   

   2 Mechanisms of Hormone Action 323

   
   

A. The lipophilic hormones (steroids and thyroid hormones)
bind to nuclear receptor proteins, which function as ligand-
dependent transcription factors.

1. Some steroid hormones bind to cytoplasmic receptors,
   which then move into the nucleus. Other steroids
   and thyroxine bind to receptors already in the
   nucleus.


2. Each receptor binds to both the hormone and to a region
   of DNA called a hormone-response element.


3. Two units of the nuclear receptor are needed to bind to
   the hormone-response element to activate a gene; as a
   result, the gene is transcribed (makes mRNA).
   B. The polar hormones bind to receptors located on the outer
   surface of the plasma membrane. This activates enzymes that
   enlist second-messenger molecules.


4. Many hormones activate adenylate cyclase when they
   bind to their receptors. This enzyme produces cyclic
   AMP (cAMP), which activates protein kinase enzymes
   within the cell cytoplasm.


5. Other hormones may activate phospholipase C when
   they bind to their receptors. This leads to the release
   of inositol triphosphate (IP 3 ), which stimulates the
   endoplasmic reticulum to release Ca^2 1 into the
   cytoplasm, activating calmodulin.


6. The membrane receptors for insulin and various
   growth factors are tyrosine kinase enzymes that are
   activated by binding to the hormone. Once activated,
   the receptor kinase phosphorylates signaling
   molecules in the cytoplasm that can have many
   effects.