Chapter 6 - Endocrine System

     The endocrine system uses chemical messengers called hormones for long distance communication between endocrine organs and their target cells. There are two kinds of endocrine organs:
1. Primary endocrine organs whose primary function is the secretion of hormones;
2. Secondary endocrine organs which have a nonendocrine primary function. 
Primary Endocrine Organs YouTube Video
Hypothalamus and Pituitary Gland
   Together the hypothalamus and pituitary gland regulate every body system. The hypothalamus is an important part of the brain and is a primary endocrine organ in two different ways:
1. Hypothalamus and the Posterior Pituitary
   The hypothalamus is an important part of the brain with many functions. Neurons of the hypothalamus synthesize and secrete two hormones, antidiuretic hormone (ADH) and oxytocin. The cell bodies of these neurons are found in two nuclei of the hypothalamus and have axons that descend along a thin stalk of tissue called the infundibulum and end in the posterior pituitary gland. The hormones are released at the axon terminals, diffuse into the capillaries and are distributed throughout the body in the bloodstream. 
   ADH and oxytocin are peptides and are considered neurohormones because of their secretion by neurons. The functions of ADH will be discussed with the urinary and circulatory systems. Oxytocin stimulates uterine contractions and milk letdown in the breasts. 
2. Hypothalamus and the Anterior Pituitary 
   The hypothalamus exerts control of the anterior pituitary by secreting tropic hormones which are hormones that regulate the secretion of other hormones by either stimulating or inhibiting their secretion. 
   The anterior pituitary is non-neural tissue which unlike the posterior pituitary has no direct nervous connection with the hypothalamus. The tropic hormones secreted by the neurosecretory cells of the hypothalamus reach the target cells of the anterior pituitary by the hypothalamic-pituitary portal system. 
    Tropic hormones from the hypothalamus act upon cells that secrete tropic hormones in the anterior pituitary. Except for one, all hormones secreted by the hypothalamus and anterior pituitary are peptides. These hormones include: YouTube Video: TRH and TSH
Hypothalamus Tropic Effect Anterior Pituitary Effect/ Target Organ
Prolactin releasing hormone (PRH) + Prolactin Mammary gland development/milk secretion
Prolactin inhibiting hormone (dopamine) -
Thyrotropin releasing hormone (TRH) + Thyroid stimulating hormone (TSH) Secretion of thyroid hormone by thyroid gland
Corticotropin Releasing Hormone (CRH) + Adrenocorticotropic hormone (ACTH) Adrenal Cortex
Growth hormone releasing hormone (GHRH) + Growth hormone Growth and energy metabolism/secretion of insulin-like growth factors by liver
Growth hormone inhibiting hormone (GHIH) - Growth hormone
Gonadotropin releasing hormone (GNRH) + Follicle stimulating hormone (FSH)/Luteinizing hormone (LH) FSH: development of sex cells/LH: secretion of sex hormones and stimulation of ovulation
  
Pineal Gland
     The pineal gland which is located in the brain secretes the hormone melatonin. Melatonin appears to regulate circadian rhythms (day-night cycles).
Thyroid and Parathyroid Glands
     The thyroid gland is located on the anterior surface of the trachea. It secretes tetraiodothyronine (T4), also known as thyroxine, and triiodothyronine (T3) which regulate the body's metabolic rate and calcitonin which regulates calcium levels in the blood. YouTube Video
     There are four pea-sized parathyroid glands embedded in the posterior side of the thyroid gland. It secretes parathyroid hormone (PTH) which is an important regulator of calcium levels in the blood. YouTube Video
Thymus
     The thymus lies close to the heart and is critical for normal immune function because T-lymphocytes mature here (which is the reason they are called T (T for thymus) - lymphocytes). The thymus also secretes the hormone thymosin which regulates T cell function. 
Adrenal Glands
     The adrenal glands are similar to the pituitary gland in consisting of two structurally and functionally different parts, the adrenal cortex and adrenal medulla. 
  Adrenal Cortex
     The adrenal cortex is the thick (about 80%) outer layer of the adrenal gland. It can be further divided into three layers:
1. Zona glomerulosa - The outer layer. Mineralocorticoids are secreted by cells of this layer. Aldosterone is the primary mineralocorticoid and it regulates sodium reabsorption and potassium secretion by the kidney. 
2. Zona fasiculata - The middle layer. Glucocorticoids (primarily cortisol) is the main hormone secreted by the cells of this layer. Glucocorticoids regulate the body's response to stress; protein, carbohydrate and lipid metabolism; and blood glucose levels. Some sex hormones are also secreted here. 
3. Zona reticularis - The inner layer. Sex hormones (primarily androgen) and some glucocorticoids are secreted here. Sex hormones regulate reproductive function. In males a small amount of androgens secreted compared to the testes, and in females the small amount of androgens secreted may stimulate the sex drive.
  Adrenal Medulla 
     The adrenal medulla contains chromaffin cells that secrete epinephrine (about 80%) and norepinephrine (about 20%). The functions of these "stress" hormones will be discussed later. YouTube Video: Action of Epinephrine
Pancreas YouTube Video
     The pancreas is both an exocrine and endocrine glands. The endocrine portion of the gland resides in the cells of the islets of Langerhans scattered throughout the more numerous exocrine cells.
     Two major hormones secreted by the pancreas are insulin which is secreted by the beta cells and glucagon which is secreted by alpha cells. Both these hormones regulate energy metabolism and blood glucose levels. YouTube Video: Shows the role insulin plays in fat metabolism.
     Two other hormones secreted by the pancreas are somatostatin secreted by the delta cells and pancreatic polypeptide secreted by F cells.
Gonads
     The ovaries and testes in addition to producing the sex cells are major sites for the production of sex hormones.
Secondary Endocrine Organs
     A number of organs, in addition to their primary functions, secrete hormones or participate in their activation. These secondary endocrine organs include:
Heart - Secretes atrial natriuretic peptide which regulates sodium reabsorption by the kidneys.
Kidneys - Secretes erythropoietin (EPO) which stimulates production of red blood cells in the bone marrow.
Digestive organs - Secrete several hormones that regulate digestion and absorption of food.
Liver - Secretes insulin-like growth factors (IGF's) which promote tissue growth.
Skin and Kidneys - Activate vitamin D3 which regulates blood calcium levels.