The endocrine system

The endocrine system is a series of glands that works with the nervous system to maintain homeostasis.  Each gland secretes specific hormones that pass through the blood to the appropriate receptors on specific organs.  The endocrine system uses negative feedback and cycles for regulation.

The main glands of the endocrine system are the pituitary gland, the hypothalamus, the pancreas, the thyroid gland, and the adrenal glands.  The pituitary gland is linked to the hypothalamus to control release of pituitary hormones from the anterior pituitary lobe.  The hypothalamus monitors levels of thyroid and growth hormones in the blood.  The thyroid is located in the neck and controls metabolic rate, regulates growth and development, and regulates the onset of maturity.   Thyroid hormones target almost all body cells (Farabee, 2001).  The pancreas secretes insulin and glucagon.   The adrenal glands are located at the top of the kidneys and secrete epinephrine and norepinephrine, the “fight or flight” hormones.

The hormones used in the endocrine system are grouped into three classes:  steroids, peptides, and amines.  Steroid hormones pass through the plasma membrane to bind to the nuclear membrane receptors.  This produces an activated hormone-receptor complex that binds to DNA and activates specific genes to increase the production of proteins (Faradee, 2001).   The steroids are not stored in cells and are controlled by rate of synthesis.  Non-steroid hormones, on the other hand, are water-soluble and bind to the plasma membrane receptors instead of entering the cell.  Non-steroid hormones generate a chemical signal inside the target cell to trigger second messengers to activate other intracellular chemicals in order to produce an appropriate response (Farabee, 2001).

Peptides and Amines are short chains of amino acids secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.  The thyroid and adrenal medulla secrete amines derived from the amino acid tyrosine (Farabee, 2001).  Peptides and amines are stored in secretory granules and cytoplasm to be used when needed.  The hormones secreted by the endocrine system cross over into other systems in the human body.  If any of the glands in the endocrine system are not secreting the proper amounts of a particular hormone, then it could affect one or more of the other systems.  If any of these hormones are lacking or too plentiful, the body is thrown out of homeostasis.

In the Journal of Biological Physics, Conrad et al (2009) propose using positive and negative feedback loops for restoring homeostasis in the hypothalamus-pituitary-adrenal system.  This system is responsible for controlling stress levels and is stimulated in the second half of the night, when a person is in deep sleep.  If a person cannot reach deep REM sleep, the HPA system cannot restore homeostasis, and that person could suffer depression or obesity.

Just as the HPA system can affect mental health and weight, the pancreas can affect the body’s ability to utilize insulin.   There are a great many studies on the effect of the endocrine system and Diabetes Mellitus.   The pancreatic islets contain alpha cells (A cells) that secrete glucagon and beta cells (B cells) that secrete insulin.   Glucagon controls gycogenolysis; this increases blood sugars.  These elevated blood sugars trigger the pancreas to release insulin to lower the blood sugar levels.  In type 1 diabetes mellitus, the pancreatic islets secrete too little insulin; in type 2 diabetes mellitus, the pancreatic islets secrete insulin but there is an abnormality of insulin receptors (Thibodeau and Patton, 2008). 

In Nutrition Reviews, Wang et al (2012) discuss the protein PANDER and its affect on beta-cell function.  This could lead to repressed glucose-stimulated insulin secretion, leading to Type 2 diabetes.  Finding a way to increase the effect of PANDER on B cell functions could help type 2 diabetics with their insulin production and effectiveness.

In Clinical Endocrinology, Wang et al (2012) also studied the effect of pancreatic secretions on diabetics.  In this study, they studied the glycemic variability of non-diabetic individuals and newly diagnosed type 2 diabetics.  They found more intraday glucose fluctuations in newly diagnosed type 2 patients and patients with impaired glucose regulation than non-diabetic individuals (Wang et al, 2012).

These articles teach us that the endocrine system must work properly for the body to maintain homeostasis and avoid mild to serious diseases.  Current research is still working on finding answers to the best way to utilize the knowledge scientists already have about the functioning of the endocrine system to help combat these diseases.  The first step is to understand how the endocrine system functions with the rest of the body to maintain homeostasis.

References:

Conrad, Matthias; Hubold, Christian; Fischer, Bernd; Peters, Achim. (2009)  Modeling the hypothalamus-pituitary-adrenal system:  homeostasis by interacting positive and negative feedback.  Journal of Biological Physics, (2009) 35: 149-162.  Springer Science & Business Media

Farabee, M.J. (2001)  The Endocrine System.  Retrieved on March 11, 2012, from www.emc.maricopa.edu/faculty/farabee/biobk/biobookendocr.html

Thibodeau, Gary & Patton, Kevin (2008)  Structure & Function of the Body, 13^th Edition.  Mosby Elsevier

Wang, Chun; Lu, Lifang; Yang, Yanzhi; Chen, Dawei; Liu, Guanjian; Chen, Lihong; Song, Yuanxia; He, Liping; Li, Xiujun; Tian, Hasming; Jia, Weiping; Ran, Xingwu (2012).  Glucose fluctuations in subjects with normal glucose tolerance, impaired glucose regulation and newly diagnosed type 2 diabetes mellitus.  Clinical Endocrinology, Vol 76, Issue 6: 810-815.  Wiley-Blackwell.

Wang, Chunjiong; Burkhardt, Brant R; Guan, Youfei; Yang, Jichun (2012)  Role of pancreatic-derived factor in type 2 diabetes:  evidence from pancreatic Beta cells and liver.  Nutrition Reviews Vol. 70(2):  100-106.  Wiley-Blackwell.