A closer look at the Endocrine system

The Endocrine system is composed of several glands including pineal, pituitary, hypothalamus, parathyroid, thyroid, thymus, adrenal, pancreas, testes and ovaries. These endocrine glands can synthesize and release hormones directly into the blood to respond to specific signals (McCance & Huether, 2017). Endocrine glands secretions can be regulated by a hormone manufactured by another gland or nervous system input (Mallya & Ogilvy-Stuart, 2018). In fact, the endocrine system works together with the nervous system to accomplish and maintain homeostasis. The mechanism usually used by the endocrine system to regulate biologic control is the negative feedback loop (Patton & Thibodeau, 2013). The negative feedback loop tends to reverse any alteration of the internal environment (Mallya & Ogilvy-Stuart, 2018).

There are three types of stimuli that usually initiates synthesizing and releasing of hormones; the hormonal, humoral and neural mechanism. The hormonal is control by a hormone from one endocrine gland regulating another endocrine gland (McCance & Huether, 2017). The humoral is activated by changes in blood levels of ion and nutrients, such as glucose or calcium (McCance & Huether, 2017). Lastly, the neural control, such as the sympathetic nervous system release of catecholamines from adrenal medulla during stress (Patton & Thibodeau, 2013). The hormones are released into the blood but only affect the intended target cells with specific hormone receptors for that hormone (McCance & Huether, 2017). The sensitivity of the target cell depends partially on the number of receptors and factors that alter signal transcription (Patton & Thibodeau, 2013). Increased number of hormone receptors increases sensitivity ( up-regulation), while a decreased number of hormone receptors decreases sensitivity, named down-regulation (McCance & Huether, 2017). Negative feedback is fundamental in keeping hormone levels within an appropriate physiological range (McCance & Huether, 2017). 

The pituitary gland is located with the sella turcica and is connected to the hypothalamus by a stalklike infundibulum (Patton & Thibodeau, 2013). The pituitary gland is made of two separate glands, the anterior pituitary gland (adenohypophysis) and the posterior pituitary gland (neurohypophysis). The hypothalamus is located at the base of the brain, near the pituitary gland (McCance & Huether, 2017). The negative feedback mechanism is the instrument used by the hypothalamus to regulate the secretion of the adenohypophysis, and the adenohypophysis adjusts the secretions of its target glands, which in turn alter the activity of their target tissues (Patton & Thibodeau, 2013). 

The hormones from the hypothalamus have releasing and inhibiting effects of the adenohypophysis. For example: the growth hormone-releasingg hormone (GHRH) prompts release of growth hormone (GH) by the adenohypophysis, growth hormone-inhibiting hormone (GHIH) inhibits secretion of growth hormone by the adenohypophysis, corticotropin-releasing hormone (CRH) prompts release of adrenocorticotropic hormone (ACTH) by the adenohypophysis, Gonadotropin-releasing hormone (GnRH) stimulates release of gonadotropins (FSH and LH) by the adenohypophysis, etc. (McCance & Huether, 2017). The GH promotes the growth of bone, muscle, and other tissues. In addition to stimulating fat metabolism and act as an insulin antagonist. The ACTH stimulates the adrenal cortex to secrete corticosteroids, which help regulate metabolism and immune response. The FSH and LH stimulate the growth and maintenance of the gonads. These examples show the importance of the negative feedback system in keeping hormone levels within an appropriate physiological range to avoid diseases (Mallya & Ogilvy-Stuart, 2018). 

References

Mallya, M., & Ogilvy-Stuart, A. L. (2018). 2: Thyrotropic hormones. Best Practice & Research Clinical 

Endocrinology & Metabolism, 32(SI: Hormones in milk - Part II), 17-25. doi:10.1016/j.beem.2017.10.006

McCance, K. L., & Huether, S. E. (2017). Pathophysiology: the biologic basis for disease in

              adults and children. St. Louis, MO: Elsevier.

Patton, K. T., & Thibodeau, G. A. (2013). Anatomy & physiology. St. Louis, MO: Mosby/Elsevier.