Chapter Summary:
In this chapter, the functions of the endocrine system were explained. The endocrine is a complex system that includes the major glands of the body (pancreas, pituitary, thyroid, gonads, etc.) and also the hormones secreted by these glands. The first part of this chapter gave an overview of what hormones are and how they work in our bodies. The second part of the chapter covered hormone actions and how hormones use messengers to communicate within the human body. The chapter then went on to cover the pituitary gland, adrenal glands, thyroid gland, parathyroid galnd, pancreas and regulation of the endocrine system using autocrine and paracrine methods. The endocrine system is a complex system that works on many different areas of the body. Even our sleep cycle is influenced by the endocrine system. The transition into puberty is also done by functions of the endocrine system.

Hormones; we all love and need them. Hormones are the things that are responsible for our growth, they stimulate reproductive organs to allow males to create sperm and for females to ovulate, among many other functions. Hormone are classified under many different categories. They can be steroids, amine, polypeptides and glycoproteins. Each category of hormones work on a different part of the body and have very different functions. Hormones are secreted into the body via the glands of the endocrine system. The blood will carry hormones throughout the body to the target cells that need the hormones. Hormones will only act on specific target cells that are specialized to receive that specific hormone. If a cell does not have a target receptor protein for a specific hormone, that hormone will not act on that cell. The picture below illustrates how the hormone secreted works on target cells.
Pituitary Gland:
The pituitary gland is located in the brain just below and slightly forward of the hypothalamus. The pituitary gland is about the size of a pea, and is divided into two lobes. These lobes are called the anterior (front) pituitary and the posterior (back) pituitary. Not only are these regions structural divisions, they are functionally different as well. Although quite small, the pituitary gland has many very different and very important functions. The pituitary gland regulates the release of hormones that are responsible for growth, reproduction and metabolism, just to name a few. The posterior and anterior portions release different hormones. The anterior pituitary releases six different hormones that are said to be trophic hormones. Trophic hormones stimulate other glands to release their specific hormones. The hormones that are released by the posterior pituitary are formed because of sensory stimulation. The hypothalamus forms these hormones. The posterior pituitary then accepts these hormones that are released by the hypothalamus and stores them until they are needed. This process is explained in more detail below.

The pancreas is the largest gland in the body. It is located below the stomach and is encircled by the duodenum. The pancreas is both an endocrine gland as well as an exocrine gland. The endocrine function of the pancreas involves secretion of two hormones; insulin and glucagon. Insulin and glucagon play a vital role in the regulation of blood sugar levels. Within the body and tail of the pancreas are structures called islets. These islets contain beta cells that secrete insulin when they are stimulated by a rise in the sugar levels of the blood. The islets contained in the pancreas also contain alpha cells. These alpha cells are responsible for secreting glucagon and are triggered by a decrease in the level of sugar in the blood. Below is a picture showing these alpha and beta cells of the pancreas. The pancreas also has exocrine functions that work on the digestive system, but that will be discussed at a later time. The pancreas is a perfect example of the negative feedback loops that were discussed in chapter one. An increase in blood glucose levels stimulates the beta cells to release insulin which decreases the level of blood glucose. In the reverse of this action, a decrease in blood glucose levels stimulates alpha cells to release glucagon which increases blood glucose levels and decreases insulin levels. It is a "push-pull" system.

Application to Nursing:
Endocrinology has endless applications to the field of nursing. One very important aspect that the nurse must understand in order to properly care for their patients is the sleep cycle. The sleep cycle is actually quite a complex system. The retina of the eyes detect changes in light. The eyes send this message to the superior cervical ganglion. The superior cervical ganglion then sends the message of either increasing or decreasing light to the pineal gland. Based on this information, the pineal gland will then either increase secretion of melatonin to encourage the body to go to sleep or it will decrease melatonin production to encourage the body to wake up. When patients are in the hospital, often their sleep cycle is disturbed. This can be a result of many different things including a change of environment, anxiety due to health condition, etc. A nurse who understands how the sleep cycle works can help get patients the rest that they need by ensuring that the amount of light being let into a patient's room is low enough to encourage sleep. Also when a patient has to be awake, the nurse can open up window shades and door to let in more light to help keep the patient from sleeping.

Case Study/Essential Question:
"Describe how the hypothalamus regulates the action of the posterior pituitary and the anterior pituitary. Describe one hormone that the posterior pituitary and anterior pituitary makes and describe the action of that hormone. What are trophic hormones and their role in hormone communication?"The control that the hypothalamus has on the posterior pituitary gland is driven by sensors in the body. The posterior pituitary only produces two hormones. Oxytocin is regulated by sensory nerve impules that are produced (in a female) through baby's sucking action. ADH is regulated by sensors in the blood stream that detect changes in the plasma osmolality of the blood. The hypothalamus receives information from these sensors and then secretes the respective hormone based on the sensory information. The hormones are sent via nerve axons to the posterior pituitary gland and are stored until their release is needed. The anterior pituitary is controlled in a non-neural manner. To control the anterior pituitary, the hypothalamus releases hormones that act on the anterior pituitary and stimulate it to release other hormones that stimulate target organs. When the hypothalamus releases hormones that are intended for the anterior pituitary, they are transferred down nerve axons that end at the base of the hypothalamus. These nerve endings release their hormones into a capillary bed system that is in contact with the anterior pituitary. The anterior pituitary receives their hormonal messages through this capillary bed system between the hypothalamus and the anterior pituitary which is called the hypothalamo-hypophyseal portal system.
The posterior pituitary gland only secretes two different hormones. They are the antidiuretic hormone and oxytocin. The antidiuretic hormone is responsible for the kidney ability to retain water or excrete increased amounts of water as needed. When a person is dehydrated, the secretion of ADH is increased and the body retains water. When there is an excess of fluid of water in the body, stimulation of ADH is decreased and more water is released from the body. Oxytocin is mainly a female hormone. An increase in Oxytocin produces an in crease in uterine contraction during childbirth. It also stimulates milk ejection in women who are lactating. The significance of oxytocin in men remains a mystery. The anterior pituitary gland secretes trophic hormones. There are six of them and they are growth hormone, thyroid stimulating hormone, adrenocorticotrophic hormone, follicle-stimulating hormone, luteinizing hormone and prolactin. Trophic hormones stimulate their target organs to produce hormones specific to those organs. When trophic hormones are present in increased levels, hypertrophy of their target organs is the result. When they are present in decreased levels, atrophy of their target organs is the result. For example, growth hormone is a trophic hormone. If there is an overabundance of growth hormone, a person becomes much larger in size than they normally should be. This is present in conditions such as giantism. If there is not enough growth hormone, a person becomes smaller in size than they normally should be. This is present is conditions such as dwarfism.