Endocrine System

• This organ system is the endocrine system, and it utilizes

  substances called hormones to coordinate the functions of almost all other organ systems.
• General Endocrine Functions

  The endocrine system mostly functions to synthesize, store, and release hormones that serve as chemical signals for other body tissues to perform specific tasks. A vast majority of body tissues are receptive to hormones. Thus, the endocrine system’s indirect functions are highly diverse if the responses of these tissues are taken into account.
• Major Glands of the Endocrine System

  The endocrine system is made up of multiple glands that release hormones directly into the bloodstream. The major glands of the endocrine system are discussed in the succeeding sections.
• The hypothalamus, which is located deep within the brain tissue (as shown in Fig. 12.3.1), is one of the major endocrine organs that maintain homeostasis in the human body. Recall that homeostasis is the maintenance of a state of balance in the body, and it involves processes such as pH,
• Major Glands of the Endocrine System

  
• Pituitary Glands

  The pituitary glands, also known as the master gland, control many other endocrine glands’ functions and activities. It is a small gland located at the base of the brain (also shown in Fig. 12.3.1). It has two distinct lobes, namely, the anterior and posterior pituitary lobes. Though the pituitary is known as the master gland, many of its functions are influenced by the

  hypothalamus, which the pituitary has a direct anatomical connection with.
• Pineal Gland

  The pineal gland is another endocrine gland that is located in the brain (also shown in Fig. 12.3.1). This small, pea-sized gland is important for the hormone melatonin that it produces.
• Melatonin plays an important role in regulating sleep-wake patterns or an animal’s circadian rhythm. The circadian rhythm is also known as the body clock, which is a repeating daily pattern of sleep-wake cycles in humans. To exert its effects, the pineal gland typically increases its melatonin secretion at night to promote sleep.
• Thyroid and Parathyroid Glands

  The thyroid gland is located near the base of the human neck (as shown in Fig. 12.3.2). This gland produces important hormones that help regulate the metabolic rate of the body. Thus, their effects influence almost all tissues of the body, particularly bones and muscles.

  
• Two of the most important hormones produced by the thyroid gland are the triiodothyronine and thyroxine, and they are collectively referred to as

  thyroid hormones. These two hormones perform some of the most important functions of the thyroid glands, like metabolic regulation

  (particularly boosting an individual’s basal metabolic rate), nervous system development, and heightening of protein synthesis rate.
• Calcitonin is a hormone produced by the thyroid gland
• Calcitonin regulates the body's calcium, phosphate, and potassium levels
• Calcitonin reduces calcium levels in the blood by inhibiting bone resorption by osteoclasts
• The thyroid gland needs an adequate amount of iodine from the bloodstream to synthesize thyroid hormones
• An insufficient amount of iodine can lead to abnormalities in thyroid function
• Iodine deficiency can lead to the accumulation of the colloidal thyroid hormone precursor in the thyroid glands
• This accumulation can cause goiter, an abnormal enlargement of the thyroid glands
• The parathyroid glands are endocrine glands that are located close to the thyroid gland. Its major purpose is to control calcium and phosphorus levels by synthesizing and regulating the secretion of the parathyroid hormone (or PTH). This hormone has an effect that is opposite from that of the calcitonin from the thyroids. While calcitonin decreases blood calcium levels, PTH acts to increase them. These opposite actions are important features of the negative feedback mechanism, which will be discussed in detail later.
• Thymus

  In humans, the thymus is located behind the sternum (as shown in Fig.

  12.3.4). The thymus is an organ that functions at the intersection of the

  endocrine system and the immune system. The thymus is an important organ that plays a role in the maturation of T cells (or T lymphocytes) of the immune system after migrating from the bone marrow. Aside from immune functions, the thymus also produces hormones that are required for normal bodily functions.

  
• Thymopoietin is a hormone that plays a role in T cell development and

  differentiation, as well as in the maintenance of the aging process of various cells.
• Thymulin also promotes the differentiation and normal function of T cells, as well as regulates the release of other hormones in the body.
• Thymosin is a hormone that plays a role in stimulating T cell production. Recent studies show that thymosin can also be found in many other parts of the body, but was named such because it was first isolated from thymic tissues.
• Adrenal Glands

  The adrenal glands, which are shown in Fig. 12.3.5, are named such because they are located on top of the human kidneys (“renal” refers to the kidneys), and they occur as a pair for every individual. The adrenal glands synthesize and regulate the secretion of hormones that serve diverse functions.

  
• Adrenaline is a hormone that is closely associated with the “fight-or-flight”

  response. Some of its effects include increasing the heart rate, redirecting the blood to muscles, increasing respiratory rate, and enlarging the pupils. Adrenaline also plays a role in maximizing energy utilization by the body and the brain. These effects allow a more efficient reaction of the human body to danger, thereby facilitating fight-or-flee reactions more efficiently.
• Cortisol is the “stress hormone” released by the adrenal glands. This hormone helps in regulating responses associated with stress, as well as in maintaining immune and metabolic functions.
• Aldosterone helps in the regulation of water and salt balance in body tissues. It also plays a very important role in the maintenance of proper blood pressure.
• The gonads, which include the testes in males and the ovaries in females (their relative locations are shown in Fig. 12.3.6), are normally associated with the reproductive system. However, they also serve very important endocrine functions.

  
• The ovaries secrete the steroid hormones estrogen and progesterone. Estrogen plays a major role in females’ sexual development, particularly in the development of secondary sex characteristics. In addition, it plays an important role in the menstrual cycle by helping initiate the growth of the uterine lining. Dips in estrogen levels trigger menstruation in females.
• Progesterone is another major steroid hormone secreted by the ovaries. This hormone helps maintain the uterine lining during the menstrual cycle and prepares the uterus for a possible pregnancy.
• The major hormone produced by the testes, on the other hand, is testosterone. Much like estrogen, testosterone influences the development of secondary sex characteristics in males. Other functions of testosterone include the regulation of bone mass, muscle mass, and sperm production.
• Pancreas

  The pancreas, which is shown in Fig. 12.3.7, has both exocrine and endocrine functions. The pancreas’ exocrine function primarily involves the digestive system through the secretion of enzymes that can break down specific food molecules. These include the proteases trypsin and chymotrypsin for digesting proteins, amylase for breaking down carbohydrates, and lipases for fat digestion.

  
• The endocrine functions of the pancreas, on the other hand, involves the hormones insulin and glucagon. These hormones have antagonistic functions that influence blood sugar levels via a negative feedback mechanism, which will be discussed further in this lesson.
• Feedback Mechanisms

  The body detects stimuli from both the internal and external environments in order to ensure that the appropriate responses are performed. These responses are often made through feedback mechanisms, which exist in two major types. These two types will be briefly discussed below but will be focused on in greater detail in a future unit.
• During a positive feedback mechanism, the body responds to a stimulus by promoting it to further increase in intensity
• This trend means that if the stimulus leads a body function to deviate from its normal state, the organs involved in the positive feedback will further push that body function to a greater degree of deviation
• A good example of positive feedback occurs during childbirth
• A woman undergoing labor experiences the release of the hormone oxytocin from the pituitary gland
• Oxytocin promotes uterine contractions that help push the baby from the uterus to the vagina
• This effect continues and amplifies through the continuous release of oxytocin until the baby is finally expelled from the mother’s womb
• Negative Feedback

  Negative feedback involves mechanisms that reverse the effect of the stimulus that is detected by the body. This feature is in contrast with positive feedback that promotes or heightens the effects of the detected stimulus.