The nervous system isn’t the only message-relaying system of the human body. The endocrine system also carries messages. The endocrine system, is a system of glands that release chemical messenger molecules into the bloodstream. Other endocrine glands includes; the thyroid gland which is a large gland in the neck or also called trachea. These hormones increase the rate of metabolism in cells throughout the body, influence metabolism, growth and development, and body temperature. The main hormone is thyroxine, also called T4. During infancy and childhood, adequate thyroid hormone is crucial for brain development. They also control how quickly cells use protein and make proteins. The pineal gland is a tiny gland located at the base of the brain and it secretes the hormone melatonin. This hormone controls sleep-wake cycles and several other processes. The pineal gland connects the endocrine system with the nervous system in which it converts nerve signals from the sympathetic systems of the peripheral nervous system into hormone signals. The pancreas is located near the stomach. Its hormones include insulin and glucagon. These two hormones work together to control the level of glucose in the blood. Insulin causes excess blood glucose to be taken up by the liver, which stores the glucose as glycogen (a group of polysaccharide of glucose). Glucagon stimulates the liver to break down glycogen into glucose and release it back into the blood. The pancreas also secretes digestive enzymes into the digestive tract. The two adrenal glands are located above the kidneys. Each gland has an inner and outer part. The outer part, called the cortex, secretes hormones such as cortisol (which helps the body to deal with stress), and aldosterone (which helps regulate the balance of minerals in the body). For example; adrenaline increases the amount of oxygen and glucose going to the muscles. The last endocrine gland is the gonads which secrete sex hormones. The male gonads are called testes and they secrete the male sex hormone testosterone. The female gonads are called ovaries and they secrete the female sex hormone estrogen. Sex hormones are involved in the changes of puberty. They also controls the production of gametes by the gonads. The messenger molecules are hormones. Hormones act slowly compared with the rapid transmission of electrical messages by the nervous system. They must travel through the bloodstream to the cells they affect, and this takes time. On the other hand, because endocrine hormones are released into the bloodstream, they travel throughout the body. As a result, endocrine hormones can affect many cells and have body-wide effects. The hypothalamus is actually part of the brain, but it also secretes hormones. Some of its hormones that “tell” the pituitary gland to either secrete or stop secreting its hormones. In this way, the hypothalamus provides a link between the nervous and endocrine systems. The hormones include antidiuretic hormone, which stimulates the kidneys to conserve water by producing more concentrated urine and oxytocin, which stimulates the contractions of childbirth among other functions. The hypothalamus also produces hormones that directly regulate different body processes. These hormones travel to the pituitary gland, which stores them until they are needed. The hypothalamus and pituitary gland are located close together at the base of the brain. How do the endocrine hormones works? Well, endocrine hormones travel throughout the body in the blood. However, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to the bloodstream until it finds a target cell with a matching receptor it can blind to. After this happens, it causes a change within the cell. Exactly how its works depends on whether the hormone is a steroid hormone or a