In multicellular organisms thousands of cells are present. These cells in definite groups form tissue, organ and organ system. Each organ contains specialised cells for a particular function, kidney cells are specialised for excretion, ovary cells are specialised for the formation of ova, etc. All these organ systems should work in coordination for the maintenance of life otherwise the organism may die, metabolic reactions take place in every cell and nitrogenous waste materials are formed which are harmful to the body. Blood collects these substances and takes them in live. Liver converts them to urea which is again taken by the blood to kidney where it is removed from blood and passed out of the body in the form of urine. There are many such examples in our body where many systems participate for completing one function. While we are running, our breathing rate automatically goes up and we get more oxygen to produce more energy. Shortly after we stop running, our breathing rate returns to normal. Our eyes, nose and hands work in coordination in taking the food upto mouth. Eyes help in locating, nose help in smelling and the hands bring the food upto mouth. Teeth, jaw muscles and salivary glands help in chewing and swallowing the food. Disgestive system including oesophagus, stomach, intestine, liver, pancreas play important role in digestion, absorption and egestion. Thus, all these organs require co-ordination in completing one activity. Without this coordination you cannot do many things that you can do now. In multicellular organisms there are two systems which regulate and coordinate the activities. These are nervous system and endocrine system. Endocrine System: Much of the body internal activity like growth, reproduction, metabolic activities, are controlled through the secretion of endocrine glands. Exocrine glands: These glands have ducts. Their secretions are carried by their ducts and poured on the surfaces of concerned epithelia, gastric gland, sweat gland, salivary gland, sebaceous gland, etc. Endocrine glands: These glands are without ducts. Their secretions are poured directly into the blood, Thyroid, Adrenal gland, etc. Mixed glands: These are duct glands with major exocrine part and some endocrine cells, Pancrease. Hormones are chemical substances secreted by endocrine glands and are taken up by the blood stream and carried to some part of the body where they initiate a specific response. Hormones are synthesized in the body in endocrine glands. They are not found in food. Hormones are organic compounds (proteins, steroids, amino acid derivatives and eicosanoids). Hormones are secreted in small amount in response to body requirement. Hormones generally affect a specific type of cells in a definite part of body. Hormones do not take part in metabolism. Most hormones do not possess species specificity. Each hormone has a fixed, specific effect on the activity of its target cell. Hormones control and regulate activity levels of target cells. Hormones affect metabolism of target cells in following ways: Hormones change membrane permeability. Some neurotransmitters which act as local hormones change permeability of membranes. Some hormones modify cellular metabolism through second messengers. Second messengers are cyclic adenosine monophosphate which stimulate some enzymes, Adrenaline. Some hormones modify cellular metabolism through genes. They regulate RNA formation in nucleus, cortisol. Important endocrine glands are thyroid, parathyroid, adrenal, pituitary, islets of Langerhans in pancreas and gonads. Secretions of these glands stimulate to activate and regulate the functions of distant organs to which they are carried through the blood stream. Thus, they maintain the homeostasis and regular control on the functions of the internal organs as well as chemical co-ordination. Thyroid gland: It is located along the side of the trachea just below the larynx in the neck. Thyroid is H-shaped gland. Its two lobes are connected by a narrow isthmus. It secretes two important hormones.