• The neural system provides a point-to-point rapid coordination among organs.
• The neural coordination is fast but short-lived.
• As the nerve fibres do not innervate all cells of the body and the cellular functions need to be continuously regulated; a special kind of coordination and integration must be provided. This function is carried out by hormones.
• The neural system and the endocrine system jointly coordinate and regulate the physiological functions in the body.
Endocrine glands and hormones
• Endocrine glands lack ducts and are hence, called ductless glands. Their secretions are called hormones.
• The classical definition of hormone as a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ has current scientific definition as follows: Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
• The new definition covers several new molecules in addition to the hormones secreted by the organised endocrine glands.
• Invertebrates possess very simple endocrine systems with few hormones whereas many chemicals act as hormones and provide coordination in the vertebrates.
• The human endocrine system is described here.
Human Endocrine System
• The endocrine glands and hormone producing diffused tissues/cells located in different parts of our body constitute the endocrine system.
• Pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis in males and ovary in females) are the organised endocrine bodies in our body. In addition to these, some other organs, e.g., gastrointestinal tract, liver, kidney, heart also produce hormones. A brief account of the structure and functions of all major endocrine glands and hypothalamus of the human body is given in the following sections.
The Hypothalamus
• The hypothalamus is the basal part of diencephalon, forebrain and it regulates a wide spectrum of body functions.
• It contains several groups of neurosecretory cells called nuclei which produce hormones.
• These hormones regulate the synthesis and secretion of pituitary hormones.