Hormones and the pituitary gland

The body is coordinated by the nervous system and by hormones. Endocrine glands have no ducts: they pour their hormones straight into the blood. A hormone is a chemical made by an endocrine gland and carried by the blood to another organ, where it changes how the organ works or grows, very often by stimulating other glands or organs. Too much or too little of a hormone disturbs the body. The first hormone idea came from the work of Starling in 1905. He found that the pancreas starts to release its juice as soon as food reaches the duodenum, even when the nerves to the pancreas have been cut. So the signal could not be a nervous one. A chemical made by the lining of the duodenum must pass into the blood and stimulate the pancreas. He named such substances hormones, from a Greek word meaning activators. Plants have hormones too, but no special glands. In 1913 Boysen-Jensen showed that something made at the tip of a shoot passes down the stem and makes the part below bend towards light. Such substances are the auxins, for example indole acetic acid, made at the tip of the shoot and in buds, which then act on other parts. Auxins organize the growth and differentiation of tissues and take part in flowering and in the forming of fruit. Hormones are organic substances (proteins, amino acids or steroids), made in tiny amounts of micrograms. Their jobs include keeping the internal environment balanced, growth, sexual maturity, metabolism and development. Hormone levels also affect mood and growth, so anyone with a concern about this should ask a doctor.