Physiological functions of our human body is regulated and coordinated by both neural and endocrine systems. The endocrine system influences the metabolic activities by means of hormones (hormone means to excite) which are chemical messengers released into the blood and circulated as chemical signals and acts specifically on certain organs or tissues called target organs or target tissues. Hormones may speed up or slow down or alter the activity of the target organs. The hormones secreted do not remain permanently in the blood but are converted by the liver into inactive compounds and excreted by the kidneys.

Hormones are chemical messengers because they act as organic catalysts and coenzymes to perform specific functions in the target organs. The target organs contain receptor molecules either on the surface or within the cell. Although different hormones come in contact with cells, only

the cells that contain receptor molecules specific for the hormone are physiologically activated. A single hormone may have multiple effects on a single target tissue or on different target tissues.

Many hormones exhibit long term changes like growth, puberty and pregnancy. Hormones often influence many organs and organ systems at the same time. Serious deficiency or excess secretion of hormones leads to disorders. Hormones coordinate different physical, physiological, mental activities and

Table 11.1. Chemical nature of hormones Class Chemical properties

Amines Small, water soluble derived from tyrosine or tryptophan

Protein/Peptides Water soluble Steroids Derived from cholesterol

mostly lipid soluble

maintain homeostasis. Hormones are composed of water soluble proteins or peptides or amines or fat soluble steroids.

Homeostasis: Maintenance of constant internal environment of the body by the different coordinating system.