Hormones are the chemical messengers of the body. They are defined as organic substances secreted into blood stream to control the metabolic and biological activities. These hormones are involved in transmission of information from one tissue to another and from cell to cell. These substances are produced in small amounts by various endocrine (ductless) glands in the body. They are delivered directly to the blood in minute quantities and are carried by the blood to various target organs where these exert physiological effect and control metabolic activities. Thus frequently their site of action is away from their origin. Hormonesare required in trace amounts and are highly specific in their functions. The deficiency of any hormones leads to a particular disease, which can be cured by administration of that hormone.
Steroids on which the above classification is made are compounds whose structure is based on four-ring network, consisting of 3 cyclohexane rings and 1 cyclopentane ring.
Classification of Hormones
Hormones are classified on the basis of
(i) their structure (ii) their site of activity in the cell.
Hypothalamus functions as master co-ordinator of hormonal action. It produces atleast 6 releasing factors or hormones.
- Thyrotropin releasing hormone (TRH)
- Corticotropin releasing hormone (CRH)
- Gonadotropin releasing hormone (GnRH)
- Growth hormone releasing hormone (GRH)
- Growth hormone release inhibiting hormone (GRIH)
- Prolactin release inhibiting hormone (PRIH).
Functions of steroid hormones
1. Sex hormones - are divided into 3 groups
(i) male sex hormones or androgens(ii) female sex hormones or estrogens
(iii) pregnancy hormones or progestinesTestosterone is the major male sex hormone produced by testes. It is responsible for male characteristics (deep voice, facial hair, general physical constitution) during puberty. Synthetic testosterone analogos are used in medicine to promote muscle and tissue growth in patients with muscular atrophy.
Estradiol is the main female sex hormone. It is responsible for the development of secondary female characteristics and participates in control of menstrual cycle.Progesterone is an example of progestin and is responsible for preparing the uterus for implementation of the fertilized egg. It also has an important role as birth control agents.
Hormones of Adrenal Cortex and Adrenal Medulla
Adrenal cortex is a small organ. The hormones of adrenal cortex are Adreno corticosteroids.
a) Minerals Corticoids, made by different cells in the adrenal cortex are concerned with water salt balance in the body. They regulate NaCl content of blood and cause excretion of potassium in urine.b) Glucocorticoids made by adrenal cortex, modify certain metabolic reactions and have an anti-inflammatory effect.
Functions of non steroid hormones
1. Peptide hormones
Insulin has a profound influence or carbohydrate metabolism. It facilitates entry of glucose and other sugars into the cells by increasing penetration of cell membranes and augmenting phosphorylation of glucose. This decreases glucose concentration in blood and insulin is commonly known as hypoglycamic factor. It promotes anabolic processes and inhibits catabolic ones. Its deficiency in human beings causes diabetes mellitus. Insulin isolated from islets of Langerhens or islets tissue of pancreas was the first hormone to be recognized as protein. Sanger determined the structure of insulin and was awarded the Nobel prize in 1948 for this achievement.2. Amino acid derivatives
The thyroidal hormones e.g., thyroxin and tri-iodothyronine affect the general metabolism, regardless of their specific activity. It is for this reason why thyroid gland is known as pace setter of the endocrine system.Based on the site of activity in the cell hormones are divided into two categories 1st category of hormones effect the properties of plasma membranes. These include all peptide hormones e.g., insulin and hormones of pituitary gland.
In the 2nd category of hormones - hormones are taken into the cell and transported to the nucleus where they influence the native and rate of gene expression.