Hormones
Definition:
A hormone may be defined as a specific organic substance of an endocrine gland
secreted into the blood which carries it to specific part of the body where it produces a
definite physiological effect. This effect may be either excitatory or inhibitory in its
action.
Endocrine glands have no ducts and liberate hormones directly into the bloodstream.
For this reason, the endocrine glands are also referred to as the ductless glands.
holocrine glands:The endocrine glands which secrete only hormones are called the
holocrine glands (e.g., thyroid, parathyroid, adrenals, and pituitary glands).
heterocrine glands :The glands which have dual functions (secretion of hormones
and some other functions) are termed the heterocrine glands (e.g., pancreas, testes
ovaries, etc.)
Properties of Hormones:
1. Hormones are produced by a specialized glands and are secreted directly into the
blood stream.
2. Hormones are transported by the blood stream from the endocrine cells to serve
as ‘chemical messenger’ which act on target cells or organs by regulating the rates
of specific metabolic reactions.
3. 3. Hormones exert their effects not where they are locally produced, but on other
tissues of the body.
4. 4. Hormones are required in very small amounts and their effect may be
excitatory or inhibitory depending upon their concentration and the physiological
state of the responding tissue.
5. 5. Hormones belong to different types of chemical structure. They may be
steroids, proteins, peptides or amino acid derivatives.
Chemical Nature of Hormones: All the hormones are organic substances
of varying structural complexity. Chemically, they may belong to any of the
following categories.
(a) Steroid Hormones: The hormones secreted by the adrenal cortex, testes, ovaries and
placenta are composed of steroids e.g., cortisone, aldosterone, testosterone, estrogen,
progesterone.
(b) Proteinous Hormones: somatotrophic, thyrotrophic and gonadotrophic hormones
secreted by the anterior lobe of pituitary gland and insulin hormone secreted by pancreas.
(c) Catecholamine: e.g., adrenaline and noradrenaline secreted by the adrenal medulla.
(d) Amino acid derivative: e.g., thyroxine hormone secreted by the thyroid gland.
(e) Peptide Hormones: e.g., melanocyte stimulating hormone, the hormones oxytocin and
vasopressin, adrenocorticotrophic hormone, calcitonin and parathormone.
Regulation: Hypothalamus- Pituitary axis
Hormones Target Organ Functions
Secretion &
ACTH Adrenal Cortex Growth
of Adrenal Cortex
Secretion &
TSH Thyroid Growth
of Thyroid
Secretion of
Ovaries Hormones
FSH
Anterior Testes Follicle Growth
Pituitary Spermatogenesis
Secretion of
Hormones
Ovaries
LH Follicle
Testes
Luteinization
Ovulation
Prolactin Mammary Gland Milk Production
Growth
Most Tissues Body Growth
Hormone
ADH Kidney Water Retention
Posterior
Pituitary Mammary Gland Milk Ejection
Oxytocin
Uterus Childbirth
The Hypothalamus is a Major Control Center for Homeostasis
The hypothalamus constantly measures the condition of the body and regulates functions
using both nerves and hormones
Some of the functions controlled are: body temperature, hunger & thirst, sex drive, sleep,
fight or flight reaction, levels of hormones
Hormonal control is exerted through the anterior and posterior pituitary
The Hypothalamus Controls Both Lobes of the Pituitary Gland
Anterior pituitary is formed in the embryo from Rathke's pouch (part of oral cavity)
Posterior pituitary is a projection from the hypothalamus
Connections to the anterior and posterior lobes are different:
Anterior: controlled by set of hormones
Posterior: connected directly to hypothalamus by nerves
The Hypothalamus Controls the Anterior Pituitary Through a Series of Releasing
Hormones
Hypothalamus makes a set of releasing hormones (RHs)
RHs travel in blood (portal system) to ant. pituitary
RHs cause ant. pituitary to secrete hormones (such as TSH and LH)
There are also inhibitory hormones (IHs) that block pituitary secretion
The hormones of the ant. pituitary are made in the pituitary
The Anterior Pituitary Produces 6 Major Hormones
Anterior pituitary is sometimes called the "master gland" of the body
4 ant. pituitary tropic hormones regulate the secretion of other endocrine glands (ACTH,
TSH, LH, FSH:
2 ant. pituitary hormones have direct effects on non-endocrine organs:
Prolaction- stimulates milk production in mammary glands
Growth hormone- promotes body growth
Several minor hormones: lipotropin, endorphins, MSH (hCG)
