Introduction
•Cells of the body require specific conditions to
survive and function
•Maintenance of body conditions in a stable
steady state is called homeostasis
- Body homeostasis is chiefly controlled by
Nervous control
Chemical control
- Endocrine secretion – substance released by cell into
blood stream that affects distant cells - Exocrine secretion – substance released by cell into a
duct that leads to epithelial surface (onto skin or into
gut). Action doesn’t depend on receptors in target tissue. - Endocrine and exocrine secretions are glandular
secretions; they come from specialized secretory cells that
are clumped together to form a gland.
Chemical Messengers
Chemical messenger systems regulates multiple activities of
tissues:
- Neurotransmitters– released by axon terminals of neurons–
act locally to control nerve cell functions - Endocrine hormones– released by glands or specialized cells
into circulating blood– act on target cells - Neuroendocrine hormones– secreted by neurons into the
circulating blood– acts on target cells
- Paracrines– secreted by cells into extracellular
fluid and affect neighboring target cells of a
different type - Autocrines– secreted by cells into the ECF and
affect the function of the same cells that
produced them - Cytokines– secreted by cells into ECF &
function as autocrines, paracrines, or endocrine
hormones. Examples of cytokines– interleukins
& lymphokines
Examples
- Neuroendocrine hormones
Hormones of posterior pituitary - Endocrine Hormones
Hormones of anterior pituitary, thyroid
hormones, adrenocortical hormones - Exocrine Hormones
Functions
- Role of hormones
Growth– growth hormone
Metabolism and development– growth hormone
& thyroid hormone
Water and electrolyte balance– aldosterone,
ADH
Reproduction– sex hormones
Behavior– thyroid hormone
Secretion of Hormone
- Variable
- May secrete within minutes after the stimulus
- Action starts within seconds and may last till
months - Short duration of action– epinephrine &
norepinephrine - Long duration of action– thyroxine
- Concentration and secretion of hormone is usually
small - From picograms to micrograms
Control of Hormone Secretion
- Plasma concentrations of hormones depends upon various
stimuli - Mostly controlled by negative feedback mechanisms that
ensure a proper hormone activity at target tissue - After a stimulus causes release of the hormone
- Response resulting from the action of the hormone tend to
suppress its further release - Thus negative feedback effect to prevent oversecretion or
overactivity at the target tissue
Control of Hormone Secretion
- Control is not always the secretory rate of hormone, it
can be degree of activity of the target tissue - When target tissue activity rises to an appropriate level
will feedback signals to decrease secretion of hormone - Feedback regulation can occur at all levels
Gene transcription and translation steps involved in
the synthesis of hormones
Steps involved in processing hormones or releasing
stored hormones
Control of Hormone Secretion
- Positive feedback occurs when action of hormone requires
additional secretion - Example–luteinizing hormone (LH) surge
- Occurs as a result of stimulatory effect of estrogen on anterior
pituitary before ovulation - LH acts on ovaries to stimulate more secretion of estrogen,
which in turn causes more secretion of LH - Concentration is achieved and negative feedback is exerted
Variations in Hormone Release
- Cyclical Variations also occur in hormone release.
- Periodic variations occur in hormone release influenced by
Seasonal changes
Various stages of development and aging
Diurnal (daily) cycle
Sleep - For example, growth hormone is increased in early period
of sleep and reduced in later stages of sleep - Cyclical variations occur due to changes in activity of neural
pathways in controlling hormone release
Transport
- Water-soluble hormones (peptides and catecholamines)
Dissolved in plasma
Diffuse out of the capillaries, into the interstitial fluid, and
ultimately to target cells - Steroid and thyroid hormones,
Circulate in the blood mainly bound to plasma proteins easily
diffuse across the capillaries and reach target sites
10 percent of steroid or thyroid hormones exist free in solution - Large amounts of hormones bound to proteins serve as reservoirs
- Binding of hormones to plasma proteins slows clearance from the
plasma
Clearance
- Determined by
Rate of secretion of hormone
Rate of removal of hormone - Hormone is cleared by
Rate of destruction in target tissues
Binding with the tissues
Metabolism by liver
Removal by kidneys
•Water soluble hormones
Degraded by enzymes in blood and tissues
Cleared by liver/kidneys
Angiotensin
•Lipid soluble hormones
Protein bound
Slowly cleared
Thyroid and steroid
Metabolic Clearance Rate= Disappearance of hormone from plasma / Concentration of Hormone
Measurement of Hormones
- Hormone concentration can be measured by
- RIA (Radioimmunoassay)
- ELISA (Enzyme-Linked Immunosorbent
Assay)