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BS2050 Human Physiology
Basic Principles of Endocrinology
An Endocrine Gland is a collection of specific cells (endocrine cells) organised into a tissue
whose major function is to produce hormones in response to a particular physiological signal
or signals. The hormones may be stored in the endocrine gland in secretory granules as is the
case with catecholamine, peptide and polypeptide hormones or synthesised on demand as is
the case for steroid and thyroid hormones.
A Hormone is a specific chemical substance produced in specialized endocrine cells and is
released into the bloodstream where it can exert its effects on different cell types in other
tissues. These are referred to as Target Cells and are characterized by the presence of a
specific hormone receptor which is involved in the transduction of the hormonal signal.
Hormonal effects may be general i.e. many different types of cell may be affected, often in
different ways (e.g. insulin), or they may be quite specific effect on a single target cell (e.g.
the effects of LH on the Leydig cells in the testis).
Hormones have a diversity of chemical structure and size ranging from the glycoprotein
hormones (LH, FSH, hCG, TSH), larger polypeptides (insulin, ACTH, Gastrin, CRH),
smaller modified peptides (TRH, GnRH), the lipophilic steroid hormones (testosterone,
oestradiol, cortisol , aldosterone), hormones derived from amino acids (adrenaline and
thyroxine) and those derived from fatty acids like prostaglandins.
Hormones are produced and secreted in small quantities (ng – mg range per day) and the
blood plasma concentration is low (1 pM – 10nM depending on the hormone) but changes
may be 10 –100 fold in response to a physiological signal. The physiological or metabolic
response to a hormonal signal is greatly amplified compared to the change in hormone
Hormones are subject to rapid turnover, so that hormone levels rapidly rise in response to a
physiological signal and as soon as the need for the signal disappears, the hormone is rapidly
removed from the system. The physiological effects of the hormone may not appear until well
after the hormone levels have returned to their original level.
The concentration of hormone in the blood is normally determined by its rate of secretion
by the endocrine gland. The rate of degradation or modification of a hormone, and thus its
inactivation, is usually a fairly constant process and can occur in different sites within the
body (e.g. liver and kidney).
The effect of a hormone on a target cell is dependent on two factors:
 The capacity of the tissue to respond to the hormone which, in turn, is dependent on the
presence of a high affinity receptor, which binds hormone at low concentrations of the
ligand (hormone). These receptors may be associated with the outside of the plasma
membrane (e.g. peptide, polypeptide and catecholamine receptors) or in the cell
cytoplasm or nuclei, e.g. steroid and thyroid hormones). The basic principles of kinetic
analysis (e.g. Scatchard Analysis) of ligand-binding receptor proteins can also be applied
to hormone-receptor interactions.
 The concentration of hormone in the blood which, as mentioned can change about 10 –
100 fold. This change is normally induced by a small range of concentrations on either
side of the dissociation constant (Kd) for the hormone-receptor interaction.
There is normally a sigmoidal relationship between the hormonal concentration and its
effect. At low concentrations the hormone level can change without much effect, until it
reaches a threshold concentration where the magnitude of the physiological effect is directly
proportional to the hormone concentration. At higher concentrations this proportionality
disappears and the physiological effect reaches a maximum. An useful measurement is the
concentration of hormone at which half of the maximal physiological effect is found. (This is
often estimated by plotting the magnitude of the physiological response against the
Log10[Hormone]). This is usually closely related to the Kd of the hormone-receptor
Note: The same hormone can have different qualitative and quantitative effects on different
types of cell. Also the same hormone can have different effects on one type of cell at different
Hormones regulate one or more existing cellular functions:
 Metabolism via alterations in enzyme activities (positive or negative effects)
 Membrane permeability to specific ions and metabolites
 Gene transcription of specific gene products, in turn regulating cell-specific protein
 induce secretory activity of cell
 mitotic cell division
There is a shift in the timeframe between the change in hormone concentration which may be
all over within seconds and the ensuing physiological effects which may take several minutes,
hours or sometimes days to manifest themselves.
The functions of hormones are many and varied but some general examples are given below:
 The maintenance of homeostasis i.e. the maintenance of the composition of the tissues
and body fluids for the benefit of the organism as a whole (e.g. insulin and glucagon
regulate blood glucose within strict limits by integrated effects on glucose metabolism
various tissues).
 Hormones allow an appropriate response to external stimuli (e.g. the effect of adrenaline
in the fight or fright response).
 Some hormones control cyclic and developmental changes (e.g. they regulate the
growth of the individual, circadian rhythms or sexual function such as the menstrual
cycle, pregnancy, spermatogenesis etc)
 Hormones can cause changes in brain function and behaviour
 Hormones often have synergistic effects – on their own they are inactive or poorly active
but, at the same concentration, in combination with one or more other hormones, they
produce a full physiological effects.
The most common regulatory mechanism for controlling hormone secretion and thus the
hormone concentration is the Negative Feedback Loop. This is where a hormone or
metabolite, produced as part of the physiological response by the target tissue, is released into
the bloodstream, is transported to the endocrine cell where it limits the further secretion of
D R Davies Jan 2005