Module 1: The endocrine system
Hormones are defined as chemical messengers that travel in blood, and therefore
are distributed throughout the body. However there is also important short distance
signalling within tissues or organs, via paracrine or autocrine signalling. In both
scenarios, the physiological response of cells to chemical signals depends on the
presence of specific receptors. Too much or too little hormone activity results in
dysregulation and endocrine disease.
Learning objectives
By the end of this unit, you should be able to:
Describe cell signals that act locally, within a tissue.
Explain the factors that influence the circulating concentrations of free,
biologically active hormones.
Describe how hormones result in physiological responses in cells.
Differentiate the fundamental causes of endocrine conditions.
Cell signals
By definition, a hormone is a signaling molecule that is secreted by an endocrine
gland into circulation and acts on a distant target cell - this is illustrated in the figure
below. In general, hormones are long-ranged messenger molecules.
Image source: Kenneth Lopez-Loo.
A key point about hormones, since they travel via the blood circulation, is that they
can act throughout the body. Indeed, they often act on multiple tissues at the same
time. What gives hormones specificity of action is whether cells express receptors
or not for that hormone. The hormone molecules interact with these receptors to
result in a cellular response within target cell. Cells that do not express the
appropriate receptor for a particular hormone will therefore be unaffected by that
endocrine signal.
In the body, there is also important short distance cell signalling. Here, the cell
signalling molecules do not circulate in the blood to reach a distant target tissue.
Rather, the signalling molecule is secreted out into interstitial fluid and acts on
neighbouring cells, or even on the secreting cell itself. This type of cell
communication is called a local effect. The cell signal here is not endocrine, but
rather paracrine or autocrine.
Image source: Kenneth Lopez-Loo.
Paracrine signalling is where signalling molecules secreted by the cells exert their
effects on neighbouring cells in their immediate vicinity. The "para-" prefix stems
from ancient Greek, which means "besides, or next to". The suffix "-crine" means "to
secrete".
Autocrine signalling occurs on an even more localized level, where the signal
secreted by a cell acts on its own receptors. The "auto-" prefix means "self"; like
autopilot, or autobiography.
Note that both paracrine and autocrine signals are dispersed through simple
diffusion, working within interstitial fluid in a tissue to have their effects. This
distinguishes such signalling from classical endocrine signalling via the circulation.
However, specificity of action with paracrine or autocrine signals is still determined
by whether neighbouring cells express receptors (or not).
Physiological responses to hormones
Watch the video below to gain further insight into how hormones bring
about physiological responses.
Physiological responses to hormones
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end.
So I'm going to use this diagram
to describe the physiological
response to hormones and one
of the things that matters with
hormone action is the levels or
concentrations of the free
biologically active hormone.
So when you take a blood
Endocrine diseases
Hormones are particularly important in maintaining homeostasis in the body.
Abnormal hormone activity can lead to pathophysiological outcomes. Watch
the following video to understand the fundamental causes of endocrine
disease states.
Endocrine diseases
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end.
No playable video sources found.
When we think of what can
potentially go wrong when we
in endochronology usually we've
got two scenarios which we talk
about in sort of
pathophysiological terms.
