Re: What are the mechanisms triggered by Hypovolaemic Shock (Hemorrhage)

The compensatory mechanisms are many and happen almost at the same time 
after
the initial reflex response by the body. Below is a "quickie" in shock
education. The mechanisms that occur are far beyond the scope of information
I am able to present here.
The "trigger" factor in hypovolemic shock is decreased circulatory volume 
and
inadequate oxygenation (hypoxia) to tissue/organs.
Decreased volume initiates the early phase or reflex reaction and begin with
arteriolar constriction, increased heart rate, and increased contractile
force of the heart. These mechanisms can maintain a stable blood pressure as
well as adequate tissue oxygenation for up to as much as 10% volume loss.

For how long? The time factor is partially dependent on the type of injury 
or
bodily dysfunction, the age and preexisting health of the person before the
assult/alarm occurs. A healthy young person may be successfully compensated
in the initial phase for longer periods of time than someone that has a
disease process in progress or has -- lets say a small ulcer bleed vs
internal bleed from a car accident.

As the volume percentage lost increases to 15% -- 25% these mechanisms lose
their ability to compensate and rapidly become inadequate.
As blood pressure begins to drop baroreceptors (pressure/streach receptors)
in the heart, and the great vessels  "feel"  the pressure drop and initiate
the sympathatic/autonomic  nervous system (fight or flight reaction) to kick
in.
This system releases norepinephrine and epinephrine (a powerful
vasoconstrictor and heart rate increaser).
In other words these two squeeze the vessels and kick up the speed of the
heart to work even harder to oxygenate the "life saving organs" (heart,
brain, deep large muscles) with the little amount of blood that is left.
Blood is redirected away from peripheral tissue/vessels and non vital organs
(your skin, gut, liver, lungs and kidneys will not save your life if you are
running from a grizzly bear -- thus the name fight or flight) in an effort 
to
support the needs for energy and oxygen for the heart, brain and "running"
muscle.  In hypovolemic you would be lying down (passed out) as this is the
first response to equalize or redistribute oxygenated blood to the brain.
At the same time a hormonal response by the pituitary, hypothalamus and
adrenal cortex  release adrenocortical, mineralocorticoid and 
glucocorticoids which increase intravascular volume, via the kidney, by 
decreasing urine
output and retaining sodium/water which in turn increases blood pressure;
they stimulate a rapid increase in glucogenesis (increasing blood sugar to
fuel the heart to beat faster and muscles to run faster/diaphram to breath
deeper/faster); they attempt to maintain a sodium/water/electrolyte balance.
As dehydration increases so does the concentration of the blood (osmolality)
which triggers osmoreceptors to release yet another fluid saving chemical,
antidiuretic hormone (ADH) which in turn retains more sodium/water.
 At the same time everything else is changing, all the tissue and organs 
that
have previously had oxygen to maintain aerobic cellular metabolism are now 
in
anaerobic mode.
As a result the body is building up lactic acid (the chemical that make our
muscles so sore when we over exert our bodies) this leads to metabolic
acidosis causing our respiratory rate to increase in rate and depth to try
and "blow" off some of CO2 to decrease the acidosis.
This requires even more energy to maintain a rapid deep breath and in turn
throws us into metabolic alkalosis making the hormonal responses that have
been in progress work even harder to maintain an electrolyte level that is
compatible with life. Eventually the acid-base balance mechanisms fail
causing cellular damage and death. Without life support and drugs that are
given IV immediately the body gets to the irreversible stage and body death
occurs.
The following references have charts and diagrams to help you understand how
all these things can interact to attempt compensatory maintenance of the 
body
in hypovolemic shock.

Leavelle, D. Professional Guide to Diseases (3rd ed.). Cardiac 
Complications.
(pp. 1083). Springhouse.

Luckmann and Sorensen (1987), Medical Surgical Nursing (3rd ed.). 
Physiologic
Shock: Basic Concepts and Intervention. (pp. 220-225). W.B. Saunders 
Company.