H: Hypovolemia

There are two types of
Hypovolemia:

1) Absolute-meaning loss of blood or a blood component from the body

2) Relative-meaning pooling or compartmentalizing of blood or a blood component into one area of the body.

In absolute hypovolemia, blood volume has actually been expelled. The two most common causes are hemorrhage and diuresis.

When I think of hemorrhage, I usually think of laceration wound or car accident-blunt trauma, even blood loss that has been documented and monitored from surgery can cause problems if it is not replaced.

But I've also seen many cases of gastrointestional bleeding that caused hypovolemia. Sometimes people have these ulcerations along their digestive tract and even if they've been there for a long time they can become super-irritated and bleed. If it's in the stomach, and the blood loss is acute, the patient might vomit frank blood or curdled blood. If it is a slower, 'chronic bleed' it might pass through the GI tract and come out the other end so it's importiant to examine stool. If its from the stomach or early in the digestive tract, it will be partially digested. Digested blood isn't red, it's dark and tarry.
If the stool has reddish streaks or you see blood then the bleed occured later in the digestive tract.

Whole blood may be lost via hemmorrage, but loss of blood components can also cause hypovolemia. This is the case with diuresis.

Interstial fluid (the fluid component with electrolytes) is lost during kidney diuresis so excessive diuretic use, or a pituitary disorder called diabetus inspitus which mimics this effect, can cause hypovolemia. It is also lost when excessive vomiting or diarrhea occurs. 
  
One other blood component that can be lost is plasma. In severe burn injuries, the protien-rich plasma is lost, contributing not only to volume loss but to REALTIVE loss of fluid from circulation.

Relative Hypovolemia-

There can be a suficient amount of fluid in the body (or even an excess of fluid) and the patient can still be dehydrated/hypovolemic. This is because blood is stuck in one area of circulation and is not circulating as it should.

Third spacing-If systemic blood pressure is insufficient (CHF, Hyper/hypotension), if electrolytes are unballanced (Hypo/hypernatremia), if plasma is lost (severe burns), if an organ is infected or suffering from inflammation (ascites, bowel obstruction, peritonitis et all)...fluid will accumulate outside of the blood vessels.

Internal bleeding-Internal injury (such as fracture, organ rupture,  can cause hemmorrhage that causes blood to accumulate outside of the blood vessels.

In addition, Massive Vasodillation (such as that triggered by sepsis) can trap blood inside the larger vessels, so blood can't perfuse the capillary bed. 

Some of the signs and symptoms of hypovolemia will depend on the cause (Eg. Diarrhea, low urine output, etc) but some hallmarks are to keep in mind are:

Complaints of thirst-
Elevated heart rate-
Elevated respiratory rate-
Decreased urine output-
Decreased skin turgor/flat neck veins/dry tongue

LABS-Dehydration will show a high degre of solutes (hemglobin/hematocrit, electrolytes, etc.) and a low degree of solution. Also look for sirum protiens outside of normal limits.