Dehydration is not shock. Shock is the global failure of cellular oxidative metabolism. One form of shock is hypovolemic shock in which the circulating blood volume is low. Dehydration may cause shock but is not synonymous with shock. Other forms of shock may have normal blood volume but the volume is mal distributed, e.g. anaphylactic shock. In some cases of shock, blood volume may in fact be increased but the heart may be unable to circulate the blood and oxygen to the tissues, e.g. cardiogenic shock.
Dehydration is the loss of total body water. Hypovolemia is a low circulating blood volume. Total body water amounts to about 0.6 liters per Kg body weight. For the typical 70 kg person, this means a total body water of 42 l. About 2/3 of this (about 28 l) is in the cells (intracellular water) and about 1/3 (14 l) is outside of the cells (extracellular water). About 2/3 of the extracellular water (9 l) is in the space between the cells (interstitial fluid). About 1/3 (4 l) is in the vascular space (intravascular water). Usually when we lose fluid, we lose it from the intravascular space. For example, sweat glands take fluid from the blood and put it on the skin or the kidneys take fluid from the blood and produce urine. Thus, most of our fluid losses are from the intravascular space.
If fluid loss is slow, the body is able to move fluid, first from the interstitial space and then more slowly from the intercellular space, into the intravascular space to maintain the intravascular volume. If we lose fluid suddenly, then there is no time for the body to so this. The body can move a maximum of 1-2 l of fluid per day from the interstitial and intercellular spaces into the intravascular space. If you dehydrate slowly, you will not display symptoms until you have lost about 5-10% of your total body water (2-4 l). At that point, if you suddenly stand up, your heart rate will increase but your blood pressure will remain normal. If you lose 10-15% (4-6 l) and suddenly stand up, your blood pressure will drop and you may feel lightheaded. If you lose 15-20% (6-8 l), your blood pressure will be low even lying down. At this point, you would probably meet diagnostic criteria for hypovolemic shock. If you suddenly lose fluid from your intravascular space without time to move fluid from other areas back into the space, you will have symptoms at lower levels of fluid loss. This is why some people feel lightheaded when they stand up after donating only a half-liter of blood.