When you're dehydrated, your body doesn't have enough fluid to work properly. An average person on an average day needs about 3 quarts of water. But if you're out in the hot sun, you'll need a lot more than that. Most healthy bodies are very good at regulating water. Elderly people, young children and some special cases - like people taking certain medications - need to be a little more careful.
Signs of dehydration in adults include
Signs of dehydration in babies and young children include a dry mouth and tongue, crying without tears, no wet diapers for 3 hours or more, a high fever and being unusually sleepy or drowsy.
If you think you're dehydrated, drink small amounts of water over a period of time. Taking too much all at once can overload your stomach and make you throw up. For people exercising in the heat and losing a lot of minerals in sweat, sports drinks can be helpful. Avoid any drinks that have caffeine.
Dehydration, i.e. a reduction in fluid volume, could theoretically lead to lower blood pressure if the body’s compensatory mechanisms were to fail. Put simply, blood pressure is the force of circulating blood on the walls of blood vessels. With all other variables remaining constant, a lower blood volume would lead to lower blood pressure. However, blood pressure is regulated and maintained by numerous processes in the body, namely the baroreceptor reflex and the renin-angiotensin-aldosterone system (RAAS). RAAS is also a major control system for fluid balance. In a healthy person, these mechanisms would react to dehydration and a drop in blood volume to maintain a normal blood pressure through a cascade of events. References 1. Sparks MA, Crowley SD, Gurley SB, et al. Classical Renin-Angiotensin System in Kidney Physiology. Compr Physiol. 2014 Jul; 4(3): 1201-1228. URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4137912/. Accessed April 22, 2018.