Water and Hydration

Why Drinking Water Is So Important

Two-thirds of the water in your body is in the intracellular space (inside the cells) and the other one-third is in the extracellular space.  You experience thirst when there is not enough water in your body but that thirst is different depending on whether you need water inside or outside of your cells.  In the case of blood loss you may need electrolytes in addition to water to replenish your blood volume.  Water in the body can be lost in many ways- through urination, sweating, bleeding, crying, breathing, etc.  Loss of water also involves loss of electrolytes, which are the salts used by our bodies to regulate where water goes, and must also be replenished.

Factors that contribute to your daily need for water include altitude, your age, menstruation, level of alcohol consumption and physical activity to name some of them.  Most of us find it too complex to try to answer all of those questions to arrive at the correct amount of water to drink each day, so instead you can build the habit of drinking water frequently and then fine tune the amount if you need to based on how you feel.  Dehydration inside of your cells reduces your cells ability to function, resulting in symptoms such as headaches, dryness in mucous membranes such as mouth and eyes, confusion, fatigue, and when very far progressed, delirium and death.

The hypothalamus and Pituitary glands are the structures in the brain responsible for monitoring hydration levels and creating the sensation of thirst.  These two glands are where communication happens between your neurological system (your brain) and your endocrine system (glands and hormones). 

Homeostasis is the drive in the body and brain to maintain an equilibrium.  Osmosis is the movement of water between areas based on electrolyte concentration.  Electrolytes include sodium, chloride, potassium, etc.  They are essential for maintaining homeostasis.  The hypothalamus contains cells called osmoreceptors.  Osmoreceptors monitor hydration levels by stretching or shrinking caused by their own water content.  They send signals to the posterior pituitary gland which in turn releases anti-diuretic hormone.  A diuretic substance is one that causes the body to lose water through urination, so an anti-diuretic one causes the body to hold on to water by not urinating. 

Osmoreceptors are part of a larger system called osmoregulation.  Baroreceptors are cells that are also part of the osmoregulatory system.  They are located in arteries and monitor blood pressure, which is also influenced by the level of hydration in the body.  For example, bleeding can cause a decrease in blood pressure that is sensed by the baroreceptors, which then signal that more water is needed.

The kidneys are organs that also play a central role in osmoregulation.  They filter waste particles of the blood and send them out of the body in the urine.  Kidney tissue contains high levels of electrolytes which draws water in to be filtered.  When the kidneys receive signals to release water, they move salt into the renal pyramids, structures which filter and release water into the urine.  If the kidneys receive signals to retain water, the salts move into the tissues of the kidneys and returns to the vascular system rather than being excreted.