Awakening from a deep slumber in a dehydrated panic, I know immediately that I need to get some water. NOW. Day in and day out, I be sippin’ (and I’m fortunate to live in a place where I can turn on the tap and get clear, tasty H2O) The sensation of thirst happens without any thought on my end, yet somehow my body knows that water is what it needs.
I became thirsty for information.
In this article we will look at how the body senses dehydration.
Go ahead and grab yourself a glass before reading on, you’re gonna want one I promise.
Let’s dip our toe into how our body uses water by first discussing fluid distribution.
Fluids within the body can be generally categorized into 2 groups:
water inside cells, (intracellular) and
water outside of cells (extracellular)
Intracellular fluid, the water within cells, must remain generally stable, in volume and pH, as this is where most reactions in the cell occur that make the cell perform its job. Intracellular fluid makes up about 2/3 of our fluid volume and 40% of our bodyweight.
Extracellular fluid, the water between cells, serves as a liquid medium for transportation of nutrients and waste, chemical reactions and physical protection (lubrication, shock absorption, etc…). It is in constant motion throughout the body. Though it is all the same fluid, the concentrations of different substrates determine its use at a particular time in a particular area. Extracellular fluid makes up about 1/3 of our fluid volume and 20% of our body weight.
Extracellular fluid can be broken down into 3 main categories:
plasma in the blood,
lymph in lymphatic vessels and
interstitial fluid between tissue cells
Water moves passively between these intra- and extracellular compartments based on concentration of electrolytes, proteins and sugars. When an area gets salty and dehydrated, water will naturally flow to re-hydrate and bring the concentration of electrolytes and other substrates down. When there are not enough electrolytes, protein and sugars and too much fluid, water will move away to bring that substrate/water concentration into balance.
It is the relationship between these two fluid volumes (intra- and extracellular) and their concentration of substrates therein that determine thirst.The environment in and around cells is greatly effected by the presence of electrolytes (charged ions of inorganic salts, namely sodium, potassium, magnesium and calcium, among others--all necessary for cellular function) as well as other substances like amino acids, fatty acids and sugary bits. The relationship between water volume and substrate volume is called osmolality.
Our bodies sense the hydration status of both the intra and extra cellular space and respond to each in slightly different ways, though they are inextricably linked. There is a constant exchange between these two compartments, both through the “push” of hydrostatic pressures and the “pull” of osmolality. As water volume in one compartment changes, so does the ionic concentration, which alter function just as much as the presence or absence of water. So, we are driven to thirst by both the actual water content within the body AND the osmolality of the internal environment.
Water loss inside a cell:
Our bodies are particularly sensitive to the water content of the intracellular space, because each cell requires specific conditions in which its processes must occur. Therefore the body is more likely to want to hold onto water inside the cells, but in a state of dehydration, it is not always possible to keep a perfect balance. If the intracellular space becomes devoid of water, the ionic concentration within the cell, and thus the pH, changes the internal environment, making it difficult to perform all of its usual functions.
A cell will essentially sacrifice its ability to function normally by "sending" water into the extracellular space when we get dehydrated. This is not a completely selfless act on the part of the cell. If the space around the cell gets too concentrated with salty electrolytes, those substances will seep through the membrane of the cell and create an even more undesirable environment than if the cell were to just give up some of its water.
So, when the extracellular space becomes dehydrated and highly ionic, the intracellular compartment will “give” some of its water to the extracellular compartment to maintain an ionic balance, yet dehydrate itself in the process. The corresponding shrinkage of the cell is detected by the brain, which sends hormonal signals that make us thirsty. Additionally, the brain will also send messages to the kidneys, telling them hold onto water by producing a smaller volume of more concentrated urine.
Upon intracellular dehydration, drinking water alone is enough to satiate thirst.
Water loss in the extracellular space:
Through processes like sweating and breathing, extracellular fluid is the first to be lost. Additionally, different parts of the body will draw from this extracellular space to maintain fluid volume of essential areas, like cerebrospinal fluid protecting our brain and spinal cord, or serous fluid protecting our heart, lungs and abdominal cavity. (I’ll look in depth at these fluids more specifically in my next post).
Loss of water from the extracellular space is accompanied by a shift in electrolyte balance, so thirst triggered from decrease in extracellular volume is often accompanied by salt appetite. (this is why coconut water is touted as a “hangover cure”, because it contains both water and electrolytes (like sodium and potassium) that not only rehydrate you but also help to balance out ionic composition)
Extracellular fluid also determines blood volume. Blood is a mixture of formed elements (red and white blood cells) and plasma (extracellular fluid between the cells). As the body approaches a state of dehydration, plasma volume decreases, making the blood more viscous and concentrated with formed elements. As blood volume decreases and blood becomes thicker, it requires more effort of the body to keep the minimum blood pressure necessary to maintain circulation.
Here’s some ways the body can detect dehydration in the extracellular space:
SENSORS IN THE BRAIN:
1) Forebrain areas with ridiculously long names:
Changes in osmolality can be detected by two structures in the brain that actually lie outside of the blood-brain barrier, giving them access to changes in circulation. Firing of neurons in these areas is sufficient stimulus to generate thirst. These areas are located in the forebrain and are called subfornical organ and organum vasculosum of the lamina terminalis. Say those 10 times fast
2) Deep brain areas with easier to say names:
Changes in osmolality can ALSO be sensed by the anterior hypothalamus and pre-optic area that lie deeper in the brain. Neurons in these areas fire more when the salt content of the body gets too high (ie dehydrated) and decrease in firing when water from the corotid artery “irrigates” them. So wet neurons in these areas fire less, essentially conveying to ourselves that we are no longer thirsty.
This response happens in the same neurons when water load is applied to the tongue. So even the presence of water in the mouth decreases thirst-regulating neuronal firing, even if water hasn’t actually physically reached those neurons yet.
SPECIALIZED CELLS IN THE KIDNEYS:
Changes in blood pressure can be detected by the kidneys, which induce a hormonal response that work physiologically both to balance blood pressure through vessel constriction and water re-uptake by the kidneys, and behaviorally to promote drinking.
PRESSURE RECEPTORS IN THE BLOOD VESSELS AND HEART:
Changes in blood pressure can be detected by stretch-sensitive receptors that innervate the heart and blood vessels. These baroreceptors are cells of the nervous system that live next to blood vessels and communicate directly with the brain about the hydration status of the circulatory system and the brain responds accordingly through hormonal, neuronal and behavioral action.
Interestingly, thirst is regulated through mere anticipation. When we drink, it takes a while for the water to diffuse to the farther reaches of our cells and the space around them. However, our taste buds send messages to the brain about the nature of what we have just drank, like water and salt content, and the body will respond as if fluid has already reached the blood stream. The brain will put an end to our sensation of thirst preemptively based on signals that it received from the mouth.
Thirst is also anticipatorily regulated by eating. When we eat, we need more fluid to replace the saliva we are swallowing, to lubricate the foodstuffs as it passes through our gastrointestinal tract, and to counteract the increase in osmololity from the salts and other molecules in our food. So, the act of eating itself triggers thirst in an anticipation of need.
So, we have a whole system with special sensors and regulators that work specifically to balance our body fluids and work to make us thirsty…or not.
The sensation of thirst actually stimulates our sympathetic nervous system (the fight or flight version of ourselves, see “The Axis: How Stress Works” for more info on that). When we get thirsty, our blood vessels constrict, our kidneys start to hold onto water and a whole slew of hormones are released to maintain proper blood pressure and fluid balance until we can get re-hydrated. So staying hydrated actually could…make you feel less stressed? It’s a bit of a reach, but the science checks out!
Next week I’ll be launching a full investigation of each body fluid and the functions they serve. It’s gonna be a good time!
For now, go get yourself another nice tall glass of water. But sip it slowly so you don't shift your ionic concentrations too much.
“Thirst” Leib, D.E., Zimmerman, C.A., Knight, Z.A. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5957508/
“Water, Hydration and Health” Popkin, B.M., D’Anci, K.E., Rosenberg, I.H. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908954/
“Principles of Human Anatomy” Tortora, G.J., Nielsen, M.T.