Exploring our hydration needs (part 1)
One of the pieces of health advice that gets bandied around the most is ‘drink plenty of water’.
However, meeting our daily hydration needs, in most situations, is simply about responding to thirst.
In this post we will look at the components of our bodies’ self-regulation of fluid levels, which includes the thirst mechanism.
In a follow up post we will discuss the specific states of dehydration and overhydration, the role of sports drinks, whether or not we need a fluid intake plan, and specific conditions under which our thirst mechanism may not function properly.
The importance of adequate hydration
Water (H2O) accounts for about 60% of a male human body and 55% of a female.
For example, a female weighing 70kg will hold 38.5 litres of total body water (TBW). About 25 litres of her water will be found within the cells of her body (intracellular), 10.5 litres will be found between her cells (interstitial fluid), and 3 litres will be found within her blood plasma.
“Total Body Water (TBW) is the total amount of water in the human body. This includes fluids both inside (intracellular) and outside (extracellular) cells.”
Water is the solvent in which many chemical reactions occur. It plays a critical role in thermoregulation (heat redistribution and sweating), nutrient transport, joint lubrication and cellular function.
Through complex homeostatic mechanisms, our body delicately regulates its TBW.
Our body obtains water as you might expect - primarily through absorption in the digestive tract after we consume fluids or food.
We primarily lose body water excreting it in urine from the kidneys. The kidneys adjust the level of excretion depending on our body’s needs.
We also lose water by evaporation from the skin, and by exhalation from the lungs. Profuse sweating, caused by either vigorous exercise, hot weather or a high body temperature, can dramatically increase water loss through evaporation.
Vomiting and diarrhea can also lead to significant water loss, particularly during prolonged bouts.
Water level regulation
Our body constantly regulates our TBW and electrolyte levels in our blood.
Electrolytes such as sodium and potassium are dissolved within water, and so we refer to the balance of water and electrolytes as concentration levels.
There are three dependable ways in which our body auto-regulates the water and electrolyte balance:
The stimulation or suppression of thirst.
By increasing or decreasing excretion of fluid by the kidneys.
By osmosis.
For example, when the body is low in water and sodium concentration in our blood increases (increased osmolality), two critical responses occur.
Firstly, we become thirsty and drink more fluids. And secondly, vasopressin (an antidiuretic hormone) is secreted by our pituitary gland (in our brain), causing the kidneys to conserve water and excrete less urine.
The net effect is the addition of water to our TBW, which will dilute sodium levels and restore a normal balance in our blood (homeostasis).
By contrast, when sodium concentration levels are too low, thirst will be suppressed and our kidneys will secrete more water, thereby lowering TBW and increasing concentration to normal levels.
How our body’s thirst mechanism works
Our body’s thirst mechanism is a highly sophisticated and sensitive system designed to maintain fluid balance and electrolyte levels.
The key players in this process include the brain, hormones and sensors in the body, working together in a negative feedback loop.
The body’s primary ‘thirst centre’ is the hypothalamus - a deep brain structure that also regulates body temperature, sleep and appetite.
Osmoreceptors are specialised cells in the hypothalamus that detect changes in blood osmolality.
When osmolality rises, indicating dehydration, these osmoreceptors signal the brain to induce thirst.
Salivary glands in the mouth are instructed via the sympathetic nervous system to decrease serous output, resulting in a ‘dry mouth’. This not only reduces some fluid loss via exhalation, but also acts as a trigger for thirst.
Along with consuming fluid, the body seeks to reduce the pathways of fluid loss. To achieve this, the hypothalamus stimulates the posterior pituitary gland to release vasopressin (ADH) which signals the kidneys to slow fluid excretion.
Aside from a change in osmolality, we also want to maintain our TBW volume.
A reduction in TBW results in decreased blood pressure. Baroreceptors (blood pressure receptors) detect this change and ultimately increase cardiac output.
The kidneys also respond by increasing release of hormone angiotensin II, which helps to stimulate thirst, as well as the release of a subsequent hormone, aldosterone (from the adrenal glands).
Aldosterone causes the kidneys to retain sodium, which increases water retention and thereby raises blood volume back to normal levels.
This sophisticated feedback system ensures that we drink the right amount of water to meet our needs, without the need for over-regulating our fluid intake.
Our fluid levels are in a constant state of flux throughout the day, however with the combined input of the hypothalamus, the kidneys, specialised sensory receptors and the hormonal system, we are able to maintain fluid levels within very specific margins by simply responding to thirst.
The average Australian male will both consume and excrete 2.8L of fluid per day, and females 2.2L. There is quite a wide margin in this exchange, depending on age, health and activity levels.
In our next blog post we will look at the specific states of dehydration and overhydration, the role of sports drinks, whether or not we need a fluid intake plan, and specific conditions under which our thirst mechanism may not function properly.
“Fun fact: Euvolemia is the state of normal body fluid volume.”
