At one point, you’ve probably wondered what makes us truly human?
You’ll probably get various answers from people with different backgrounds. There’s consciousness, free will, having a set of morals, the idea of having a soul, and being fully aware of our own mortality.
Although these answers require further deep thought and investigation, one thing’s for sure: as much as two-thirds of your body weight is water.
Trivia: Ninety-five percent of the body of a jellyfish is water.
Even though much of our body is made up of water, the percentage of our body composition that is water changes based on our functional needs. Some of the essential functions of water in our bodies include the following:
- – It’s a building block to almost every cell in the body.
- – It regulates body temperature via sweating and respiration.
- – Carbohydrates and proteins used for energy are transported by water in the bloodstream.
- – It assists in the removal of metabolic waste, mainly through urination.
- – It acts like a shock absorber to protect our brain and spinal cord.
- – It makes up saliva and fluid that lubricates your joints.
The amount of water in your body depends on various factors including age, gender, physical activity, and even where you live. It’s often referred to as Total Body Water (TBW).
For example, infants are born with roughly 78% of their entire weight being water. By one year of age, TBW decreases to about 65% of weight. In healthy adult males, TBW averages 60% of their weight because they generally carry more lean mass. On the other hand, women will see that roughly 55% of their weight is TBW.
Most of this water comes from that lean body mass mentioned above. This includes your blood, organs, and muscle. Here’s a snapshot of the major body organs’ water content:
- – Brain and heart – 73%
- – Lungs – 83%
- – Skin – 64%
- – Muscles and kidneys – 79%
- – Bones – 31%
Your TBW can be further segmented into two compartments: extracellular water (ECW) and intracellular water (ICW).
Intracellular Water (ICW)
ICW is the amount of water found in your cells. In healthy adults, ICW makes up two-thirds of your total body water. Essential cellular processes take place in the ICW, such as creating the energy necessary to fuel each cell for their specific functions.
Extracellular Water (ECW)
ECW is the amount of water located outside of your cells. It makes roughly a third of the total body water in healthy adults. ECW helps control the movement of electrolytes, delivers oxygen to the cells, and clears waste from metabolic processes.
In terms of body composition, an increase in ICW can signify increased muscle mass. On the other hand, an increase in ECW could indicate inflammation, fluid retention, or disease. In fact, inflammation resulting from having too much visceral fat can trigger activation of the renin-angiotensin-aldosterone-system (RAAS) which in turn increases ECW. When activated, this hormone system acts to maintain blood pressure and fluid balance. However, when overstimulated, the RAAS can lead to symptoms associated with obesity, kidney disease, and the metabolic syndrome.
It’s worth noting that TBW is constantly changing with gains and losses of fluid even in healthy adults. The volume is regulated through a complex system of exchange of fluids, solutes, and ions within the compartments in the body — the RAAS and the ADH.
Here’s how the ADH system works when your body experiences a significant loss of body water:
- 1. The hypothalamus—the gland responsible for regulating our body temperature and triggering the processes that balance the fluids in our bodies—detects dehydration.
- 2. As a result, the hypothalamus signals the release of antidiuretic hormone (ADH) vasopressin which causes the kidneys to remove less water from the blood. This leads to peeing less and a darker, more concentrated urine. It also raises blood pressure.
- 3. Meanwhile, your brain also tells you that you’re thirsty, and once we sip on some water or consume something that’s made of water, TBW levels return to normal. The same mechanism happens when your body temperature rises being in a warm environment, working out, or fever. Body temperature goes down with sweating which in turns helps you cool off.
As for the cellular compartments, a water deficit leads to an increase in the electrolyte concentration in the extracellular compartment. This higher electrolyte concentration draws water from the intracellular compartment improve the electrolyte-fluid balance in the ECW, causing the cells to shrink.
The RAAS and ADH systems work interchangeably to keep our fluid balance on track and properly-timed because our brains, kidneys, various glands, and hormones work together to monitor the amount of water that you’re taking in and losing.
All of these are analogous to a radiator which heats a room. You set it to turn on at a certain temperature. When the room falls below that temperature the radiator turns on. When the room temperature increases, the radiator turns off. In this case, the RAAS activates when the room temperature is too hot (fluid is too high) and the ADH activates when room temperature is too low (fluid reduced).
In short, your body is smart and sensitive enough to detect irregularities and compensate for losses and gains to make sure that everything’s well-balanced.