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How does the body take in and expel water?

Water intake and output must be balanced for homeostasis. So how does the body manage this? Find out in this article.
person drinking from water bottle

Homeostasis requires that water intake and output are balanced. But how do we take in and expel water?

Water intake

We meet our daily requirement for water by drinking, eating and metabolising glucose into ATP, CO2 and water. Most water intake comes through the digestive tract via liquids and food, but roughly 10 percent of water available to the body is generated at the end of aerobic respiration during cellular metabolism. Here’s what that looks like: drinking 1580ml 63% food 620ml 25% metabolism 300ml 12%

Water output

We lose most of our body water through urination, then through the skin and lungs, and a small amount in our faeces. Losses through the skin and from the lungs (breathing) average about 900 ml per day or more during heavy exercise. These are called insensible losses.

Urine produced by the kidneys accounts for the largest amount of water output. The kidneys can adjust the concentration of the urine to reflect the body’s water needs, conserving water if the body is dehydrated or making urine more dilute to expel excess water when necessary. The antidiuretic hormone (ADH) is a hormone that helps the body to retain water by increasing water reabsorption by the kidneys. Here’s what our water output looks like:

Fluid balance

The body’s fluid and electrolyte balance is critical to metabolic function. The active maintenance of these is called osmoregulation. Water makes up around 60% of the body and is found within two main fluid compartments.

Intracellular The intracellular (inside a cell) fluid makes up 60-66% of the water in the body and is found within the body’s cells.
Extracellular The extracellular (outside of cells) fluid makes up the rest of the body’s water and can be divided into intravascular fluid (mostly blood) and the extravascular fluid (interstitial fluid around the cells).

In the image below you can see the make up of fluids and solids in the average person. Of the fluids, 66% is intracellular fluid (inside the cells) and 33% is extra-cellular fluid. see pdf download at bottom of step

Electrolytes

Electrolytes in body fluids are responsible for maintaining osmotic gradients and permitting ion exchanges. For example, in blood plasma, electrolytes help to maintain blood volume by keeping water moving into the capillaries.

When electrolyte (mostly Na+) levels fall, water moves out of the capillaries and into the tissues. This causes blood volume and pressure to fall and plasma to thicken.

The kidney has a major role in this. Let’s look at the kidney more next.

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Finding the Body’s Balance: Understanding Homeostasis

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