Chapter 1: How Water Works

Chapter Introduction

You have been a creature of water from the first moment you existed.

Before you had bones, you were water. Before you had a heart, you were water. Inside your mother, you floated in salt water of a specific composition that ancient oceans would have recognized. When you were born, you were more water than you will ever be again — somewhere around seventy-five percent of your newborn body was water, give or take. The percentage has gone down over the years as your bones thickened and your fat layer built and your muscles grew, but the truth has not changed. You are, today, somewhere between fifty and sixty-five percent water by mass [1]. You have less water in you than a jellyfish and more than a brick, and what your body is doing with that water, every second of every day, is more complex than what any factory on Earth could manage.

This chapter is not about how much water to drink. Not yet. This chapter is about what water actually is — the small, simple molecule that is somehow responsible for almost every chemical reaction your body has ever performed. You will learn why water is strange — why it expands when it freezes, why it dissolves things other liquids cannot dissolve, why it can carry electricity, why it climbs up the inside of plants against gravity, and why all of these strange properties matter to a body. You will learn about the three compartments of body water — the water inside your cells, the water between your cells, and the water in your blood — and why each compartment is guarded carefully. You will learn about the kidney, the most underappreciated organ in your body, which filters every drop of your blood roughly thirty times a day. You will learn about electrolytes — the salts that make water inside a body behave differently from water inside a glass. And you will meet the hypothalamus, the small region deep in your brain that knows, before you do, when you are thirsty.

The Elephant teaches water. The Elephant has lived alongside water for millions of years, longer than any human civilization has existed. The Elephant remembers water-holes that dried up generations before any human alive. The Elephant walks toward water across distances no other land animal can cross. The Elephant drinks water by the gallon, bathes in water daily, sprays water across its back to cool down, mourns at water-holes where elders once died. The Elephant is patient with water. The Elephant is not frantic. The Elephant does not gulp water in panic and does not avoid water in fear. The Elephant has a relationship with water that is older than memory. This is the animal that teaches you water.

Begin.

---

Lesson 1.1: What Water Is

Learning Objectives

By the end of this lesson, you will be able to:

• Describe the chemical structure of a water molecule and explain why water is a polar molecule
• Identify hydrogen bonding and explain why it gives water unusual properties compared to other small molecules
• Describe the three states of water (solid, liquid, gas) and the unusual fact that water expands when it freezes
• Explain the meaning of cohesion, adhesion, and surface tension in the context of water
• Identify specific heat capacity and explain why water resists changes in temperature

Key Terms

A Small, Strange Molecule

Begin with what is in your glass.

A single water molecule is made of three atoms. Two are hydrogen. One is oxygen. The chemical formula — H₂O — is one of the first formulas you ever learned. It is also one of the simplest in chemistry. There are millions of chemical compounds, and almost all of them are bigger and more complicated than water [2]. And yet water is, by a wide margin, the most important molecule in biology. Life on Earth uses water for almost every chemical reaction it performs. No known living thing exists without water. When scientists look for life on other planets, the first thing they look for is liquid water, because the assumption is that without water, the kind of chemistry that supports life simply does not happen [3].

To understand why water is so strange, you have to look at the shape of the molecule. The two hydrogen atoms are not stuck onto opposite sides of the oxygen atom. They are stuck onto the same side, at an angle of about 104.5 degrees. This gives the water molecule a bent shape — like the head of Mickey Mouse, with the oxygen as the face and the hydrogens as the ears, except smaller and the wrong scale by many orders of magnitude. That bent shape is the reason water is polar. The oxygen atom pulls more strongly on the shared electrons than the hydrogens do, so the oxygen end of the molecule has a slight negative charge, and the hydrogen end has a slight positive charge [4].

This little electrical unevenness — what chemists call polarity — is the source of nearly every interesting thing water does. A polar molecule does not just sit by itself in space. It sticks to other polar molecules. The slightly positive hydrogen end of one water molecule reaches toward the slightly negative oxygen end of the next water molecule, and a faint electrical attraction forms between them. This attraction is called a hydrogen bond. A single hydrogen bond is weak. But every water molecule is forming hydrogen bonds with several of its neighbors at once, all the time, and those bonds are constantly breaking and reforming as the molecules move around. The result is that water is not really a loose collection of separate molecules — it is a constantly shifting network of molecules holding hands [5].

Why Water Is Strange

Most small molecules with three atoms behave like gases at room temperature. Methane (CH₄) is a gas. Carbon dioxide (CO₂) is a gas. Hydrogen sulfide (H₂S, which has a similar shape to water but a sulfur atom in place of oxygen) is a gas. By the rules of basic chemistry, water "should" be a gas too at the temperatures and pressures you experience in everyday life. The fact that water is a liquid at room temperature is a chemical anomaly. It is liquid because of those hydrogen bonds — the bonds hold the molecules together against the heat energy that would otherwise spread them apart [6].

This is not the only place water breaks expectations. Most substances, when they cool down, get denser. The molecules slow down, pack more tightly, and the solid form sinks in the liquid form. Water does this too — until you get near freezing. Below about 4°C, water expands as it gets colder. By the time it freezes, the hydrogen-bond network has locked into a regular hexagonal pattern that is less dense than the liquid. This is why ice floats. This is why lakes and ponds freeze from the top down, not the bottom up, leaving liquid water underneath where fish and amphibians can still live. If water behaved like a normal substance, every cold body of water would freeze solid from the bottom up every winter, and most freshwater life as we know it would not exist [7].

Water has high cohesion — it sticks to itself. This is why a drop of water on a clean surface beads up rather than spreading flat. It is why you can fill a glass slightly above the rim and the water stays bulged up by surface tension instead of spilling. It is why small insects called water striders can walk on a pond without sinking — the surface tension of water can actually support their weight. Cohesion is the hydrogen-bond network holding itself together [8].

Water also has high adhesion — it sticks to other polar surfaces. This is why water climbs up a thin straw without you having to suck on it. This is part of why water moves up the inside of plant stems, sometimes a hundred feet up a redwood tree, against gravity, every day. The water molecules cling to the inside of the tiny tubes, and to each other, in an unbroken chain from root to leaf [9].

And water has high specific heat capacity — it absorbs a lot of energy before its temperature changes. This is part of why the ocean moderates the climate of coastal cities, why your body can dump heat into water during exercise without immediately overheating you, and why drinking very cold water on a hot day cools you down slowly rather than suddenly. The hydrogen-bond network has to absorb a great deal of energy to break apart enough to allow the molecules to move faster, which is what temperature actually is at the molecular level [10].

The Universal Solvent

The last great trick of water is that it dissolves things. When you stir salt into water, the salt — sodium chloride — does not simply sit there as undissolved crystals. The polar water molecules surround each sodium and chloride ion, with the slightly negative oxygen ends pointing at the positive sodium and the slightly positive hydrogen ends pointing at the negative chloride. The ions are pulled apart from each other and surrounded by little jackets of water molecules. The salt has dissolved. The same trick works on sugar, on most acids and bases, on many vitamins, on many proteins, on the small charged particles called ions that your body's cells run on [11].

This is why water is sometimes called the universal solvent. The phrase is an exaggeration — water cannot dissolve everything, and some substances like oils and fats are famously not dissolved by water — but water dissolves more substances than any other common liquid. And that property is the reason life uses water. Almost every chemical reaction in your body happens in water. The chemicals that need to react have to be able to move around and bump into each other, and they can only do that if they are dissolved. The blood traveling through your veins is mostly water carrying dissolved substances. The fluid inside your cells is mostly water carrying dissolved substances. The fluid in your spine, in your eyes, in your joints, in your gut — all of it is water carrying dissolved substances. Take away the water and the chemistry stops [12].

The Elephant knows what water is now. So do you. The next lesson asks where, exactly, all of it is in you.

Lesson Check

1. What two atoms make up a water molecule, and in what proportion?
2. Why is water called a polar molecule?
3. Name two unusual properties of water that hydrogen bonding helps explain.
4. What does specific heat capacity mean, and why does it matter for biology?
5. Why is water sometimes called the universal solvent?

---

Lesson 1.2: The Water in You

Learning Objectives

By the end of this lesson, you will be able to:

• Estimate the percentage of body mass that is water in an adolescent and identify how that percentage varies by age, sex, and body composition
• Identify the approximate water content of several major tissues, including muscle, fat, bone, blood, and brain
• Distinguish between intracellular fluid (ICF) and extracellular fluid (ECF), and identify interstitial fluid and blood plasma as the two main parts of extracellular fluid
• Describe how water moves between body compartments by osmosis
• Identify approximately how much body water an average adolescent loses and gains each day

Key Terms

How Much Water Are You?

You are mostly water. Estimates vary based on the measuring technique, but for a typical healthy adolescent, total body water is somewhere between fifty and sixty-five percent of body mass [13]. A 150-pound (about 68-kilogram) teenager carries something on the order of 35 to 45 liters of water — roughly nine to twelve gallons. This is not a lot by household plumbing standards. It is a great deal by what-fits-inside-a-human standards.

The exact percentage depends on several factors:

• Age. Newborns are about seventy to seventy-five percent water. By adolescence, that figure drops to roughly sixty percent. By old age, it tends to drop further, into the fifty percent range or below. Some of this drop is the build-up of fat tissue (which holds less water) over the lifespan; some of it is changes in the cells themselves [14].
• Sex. On average, adult males carry a slightly higher percentage of body water than adult females, mostly because of differences in average body composition (more muscle, less fat). The difference becomes more apparent after puberty [15].
• Body composition. Muscle is about seventy-five percent water. Fat tissue is about ten to twenty percent water. Two people of the same weight can have meaningfully different total body water depending on how much of that weight is muscle versus fat.
• Hydration status. People who have been drinking enough fluids carry more water than people who have not. People who are sick, or who have lost a lot of fluid through sweat or illness, carry less.

This is descriptive. It is not a target. There is no "correct" percentage to aim for, and no behavior — water loading, water restriction, sauna sweating to drop water weight — produces a better long-term outcome than letting your body manage its own water content. The Elephant is patient about this. Your body has been keeping its water balance for longer than you have been conscious, and it does the job quite well when you let it.

Where the Water Actually Lives

The water in you is not all in one place. It is spread across two main compartments, separated by the membranes that form the walls of every cell in your body.

The intracellular fluid (ICF) is the water inside cells. It is the larger compartment — about two-thirds of all the water in your body lives inside your cells. The water inside a cell is not pure water; it carries dissolved salts (especially potassium), proteins, enzymes, sugars, and the thousand other chemicals that the cell uses to live. Every one of the trillions of cells in your body is, essentially, a tiny bag of saltwater with machinery inside it [16].

The extracellular fluid (ECF) is the water outside cells. It is the smaller compartment — about one-third of body water. The ECF has two main parts. The interstitial fluid is the water in the spaces between cells, bathing every cell from the outside. The blood plasma is the liquid part of blood. Plasma is technically a kind of extracellular fluid in motion — it travels through the blood vessels carrying nutrients, oxygen, hormones, and signals to every part of the body. Interstitial fluid does not travel; it sits between cells, exchanging substances with the cells it surrounds and with the blood plasma that flows through nearby capillaries. The salt composition of extracellular fluid is different from intracellular fluid — extracellular fluid is high in sodium and chloride; intracellular fluid is high in potassium [17]. This salt difference is one of the most important biological facts in your body, and the next chapter on electrolytes will return to it.

A smaller portion of extracellular water sits in places like the fluid around your brain and spinal cord (cerebrospinal fluid), the fluid in your eyes (aqueous and vitreous humor), the fluid in your joints (synovial fluid), and the fluid in your digestive tract. These specialized fluids each have their own composition, but they all come, originally, from the same total body water pool [18].

The Water Content of Tissues

If you imagine each kind of tissue in your body, the water content varies widely.

• Blood: about 80% water by mass. (The other twenty percent is mostly red blood cells and proteins.)
• Brain: about 75-78% water. (Despite popular wording, the brain is not a "fatty organ" — it has a high lipid content in cell membranes, but most of its mass is water.)
• Skeletal muscle: about 70-75% water.
• Kidneys: about 80% water.
• Lungs: about 80% water.
• Skin: about 60-70% water.
• Liver: about 70% water.
• Adipose (fat) tissue: about 10-20% water.
• Bone: about 20-30% water (mostly in the marrow and in living bone cells; the mineral matrix itself is essentially anhydrous).
• Tooth enamel: about 4% water — the driest tissue in your body, and also the hardest.

Two patterns stand out [19]. First, the parts of you that do the most chemistry — brain, blood, kidney, lung, liver — are the wettest. Chemistry needs water. Second, the parts of you that store energy or provide structure — fat and bone — are the driest. Storage and structure can be done with less water in the way.

This is a long roundabout for one practical point. When researchers describe "dehydration" in a general way, they are usually describing a loss of water from the extracellular fluid first — blood plasma, interstitial fluid — because that is the compartment that loses water first when sweat or urine losses exceed intake. The brain, the kidneys, the muscles, the gut: every one of these organs is sensitive to small changes in the surrounding fluid, and your body has elaborate systems to defend that fluid against changes. Those defenses are what the next two lessons are about.

Osmosis: How Water Moves Between Compartments

The boundary between intracellular and extracellular fluid is the cell membrane. Cell membranes are partially permeable — they let some substances through freely, others through with the help of specific protein channels, and still others not at all. Water can move through cell membranes, but the direction of its movement is controlled by the dissolved-particle concentrations on either side [20].

The principle is called osmosis. Water moves from regions of lower dissolved-particle concentration to regions of higher dissolved-particle concentration, across a partially permeable membrane. The water is not "pulled" by the dissolved particles; the dissolved particles cannot cross the membrane easily, and the water rearranges itself until the concentrations on both sides are balanced.

The implication for your cells is this. If the fluid around a cell becomes more dilute than the inside of the cell (a hypotonic environment), water will flow into the cell. The cell will swell. If the swelling is severe enough, the cell can burst. If the fluid around a cell becomes more concentrated than the inside of the cell (a hypertonic environment), water will flow out of the cell. The cell will shrink. If the shrinking is severe enough, the cell cannot function. When the fluid around a cell matches the dissolved-particle concentration inside the cell (an isotonic environment), water does not move on net, and the cell holds its shape. Your body's "default" extracellular fluid is roughly isotonic to the inside of your cells. That match is not an accident — it is what your kidneys, your brain, and your hormones are constantly working to maintain [21].

This is why dehydration is dangerous and why over-hydration (water intake that overwhelms the body's ability to excrete the excess) is also dangerous. Both states disturb the dissolved-particle balance and force water into or out of cells. Brain cells are especially sensitive because they are packed tightly inside the rigid case of the skull, with little room to swell. The next chapter, in Grade 10, will return to this in the discussion of hyponatremia. The principle, though, you already have. Water follows particles. Cells respond.

How Water Comes In and Goes Out

A typical adolescent gains and loses roughly two to two and a half liters of water per day [22], though the exact figure varies widely with temperature, exercise, diet, body size, and altitude. The water in is roughly equal to the water out, every day. Out of balance, even by a small amount, you would notice within a day or two.

Water in comes from three sources:

• Beverages. Water, milk, tea, juice, broth, soup — anything liquid that you drink. This is the largest source for most people, often around half to two-thirds of daily intake.
• Water in food. Fruits, vegetables, soups, stews, yogurt, eggs, meat — most foods contain considerable water. Cucumbers and watermelon are nearly all water. Even a steak is roughly seventy percent water. This is often a quarter to a third of daily intake.
• Metabolic water. When your cells break down carbohydrates, fats, and proteins for energy, water is produced as a chemical byproduct. This is a small but real contribution — often around 250-350 milliliters per day [23].

Water out leaves through four main routes:

• Urine. Usually the largest output, somewhere around a liter to a liter and a half per day for an average adolescent.
• Sweat. Highly variable. Minimal in cool conditions at rest; can exceed a liter per hour during heavy exercise in heat.
• Breath. Every breath you exhale contains water vapor. Over a day, this is typically 300-500 milliliters.
• Stool. Usually a small contribution, around 100-200 milliliters per day, though much larger during illness with diarrhea.

There is also a smaller route: water lost through the skin even when you are not actively sweating, called insensible water loss. This adds another few hundred milliliters per day.

The balance is what your body is constantly managing. Drink more than your body needs, and the kidney excretes the extra. Drink less, and the kidney holds water back. Lose extra through sweat, and the hormones adjust. Eat a salty meal, and the brain notices. The system is responsive on a timescale of minutes. The next two lessons are about the machinery that pulls this off.

Lesson Check

1. Roughly what percentage of an adolescent's body mass is water? How does this compare to a newborn and to an elderly adult?
2. Which body water compartment — intracellular or extracellular — is larger?
3. Identify three tissues with high water content and one tissue with low water content.
4. What does osmosis mean, and in which direction does water move?
5. Name three routes by which water leaves your body each day.

---

Lesson 1.3: The Kidney and the Balance

Learning Objectives

By the end of this lesson, you will be able to:

• Identify the location and major function of the kidneys
• Describe the nephron as the functional unit of the kidney and explain its three main processes: filtration, reabsorption, and secretion
• Define antidiuretic hormone (ADH/vasopressin) and explain its role in regulating urine output
• Describe how the kidney adjusts urine volume and concentration in response to hydration status
• Identify urine color and volume as visible signals of hydration

Key Terms

Two Small Organs, an Enormous Job

You have two kidneys. Each is about the size and shape of a clenched fist, sitting at the back of your abdomen at roughly the level of your lowest ribs, one on each side of your spine. They are heavily protected — by the ribs above them, by a thick layer of fat around each one, and by muscle behind them. They are deeply unspectacular to look at and quite spectacular to examine [24].

A short list of what your kidneys do, every minute of every day:

• Filter your entire blood supply about thirty times per day.
• Adjust the amount of water in your body to within about one percent of where it should be.
• Adjust the amount of sodium, potassium, chloride, calcium, and several other dissolved substances to similarly tight precision.
• Excrete the waste products of metabolism — urea from protein breakdown, creatinine from muscle activity, dozens of other compounds.
• Regulate the acidity of your blood within a tiny range.
• Help control your blood pressure.
• Produce hormones that signal bone marrow to make red blood cells.
• Activate vitamin D into its usable form.

This is one organ system, and it has more responsibilities than the lungs and heart combined. When kidneys fail, every other system in the body begins to fail with them. Dialysis — the medical procedure that filters blood when kidneys cannot — runs blood through an artificial filter for many hours a week and still does not replicate every kidney function. The kidneys are doing things that are hard for medicine to replace.

This lesson focuses on one of those jobs: the regulation of body water.

The Nephron: A Filter and a Recovery System

Inside each kidney are about one million microscopic structures called nephrons. The nephron is the functional unit of the kidney — the working part. Each nephron is a tiny tube, several centimeters long when stretched out, twisted into a specific shape and threaded with blood vessels [24]. If you laid out all the nephrons in your two kidneys end to end, they would stretch about 50 to 70 miles.

The nephron has three jobs, performed at three different parts of its length [25]:

1. Filtration. At one end of the nephron is the glomerulus — a tight knot of capillaries where blood enters under pressure. The capillary walls are leaky in a controlled way; water and small dissolved particles (salts, glucose, urea, amino acids) are pushed through the wall and into the nephron tubule. Larger structures (red blood cells, proteins) stay in the blood. The pushed-through fluid is called filtrate — essentially blood plasma minus the large molecules. Your kidneys produce about 180 liters of filtrate per day. Most of it does not become urine.

2. Reabsorption. As the filtrate travels down the tubule, the nephron actively pulls back the things the body wants to keep. Glucose, amino acids, and most of the salts are reabsorbed into the surrounding blood vessels. Water follows the salts by osmosis. By the time the filtrate has traveled most of the length of the nephron, more than 99 percent of the water has been reabsorbed. What remains is a small volume of fluid — about 1 to 2 liters per day — that becomes urine.

3. Secretion. Some substances are too important to be left to filtration alone, so the nephron actively pumps them from the blood into the tubule along the way. This is how the kidney fine-tunes the elimination of acids, potassium, and certain medications.

The result of all three processes is urine: a concentrated solution of waste products dissolved in a small volume of water. The composition of urine — how much water it contains, how concentrated the salts are, how acidic it is — is the kidney's main way of adjusting the body's internal environment. Every change you make to your body's water balance shows up, almost immediately, in your urine.

ADH: The Water Knob

The kidney does not decide on its own how much water to reabsorb. It takes orders from the brain.

The hypothalamus, deep in the brain, is constantly monitoring the dissolved-particle concentration of the blood — the osmolality. If the osmolality rises (the blood is getting too concentrated, which happens when you are losing water or eating salty food), the hypothalamus signals the pituitary gland to release a hormone called antidiuretic hormone (ADH), also known as vasopressin [26].

ADH travels through the blood to the kidney. There, it changes the wall of the final part of the nephron — the collecting duct — to make it more permeable to water. With more ADH present, more water is reabsorbed back into the body. The urine that remains is smaller in volume and more concentrated. Your body has held onto water.

If the osmolality falls (the blood is too dilute, which happens when you have been drinking a lot of fluid), the hypothalamus signals less ADH. The collecting duct becomes less permeable to water, more water is left in the urine, and you produce a larger volume of pale urine. Your body has let extra water go.

This is a feedback loop that runs continuously. Every minute of every day, the hypothalamus is reading the blood, and the pituitary is adjusting the ADH signal, and the kidney is adjusting how much water to keep. The result is that the dissolved-particle concentration of your blood stays within an astonishingly narrow range — typically 285 to 295 milliosmoles per kilogram — under almost any circumstances. Hike a desert. Run a marathon. Drink a quart of water. Eat a salty meal. The hypothalamus, pituitary, and kidney adjust. The blood stays close to where it should be [26].

Diuretics: Things That Make You Pee

The opposite of ADH's water-conserving action is called diuresis — increased urine output. Substances or conditions that cause increased urine output are called diuretics [27]. Caffeine in coffee, tea, and many sodas is a mild diuretic. Alcohol is a moderate diuretic and actually inhibits ADH release directly, which is part of why dehydration follows heavy drinking. Some prescription medications are intentionally strong diuretics, used to manage blood pressure or fluid buildup in specific medical conditions.

Diuresis is not always a problem. After a large drink of water, your body will produce more urine to excrete the excess, which is appropriate. After several cups of coffee in the morning, you may notice you visit the bathroom more often, but research suggests that for habitual caffeine users, the diuretic effect of caffeine is modest and the net hydration effect of caffeinated drinks is mostly positive [28]. Coffee is not the dehydrator some folklore suggests.

The point is not to memorize a list of diuretics. The point is to understand the mechanism: anything that suppresses ADH, or that adds to the load of dissolved particles your kidney has to excrete, will tend to increase urine output. Hydration is a balance, and the inputs and outputs are both adjustable.

What Your Urine Is Telling You

Your urine is a visible, real-time signal of how the system is doing.

• Color. Urine color comes from a pigment called urochrome, which is the breakdown product of old red blood cells. Urochrome is produced at a roughly constant rate. The color of your urine therefore depends mostly on how much it is diluted by water. Pale yellow (about the color of straw or lemonade) suggests adequate hydration. Darker yellow or amber suggests more concentrated urine — the kidney is conserving water. Very dark or brown urine, especially with other symptoms, warrants a conversation with a healthcare provider. Very clear urine, repeatedly, may indicate overhydration [29].
• Volume. A typical adolescent produces about a liter to a liter and a half of urine per day, spread across four to seven bathroom visits. Much less or much more than this can be normal in response to fluid intake; consistent extremes outside this range are worth noticing.
• Frequency. Most people urinate every two to four hours during the day. Rarely needing to urinate, or urinating only at very long intervals, often correlates with low fluid intake.

Some things shift urine color away from this baseline. Beets can turn urine pink or red. B-vitamin supplements can turn urine bright fluorescent yellow. Asparagus can give urine a distinctive smell. Some medications change color or scent. Most of these effects are temporary and harmless.

The Elephant suggests you simply notice. Not measure. Not track. Just glance, when you are at the sink. The body is sending a small, free, visible signal every few hours, and most humans never look at it.

Lesson Check

1. What is a nephron, and roughly how many are in each kidney?
2. Describe what happens at each of the three main parts of the nephron (filtration, reabsorption, secretion).
3. What does ADH do, and where is it released?
4. Why does alcohol tend to cause dehydration?
5. What does urine color tell you about hydration status?

---

Lesson 1.4: Electrolytes and Thirst

Learning Objectives

By the end of this lesson, you will be able to:

• Define electrolytes and identify the five most important electrolytes in the human body: sodium, potassium, chloride, magnesium, and calcium
• Describe the general role of each major electrolyte
• Identify the osmoreceptors in the hypothalamus and explain how they detect changes in body water balance
• Distinguish between osmotic thirst (triggered by changes in dissolved-particle concentration) and hypovolemic thirst (triggered by changes in blood volume)
• Identify food sources of each major electrolyte without prescribing intake amounts

Key Terms

The Salts That Run You

Water by itself does not conduct electricity well. Pure distilled water is actually a fairly poor conductor. But water with salts dissolved in it conducts electricity quite well — well enough to be a hazard around electrical equipment, and well enough for your nervous system to run on it [30].

The salts dissolved in your body fluids are called electrolytes because, when they dissolve in water, they separate into charged particles called ions. Sodium chloride (table salt, NaCl) separates into sodium ions (Na⁺) and chloride ions (Cl⁻). Potassium chloride (KCl) separates into K⁺ and Cl⁻. Calcium carbonate (CaCO₃) separates into Ca²⁺ and CO₃²⁻. Each of these charged particles can move under the influence of an electric field. Each of them can be pumped through specialized channels in cell membranes. Each of them is part of the machinery that allows your body to do work.

Five electrolytes do most of the heavy lifting in human biology.

Sodium (Na⁺) is the most abundant cation in your extracellular fluid. Most of the dissolved-particle concentration of your blood and interstitial fluid is sodium and the chloride ion that usually travels with it. Sodium plays a central role in nerve impulse transmission, in the regulation of blood pressure, and in moving water around the body. Research has observed that very low sodium states — usually caused by excessive water intake that dilutes the sodium, or by certain illnesses — can cause confusion, headache, seizures, and in severe cases coma [31]. Sodium is found abundantly in nearly all processed and prepared foods, and is added to most home-cooked dishes as table salt.

Potassium (K⁺) is the most abundant cation inside your cells. While extracellular sodium is high and intracellular sodium is low, the inverse is true of potassium — high inside, low outside. The difference is maintained by a small protein pump in every cell membrane called the sodium-potassium pump, which uses energy to push sodium out of cells and pull potassium in. Every cell in your body runs this pump every second. This pump is, by some estimates, responsible for nearly a third of the total energy your body burns at rest — just to maintain the sodium and potassium imbalance across cell membranes [32]. Potassium is found in fruits, vegetables, legumes, animal proteins, and dairy. Bananas, potatoes, avocados, beans, salmon, and yogurt are all good sources.

Chloride (Cl⁻) is the major extracellular anion, usually paired with sodium. It is essential for fluid balance, for the production of stomach acid (hydrochloric acid, HCl), and for the function of red blood cells. Chloride enters the body almost entirely through dietary salt [33].

Magnesium (Mg²⁺) is mostly inside cells, where it is essential for the function of hundreds of enzymes — including every enzyme that uses ATP, the body's energy currency. Magnesium is essential for muscle relaxation (the lack of magnesium contributes to muscle cramps), nerve transmission, and the regulation of heart rhythm. Magnesium is found in green leafy vegetables, nuts, seeds, whole grains, legumes, and dark chocolate [34].

Calcium (Ca²⁺) is the most abundant mineral in your body, by mass — but most of it is locked into bones and teeth. The small fraction of calcium that is dissolved in body fluids is essential for muscle contraction, nerve signaling, blood clotting, and many cellular processes. Calcium is tightly regulated by several hormones because both very low and very high blood calcium are dangerous. Dietary sources include dairy products, sardines and salmon with bones, leafy greens, sesame seeds, and fortified plant milks [35].

These five — sodium, potassium, chloride, magnesium, and calcium — are the main electrolytes a healthy diet supplies. There are others (phosphate, bicarbonate, sulfate) that the kidney also regulates carefully, but the five above are the ones most relevant to the Coach Water domain.

Where Thirst Comes From

You feel thirsty. Why?

The answer is more interesting than "your body needs water." Your body has at least two distinct systems for triggering thirst, and they respond to two different kinds of signals [36].

The first system is osmotic thirst, and it begins with cells called osmoreceptors in the hypothalamus. These specialized neurons are sensitive to the dissolved-particle concentration of the blood that flows past them. When the blood becomes more concentrated — when you have lost water through sweat or breath, or when you have just eaten a salty meal — the osmoreceptors detect the change. They are physically sensitive to it: when the surrounding fluid is too concentrated, water leaves the osmoreceptors (by osmosis) and the cells shrink slightly. The shrinkage triggers them to fire. They send two simultaneous signals: one to the pituitary to release ADH (so the kidney holds onto water), and one to the conscious-experience part of the brain to generate the sensation of thirst.

Osmotic thirst is sensitive. Your osmoreceptors can detect a change of about one to two percent in the dissolved-particle concentration of the blood [36]. That is a tiny change. By the time you consciously notice you are thirsty, this system has already been signaling for some time.

The second system is hypovolemic thirst — thirst triggered by a drop in blood volume rather than a change in concentration. If you lose a lot of blood, or a lot of fluid through severe sweating, vomiting, or diarrhea, the volume of fluid in your blood vessels falls. Pressure sensors in your blood vessels (called baroreceptors) detect the drop and signal the brain. The kidney also detects low blood pressure and releases an enzyme called renin, which sets off a cascade that produces a hormone called angiotensin II. Angiotensin II directly triggers thirst in the brain [37]. This system is for emergencies — large blood loss, severe dehydration. Most everyday thirst is osmotic, not hypovolemic.

The point of these two systems together is that your body is paying attention to two different things: how salty your blood is, and how much blood you have. Both matter. Both can change. The brain has a different alarm for each.

Thirst Is a Signal, Not a Schedule

This is where the Elephant becomes patient.

There are competing ideas in modern wellness culture about how to handle thirst. One says: "Drink before you are thirsty. By the time you are thirsty, you are already dehydrated." Another says: "Drink to thirst. Your body knows." The truth, as best as research has been able to determine, is closer to the second — but with some careful qualifications [38].

For most people, in most conditions, thirst is a reliable signal. The osmoreceptor system is sensitive, fast, and tuned by evolution to a high standard. If you wait until you feel thirsty and then drink, you will rehydrate quickly and your body will excrete any excess in urine within a few hours.

There are conditions where thirst is less reliable. Older adults have a blunted thirst response, and may not feel thirsty even when their bodies need water — which is part of why older adults are at higher risk of dehydration. Some illnesses interfere with thirst. Heavy endurance exercise in heat can produce sweat losses that outpace the thirst response in the short term, requiring deliberate replacement. And during periods of intense concentration — long study sessions, gaming, sport — many people simply tune the thirst signal out and only notice they were thirsty when they finally look up.

The Elephant's posture on this: notice the signal. Trust the signal in normal conditions. Pay extra attention in heat, in hard exercise, in long hours of focused work, and around alcohol. Do not force water in past the body's signal — your kidney can handle some excess, but persistent water intake well beyond thirst (especially without electrolyte intake) carries real risk that Grade 10 will return to. Your body has been keeping water balance for a very long time. Cooperate with it.

Lesson Check

1. Name the five major electrolytes in human biology.
2. Which electrolyte is most abundant in extracellular fluid? Which is most abundant inside cells?
3. What does the sodium-potassium pump do, and why does it use so much energy?
4. What are osmoreceptors, and what do they detect?
5. Why is thirst generally a reliable signal, and in what situations might it be less reliable?

---

End-of-Chapter Activity: The 24-Hour Water Trace

For one full day, observe your own water in and water out without changing your behavior. This activity is about noticing, not optimizing.

Materials

• Notebook or simple printed log sheet
• A pen
• A measuring cup or marked water bottle (optional, for rough estimates)

Procedure

1. At the start of the day, write down the date and your weight (if a scale is available). Note your urine color first thing in the morning — many people produce the most concentrated urine of the day right after waking, because no water has come in for eight hours.

2. Throughout the day, jot down a short note each time you:

   • Drink anything (water, milk, tea, juice, soda, sports drink, broth). Estimate the amount (a glass, a bottle, an ounce or a milliliter — rough is fine).
   • Eat a meal or snack that is high in water (soup, fruit, vegetables, yogurt). You do not need to estimate amounts — just note "high-water snack."
   • Use the bathroom. Note urine color (pale, medium yellow, dark yellow, amber). Do not measure volume.
   • Sweat noticeably (exercise, hot weather). Note duration and intensity.

3. At the end of the day, look at your log. Answer these questions in 2-3 sentences each:

   • Approximately how much fluid did you drink (in cups, bottles, or volume units)?
   • What was the dominant urine color across the day? Did it shift?
   • Were there long periods (more than four hours) without any fluid intake?
   • Did your thirst align with your behavior — did you notice thirst before drinking, or did you mostly drink out of habit?

4. Reflection (one paragraph). Write 4-6 sentences about what you noticed. Were any patterns surprising? What does your day look like in the context of what this chapter has taught you about water in and water out? Do not propose a new plan. The Elephant is patient. Just notice.

Submission

Turn in:

• Your one-day log sheet
• Your end-of-day answers
• Your reflection paragraph

Total writing: approximately 200-300 words.

---

Vocabulary Review

---

Chapter Quiz

Multiple Choice (10 questions, 2 points each)

1. A water molecule is made of which atoms? A. One hydrogen and two oxygen B. Two hydrogen and one oxygen C. Two hydrogen and two oxygen D. One hydrogen and one oxygen

2. Water is called a polar molecule because: A. It has only one charged atom B. It freezes at high temperatures C. Its electrical charges are unevenly distributed, with a slightly negative end and a slightly positive end D. It is found at the polar regions of the Earth

3. Which of the following is an unusual property of water that hydrogen bonding helps explain? A. Water expands when it freezes B. Water dissolves more substances than any other common liquid C. Water has a high specific heat capacity D. All of the above

4. Approximately what percentage of an adolescent's body mass is water? A. 20-30% B. 50-65% C. 80-90% D. Almost 100%

5. Which body water compartment is the largest? A. Blood plasma B. Interstitial fluid C. Intracellular fluid D. Cerebrospinal fluid

6. In osmosis, water moves from regions of: A. Higher to lower dissolved-particle concentration B. Lower to higher dissolved-particle concentration C. Cooler to warmer temperature D. Higher to lower oxygen content

7. What is the functional unit of the kidney? A. The glomerulus B. The bladder C. The nephron D. The collecting duct

8. Antidiuretic hormone (ADH) causes the kidney to: A. Reabsorb more water, producing less and more concentrated urine B. Reabsorb less water, producing more and more dilute urine C. Stop producing urine entirely D. Filter blood faster

9. Which electrolyte is the major intracellular cation? A. Sodium B. Potassium C. Chloride D. Calcium

10. Osmoreceptors in the hypothalamus detect changes in: A. Blood temperature B. Blood oxygen level C. The dissolved-particle concentration (osmolality) of the blood D. Heart rate

Short Answer (5 questions, 4 points each)

11. Explain why water expands when it freezes and identify one biological or ecological consequence of this fact.

12. Describe the three main processes of the nephron (filtration, reabsorption, and secretion) and identify approximately how much filtrate the kidney produces per day versus how much urine.

13. Identify the five major electrolytes and give one main role for each.

14. A friend insists that they "shouldn't wait to be thirsty" before drinking. Based on what you have learned in this chapter, write a 3-4 sentence response that engages with both the value and the limits of thirst as a signal.

15. Compare osmotic thirst and hypovolemic thirst. What does each system detect, and which is responsible for most everyday thirst?

---

Teacher's Guide

Pacing Recommendations

This chapter is designed for 8-10 class periods. Suggested distribution:

If shorter pacing is required, Lessons 1.3 and 1.4 can be combined into a single 2-period unit, with the end-of-chapter activity assigned as homework over a weekend.

Lesson Check Answers

Lesson 1.1:

1. Two hydrogen atoms and one oxygen atom (H₂O). 2. Because the electrical charges are unevenly distributed — the oxygen end is slightly negative, the hydrogen end is slightly positive. 3. Any two of: ice floats because water expands when it freezes; high surface tension; high specific heat capacity; water as a universal solvent; water climbs against gravity in plants. 4. Specific heat capacity is the energy required to raise a substance's temperature by one degree. Water's high specific heat capacity means oceans moderate climate and bodies can dump heat into surrounding water without overheating immediately. 5. Because it dissolves more substances than any other common liquid; nearly every chemical reaction in living things happens in water.

Lesson 1.2:

1. Roughly 50-65% for adolescents; newborns are 70-75%; older adults often drop to 50% or below. 2. Intracellular fluid (about two-thirds of total body water). 3. High-water tissues include blood, brain, muscle, kidney, lung, liver; low-water tissues include bone and adipose (fat) tissue. 4. Osmosis is the movement of water across a partially permeable membrane from a region of lower dissolved-particle concentration to a region of higher dissolved-particle concentration. 5. Urine, sweat, breath, stool, and insensible loss through skin.

Lesson 1.3:

1. A nephron is the microscopic functional unit of the kidney; each kidney has about one million. 2. Filtration: blood is filtered under pressure at the glomerulus, sending water and small solutes into the nephron. Reabsorption: the nephron pulls back water, glucose, amino acids, and most salts into the blood. Secretion: the nephron actively pumps certain substances from the blood into the tubule for excretion. 3. ADH signals the kidney to reabsorb more water, producing less but more concentrated urine; it is released from the pituitary gland. 4. Alcohol inhibits ADH release, so the kidney produces more dilute urine and the body loses more water. 5. Pale yellow suggests adequate hydration; dark yellow or amber suggests the kidney is conserving water; very clear repeatedly may suggest overhydration.

Lesson 1.4:

1. Sodium, potassium, chloride, magnesium, calcium. 2. Sodium is most abundant in extracellular fluid; potassium is most abundant inside cells. 3. The sodium-potassium pump moves sodium out of cells and potassium into cells; it uses energy because it pumps ions against their concentration gradient, and it accounts for a large fraction of resting energy use. 4. Osmoreceptors are specialized neurons in the hypothalamus that detect changes in the dissolved-particle concentration (osmolality) of the blood. 5. Thirst is generally reliable because the osmoreceptor system is sensitive (detects ~1-2% changes in osmolality); it can be less reliable in older adults, certain illnesses, heavy endurance exercise in heat, and during long periods of intense focus.

Quiz Answer Key

Multiple Choice: 1.B 2.C 3.D 4.B 5.C 6.B 7.C 8.A 9.B 10.C

Short Answer (sample target responses):

1. Water expands when freezing because the hydrogen bonds lock into a regular hexagonal pattern in ice that is less dense than the random arrangement in liquid water. Consequence: ice floats; lakes freeze top-down rather than bottom-up; aquatic life survives winter in the unfrozen water below.

2. Filtration at the glomerulus pushes water and small solutes from blood into the nephron tubule (~180 L/day of filtrate); reabsorption pulls back most of this water and the substances the body wants to keep; secretion pumps additional waste into the tubule from the blood. The kidney produces about 180 L of filtrate per day but only about 1-2 L of urine, because more than 99% of the water is reabsorbed.

3. Sodium — major extracellular cation, sets blood osmotic balance and nerve transmission. Potassium — major intracellular cation, nerve and muscle function. Chloride — major extracellular anion, fluid balance and stomach acid. Magnesium — intracellular, essential for ATP-using enzymes and muscle relaxation. Calcium — bone structure plus blood fraction for muscle, nerve, and clotting.

4. The friend has a partial point: thirst is a real signal and is sensitive (osmoreceptors detect ~1-2% changes), and by the time you feel thirsty the system has been signaling for a little while. However, for most healthy people in ordinary conditions, drinking to thirst is reliable, and the kidney will excrete any excess water you drink in advance. The places thirst is less reliable are heavy exercise in heat, older age, illness, and long stretches of focused work — those are where deliberate drinking can help.

5. Osmotic thirst detects changes in the dissolved-particle concentration of the blood (osmolality); osmoreceptors in the hypothalamus shrink slightly when the blood is too concentrated, triggering thirst and ADH release. Hypovolemic thirst detects changes in blood volume; baroreceptors and the kidney's renin-angiotensin system respond to falls in blood pressure. Most everyday thirst is osmotic.

Discussion Prompts

1. What surprised you most about water's properties as a chemical substance? Why?
2. The Elephant's posture is that thirst is generally a reliable signal. Where in modern life — school, sport, screens, social pressure — might that signal be tuned out?
3. Older adults have a blunted thirst response. What does that mean for someone caring for a grandparent or elderly relative?
4. Why do you think water is sometimes called "the molecule of life"? Could life as we know it exist with a different solvent?
5. The kidney does an enormous amount of work that goes completely unnoticed. What other systems in your body work hardest when you are not paying attention?
6. The chapter described two thirst systems — osmotic and hypovolemic. What might be the advantage of having two different ways to detect a need for fluid?
7. The Elephant has lived alongside water for millions of years. What might modern humans relearn from animals that have a long, patient relationship with water sources?
8. How does what you have learned in this chapter change (or not change) how you think about a glass of water?

Common Student Questions

• "How much water should I drink each day?" This chapter does not give a number, and the curriculum doesn't prescribe one. The Elephant's answer: drink to thirst, notice urine color, pay extra attention in heat and during hard exercise. Specific intake numbers vary widely by body size, climate, activity, and diet, and any blanket recommendation will fit poorly for some people. Grade 10 will dig deeper.
• "Is sparkling water as hydrating as still water?" Yes. The carbonation does not change the water content meaningfully.
• "Does coffee count toward hydration?" For habitual caffeine users, yes. Research suggests the diuretic effect of coffee is modest, and coffee provides net positive fluid for most regular drinkers.
• "Is drinking too much water dangerous?" Yes, in some situations. The next chapter (Grade 10) will discuss hyponatremia — a dangerous condition that can result when water intake overwhelms the kidney's ability to excrete the excess, especially in endurance athletes drinking only plain water in heat.
• "Why is my urine bright yellow after I take a multivitamin?" Most multivitamins contain B vitamins, which color urine bright fluorescent yellow when excess is excreted. This is harmless.
• "My grandfather doesn't drink much water. Should I be worried?" Older adults have a blunted thirst response, so they often do not feel thirsty even when their bodies could use more water. A respectful conversation with the grandfather and his healthcare provider would be appropriate.

Parent Communication Template

Dear Parents,

This week, your student begins the Coach Water chapter of the CryoCove Library curriculum. The Coach Water domain is taught by the Elephant — an animal with one of the longest and most patient relationships with water on Earth.

The Grade 9 chapter covers the chemistry of water (why this small molecule is so unusual), the water in the body (how much you carry and where), the kidneys (the underappreciated organs that filter your blood thirty times a day), and the science of thirst (the surprisingly sensitive systems that tell your brain when to drink).

The chapter is descriptive — it teaches what water does in the body without prescribing how much your student should drink, and without making any product or supplement recommendations. The end-of-chapter activity asks your student to observe their own water intake for one day without changing their behavior, then reflect on what they noticed.

If you have questions, please reach out to your student's teacher. We are glad to walk through the materials with you.

Warmly, The CryoCove Curriculum Team

---

Illustration Briefs

Lesson 1.1 — A Small, Strange Molecule Placement: After the first paragraph describing the bent shape of the water molecule. Scene: Coach Water (Elephant) standing knee-deep at the edge of a calm watering hole at golden hour, one ear tilted back, trunk curled in the water. Above the Elephant, an educational overlay shows a magnified single water molecule — bent V shape, navy oxygen with "δ−", cyan hydrogens with "δ+", faint dashed hydrogen bonds reaching to neighbors. Mood: patient, ancient, scientifically curious. Aspect ratio: 16:9 web, 4:3 print.

Lesson 1.2 — Body Water Compartments Placement: After the description of intracellular vs. extracellular fluid. Scene: A clean cross-section diagram of a gender-neutral human figure in navy/cyan, divided into fluid compartments — ICF (deep cyan, ~two-thirds), interstitial fluid (lighter cyan, ~25%), blood plasma (coral, traced through major vessels). Coach Water (Elephant) stands to the side gesturing toward the diagram with the trunk. Mood: warm, instructional, patient. Aspect ratio: 16:9 web, 4:3 print.

Lesson 1.3 — The Nephron (optional supplementary illustration) Placement: After the description of the three nephron processes. Scene: A schematic of a single nephron, simplified into three labeled zones — the glomerulus (filtration), the convoluted tubule and loop of Henle (reabsorption), and the collecting duct (final water adjustment under ADH control). Arrows show blood entering, filtrate forming, water being reabsorbed back to the bloodstream, urine leaving. Color-coded: blood in coral, filtrate in cyan, urine in pale yellow. Mood: clear, educational, like a quality biology textbook diagram. Aspect ratio: 4:3 print, 16:9 web.

Lesson 1.4 — The Five Electrolytes Placement: After the descriptions of the five major electrolytes. Scene: Five circular icons in a horizontal row — Sodium (Na⁺, coral, salt shaker), Potassium (K⁺, cyan, banana and potato), Chloride (Cl⁻, navy, salt crystal), Magnesium (Mg²⁺, coral, greens and almonds), Calcium (Ca²⁺, cyan, fish and milk). The Elephant's trunk extends across the row above the icons, as if gathering them. Mood: clear, friendly, educational. Aspect ratio: 16:9 web, 4:3 print.

---

Citations

1. Jéquier, E., & Constant, F. (2010). Water as an essential nutrient: the physiological basis of hydration. European Journal of Clinical Nutrition, 64(2), 115-123.

2. Ball, P. (2008). Water as an active constituent in cell biology. Chemical Reviews, 108(1), 74-108.

3. McKay, C. P. (2014). Requirements and limits for life in the context of exoplanets. Proceedings of the National Academy of Sciences, 111(35), 12628-12633.

4. Brini, E., Fennell, C. J., Fernandez-Serra, M., Hribar-Lee, B., Lukšič, M., & Dill, K. A. (2017). How water's properties are encoded in its molecular structure and energies. Chemical Reviews, 117(19), 12385-12414.

5. Bagchi, B. (2013). Water in Biological and Chemical Processes: From Structure and Dynamics to Function. Cambridge University Press.

6. Chaplin, M. (2006). Do we underestimate the importance of water in cell biology? Nature Reviews Molecular Cell Biology, 7(11), 861-866.

7. Petrenko, V. F., & Whitworth, R. W. (1999). Physics of Ice. Oxford University Press.

8. Niven, J. (2012). Pollen presented by water striders. Journal of Experimental Biology, 215, 4079.

9. Tyree, M. T., & Zimmermann, M. H. (2002). Xylem Structure and the Ascent of Sap (2nd ed.). Springer.

10. Maughan, R. J., & Shirreffs, S. M. (2010). Dehydration and rehydration in competative sport. Scandinavian Journal of Medicine & Science in Sports, 20(s3), 40-47.

11. Marcus, Y. (2009). Effect of ions on the structure of water: structure making and breaking. Chemical Reviews, 109(3), 1346-1370.

12. Levitt, D. G., & Levitt, M. D. (2017). A model of blood-ammonia homeostasis based on a quantitative analysis of nitrogen metabolism in the multiple organs involved in the production, catabolism, and excretion of ammonia in humans. Clinical and Experimental Gastroenterology, 10, 27.

13. Watson, P. E., Watson, I. D., & Batt, R. D. (1980). Total body water volumes for adult males and females estimated from simple anthropometric measurements. American Journal of Clinical Nutrition, 33(1), 27-39.

14. Ritz, P., Vol, S., Berrut, G., Tack, I., Arnaud, M. J., & Tichet, J. (2008). Influence of gender and body composition on hydration and body water spaces. Clinical Nutrition, 27(5), 740-746.

15. Chumlea, W. C., Guo, S. S., Zeller, C. M., Reo, N. V., & Siervogel, R. M. (1999). Total body water reference values and prediction equations for adults. Kidney International, 56(1), 244-252.

16. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Bretscher, A., Ploegh, H., Amon, A., & Martin, K. C. (2016). Molecular Cell Biology (8th ed.). W. H. Freeman.

17. Boron, W. F., & Boulpaep, E. L. (2017). Medical Physiology (3rd ed.). Elsevier.

18. Hall, J. E., & Hall, M. E. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.

19. Wang, Z., Pi-Sunyer, F. X., Kotler, D. P., Wielopolski, L., Withers, R. T., Pierson, R. N., Jr., & Heymsfield, S. B. (2002). Hydration of fat-free body mass: review and critique of a classic body-composition constant. American Journal of Clinical Nutrition, 76(5), 911-915.

20. Verkman, A. S. (2012). Aquaporins in clinical medicine. Annual Review of Medicine, 63, 303-316.

21. Verbalis, J. G. (2003). Disorders of body water homeostasis. Best Practice & Research Clinical Endocrinology & Metabolism, 17(4), 471-503.

22. EFSA Panel on Dietetic Products, Nutrition, and Allergies. (2010). Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 8(3), 1459.

23. Popkin, B. M., D'Anci, K. E., & Rosenberg, I. H. (2010). Water, hydration, and health. Nutrition Reviews, 68(8), 439-458.

24. Sands, J. M., & Layton, H. E. (2009). The physiology of urinary concentration: an update. Seminars in Nephrology, 29(3), 178-195.

25. Eaton, D. C., & Pooler, J. P. (2018). Vander's Renal Physiology (9th ed.). McGraw-Hill.

26. Robertson, G. L. (2016). Diabetes insipidus: differential diagnosis and management. Best Practice & Research Clinical Endocrinology & Metabolism, 30(2), 205-218.

27. Armstrong, L. E., Casa, D. J., Maresh, C. M., & Ganio, M. S. (2007). Caffeine, fluid-electrolyte balance, temperature regulation, and exercise-heat tolerance. Exercise and Sport Sciences Reviews, 35(3), 135-140.

28. Killer, S. C., Blannin, A. K., & Jeukendrup, A. E. (2014). No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS ONE, 9(1), e84154.

29. Kavouras, S. A. (2002). Assessing hydration status. Current Opinion in Clinical Nutrition and Metabolic Care, 5(5), 519-524.

30. Light, T. S., Licht, S., Bevilacqua, A. C., & Morash, K. R. (2005). The fundamental conductivity and resistivity of water. Electrochemical and Solid-State Letters, 8(1), E16-E19.

31. Adrogué, H. J., & Madias, N. E. (2000). Hyponatremia. New England Journal of Medicine, 342(21), 1581-1589.

32. Rolfe, D. F. S., & Brown, G. C. (1997). Cellular energy utilization and molecular origin of standard metabolic rate in mammals. Physiological Reviews, 77(3), 731-758.

33. Berend, K., van Hulsteijn, L. H., & Gans, R. O. B. (2012). Chloride: the queen of electrolytes? European Journal of Internal Medicine, 23(3), 203-211.

34. Volpe, S. L. (2013). Magnesium in disease prevention and overall health. Advances in Nutrition, 4(3), 378S-383S.

35. Weaver, C. M., & Heaney, R. P. (2006). Calcium in Human Health. Humana Press.

36. McKinley, M. J., & Johnson, A. K. (2004). The physiological regulation of thirst and fluid intake. News in Physiological Sciences, 19(1), 1-6.

37. Fitzsimons, J. T. (1998). Angiotensin, thirst, and sodium appetite. Physiological Reviews, 78(3), 583-686.

38. Armstrong, L. E., & Johnson, E. C. (2018). Water intake, water balance, and the elusive daily water requirement. Nutrients, 10(12), 1928.