Anatomy & Physiology · Chapter 26

Fluid, Electrolyte & Acid-Base Balance

You are, quite truly, mostly water. Staying alive is largely about keeping three things in very narrow ranges. These are that water, the salts dissolved in it, and how acidic it is. This chapter follows the fluid. First you will see the spaces, or compartments, where it lives. Then you will see the daily balance of what comes in and what goes out. Next come the electrolytes, the charged particles that float in the fluid. Then comes pH, a measure of how acidic a fluid is. Every enzyme in your body depends on it.

You will meet the buffers that catch stray acid in an instant. You will see the lungs and kidneys finish the job over minutes and days. You will also meet the four classic acid-base disorders that show up when the whole system slips. By the end, you will be able to read a set of blood numbers. You will be able to say plainly what has gone wrong and how the body is fighting back.

By the end of this chapter you can

Name the body's fluid compartments. Estimate how much of the body's water is in each. Describe how their ions differ.
Balance a day's water intake against output. Name the ways water comes in and goes out, including insensible loss (water you lose without noticing).
Explain how thirst and antidiuretic hormone keep total body water steady.
List the main electrolytes. Say where each is found, what it does, and which hormone controls it.
Write the bicarbonate buffer equation. Explain how a buffer fights a change in pH.
Rank the body's three lines of acid-base defense by speed. Describe how each one works.
Sort the four acid-base disorders by cause and direction. Predict how the body makes up for each.
Read a simple set of blood gas numbers and name the acid-base problem.

Section 1The fluid compartments

The spaces in the body where fluid is found — **Reference.** The body's fluid compartments — OpenStax, CC BY 3.0 (via Wikimedia Commons)

How much of you is water

In a healthy adult, about 60% of body mass is water. That is about 42 liters in a person who weighs 70 kilograms. The number is not fixed. It is higher in babies, near 75%. It is lower in older adults.

Fat tissue holds very little water. So a leaner body is a wetter body. On average, women carry a bit more fat for their size than men. So their water share is a few points lower. This total body water, all the water in your body, is not one big puddle. It is split into compartments. Living membranes separate them. Fluid balance is really about how water and dissolved substances move between these compartments.

Two great pools, and a smaller subdivision

The first and biggest split is made by the cell membrane. About two-thirds of your water is inside your cells. This is the intracellular fluid, or ICF. (Intra means "inside.") The other one-third is outside the cells. This is the extracellular fluid, or ECF. (Extra means "outside.")

The walls of the blood vessels split the ECF again. Most of it bathes the tissues directly. This is interstitial fluid, the water in the gaps between cells. A smaller share is the plasma, the fluid part of blood inside the vessels. A tiny extra part is the transcellular fluid. It includes special pools like the fluid around the brain and spinal cord, joint fluid, and the fluid in the front of the eye.

Compartment	Share of total body water	Where it is
Intracellular (ICF)	About 2/3 (about 40% of body weight)	Inside all the cells
Extracellular (ECF)	About 1/3 (about 20% of body weight)	Outside the cells. It is split into the two parts below.
· Interstitial	About 3/4 of the ECF	In the spaces between cells
· Plasma	About 1/4 of the ECF	The fluid part of blood, inside vessels
Transcellular	A small extra pool	CSF (brain and spinal fluid), joint fluid, eye fluid, gut juices

Same water, very different salt

What makes the compartments different is not the water. It is the solutes, the things dissolved in it. The ICF and ECF have very different mixes of ions. (Ions are particles with an electric charge.) The cell membrane and its pumps keep them apart.

The ECF is a salty sea, a lot like seawater. It is full of sodium (Na⁺) and chloride (Cl⁻). The ICF, on the other hand, is rich in potassium (K⁺), magnesium, phosphate and proteins with a negative charge. Every cell membrane has a sodium-potassium pump. It works around the clock to keep this difference. It pushes Na⁺ out and pulls K⁺ in. That stored difference works like a charged battery. It powers nerve signals, muscle squeezing and much of the traffic across membranes.

Water follows dissolved particles by osmosis. So the amount of dissolved particles in each compartment decides where the water goes. This amount is called osmolarity. Add salt to the ECF, and it becomes more concentrated. It pulls water out of the cells, and they shrink. Make the ECF more watery, and water rushes in. Then the cells swell. Sodium is the most common particle in the ECF. That is why it is the single biggest lever on fluid movement in the body.

The 60–40–20 rule. Water is about 60% of body weight. Of that, about 40% of body weight is inside cells, and about 20% is outside them. Fat has very little water. So the leaner the person, the higher the percentage. This is useful to remember. A very lean athlete and a heavier patient of the same weight can hold very different amounts of water.

Tap each compartment in the interactive below to see where the water lives.

Interactive About 60% of your body is water. It is split into compartments separated by cell membranes. Tap each one to see where it lives. Open full ↑

Key points

Water is about 60% of body mass. Leaner bodies and babies hold more. Older adults hold less.
Two-thirds of body water is intracellular, inside cells. One-third is extracellular, outside cells. The ECF splits into interstitial fluid and plasma.
ECF is high in Na⁺/Cl⁻. ICF is high in K⁺ and proteins. The sodium-potassium pump keeps this difference.
Water follows dissolved particles by osmosis. So ECF sodium is the main control on where fluid moves.

Section 2Water in, water out

Note. How much water comes in and goes out each day

A ledger that must balance

Total body water stays very steady from day to day. The reason is simple bookkeeping. Over any 24 hours, water in must equal water out. A typical adult moves about 2.5 liters through this "bank account" each day.

On the intake side, most comes from what you drink, about 1.5 L. A good amount comes from water hidden in food, about 0.7 L. Fruit, vegetables and even bread are wetter than you might think. A small but real amount is made inside you. Each time a cell burns sugar or fat for energy, water is a by-product. This metabolic water adds about 0.3 L a day, without you drinking a drop.

The four ways out

Water leaves by four routes. Only one of them is closely controlled. The kidneys make urine, about 1.5 L. It is the largest output and the only one the body tightly controls.

Much of the rest is insensible loss. This is water you lose without noticing. It dries off your skin all the time. It also leaves as vapor in every breath from your lungs. Together, these add up to about 0.9 L. A small amount leaves in the feces, or poop. More pours out as sweat when you are hot or working hard. Fever, heat, exercise, throwing up and diarrhea can all make these losses soar. That is why they can upset the balance so fast.

Direction	Route	Rough daily amount
In	Fluids you drink	About 1.5 L
In	Water in food	About 0.7 L
In	Metabolic water (made by your cells)	About 0.3 L
Out	Urine (controlled)	About 1.5 L
Out	Skin & lungs (insensible, not noticed)	About 0.9 L
Out	Feces	About 0.1 L

Thirst and the water-saving hormone

You choose when to drink, but much of your water loss cannot be stopped. So the body controls both ends. When you lose water, the ECF gets more concentrated. Its osmolarity goes up. Tiny sensors in the hypothalamus of the brain notice this. They are called osmoreceptors. They do two things.

First, they cause thirst. This drives you to drink, which raises intake. Second, they signal the back part of the pituitary gland to release antidiuretic hormone (ADH), also called vasopressin. ADH travels to the kidney. It tells the collecting ducts to put in water channels called aquaporins. Water flows back into the blood through them. The result is a small amount of concentrated urine.

Drink too much, and the reverse happens. Osmolarity falls, and ADH switches off. The kidney then lets a large amount of watery urine flow. Thirst pulls water in, and ADH works like a tap that holds water back. Together, they keep total body water within about one percent from one day to the next.

The kidney can concentrate, but it cannot make water. Even when you are badly dried out, the kidney must still pass some urine. This obligatory urine volume is about half a liter. It is needed just to flush out the day's wastes. That is why fluids can never be shut off fully. It is also why a person with no water keeps losing it. They lose it through breath, skin and this urine that cannot be avoided.

Tap each route in the interactive. See how the body keeps water in and water out equal.

Interactive Each day you take in about 2.5 L of water and lose the same amount. Tap each route to see how the body keeps in and out equal. Open full ↑

Key points

Over a day, intake ≈ output ≈ 2.5 L. Intake is drink + food + metabolic water.
Output is urine, the only tightly controlled route. It also includes insensible loss from skin and lungs, plus feces and sweat.
Thirst raises intake. ADH (vasopressin) lowers output by making the kidney take back water.
A minimum obligatory urine volume must always be passed to clear wastes.

Section 3The electrolytes

**Reference.** The main electrolytes and their jobs — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Charged particles that do the work

An electrolyte is any substance that breaks apart into charged ions when it dissolves. This lets a fluid carry an electric current. Electrolytes are the reason body fluids can carry current at all. They do much more than just float there. Their uneven spread drives every nerve impulse and every muscle contraction. It also drives every shift of water between compartments.

These jobs are very sensitive. So the body controls electrolyte levels tightly. It measures them in milliequivalents per liter (mEq/L). This unit counts electric charge, not plain weight. Ions with a positive charge are cations (Na⁺, K⁺, Ca²⁺, Mg²⁺). Ions with a negative charge are anions (Cl⁻, bicarbonate, phosphate).

The big four cations

Sodium (Na⁺) is the main cation of the ECF. It is the boss of fluid volume, because where sodium goes, water follows. Its normal level is 135–145 mEq/L. It is guarded mainly by aldosterone. This hormone from the adrenal gland tells the kidney to take back sodium.

Potassium (K⁺) is the main cation of the ICF. It sets the resting membrane potential, the small electric charge across the membrane of nerve and muscle cells at rest. Its normal range is narrow, 3.5–5.0 mEq/L. This matters a lot to the heart. Too much or too little potassium can stop it.

Calcium (Ca²⁺) is 99% stored in bone. It runs muscle contraction, blood clotting and nerve signals. Parathyroid hormone and vitamin D tune its blood level. Magnesium is a quieter partner. It helps hundreds of enzymes work. It also keeps nerves and muscles steady.

Electrolyte	Mainly in	Key job	Main regulator
Sodium (Na⁺)	ECF	ECF volume & osmolarity; nerve and muscle signals	Aldosterone, ADH, ANP
Potassium (K⁺)	ICF	Resting membrane potential; heart rhythm	Aldosterone
Calcium (Ca²⁺)	Bone / ECF	Muscle contraction, clotting, nerves	PTH, calcitriol (active vitamin D), calcitonin
Chloride (Cl⁻)	ECF	Follows sodium; balances charge; stomach acid	Follows how Na⁺ is handled
Bicarbonate (HCO₃⁻)	ECF	The body's main pH buffer	Kidneys
Phosphate	ICF / bone	ATP, DNA, bone; buffer	PTH

When the numbers drift

These ions carry electric charge. So small changes can cause big symptoms. Low sodium is called hyponatremia. (Hypo means "low," and natrium is Latin for sodium.) It draws water into brain cells. This brings confusion, headache and seizures. High sodium dries brain cells out.

Potassium problems are dangerous mainly for the heart. High potassium is hyperkalemia, and low potassium is hypokalemia. (Hyper means "high," and kalium is Latin for potassium.) Both cause arrhythmias, or uneven heartbeats, that can kill. Low calcium makes nerves too jumpy. This leads to tetany, which means muscle spasms and tingling. High calcium dulls nerves, causing weakness and confusion. Doctors check electrolytes every day for a good reason. These are the numbers that keep hearts beating and nerves firing.

Aldosterone is the salt-keeping hormone: it keeps sodium and dumps potassium. This one easy sentence explains a lot. When aldosterone rises, the kidney holds on to Na⁺. Water follows, which adds volume and raises blood pressure. At the same time, the kidney pushes K⁺ into the urine. So conditions that raise aldosterone tend to lower potassium. Drugs that block it tend to raise potassium.

Tap each electrolyte in the interactive. See where it lives and what it does.

Interactive Electrolytes are charged ions in body fluids. They drive nerve signals, muscle contraction and fluid balance. Tap each one. Open full ↑

Key points

Electrolytes are ions (cations and anions) measured in mEq/L. They drive nerves, muscles and fluid balance.
Sodium rules ECF volume. Potassium sets membrane potential and is vital to the heart. Calcium runs contraction, clotting and nerves.
Aldosterone keeps sodium and gets rid of potassium. PTH and vitamin D control calcium.
Small shifts cause big trouble: hyponatremia (brain), hyperkalemia (heart), hypocalcemia (tetany).

Section 4Blood pH and buffers

**Reference.** Blood pH and the bicarbonate buffer system — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A razor-thin window

pH measures how acidic or basic a fluid is. The scale runs from 0 to 14, and 7 is neutral. Lower numbers mean more acidic. That means more free hydrogen ions (H⁺). Higher numbers mean more basic.

Blood in the arteries is kept slightly basic. It stays in the tiny band of 7.35 to 7.45. This is not a comfort zone. It is a survival zone. Below 7.35 is acidemia, or acid blood. Above 7.45 is alkalemia, or basic blood. Life is not possible outside about 6.8 to 7.8.

The stakes are high because pH bends the shape of proteins. Enzymes are proteins. So a drifting pH slows or stops the chemistry of every cell at once. The trouble is that your body makes acid all the time. Burning fuel makes carbon dioxide, which turns into acid in water. Hard work or going without food adds lactic acid and ketoacids on top.

What a buffer does

The first defense against this flood of acid is a buffer. A buffer is a pair of chemicals. It soaks up extra H⁺ when acid is added. It gives off H⁺ when base is added. This softens the swing in either direction, like a sponge.

Buffers act in an instant. But they do not remove acid from the body. They only hold on to it for a while, until the lungs or kidneys can get rid of it. The body's most important buffer is the bicarbonate system. It is summed up in one reaction worth learning:

CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃⁻

Read it both ways. When acid floods in, bicarbonate (HCO₃⁻) grabs the extra H⁺. This drives the reaction to the left, toward carbonic acid and CO₂. The lungs can breathe the CO₂ out. When base is added, carbonic acid gives up an H⁺ to replace what was lost. This drives the reaction to the right.

What makes this buffer so strong is that it is open at both ends. The lungs can blow off the CO₂ on the left. The kidneys can change the bicarbonate on the right. So the body can shift the whole equation instead of just using up the buffer.

Buffer system	Where it works	Note
Bicarbonate	Blood / ECF	The main ECF buffer. It is linked to the lungs and kidneys, so it can be controlled.
Phosphate	Inside cells; urine	Important in the ICF and in making urine acidic
Protein	Cells & plasma	Biggest total capacity. Hemoglobin buffers the CO₂ carried in blood.

Blood pH tracks a 20-to-1 ratio. A normal pH of 7.4 matches about 20 parts bicarbonate for every 1 part carbonic acid (CO₂). It is the ratio that sets the pH, not the actual amounts. The kidney sets the bicarbonate on top of the fraction. The lungs set the CO₂ on the bottom. So each one can nudge pH by changing its side of the fraction.

Add acid or base in the interactive. Watch the bicarbonate buffer fight to hold the line.

Interactive Blood pH must stay between 7.35 and 7.45. Add acid or base and watch the bicarbonate buffer fight to hold the line. Open full ↑

Key points

Blood pH in the arteries is held at 7.35–7.45. Straying from it stops enzymes from working and puts life at risk.
A buffer soaks up or gives off H⁺ to soften pH swings right away. But it does not remove acid from the body.
The bicarbonate buffer is CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃⁻. It is central because the lungs control CO₂ and the kidneys control HCO₃⁻.
Phosphate and protein buffers back it up. Proteins have the biggest total buffering power.

Section 5The three lines of defense

The three ways the body controls blood pH — **Reference.** The three systems that defend blood pH — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Three defenders, three clocks

The body guards its pH in three ways. Each works at a very different speed. Each one covers for the limits of the others. Think of an emergency response in stages. First comes an instant reflex. Next comes a fast fix. Last comes a slow but complete repair.

The first is chemical buffering, which we just met. It acts within seconds. But it can only handle so much. It also only hides the acid instead of removing it. The second is respiratory control, by the lungs. The third is renal control, by the kidneys. Together, they let the body soak up an acid load right away and then truly clear it.

The lungs: fast and forceful

The lungs change pH by controlling how much CO₂ stays in the blood. CO₂ forms acid, so breathing works like a pH dial. When blood gets too acidic, sensors notice the rising H⁺. They tell the breathing center in the brain to breathe faster and deeper. This blows off CO₂ and pulls the buffer equation to the left. pH climbs back up within minutes.

When blood gets too basic, breathing slows. This keeps CO₂ in and lets pH fall. This lung response is quick and very strong. But it has a built-in limit. It cannot fix a problem that the lungs caused.

The kidneys: slow and complete

The last and most thorough defender is the kidney. It can actually add or remove acid and base from the body. It secretes H⁺ into the urine. It also takes back or makes bicarbonate and returns it to the blood. This resets the whole system. But it takes hours to days, not seconds.

The kidney is the slowest line of defense. It is also the only one that can fix an imbalance for good. That is why it has the final say on blood pH. Buffers buy time. The lungs buy more time. The kidneys settle the bill.

Line of defense	Speed	How it works	Limit
1. Chemical buffers	Seconds	Grab or give off H⁺ on the spot	Can only handle so much; only hides the acid
2. Respiratory (lungs)	Minutes	Change breathing to blow off or keep CO₂	Cannot fix a problem caused by the lungs
3. Renal (kidneys)	Hours to days	Get rid of H⁺; take back or make HCO₃⁻	Slow, but the only complete fix

Fast but shallow, slow but deep. The lungs can fix a pH swing in a minute. But they cannot fix a problem they caused. The kidneys need a day. But they are the only system that truly removes acid from the body. When one is the source of the trouble, the other comes to the rescue. This is the heart of compensation, in the next section.

Tap each line of defense in the interactive. Watch it act at its own speed.

Interactive The body defends its pH with three systems. They work at very different speeds, from instant to days. Tap each line of defense. Open full ↑

Key points

Three defenses, in order of speed: chemical buffers (seconds), respiratory (minutes), and renal (hours to days).
The lungs tune pH by breathing out more or less CO₂. They are fast and strong, but cannot fix problems the lungs caused.
The kidneys get rid of H⁺ and adjust bicarbonate. They are slow, but they are the only complete fix.
When one organ system causes the problem, the other compensates, or makes up for it.

Section 6The four acid-base disorders

Two questions sort every case

When pH control fails, the result is always one of four core disorders. You can name any of them by asking two questions.

First, which way did the pH go? Is it too acidic? That is acidosis, with pH below 7.35. Or is it too basic? That is alkalosis, with pH above 7.45. Second, what is the cause? Is it a lung problem that changes CO₂? That is respiratory. Or is it a problem that changes bicarbonate or other acids? That is metabolic. Put those two questions together, and you get a simple two-by-two grid. Each box holds one disorder.

Disorder	Primary change	pH	Typical causes
Respiratory acidosis	CO₂ kept in (hypoventilation, or breathing too little)	Low	COPD (a long-term lung disease), opioid overdose, chest injury
Respiratory alkalosis	CO₂ blown off (hyperventilation, or breathing too much)	High	Anxiety, high altitude, fever, pain
Metabolic acidosis	HCO₃⁻ lost or acid gained	Low	Diabetic ketoacidosis, lactic acidosis, diarrhea, kidney failure
Metabolic alkalosis	HCO₃⁻ gained or acid lost	High	Throwing up (loss of stomach acid), diuretics, too many antacids

The body fights back: compensation

When one system fails, the other steps in to soften the damage. This is called compensation. If the trouble is metabolic, the lungs respond within minutes. Take diabetic ketoacidosis, where the body builds up acids called ketones. Deep, fast Kussmaul breathing shows up. This is the lungs working hard to blow off CO₂ and fight the acid.

If the trouble is respiratory, the kidneys respond over days. They keep or dump bicarbonate to nudge pH back. Compensation can be partial, with pH still not normal. Or, given time, it can be nearly full. Then pH is back in range, but the other numbers are still off. It never overcorrects past normal. It also never cures the cause. It only buys the patient time.

Reading the numbers

To read a blood gas result, look at three values in order. First is the pH. Which way is it off? Next is the CO₂, the lung number. Last is the bicarbonate, the metabolic number. Then ask which of the two fits the pH.

Say the pH is low and the CO₂ is high. Then the lungs are to blame. This is respiratory acidosis. Now say the pH is low and the bicarbonate is low. Then metabolism is to blame. This is metabolic acidosis. The value that moves along with the problem points to its cause.

ROME: Respiratory Opposite, Metabolic Equal. In a respiratory disorder, pH and CO₂ move in opposite directions (CO₂ up, pH down = acidosis). In a metabolic disorder, pH and bicarbonate move in the same direction (HCO₃⁻ down, pH down = acidosis). These four letters let you name any acid-base picture at a glance.

Tap a box in the interactive. Explore each of the four disorders by its cause and direction.

Interactive Four core disorders, sorted by cause (lungs or metabolism) and direction (too acidic or too basic). Tap a box. Open full ↑

Key points

The four disorders come from two questions: direction (acidosis or alkalosis) × cause (respiratory or metabolic).
Respiratory problems change CO₂. Metabolic problems change bicarbonate.
Compensation uses the other organ system. The lungs help with metabolic problems (Kussmaul breathing). The kidneys help with respiratory ones.
Read the numbers with ROME. In respiratory problems, pH and CO₂ move in opposite ways. In metabolic problems, pH and HCO₃⁻ move the same way.

The chapter in one breath

Water is about 60% of body mass. It is split two-thirds intracellular and one-third extracellular (interstitial fluid plus plasma).
The compartments differ in what is dissolved in them. ECF is rich in Na⁺/Cl⁻, and ICF is rich in K⁺. Water follows dissolved particles by osmosis.
Each day, intake equals output at about 2.5 L. Thirst raises intake, and ADH lowers output. This holds total body water steady.
Electrolytes drive nerves, muscles and fluid balance. Sodium rules volume, potassium rules the heart, and calcium rules contraction. Aldosterone keeps Na⁺ and dumps K⁺.
Blood pH is held at 7.35–7.45 by the bicarbonate buffer, CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃⁻.
Three defenses guard pH: buffers (seconds), lungs changing CO₂ (minutes), and kidneys changing HCO₃⁻ (days).
When they fail, you get four disorders: respiratory or metabolic, and acidosis or alkalosis. You read them with ROME. Compensation softens them.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

Acidosis: When blood pH falls below 7.35. It is respiratory when CO₂ builds up, and metabolic when bicarbonate is lost or acid is gained.
Aldosterone: A hormone from the adrenal gland. It makes the kidney take back sodium (and water) and get rid of potassium, which raises ECF volume and blood pressure.
Alkalosis: When blood pH rises above 7.45. It is respiratory when too much CO₂ is lost, and metabolic when bicarbonate is gained or acid is lost.
Antidiuretic hormone (ADH): A hormone from the back part of the pituitary gland, also called vasopressin. It makes the kidney take back water, so urine is concentrated and body water is saved.
Bicarbonate (HCO₃⁻): The main buffer anion of the ECF. It is the metabolic part of the bicarbonate buffer system, and the kidneys control it.
Buffer: A pair of chemicals that grabs extra H⁺ or gives it off to resist changes in pH. It acts right away but does not remove acid from the body.
Compensation: When one organ system (lungs or kidneys) fights a pH problem caused by the other. It eases the problem but does not cure it.
Electrolyte: A substance that breaks into charged ions in water. It lets fluids carry current and drives nerves, muscles and fluid balance.
Extracellular fluid (ECF): The one-third or so of body water outside the cells. It is made of interstitial fluid and plasma, and it is rich in sodium and chloride.
Insensible loss: Water you lose without noticing. It dries off the skin and leaves as vapor in the breath you breathe out.
Interstitial fluid: The fluid outside cells that fills the spaces between them. It is the biggest part of the ECF.
Intracellular fluid (ICF): The two-thirds or so of body water inside the cells. It is rich in potassium, magnesium, phosphate and proteins.
Metabolic water: Water made inside cells when they burn fuel for energy. It is a small daily source of water.
Osmolarity: How many particles are dissolved in a fluid. It decides which way water moves by osmosis.
pH: A 0–14 scale of how acidic something is, based on how many hydrogen ions it has. Blood is held at 7.35–7.45.
Potassium (K⁺): The main cation inside cells. It sets the resting membrane potential and is vital for a normal heart rhythm.
Respiratory acidosis: A fall in pH caused by breathing too little (hypoventilation), which keeps CO₂ in. It happens in COPD or opioid overdose.
Sodium (Na⁺): The main cation outside cells. It is the master control of ECF volume and osmolarity, and aldosterone mainly controls it.
Thirst: The urge to drink. Sensors in the hypothalamus trigger it when the ECF gets too concentrated.
Total body water: All the water in the body, about 60% of an adult's mass. It is split between the fluid inside and outside cells.

Check yourself

1. In a typical adult, about how much of body mass is water? How is that water split between the inside and outside of cells?

About 60% of body mass. About two-thirds is intracellular and one-third is extracellular. The outside third is split again into interstitial fluid (the bigger part) and plasma. Leaner bodies and babies hold a higher percentage, because fat holds very little water.

2. Which ions fill the extracellular fluid, and which fill the intracellular fluid? What keeps the two so different?

ECF is rich in sodium (Na⁺) and chloride (Cl⁻). ICF is rich in potassium (K⁺), magnesium, phosphate and proteins. The sodium-potassium pump in every cell membrane keeps pushing Na⁺ out and pulling K⁺ in. This keeps up the difference that powers nerves and muscles.

3. The ECF gets more concentrated (its osmolarity rises). Which way does water move across cell membranes? What happens to the size of the cells?

Water moves out of the cells by osmosis, so the cells shrink. Water always moves toward the side with more dissolved particles. A concentrated ECF pulls water out. A watery ECF drives water in and makes cells swell.

4. List the three sources of water you take in each day and the four ways water leaves.

In: fluids you drink, water in food, and metabolic water. Out: urine, insensible loss from skin and lungs, feces, and sweat. Intake and output each add up to about 2.5 L a day. Only urine is closely controlled.

5. A hiker gets dried out. Trace the two-part response that raises intake and cuts output.

Osmoreceptors in the hypothalamus sense the concentrated ECF. They cause thirst (raising intake) and release of ADH (cutting output). ADH makes the kidney's collecting ducts put in aquaporins to take back water. This makes a small amount of concentrated urine.

6. Which electrolyte is the main cation inside cells? What does it set, and why are problems with it most dangerous for the heart?

Potassium (K⁺). It sets the resting membrane potential of nerve and muscle cells. Heart muscle needs that charge to fire and reset. So both hyperkalemia and hypokalemia upset the heart's rhythm. They can cause deadly arrhythmias.

7. Aldosterone goes up. What happens to sodium, potassium and blood pressure?

Sodium is kept, and water follows, so volume and blood pressure rise. Potassium is pushed into the urine, so its level falls. Aldosterone is the salt-keeping hormone. It keeps Na⁺ and dumps K⁺.

8. Write the bicarbonate buffer equation. Explain what happens to it when acid is added to the blood.

CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃⁻. Bicarbonate grabs the added acid (H⁺). This pushes the reaction left toward CO₂, which the lungs breathe out. The system works because it is open at both ends. The lungs control the CO₂, and the kidneys control the bicarbonate.

9. Name the body's three lines of acid-base defense in order of speed. Give how long each takes.

Chemical buffers (seconds), respiratory control by the lungs (minutes), and renal control by the kidneys (hours to days). Buffers hide the acid right away. The lungs blow off CO₂ quickly. Only the kidneys truly remove acid and fully fix the imbalance.

10. A patient with a bad opioid overdose is breathing very slowly. Predict the acid-base disorder and explain how it happens.

Respiratory acidosis. Breathing too little (hypoventilation) traps CO₂ in the blood. The extra CO₂ forms carbonic acid. This raises H⁺ and drops pH below 7.35. The lungs are the cause, so the lung number is off: CO₂ is high.

11. A patient with diabetic ketoacidosis starts deep, fast Kussmaul breathing. What disorder do they have? What is the breathing doing?

Metabolic acidosis, with the lungs compensating. Ketoacids use up bicarbonate and drop the pH. The lungs respond by breathing hard to blow off CO₂. This pulls the buffer equation left and pushes pH back toward normal. Compensation eases the acidosis but does not cure it.

12. A blood test shows a low pH and a low bicarbonate. Using ROME, is this respiratory or metabolic? Which way did pH and the matching value move?

Metabolic acidosis. In a metabolic problem, pH and bicarbonate move in the same direction (both down). ROME stands for Respiratory Opposite, Metabolic Equal. The bicarbonate (a metabolic number) fell along with the pH. So the cause is metabolic, not respiratory.

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