General Physiology: Cell, Body Fluids & Transport Explained

Open the first chapter of general physiology and it feels like 4 subjects crammed into one: cells, body fluids, membranes, and electricity. Most students memorise each one as a separate list, then wonder why exam questions blend them. The questions blend them because the chapter tells a single story.

Here is that story in 4 links. Your body is about 42 litres of salty water, divided into compartments. A thin membrane separates those compartments. The membrane decides what crosses and how. And because crossing is uneven, a voltage appears across it. Fluid, wall, crossing, voltage. Keep that chain in your head and every topic below slots into place.

Table of Contents

How Much of You Is Water, and Where Does It Sit?

In a 70 kg adult man, total body water is about 60% of body weight, or roughly 42 litres. About 2/3 of it, near 28 litres, sits inside cells as intracellular fluid. The remaining 14 litres is extracellular fluid, which splits into interstitial fluid around cells and plasma inside blood vessels.

That 60-40-20 pattern comes straight from Guyton and Hall's Textbook of Medical Physiology (14th edition, 2021), and the StatPearls review of body fluids gives the same numbers. Learn it as a rule: 60% of weight is water, 40% of weight is inside cells, 20% is outside. In everyday terms, you carry roughly 21 two-litre bottles of water around, and 14 of them sit locked inside your cells.

Think of it like a city. Cells are the houses, and the water inside the houses is the intracellular fluid. The streets between the houses carry the interstitial fluid. The motorway running through the whole city is the plasma. Streets and motorway together make up the extracellular fluid, the "outside" that every house has to deal with. The tree below puts numbers on every branch.

Figure 1: How total body water divides into intracellular fluid, interstitial fluid and plasma in an adult.

Books round the last 2 branches differently. Some list about 3 L of plasma and 11 L of interstitial fluid, others use the 5% and 15% split, which gives 3.5 L and 10.5 L. Both are accepted, so write the percentages in a theory answer and you stay safe.

Here's where most students slip. They hear "extracellular fluid" and picture blood. Plasma is only about 1/4 of the ECF. So where does the other 3/4 sit? Between your cells, as interstitial fluid. The 2 fluids are near twins, and the single difference that separates them is protein: plasma carries a lot, interstitial fluid very little, because capillary walls hold large proteins back.

A small leftover of 1 to 2 litres, called transcellular fluid, also counts as extracellular. It includes cerebrospinal, synovial, pleural, pericardial, peritoneal, and intraocular fluids. In clinical practice you will see these spaces fill abnormally, as in a pleural effusion.

Blood straddles 2 compartments. Its plasma is ECF, while the fluid inside red cells is ICF. Total blood volume sits near 5 L. And the 60% figure is an average for an adult man: women, older adults, and people with more body fat carry less, because fat holds very little water.

Measuring Volumes Nobody Can See

Nobody can pour a person into a measuring jug, so physiologists use the indicator dilution principle. In plain words: drop a known amount of a marker into a compartment, wait until it mixes evenly, then check how diluted it became. Heavy dilution means a big compartment.

As a formula, volume = (amount injected − amount lost) ÷ concentration. Quick illustration: inject 15 mg of dye into the blood, lose none, and measure 0.005 mg per mL after mixing. The volume is 15 ÷ 0.005 = 3,000 mL, which is 3 L, the size of the plasma compartment.

The whole trick is choosing a marker that stays in the space you want to measure.

Volume wanted	Marker or calculation	Why it works
Total body water	Heavy water (D2O), tritium water, antipyrine	Spreads through every body fluid, crossing cell membranes freely
Extracellular fluid	Radioactive sodium or chloride, inulin, thiosulfate	Crosses capillary walls but largely stays out of cells
Plasma	Evans blue dye (T-1824), radio-iodinated albumin	Binds to, or is, plasma albumin, which cannot leave the vessels easily
Blood volume	Plasma volume ÷ (1 − hematocrit)	Red cells make up the rest
Interstitial fluid	ECF − plasma	No marker stays only in this space
Intracellular fluid	Total body water − ECF	No marker stays only inside cells

Try the blood volume row: with plasma at 3 L and a hematocrit of 0.4, blood volume is 3 ÷ 0.6 = 5 L. Notice that 2 rows are subtractions. You never measure interstitial or intracellular fluid directly. They are always the leftover. Also note that radioactive sodium and chloride leak into cells a little, so the result slightly overestimates the ECF, which is why some books call it the "sodium space".

Exam Pointer Examiners love the "which marker for which compartment" table, and the follow-up "how do you find intracellular fluid?" The safe answer: total body water minus ECF, because no marker stays confined to cells.

With numbers on every compartment, the next thing to ask is what each one contains, because the contents differ sharply.

Why Is Sodium Outside and Potassium Inside?

The ECF is rich in sodium (Na+) and chloride (Cl−), while the ICF is rich in potassium (K+), phosphates, and proteins. The imbalance is deliberate. The Na+/K+ pump in cell membranes keeps pushing sodium out and potassium in, spending ATP to hold the gradient. Stop the spending and the 2 fluids would slide toward the same mix.

Ion	ECF (mmol/L, approx.)	ICF (mmol/L, approx.)	What to remember
Na+	142	10 to 14	Roughly 10 to 14 times higher outside
K+	4	140	About 35 times higher inside
Cl−	about 106	4	Mostly outside
HCO3−	24 to 28	10	Mostly outside; main blood buffer
Proteins	Low (lowest in interstitial fluid)	High	Large, charged, trapped where they are made

Ratios matter more than decimals, because books print slightly different values. Calcium is the most extreme case: free calcium inside the cytosol sits roughly 10,000 times lower than outside. That gradient is held by pumps, which is why a small calcium leak into the cell works as a loud signal.

Why would a cell keep paying ATP to hold such a lopsided arrangement? Because the arrangement does 2 jobs. It controls cell volume, since an unbalanced cell would swell with water. And it stores energy that nerve and muscle cells later spend to fire signals.

Inside the cell, large proteins and phosphates carry negative charges and cannot leave. Positive ions gather around those trapped charges, which skews the distribution of every freely moving ion. This is the Gibbs-Donnan effect, and it returns in the osmosis section as part of plasma colloid osmotic pressure.

Holding these numbers steady has a name: homeostasis, the maintenance of nearly constant conditions in the internal environment. Claude Bernard described that internal environment (the ECF) in the 19th century, and Walter Cannon popularised the word in his 1932 book The Wisdom of the Body.

Most control loops run on negative feedback: a change triggers a response that pushes the value back toward normal. Arterial pressure, body temperature, and blood glucose all behave this way. Positive feedback amplifies a change instead, and the body keeps it for events that must finish, such as childbirth contractions and blood clotting.

Quick pause. What do we have so far? 3 fluid spaces, a salty outside, a potassium-rich inside, and constant effort to keep it that way. Hold those 4 facts, because the next idea shows what water does when the solute count shifts.

Osmosis and Tonicity: Which Way Does the Water Go?

Water moves toward the side with more dissolved particles. That single line is osmosis: the net movement of water across a selectively permeable membrane, from the side with fewer solute particles to the side with more.

Think of the solute as a crowd that cannot squeeze through the doorway between 2 rooms, while water slips through easily. Water keeps drifting toward the crowded room, thinning the crowd out. A red blood cell dropped in distilled water shows the result: water rushes in, the cell swells, and it bursts (haemolysis).

Exams punish the flipped version. Many students write "water moves to where there is more water." Follow the solute instead: water chases it. Water ends up on the side with more solute particles, which is the side with less free water.

Osmotic pressure is the pressure needed to stop that water movement, and it depends on how many particles are present, whatever their size. 1 osmole is 1 mole of particles, and NaCl, which splits into 2 ions, delivers close to 2 osmoles per mole. Guyton and Hall calculate that the total osmolarity of body fluids, about 282 mOsm per litre, represents a potential osmotic pressure near 5,400 mm Hg. Compare that with a mean arterial pressure of roughly 100 mm Hg. A small shift in solute therefore moves a large amount of water.

There is a catch in the word "effective", and it trips up a lot of students. Three terms sound alike and need a contrast. Osmolarity and osmolality both count every dissolved particle. The difference is the denominator: litres of solution for osmolarity, kilograms of water for osmolality. Tonicity counts only the particles that cannot cross the cell membrane, called effective osmoles, and it describes what a solution does to cell volume.

Term	What it counts	Unit	Does the membrane matter?
Osmolarity	All dissolved particles per litre of solution	mOsm/L	No
Osmolality	All dissolved particles per kg of water; the figure laboratories report, near 280 to 300 in plasma	mOsm/kg	No
Tonicity	Only effective osmoles, the ones that cannot cross	Described as iso-, hypo-, or hypertonic	Yes

Common Mistake! Calling a solution hypotonic just because its osmolarity is lower than plasma, or isotonic because the numbers match. Urea shows why: it adds to osmolality, yet it crosses most cell membranes freely, so it adds little to tonicity. Ganong's Review of Medical Physiology (26th edition, 2019) calls the tonicity-relevant figure the effective osmolality.

Plasma proteins add a pull of their own. Because they are charged and stuck inside the vessels, the Gibbs-Donnan effect lets them hold extra positive ions nearby, and Guyton and Hall put plasma colloid osmotic pressure near 28 mm Hg: about 19 from the proteins themselves and about 9 from the Donnan effect. That pull keeps fluid inside capillaries. When albumin falls, the pull weakens and fluid collects between cells as oedema.

Tonicity predicts cell size, and the diagram below shows the 3 outcomes for the same red cell.

Figure 2: Osmosis and tonicity compared, with a cell swelling, staying the same size or shrinking by fluid type.

Fluid type	Example	Cell outcome	ECF volume	ICF volume
Isotonic	0.9% NaCl	Stays the same	Rises	No change
Hypotonic	0.45% NaCl	Swells	Rises	Rises
Hypertonic	3% NaCl	Shrinks	Rises	Falls

Clinical Note 5% dextrose is isotonic in the bag, but cells soon use up the glucose and the free water stays behind. In the body it therefore acts like a hypotonic fluid, spreading across both ECF and ICF.

In clinical practice, low plasma sodium (hyponatraemia) shows the hypotonic column at work. The ECF turns dilute, water enters cells, and brain cells swell inside a rigid skull, which is why early features are neurological, such as headache and confusion.

Water follows solutes, and the biggest pile of solutes sits inside the cell, where every organelle has its own job.

Inside the Cell: Which Organelle Does Which Job?

Peel the membrane back and a cell looks crowded, yet every structure has a main job. The fluid filling the space is the cytoplasm, and the working parts floating in it are organelles, meaning "little organs".

Take the mitochondrion, the organelle exam questions never skip. The plain version: it is the cell's power plant. The mechanism: it uses oxygen to release energy from fuel and captures that energy as ATP (the cell's energy currency) on its inner membrane, a process called oxidative phosphorylation. The payoff: heart, muscle, and nerve cells burn ATP all day, so they are packed with mitochondria, and a large share of that ATP gets spent at the membrane pump you'll meet shortly. Mitochondria also carry their own circular DNA, inherited from the mother, a fact that turns up in MCQs regularly.

Organelle	Job in plain words	Exam hook
Nucleus	Control room that stores DNA and decides which genes switch on	Nucleolus builds ribosome subunits; the envelope has pores
Ribosome	Protein-making machine	Free ones make proteins for the cytosol; bound ones make proteins for export and membranes
Rough ER	Protein workshop studded with ribosomes	Prominent in secretory cells such as pancreatic acinar cells
Smooth ER	Makes lipids and steroids, detoxifies, stores calcium	In muscle it is the sarcoplasmic reticulum, the calcium store
Golgi apparatus	Post office that modifies, sorts, and packs proteins into vesicles	Adds carbohydrate chains (glycosylation)
Mitochondrion	Power plant making ATP	Own DNA; double membrane; folded cristae
Lysosome	Digestive bag full of acid hydrolases	Internal pH about 5; digests worn-out parts and engulfed material
Peroxisome	Oxidation unit handling fatty acids and hydrogen peroxide	Catalase breaks H2O2 into water and oxygen
Cytoskeleton	Scaffolding and railway tracks	Actin filaments, intermediate filaments, microtubules

Students mix up lysosomes and peroxisomes because both are small bags of enzymes. The separating question is which reaction happens inside. Lysosomes digest, using hydrolases that work at acid pH. Peroxisomes oxidise, and they make hydrogen peroxide as a by-product, which catalase destroys on the spot.

Every organelle on that list depends on materials crossing a single boundary: the cell membrane.

The Cell Membrane: Structure and the Ways Across

Why can oxygen drift straight into a cell while a sodium ion needs a dedicated channel? The answer lives in what the membrane is made of.

In 1972, Singer and Nicolson proposed the fluid mosaic model in Science, and it still anchors how textbooks describe the cell membrane. The plain version: a thin, oily sheet with proteins floating in it. The mechanism: the sheet is a phospholipid bilayer. Each phospholipid has a water-loving head and 2 water-hating tails, so the heads face the watery fluid on both sides and the tails tuck into the middle. The payoff: that oily middle repels charged ions but lets lipid-soluble molecules pass, so what crosses depends more on solubility than on size.

By mass, Guyton and Hall describe the membrane as roughly 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. Cholesterol keeps the sheet stable and controls its fluidity. Integral proteins span the sheet and form channels, carriers, pumps, and receptors, while peripheral proteins cling to one surface, usually the inner one, and act mostly as enzymes. Short carbohydrate chains on the outer surface form the glycocalyx, which helps cells recognise each other.

Small molecules and ions cross in 4 ways, and the diagram below sorts them with 1 question: does the cell pay energy?

Figure 3: Cell membrane transport sorted into passive and active routes, from simple diffusion to secondary active transport.

Passive Transport: Downhill and Free

Simple diffusion is the net movement of molecules from high to low concentration, straight through the bilayer or through an open channel, with no carrier and no energy. 4 things speed it up: a steeper gradient, a larger surface area, a thinner membrane, and higher lipid solubility. Oxygen, carbon dioxide, and steroid hormones cross this way. Water slips through the bilayer slowly and moves quickly through aquaporins, channels built for it.

Facilitated diffusion uses a protein, either a channel or a carrier, but the substance still travels downhill and no ATP is spent. Glucose entering muscle and fat cells through GLUT4, under insulin control, is the textbook example. Carriers saturate: once every binding site is busy, a steeper gradient adds nothing. That saturation is the exam fingerprint of carrier-mediated transport, because the rate of simple diffusion keeps climbing with the gradient.

Here's the confusion: both use proteins, so students file facilitated diffusion under "active". The question that separates them is direction. Downhill with a carrier is facilitated diffusion and costs nothing. Uphill is active transport, and someone pays.

Active Transport: Uphill and Paid For

Primary active transport spends ATP directly. The star is the Na+/K+ ATPase: for each ATP, it moves 3 Na+ out and 2 K+ in, per Guyton and Hall. Because the swap is uneven, each cycle shifts 1 net positive charge outward, so the pump is called electrogenic. Other primary pumps include the Ca²⁺ ATPase and the H+/K+ ATPase of the stomach.

Secondary active transport spends no ATP itself. It borrows the sodium gradient the pump built. Think of it like a dam: the pump hauls water uphill into the reservoir, and secondary transporters let a little run back down to turn a turbine that does other work. The sodium-glucose cotransporter (SGLT) in the small intestine and kidney tubule drags glucose into the cell, uphill, as sodium flows in, downhill. A counter-transporter such as the Na+/Ca²⁺ exchanger swaps ions in opposite directions.

Imagine the pump stops. Sodium keeps leaking in and potassium keeps leaking out, with nothing pushing them back. The gradients fade, the membrane voltage fades with them, and the cell swells, because trapped negative charges inside keep attracting positive ions and water follows. Digoxin, a drug you will meet in pharmacology, partly inhibits this pump in heart cells, and that partial block underlies its action on the heart.

Large cargo travels by vesicle. Endocytosis brings material in, as phagocytosis for particles, pinocytosis for droplets, and receptor-mediated endocytosis for specific molecules. Exocytosis pushes material out, as in calcium-triggered neurotransmitter release. Both need ATP.

Type	Energy	Direction	Protein needed?	Classic example
Simple diffusion	None	Downhill	No	O2, CO2, steroid hormones
Facilitated diffusion	None	Downhill	Yes: channel or carrier; carriers saturate	Glucose via GLUT; ion channels
Primary active	ATP, directly	Uphill	Yes: pump	Na+/K+ ATPase, Ca2+ ATPase
Secondary active	Indirect: uses the Na+ gradient	Uphill for 1 substance	Yes: co- or counter-transporter	SGLT, Na+/Ca2+ exchanger
Vesicular	ATP	Bulk cargo in or out	Vesicles	Phagocytosis, neurotransmitter release

Exam Pointer Facilitated diffusion versus active transport is a standard viva comparison. Anchor your answer on 2 words, gradient and ATP, then add saturation for carriers and the 3-for-2 swap for the pump.

That unequal 3-for-2 swap does more than move ions. It helps set a voltage across the membrane, and one piece of that voltage story is still missing: why the inside ends up negative at all.

What Creates the Resting Membrane Potential?

Push a fine electrode into a resting nerve fibre and the meter reads about −90 mV, meaning the inside is negative compared with the outside. Many neurons read closer to −70 mV. Both numbers are right for the cell being measured: Guyton and Hall use −90 mV for large nerve fibres and skeletal muscle, while a StatPearls review of resting potential quotes about −70 mV.

The resting membrane potential is the steady voltage across the membrane of a cell that is not firing, measured inside relative to outside. The plain version: a small stored charge difference. The payoff: it is the starting line for every nerve impulse and muscle contraction.

To see why potassium matters most, start with its gradient: 140 inside, 4 outside. K+ leaks out through open channels, and each K+ that leaves takes a positive charge with it, so the inside turns more negative. That growing negativity pulls K+ back in. At some voltage the electrical pull exactly balances the chemical push, and net flow stops. That voltage is the Nernst (equilibrium) potential. At body temperature, Guyton and Hall write it as E = −61 × log (inside ÷ outside) for a positive ion with 1 charge.

For K+: E = −61 × log (140 ÷ 4) = −61 × 1.54 = about −94 mV. For Na+, using 14 inside and 142 outside: E = −61 × log (14 ÷ 142) = about +61 mV. Two ions, 155 mV apart, and the resting cell sits right beside one of them.

Quick pause. Hold 4 facts: K+ is high inside, Na+ is high outside, the resting membrane leaks K+ about 100 times more readily than Na+, and the pump keeps refilling the stores. The 3 steps below stack those facts into a real number.

1. K+ leak alone. If only K+ could move, the inside would sit at its Nernst level, about −94 mV.
2. Add the Na+ leak. The resting membrane is about 100 times more permeable to K+ than to Na+, so a little Na+ seeps in and the inside drifts up to about −86 mV.
3. Add the pump. The electrogenic 3-for-2 pump contributes roughly −4 mV of extra negativity, giving about −90 mV.

Figure 4: The resting membrane potential sits close to the potassium equilibrium potential and far from sodium.

The figure makes the logic visible. The resting potential sits next to E(K+) because the resting membrane is far more permeable to K+ than to anything else. When several ions leak at once, Nernst is not enough. The Goldman-Hodgkin-Katz equation weighs each ion by how readily the membrane lets it through, according to the StatPearls review, and that weighting explains why the real value lands near E(K+) and far from E(Na+).

If potassium leaks out all day, why does the cell never run out of it? The leak is small next to the stored supply, and the pump hauls K+ back in. That also corrects the usual slip, the idea that the pump makes the resting potential. In Guyton and Hall's accounting, the pump adds only about 4 mV directly. K+ leaking through open channels does the heavy lifting, and the pump's bigger job is long-term: it stops the Na+ and K+ gradients from running down.

Common Mistake Writing "the Na+/K+ pump creates the resting potential" as a full answer. The accurate version: K+ diffusion sets most of it, the Na+ leak lifts it slightly, and the pump adds a small direct share while keeping the gradients alive.

Imagine ECF potassium climbs. The K+ gradient shrinks, E(K+) moves toward zero, and the resting potential follows it, becoming less negative. In clinical practice this is hyperkalaemia, and the heart is the organ examiners ask about, because the excitability of cardiac cells changes first.

Disturb the voltage by a few millivolts and a nerve fires. How that small disturbance becomes a travelling signal is the action potential, the next step in your syllabus.

Quick Recap: High-Yield Points

• General physiology is one chain: fluid compartments, a selective membrane, uneven crossing, and the voltage that results.
• Total body water 60% (42 L): ICF 40% (28 L), ECF 20% (14 L) = interstitial about 15% + plasma about 5%.
• Markers: heavy water or antipyrine for total body water; inulin or radioactive sodium for ECF; Evans blue or radio-iodinated albumin for plasma. ICF = total body water − ECF; interstitial = ECF − plasma; blood volume = plasma ÷ (1 − hematocrit).
• ECF is rich in Na+, Cl−, HCO₃⁻; ICF is rich in K+, phosphates, and proteins. Na+ is about 142 outside versus 10 to 14 inside; K+ is about 4 outside versus 140 inside.
• Water follows solute. Osmolarity is per litre of solution, osmolality is per kg of water, tonicity counts effective osmoles only.
• Isotonic fluid raises ECF only; hypotonic raises ECF and ICF; hypertonic raises ECF and lowers ICF.
• Membrane: phospholipid bilayer, cholesterol, integral and peripheral proteins, and a carbohydrate glycocalyx (fluid mosaic model, Singer and Nicolson, 1972).
• Facilitated diffusion: downhill, protein-assisted, saturable, no ATP. Primary active: ATP directly, Na+/K+ pump moves 3 Na+ out and 2 K+ in. Secondary active: runs on the Na+ gradient.
• Resting potential about −90 mV (Guyton, large fibres and muscle): K+ diffusion −94 mV, Na+ leak brings it to −86 mV, pump adds about −4 mV. E(K+) = −94 mV; E(Na+) = +61 mV.

Frequently Asked Questions About General Physiology

What are the body fluid compartments and how much water does each hold?

Think of 3 spaces. In a 70 kg adult, total body water is about 42 litres, which is 60% of body weight. Roughly 28 litres sits inside cells as intracellular fluid. The other 14 litres is extracellular fluid, split between interstitial fluid around cells (about 10.5 to 11 litres) and plasma inside blood vessels (about 3 to 3.5 litres).

What is the difference between osmolarity and tonicity?

Osmolarity counts every dissolved particle in a litre of solution, whatever the membrane does with it. Tonicity asks a different question: what will this solution do to a cell's volume? It counts only particles that cannot cross the membrane. That's why a urea solution can match plasma osmolarity and still swell cells. Urea walks in, and water follows.

How is facilitated diffusion different from active transport?

Both use membrane proteins, so the confusion is natural. The separating question is direction. Facilitated diffusion moves a substance downhill, from high to low concentration, and costs no ATP; carriers saturate, as with glucose and GLUT. Active transport moves it uphill, against the gradient, and someone pays: ATP directly for pumps like the Na+/K+ ATPase, or the sodium gradient for secondary transporters.

Why is the resting membrane potential negative inside the cell?

Positive potassium ions leak out faster than positive sodium ions leak in. At rest, the membrane is about 100 times more permeable to K+ than to Na+, and K+ is about 35 times more concentrated inside. Each K+ that leaves takes a positive charge with it, so the inside turns negative. The Na+/K+ pump adds a few millivolts more. Guyton and Hall put the final value near −90 mV.

Why is potassium high inside cells and sodium high outside?

The Na+/K+ ATPase in every cell membrane keeps pumping, moving 3 Na+ out and 2 K+ in for each ATP. The membrane lets K+ leak out and Na+ leak in only slowly, so the pump wins and the gradients hold. Cells pay for this arrangement because it controls cell volume and powers the electrical signals of nerve and muscle.

How do you measure plasma volume and interstitial fluid volume?

Plasma volume uses a marker that stays in the blood, such as Evans blue dye or radio-iodinated albumin: volume equals the amount injected divided by the concentration after mixing. Interstitial fluid has no marker of its own, so you subtract: ECF volume minus plasma volume. In the same way, intracellular fluid comes from total body water minus ECF.

This article is written for educational purposes for health-science students. It is not medical advice and never replaces clinical training, professional judgment, or your institution's curriculum. Real clinical decisions always require proper patient assessment under qualified supervision.

References

1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
2. Singer SJ, Nicolson GL. "The fluid mosaic model of the structure of cell membranes. " Science. 1972;175(4023):720-731. DOI: 10.1126/science.175.4023.720. PMID: 4333397.
3. Barrett KE, Barman SM, Brooks HL, Yuan JXJ. Ganong's Review of Medical Physiology. 26th ed. McGraw-Hill Education; 2019.
4. StatPearls Publishing. "Physiology, Body Fluids." StatPearls [Internet]. NCBI Bookshelf. (2026).
5. StatPearls Publishing. "Physiology, Resting Potential. " StatPearls [Internet]. NCBI Bookshelf. (2024).
6. Cannon WB. The Wisdom of the Body. W. W. Norton & Company; 1932.