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A college-preparatory survey of chemistry principles that govern biological processes — from atomic structure, bonding, and pH through organic functional groups, macromolecule chemistry, enzyme kinetics, thermodynamics, and clinical IV fluid science.
Building blocks of all matter
All matter consists of atoms, and the way atoms interact determines the properties of every substance in the body. Three types of chemical bonds are essential to understand:
One atom donates electrons to another, creating charged ions (cations = positive, anions = negative). The electrostatic attraction between opposite charges forms the bond. Example: NaCl, sodium loses an electron (Na⁺) and chlorine gains one (Cl⁻). This is why NaCl dissociates in water into electrolytes critical for nerve conduction and fluid balance.
Atoms share electron pairs. This creates very stable molecules. Water (H₂O) has covalent bonds, oxygen shares electrons with two hydrogens. Organic molecules (proteins, carbohydrates, lipids, nucleic acids) are held together primarily by covalent bonds, which is why they are structurally stable.
A weak attraction between a slightly positive hydrogen and a slightly negative oxygen or nitrogen. Individually weak, but collectively they give water its unique properties (high specific heat, surface tension, solvent capability) and maintain protein/DNA structure. Breaking hydrogen bonds denatures proteins.
Ionic bonds create electrolytes that dissociate in body fluids, essential for electrical signaling. Covalent bonds create the stable molecules of life. Hydrogen bonds maintain the 3D shapes of proteins and DNA. When a fever denatures enzymes, it's disrupting hydrogen bonds that maintain protein folding.
The chemistry of body fluids
When ionic compounds dissolve in water, they dissociate into charged particles. These electrolytes are responsible for nerve impulse transmission, muscle contraction, fluid balance, and pH regulation.
Key Cations (+)
Na⁺, primary extracellular cation, drives fluid volume. K⁺, primary intracellular cation, critical for cardiac and nerve function. Ca²⁺, muscle contraction, bone structure, clotting. Mg²⁺, enzyme cofactor, neuromuscular function.
Key Anions (−)
Cl⁻, follows sodium, maintains osmolarity. HCO₃⁻, bicarbonate, the body's primary pH buffer. HPO₄²⁻, phosphate, energy metabolism (ATP), bone. These balance the cations to maintain electrical neutrality.
Water is a polar molecule, the oxygen end is slightly negative, the hydrogen end slightly positive. This polarity allows water to dissolve ionic and polar substances (hydrophilic), making it the universal solvent of the body. Non-polar substances (lipids) do not dissolve in water (hydrophobic), this is why cell membranes, made of phospholipids, form barriers in an aqueous environment.
The hydrogen ion concentration scale
The pH scale is fundamental to understanding how the body maintains the narrow range (7.35–7.45) required for normal enzyme function and cellular processes.
The pH Scale
Acids
Substances that release H⁺ ions in solution. Strong acids (HCl) dissociate completely. Weak acids (carbonic acid, H₂CO₃) dissociate partially and are important in buffering systems.
Bases
Substances that accept H⁺ ions or release OH⁻ ions. Bicarbonate (HCO₃⁻) is the body's primary base. It neutralizes excess H⁺ by combining to form carbonic acid, which is then expelled as CO₂ by the lungs.
A buffer resists changes in pH by absorbing excess H⁺ or releasing H⁺ as needed. The bicarbonate buffer system is the most important: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The lungs regulate CO₂ (acid side) and the kidneys regulate HCO₃⁻ (base side). This dual regulation is why respiratory and renal function both affect pH.
How substances are measured in clinical practice
In healthcare, solutions are described by their concentration. Understanding concentrations is critical for medication preparation, IV fluid therapy, and lab value interpretation.
Concentration Units
mg/mL, milligrams of drug per milliliter of solution (most common in medication dosing). %, grams of solute per 100 mL of solution (0.9% NaCl = 0.9 g NaCl per 100 mL = 9 g/L). mEq/L, milliequivalents per liter (used for electrolytes, accounts for ionic charge). mmol/L, millimoles per liter (used for lab values like glucose in some countries).
Dilution Reasoning
When you dilute a solution, the amount of solute stays the same but the volume increases. C₁V₁ = C₂V₂. If you have 10 mL of a 10 mg/mL solution and add 90 mL of diluent, you now have 100 mL of a 1 mg/mL solution. The total drug amount (100 mg) hasn't changed.
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Carbon chemistry: the backbone of life
Organic chemistry is the study of carbon-containing compounds. Carbon's unique ability to form 4 stable covalent bonds — and to bond with itself — allows it to create the chains, rings, and branched structures that make up all biological macromolecules. Mastering organic functional groups is essential for pharmacology: drug molecules are organic compounds, and their functional groups determine solubility, ionization, receptor binding, and metabolism.
Why Carbon is Unique
Carbon forms 4 covalent bonds, more than almost any other element in biology. It bonds to H, O, N, S, P, and to other carbons — allowing straight chains (fatty acids), branched chains (amino acid side chains), rings (glucose, steroids, aromatic drugs), and double bonds (unsaturated fats, aromatic rings). The result is virtually unlimited molecular diversity.
The –OH group forms hydrogen bonds with water, making molecules hydrophilic (water-soluble). Found in alcohols (ethanol, isopropanol used as antiseptics), sugars (glucose has 5 –OH groups), and many drugs (morphine, acetaminophen). Polar drugs with –OH groups can dissolve in blood plasma without a carrier. Enzyme reactions commonly add –OH groups to drugs during Phase I hepatic metabolism to increase water solubility for renal excretion.
A C=O group defines both ketones (carbonyl flanked by two carbons) and aldehydes (carbonyl at chain end, –CHO). Ketones appear in ketone bodies (acetoacetate, beta-hydroxybutyrate) produced in diabetic ketoacidosis and fasting. Aldehydes are reducing sugars: glucose has an aldehyde group that reduces copper ions in Benedict's test and reduces silver in older urine glucose detection methods. Steroid hormones contain a ketone group in their ring structure.
The –COOH group ionizes (–COOH ⇌ –COO⁻ + H⁺) to donate a proton, making the molecule an organic acid. Found in all amino acids (they are both amino acids AND carboxylic acids), fatty acids (stearic acid, palmitic acid), and many drugs: aspirin (acetylsalicylic acid), ibuprofen, penicillin, furosemide. At physiologic pH (7.4), carboxyl groups are predominantly ionized (–COO⁻), increasing water solubility and affecting drug distribution.
The –NH₂ group accepts a proton (–NH₂ + H⁺ → –NH₃⁺), making it basic. All amino acids contain an amino group. Many drug molecules contain amino groups: dopamine, epinephrine, amphetamines, antidepressants (SSRIs, TCAs), antihistamines, local anesthetics (lidocaine has a tertiary amino group). The ionization state (–NH₂ vs –NH₃⁺) at different pH values affects drug absorption — non-ionized forms cross membranes more easily, which is clinically exploited in urine alkalinization to trap weak bases.
Phosphate groups are central to three critical biological systems: (1) Energy storage — ATP contains three phosphate groups connected by high-energy phosphoanhydride bonds; each ATP hydrolysis releases ~7.3 kcal/mol. (2) Membrane structure — phospholipids have a phosphate head group that is hydrophilic, while fatty acid tails are hydrophobic, creating the bilayer. (3) Nucleic acids — the phosphodiester backbone of DNA and RNA links nucleotides. Cell signaling: phosphorylation (addition of phosphate to proteins by kinases) is a primary on/off switch for enzyme activity.
The –CH₃ group seems simple, but methylation reactions are pivotal in biology. DNA methylation at CpG islands silences gene expression (epigenetics) and is disrupted in many cancers. Histone methylation regulates chromatin packing and gene transcription. In drug metabolism, N-methylation (adding –CH₃ to an amino group) is a Phase II detoxification reaction (COMT enzyme methylates catecholamines like dopamine and epinephrine for inactivation). The amino acid methionine donates methyl groups via S-adenosylmethionine (SAM), one of the body's primary methyl donors.
Isomers & Polymer Chemistry
Isomers share the same molecular formula but differ in structure. Structural isomers differ in connectivity. Stereoisomers have the same connectivity but different 3D arrangement — including enantiomers (non-superimposable mirror images, like your hands). Enantiomers are designated L or D (amino acid convention) or R and S (IUPAC). Nearly all amino acids in the human body are L-isomers; D-amino acids appear in some bacterial cell walls (antibiotic target).
Polymer synthesis occurs by dehydration synthesis (condensation): two monomers join and release one H₂O molecule. Polymers are dismantled by hydrolysis: water is added across the bond to break it. Every digestion reaction is hydrolysis (amylase hydrolysis of starch, pepsin hydrolysis of protein peptide bonds, lipases hydrolysis of ester bonds in triglycerides).
Drug Chirality — When One Mirror Image Heals and One Harms
Many drug molecules have a chiral center (asymmetric carbon with 4 different substituents), producing mirror-image enantiomers. The two forms can have dramatically different biological effects because enzymes and receptors recognize specific 3D shapes. Thalidomide is the classic cautionary example: the R-enantiomer was therapeutic for morning sickness, while the S-enantiomer caused severe limb malformations (phocomelia) in thousands of newborns. Because enantiomers can interconvert in the body (racemization), separating them may not always be a complete solution.
From amino acids to quaternary structure
Proteins are the workhorses of biology: enzymes, structural scaffolds, transporters, hormones, antibodies, and channels are all proteins. Their function depends entirely on their 3D shape, which emerges from a hierarchy of structural levels — each built on specific chemical bonds.
Amino Acid Structure
Every amino acid has a central alpha-carbon bonded to: (1) an amino group (–NH₂), (2) a carboxyl group (–COOH), (3) a hydrogen atom, and (4) a variable R group (side chain) that determines the amino acid's identity and chemical properties. With 20 standard amino acids, variation in R groups provides remarkable chemical diversity within a single molecule class.
R Group Classifications
Peptide Bond Formation
A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water (dehydration synthesis / condensation reaction). The resulting –CO–NH– linkage is a covalent bond with partial double-bond character (resonance), making it planar and rigid.
Ribosomal peptide bond synthesis consumes 2 GTP molecules per amino acid added — a reason why protein synthesis is metabolically expensive.
Four Levels of Protein Structure
Primary (1°): The linear sequence of amino acids linked by covalent peptide bonds. Encoded by DNA. Determines everything above it. Sickle cell disease is caused by a single amino acid substitution (Val for Glu at position 6 of hemoglobin beta chain) — one bond change, catastrophic consequences.
Secondary (2°): Local folding patterns stabilized by hydrogen bonds between backbone –NH and –C=O groups (not side chains). Alpha-helix (right-handed coil, 3.6 residues/turn) and beta-pleated sheet (parallel or antiparallel strands). Found in collagen (triple helix), keratin (alpha-helix), and silk (beta-sheet).
Tertiary (3°): Overall 3D folding of the entire polypeptide chain driven by R-group interactions: H-bonds (polar R groups), disulfide bonds (–S–S– between Cys residues, covalent), ionic bonds (opposite charges), van der Waals forces. The folded shape creates the active site of enzymes and the binding regions of receptors.
Quaternary (4°): Assembly of two or more polypeptide chains (subunits) into a multi-subunit protein using the same non-covalent forces as tertiary structure. Hemoglobin: 2 alpha (α) + 2 beta (β) subunits — tetrameric. Cooperative O₂ binding arises from quaternary structural changes between subunits. Collagen: triple helix of three chains. Antibodies (IgG): two heavy + two light chains.
Glycoproteins: Protein + Carbohydrate
Many membrane and secreted proteins have covalently attached carbohydrate chains (oligosaccharides) — these are glycoproteins. The sugar chains project outward from cell surfaces, forming the glycocalyx. Blood type (ABO) is determined by glycoprotein antigens on red blood cell surfaces: Type A has A antigens, Type B has B antigens, Type AB has both, Type O has neither. Mismatched transfusion triggers antibody-mediated hemolysis because the immune system recognizes foreign glycoprotein antigens.
Denaturation: Structure Lost, Primary Sequence Intact
Denaturation unfolds a protein by disrupting the non-covalent interactions (hydrogen bonds, ionic bonds, van der Waals forces) that hold tertiary and quaternary structure together. The peptide bonds of the primary structure remain intact — the sequence of amino acids is unchanged. This is why a denatured enzyme loses its catalytic activity (active site shape is lost) but cooking an egg doesn't destroy amino acid nutrition (the primary structure is still digestible). Fever-induced denaturation of critical enzymes is why body temperatures above 41°C (106°F) are life-threatening.
Which bond type holds the PRIMARY structure of a protein together?
How enzymes are regulated — and how drugs exploit this
Enzymes are biological catalysts that lower activation energy without being consumed. Every metabolic reaction in the body requires an enzyme. Understanding enzyme kinetics explains how drugs target metabolic pathways, why some poisons are rapidly lethal, and how the body regulates its own chemistry through feedback inhibition.
Michaelis-Menten Kinetics
The rate of an enzyme-catalyzed reaction increases hyperbolically as substrate concentration [S] rises, eventually plateauing at maximum velocity (Vmax) when all enzyme active sites are occupied (saturation).
Km (Michaelis constant): The substrate concentration at which reaction velocity = ½ Vmax. Km is inversely related to enzyme-substrate affinity — low Km means tight binding (high affinity), high Km means loose binding (low affinity). Different enzymes for the same substrate can have different Km values (e.g., hexokinase vs glucokinase in glucose metabolism).
Allosteric Regulation & Feedback Inhibition
Allosteric regulation: A regulatory molecule binds a site separate from the active site (allosteric site), inducing a conformational change that increases or decreases enzyme activity. Does not compete with substrate.
Feedback inhibition: The end product of a metabolic pathway inhibits an upstream enzyme, preventing overproduction and conserving resources. Example: excess ATP inhibits phosphofructokinase-1 (PFK-1), slowing glycolysis when energy is sufficient.
The inhibitor resembles the substrate and binds reversibly to the active site, blocking substrate entry. Effect on kinetics: Vmax is UNCHANGED (can overcome by adding enough substrate), but apparent Km INCREASES (need more substrate to reach half-Vmax). Clinical example: Statins (atorvastatin, simvastatin) competitively inhibit HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis. Methotrexate competitively inhibits dihydrofolate reductase (DHFR), blocking folate metabolism in cancer cells. Sildenafil (Viagra) competitively inhibits phosphodiesterase type 5 (PDE5).
The inhibitor binds an allosteric site (not the active site), inducing a shape change that reduces enzyme activity regardless of substrate concentration. Effect on kinetics: Vmax DECREASES, Km is UNCHANGED. Cannot be overcome by adding more substrate because the active site geometry is permanently altered in the inhibited enzyme-inhibitor complex. Clinical examples: some antibiotics exploit non-competitive inhibition of bacterial enzymes. Heavy metal poisoning (lead, mercury) inactivates enzymes non-competitively by binding to –SH groups of cysteine residues.
The inhibitor forms a covalent (permanent) bond with a residue in the active site, permanently inactivating the enzyme. Recovery requires synthesis of new enzyme protein. Examples: (1) Aspirin irreversibly acetylates serine-530 in the active site of COX-1 and COX-2 (cyclooxygenase enzymes), blocking prostaglandin and thromboxane synthesis. This is why aspirin's antiplatelet effect lasts the entire platelet lifespan (7–10 days) — platelets cannot synthesize new COX. (2) Clopidogrel irreversibly blocks P2Y12 ADP receptor on platelets. (3) Proton pump inhibitors (omeprazole) irreversibly inhibit the H+/K+-ATPase pump.
Organophosphates (malathion, parathion, sarin, VX) irreversibly inhibit acetylcholinesterase (AChE) by forming a covalent phosphate bond with the serine residue in AChE's catalytic triad. Acetylcholine (ACh) accumulates at all cholinergic synapses. Muscarinic effects (SLUDGE: Salivation, Lacrimation, Urination, Defecation, GI cramps, Emesis) + nicotinic effects (fasciculations, weakness, paralysis) + CNS effects (seizures, coma). The covalent bond then undergoes 'aging' — a permanent conformational change over roughly 24–48 hours after which the inhibition cannot be chemically reversed at all. That time dependence is a property of the bond itself, and it is the clearest illustration of why reversible and irreversible inhibition are genuinely different chemistry rather than a matter of degree.
Zymogen Activation
Some enzymes are synthesized as inactive precursors (zymogens or proenzymes) that require proteolytic cleavage to become active. This prevents destructive enzyme activity in the wrong place. Examples: pepsinogen → pepsin (activated by stomach HCl and pepsin itself); trypsinogen → trypsin (activated by enteropeptidase in duodenum); blood clotting factors (Factors II, VII, IX, X) are zymogens — the clotting cascade is a sequential zymogen activation amplification system. Acute pancreatitis occurs when digestive zymogens activate within the pancreas instead of the intestine, digesting the pancreatic tissue itself.
Organophosphate Poisoning: Time-Critical Antidote Window
Organophosphate compounds (certain pesticides such as malathion, and nerve agents such as sarin) irreversibly inhibit acetylcholinesterase by forming a covalent bond with the serine residue in its active site. Acetylcholine accumulates at nerve-muscle junctions and autonomic synapses, causing SLUDGE: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis — plus bradycardia, bronchospasm, and muscle paralysis. This is why the bond type matters: because the inhibition is covalent rather than reversible, the enzyme cannot simply out-compete the poison, and the effect persists until new enzyme is made. Emergency treatment for these exposures exists and is directed by the clinical team — as a pre-nursing learner your goal is to understand why an irreversible covalent inhibitor behaves so differently from a reversible one.
Energy flow in biological systems
All cellular processes obey the laws of thermodynamics. Understanding free energy, coupled reactions, redox chemistry, and equilibrium explains how cells harvest energy from nutrients, why metabolism is directional, and how the lungs and kidneys cooperate in acid-base balance.
Exergonic Reactions (ΔG < 0)
Release free energy, proceed spontaneously, thermodynamically favorable. Catabolic reactions (breaking down molecules) are generally exergonic. Complete oxidation of one glucose molecule releases ~686 kcal/mol. Cellular respiration is a series of exergonic reactions that captures this energy in ATP. Spontaneous does NOT mean instantaneous — activation energy can still be a barrier (addressed by enzymes).
Endergonic Reactions (ΔG > 0)
Require energy input, not spontaneous. Anabolic reactions (building molecules: protein synthesis, gluconeogenesis, fatty acid synthesis) are endergonic. Cells drive endergonic reactions by coupling them to ATP hydrolysis. The overall reaction (endergonic + ATP hydrolysis) has a net negative ΔG, making it thermodynamically favorable as a coupled process.
ATP: The Universal Energy Currency
Structure: Adenosine (adenine + ribose) + 3 phosphate groups. The bonds connecting the second and third phosphates are high-energy phosphoanhydride bonds created by condensing two negatively charged phosphate groups — the bond stores energy as electrostatic strain.
Hydrolysis: ATP + H₂O → ADP + Pᵢ + ~7.3 kcal/mol (−7.3 kcal/mol ΔG under standard conditions; −11 to −13 kcal/mol under physiological conditions due to Mg²⁺ chelation and concentration gradients). This energy drives muscle contraction (myosin ATPase), active transport (Na⁺/K⁺-ATPase, Ca²⁺-ATPase), biosynthesis, and signaling.
Redox Reactions: Electron Carriers in Metabolism
Oxidation: Loss of electrons (or H atoms). Reduction: Gain of electrons. Mnemonic: "LEO the lion says GER" — Loses Electrons = Oxidized, Gains Electrons = Reduced. Oxidation and reduction always occur together (redox reactions).
NAD⁺/NADH: Nicotinamide adenine dinucleotide. NAD⁺ accepts 2 electrons + 1 H⁺ → NADH (reduced form). NADH carries electrons to the electron transport chain (ETC) for oxidative phosphorylation, generating ~2.5 ATP per NADH.
FAD/FADH₂: Flavin adenine dinucleotide. FAD accepts 2 electrons → FADH₂. Carries electrons to ETC at a lower energy level than NADH, generating ~1.5 ATP per FADH₂. Both carriers are reduced in the citric acid cycle and deliver electrons to Complex I (NADH) or Complex II (FADH₂) of the ETC.
Le Chatelier's Principle in Acid-Base Physiology
Chemical equilibrium: for a reversible reaction A + B ⇌ C + D, Le Chatelier's principle states that if a stress (concentration change, pressure change) is applied to a system at equilibrium, the reaction shifts to relieve that stress. Applied to the bicarbonate buffer: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The lungs remove CO₂ by exhaling (driving equilibrium LEFT — decreasing H⁺, raising pH). Hyperventilation causes respiratory alkalosis; hypoventilation causes respiratory acidosis. The kidneys excrete H⁺ and reabsorb HCO₃⁻ to shift equilibrium — renal compensation takes hours to days, pulmonary compensation takes seconds to minutes.
Why the Body Cannot Store ATP Long-Term
ATP is a highly charged, bulky molecule that cannot cross cell membranes — each cell must produce its own. Cellular ATP stores are exhausted within seconds of maximal exercise or oxygen deprivation. A resting adult turns over roughly 40 kg of ATP per day (the entire body weight in ATP!), recycling each ADP back to ATP hundreds of times. This continuous demand explains why ischemia (interrupted blood flow) causes cell death within minutes: without O₂, oxidative phosphorylation stops, ATP falls, ion pumps fail, cells swell, and necrosis begins.
A reaction with ΔG < 0 is best described as:
IV fluids, osmolality, and tonicity
Clinical chemistry of solutions goes far beyond basic concentration calculations. Understanding molarity, equivalents, osmolality, and IV fluid tonicity directly determines patient safety in fluid management — one of the most common and potentially dangerous nursing interventions.
Molarity (M) and Equivalents
Molarity (M): moles of solute per liter of solution. Used in laboratory chemistry and pharmacokinetics. 1 M NaCl = 58.44 g NaCl per liter.
Equivalents (Eq) / milliequivalents (mEq): For electrolytes, accounts for ionic charge. 1 Eq = 1 mole of charge. For monovalent ions (Na⁺, K⁺, Cl⁻): 1 mmol = 1 mEq. For divalent ions (Ca²⁺, Mg²⁺): 1 mmol = 2 mEq (since each ion carries 2 charges). Normal saline (0.9% NaCl) = 9 g NaCl/L = 154 mEq/L Na⁺ = 154 mEq/L Cl⁻ = ~308 mOsm/L total.
Osmolality vs Osmolarity
Osmolarity: milliosmoles per liter of solution (mOsm/L) — a calculated value. Osmolality: milliosmoles per kilogram of water (mOsm/kg H₂O) — measured by the laboratory using freezing point depression. Osmolality is more clinically reliable because it does not change with temperature.
Normal serum osmolality: 280–295 mOsm/kg
Serum osmolality formula: 2[Na⁺] + [glucose (mg/dL) ÷ 18] + [BUN (mg/dL) ÷ 2.8]. Na⁺ dominates (accounts for ~90% of osmolality) because it is the primary extracellular cation and chloride follows it. An osmolal gap >10 mOsm/kg suggests unmeasured osmoles (methanol, ethylene glycol, mannitol).
Colloid Osmotic Pressure vs Crystalloid Osmotic Pressure
Crystalloid osmotic pressure: generated by all dissolved particles (electrolytes, glucose). Draws water across semipermeable membranes but does not keep water in the vascular compartment long-term because small molecules equilibrate freely. Colloid osmotic pressure (oncotic pressure): generated specifically by large molecules (primarily albumin, ~80% of plasma oncotic pressure) that cannot cross the capillary wall. Keeps fluid in the vasculature. Hypoalbuminemia (liver failure, nephrotic syndrome, malnutrition) reduces oncotic pressure → edema as fluid leaks to interstitium.
Composition: 154 mEq/L Na⁺ + 154 mEq/L Cl⁻. Isotonic to plasma (~308 mOsm/L). Stays in extracellular fluid (ECF); does not shift water into or out of cells. Uses: hypovolemia (trauma, hemorrhagic shock, sepsis initially), medication dilution/flush, hyperkalemia (no K⁺), metabolic alkalosis. Caution: large volumes cause hyperchloremic metabolic acidosis (excess Cl⁻ consumes HCO₃⁻) and can worsen fluid overload in CHF or renal failure. Does NOT contain potassium — monitor K⁺ levels.
Composition: 77 mEq/L Na⁺ + 77 mEq/L Cl⁻. Hypotonic to plasma. After infusion, water shifts from ECF into cells by osmosis — distributes to all body fluid compartments (intracellular + extracellular). Uses: cellular dehydration (hypernatremia with intact ADH response), diabetic ketoacidosis after initial NS resuscitation, nasogastric fluid replacement. Cautions: can cause cerebral edema if infused too rapidly (water shifts INTO brain cells); contraindicated in increased intracranial pressure, hyponatremia, third-spacing; should not be used for resuscitation of hypovolemia (distributes away from vasculature).
Composition: 513 mEq/L Na⁺ + 513 mEq/L Cl⁻. Strongly hypertonic. Draws free water out of cells into ECF and vasculature. Uses: severe symptomatic hyponatremia (seizures, obtundation), cerebral edema (traumatic brain injury), SIADH. Cautions: MUST be administered via central venous catheter (peripheral infusion causes phlebitis and tissue necrosis); rate is strictly controlled — overcorrection of hyponatremia faster than 8–10 mEq/L per 24 hours risks osmotic demyelination syndrome (central pontine myelinolysis). Requires frequent serum sodium monitoring.
Composition: 130 mEq/L Na⁺, 109 mEq/L Cl⁻, 4 mEq/L K⁺, 2.7 mEq/L Ca²⁺, 28 mEq/L lactate (metabolized to HCO₃⁻ by liver). Slightly hypotonic to plasma (273 mOsm/L) but behaves as isotonic in practice. More physiologically similar to plasma than NS because it has lower Cl⁻ and contains K⁺, Ca²⁺, and lactate buffer. Preferred for: trauma resuscitation, burns, surgical fluid replacement, DKA (avoids hyperchloremic acidosis of NS). Cautions: contains K⁺ — avoid in hyperkalemia; contains Ca²⁺ — do not mix with blood products (calcium chelates citrate anticoagulant → clotting); lactate may accumulate in severe liver failure.
Composition: 50 g dextrose (glucose) per liter in free water. Initially isotonic (~252 mOsm/L). However, cells rapidly metabolize the glucose → after absorption, the solution effectively becomes free water distributed to ALL compartments (2/3 intracellular, 1/3 extracellular). Thus, D5W is NOT an effective volume expander for hypovolemia — only 1/12 of the infused volume stays in the vasculature. Uses: providing free water for hypernatremia correction, vehicle for IV medication delivery, hypoglycemia (D50W in emergency). Cautions: can worsen cerebral edema (same as free water), causes hyponatremia with large volumes, provides minimal electrolytes. Should NOT be used for resuscitation.
Wrong IV Fluid Tonicity Can Be Fatal
Administering the wrong tonicity IV fluid can be fatal. Giving a large volume of free water (D5W or hypotonic saline) too rapidly to a patient with hyponatremia can worsen cerebral edema. Conversely, infusing hypertonic saline too quickly in chronic hyponatremia can cause osmotic demyelination syndrome (central pontine myelinolysis), as neurons shrink faster than they can adapt. Both directions of harm come from the same principle you already know: water follows solute across a semipermeable membrane, and brain cells tolerate a slow osmotic shift far better than a fast one. Choosing and titrating IV fluids is a licensed clinician's decision made against a specific patient's laboratory values — what belongs to you now is the underlying osmosis, tonicity, and why rate of change matters at all.
What type of bond holds NaCl together in its solid crystal form?
