Module 5 — Principles of Dialysis

Sources: Condensed study notes based on Core Curriculum for the Dialysis Technician, 7th ed. (MEI), plus free CDC/CMS/NIDDK resources.

Objectives and big picture

Study guide

Dialysis uses a semipermeable membrane and controlled fluid movement to replace selected kidney functions: remove wastes, remove excess water, rebalance electrolytes, and help correct acid–base status.
The module’s core processes are diffusion, osmosis, convection, adsorption, filtration/ultrafiltration, and fluid dynamics.
PD applies these principles inside the abdomen; HD applies them across a dialyzer. The nephrologist prescribes the treatment; the technician delivers it safely and accurately.

Why answers are correct

A treatment works because blood/body fluid and dialysate are separated while gradients and pressure are deliberately controlled.

Limitations / traps

“Dialysis removes everything from the body” is wrong: dialysis directly reaches circulating blood, while much water and waste is temporarily elsewhere in body-fluid compartments.
Principles explain treatment; they do not authorize independent changes to prescription, temperature, electrolyte, or UF settings.

Clinical pearl / safety

Connect symptoms and machine observations to the responsible nurse/nephrologist promptly; do not diagnose or self-adjust the prescription.

Exam cue

Know what moves, across what barrier, in which direction, and what force drives it.

Solutions, solubility, and concentration

Study guide

Solution: solvent plus dissolved solute. The solvent is the fluid; the solute is the substance dissolved in it.
Solubility: how completely a solute dissolves. Important influences are concentration, pH, solvent temperature, and other solutes.
Concentration: amount of dissolved solute relative to a given volume. A saturated solution cannot dissolve more of that solute under the current conditions.
Common units include mg/L (also ppm), mg/dL, and mEq/L. Read the unit before interpreting a value.
pH: reflects the relative amount of acidic versus basic ions: 7 is neutral, below 7 acidic, and above 7 alkaline.
Warmer solvent generally dissolves more solute. Interacting solutes can reduce solubility and create a precipitate; excess calcium and phosphate together can form damaging crystals.

Why answers are correct

Dialysis depends on carefully selected solute concentrations to create gradients and avoid adding unwanted precipitation or imbalance.

Limitations / traps

Concentration, amount, and volume are not interchangeable. A warmer or more concentrated solution is not automatically safer.

Clinical pearl / safety

Treat visible precipitate, unexpected solution appearance, or a questionable mix as a safety issue and follow facility procedure.

Exam cue

Solvent = fluid; solute = dissolved substance; pH and temperature change solubility.

Membranes and transport forces

Study guide

A permeable membrane allows water and particles through; a semipermeable membrane allows selected particles—often based on size—to pass.
Diffusion: solutes move down a concentration gradient, from higher to lower concentration, until equilibrium is approached. It can occur with or without a membrane.
Diffusion is affected by concentration gradient, temperature, pressure, solute size, fluid viscosity, distance, surface area, barrier permeability, solubility, and dissociation.
Small, soluble particles generally diffuse faster than large or protein-bound particles. In dialysis, a larger membrane area and a thinner/more suitable barrier improve clearance.
Convection: moving water carries dissolved solutes along with it. This can help clear larger molecules that diffuse poorly.
Osmosis: solvent (water) crosses a semipermeable membrane toward the side with greater effective solute concentration. Osmotic pressure is the pull created by that gradient.
Adsorption: molecules adhere to a surface and form a film; this differs from absorption, which is movement into a material.
Hydraulic pressure from gravity or pumps can move solvent across a membrane and can oppose or reinforce osmotic pressure.

Why answers are correct

“Diffusion = solutes” and “osmosis = solvent” is the load-bearing distinction. Both tend toward balance, but osmosis requires a membrane.

Limitations / traps

Equilibrium does not mean no movement; it means no net concentration difference. A membrane’s pores restrict some particles but do not create an absolute wall for water.

Compare / contrast

Process	What moves?	Membrane required?	Dialysis role
Diffusion	Solutes	No	Clears small solutes down a gradient
Osmosis	Solvent/water	Yes	Moves water toward higher effective solute
Convection	Water carrying solutes	Usually a porous barrier	Adds clearance, especially for larger solutes
Adsorption	Molecules attach to a surface	No fixed requirement	Can remove protein-bound material; can also slow transport

Exam cue

Ask: particle or water? Gradient or pressure? Crossing the membrane or sticking to it?

Fluid dynamics, filtration, and body compartments

Study guide

Flow rate: volume passing a point per unit time. Flow velocity: how fast fluid travels through a tube; with constant flow, a narrower cross-section raises velocity. Resistance: opposition from friction or narrowing; greater resistance raises pressure.
Filtration: a filter retains particles as fluid passes through. Ultrafiltration (UF): removes very small particles in general use; in dialysis, UF specifically means removing water from blood by pressure.
Body water is divided into compartments: about 57% intracellular, about 43% extracellular; extracellular water includes interstitial fluid (~20%), vascular fluid (~6.7%), and other spaces such as bone/fat (~16%). Percentages are approximate teaching values.
Homeostasis depends on water and solutes shifting between compartments. During HD, water is first removed from vascular blood, then interstitial water refills the vascular space, and intracellular water refills the interstitial space.

Why answers are correct

A patient’s scale change is not automatically all water. Clothing, food, muscle, fat, and seasonal behavior can also change weight.

Clinical pearl / safety

Compare similar clothing/shoes when possible and ask about the patient’s usual body changes; report unexpected symptoms or inability to tolerate UF.

Exam cue

UF removes vascular water first; refill from other compartments takes time.

Applying principles to peritoneal dialysis (PD)

Study guide

PD uses the living peritoneum and its capillary blood supply as the exchange membrane. A catheter fills the abdominal cavity with sterile dialysate by gravity; after a dwell, used fluid drains by gravity.
The four PD mechanisms are diffusion, osmosis, convection, and adsorption.
Diffusion in PD: blood has wastes; fresh dialysate has little or none. Wastes and electrolytes move into dialysate. As the dwell approaches equilibrium, diffusion slows; draining and refilling restores the gradient.
An exchange consists of drain plus fill. Manual exchanges occur during the day; a cycler can perform exchanges overnight. The prescriber selects number, dwell length, dialysate volume, and solute concentration.
Osmosis in PD: dextrose usually makes dialysate more concentrated than blood, pulling water from blood into dialysate. Higher osmotic concentration generally removes more water, but the prescription is individualized.
Patients can absorb substantial dextrose from PD fluid, adding calories and potentially affecting weight and glucose. Lower dextrose exposure is preferred when clinically adequate.
Heating dextrose can create glucose-degradation products that injure the peritoneum and reduce its water-removal ability. Icodextrin is a non-dextrose option for some long dwells; it can interfere with non-specific glucose meters, so compatibility matters.
Convection in PD: water flow through pores can carry larger wastes; more dextrose or icodextrin generally increases water removal.
Adsorption in PD: may help remove protein-bound wastes. Fibrin can clump and obstruct a catheter lumen; adsorption can also matter when intraperitoneal insulin is used.

Limitations / traps

More concentrated dialysate is not a technician-selected “stronger is better” solution. It may add glucose burden and contribute to membrane injury.
PD fluid is not simply “water in the belly”; sterility, dwell, drain, and prescribed concentration all matter.

Clinical pearl / safety

Report cloudy drainage, poor inflow/outflow, abdominal symptoms, or glucose-meter incompatibility concerns through the PD team’s protocol.

Exam cue

PD sequence: fill → dwell → drain; fresh gradient restores diffusion.

Applying principles to hemodialysis (HD)

Study guide

HD keeps blood and dialysate on opposite sides of a semipermeable dialyzer membrane. Blood flows inside hollow fibers; dialysate surrounds the fibers.
Plasma is the liquid solvent carrying electrolytes, hormones, proteins, and cells. Red cells carry oxygen, white cells support immunity, and platelets support clotting. Electrolytes are essential to nerve and muscle—especially cardiac—function.
Relative terms: hypotonic means lower solute concentration than blood, isotonic means similar, and hypertonic means higher.
Dialysate uses treated water plus prescribed solutes. Acid concentrate supplies many electrolytes; bicarbonate concentrate supplies buffer (some systems use lactate that is converted to bicarbonate). Dialysate should contain needed solutes but little of the wastes being removed; its overall osmolality is prescribed to approximate healthy blood.
Dialyzer clearance depends on membrane pores/design, surface area, flow geometry, temperature, and solute size. Countercurrent flow keeps a gradient: blood and dialysate travel in opposite directions. Same-direction concurrent flow is less efficient and may be used only for a deliberate, gentler removal goal.
Larger surface area generally improves diffusion. Dialysate is usually maintained around 36–37.5°C; too warm can injure blood cells, while cooler prescribed dialysate can reduce hypotension risk in selected patients.
Solute-size groups: small water-soluble compounds, small fat-soluble protein-bound compounds, and middle molecules such as beta-2-microglobulin. Standard membranes are less effective for some protein-bound and middle-molecule solutes.

Why answers are correct

Repeated blood passes and continuous fresh dialysate are needed because diffusion is gradual. Longer treatment or suitable increases in flow/clearance can remove more waste, but settings remain prescriber-directed.

Limitations / traps

Dialysate does not contact blood directly; a leak is abnormal. Treated dialysate water need not be sterile, but any fluid infused into blood—such as HDF replacement fluid—must be sterile.

Clinical pearl / safety

A temperature, dialysate, pressure, or flow alarm is not merely a nuisance; verify safely and notify the licensed team according to policy.

Exam cue

Inside fibers = blood; outside fibers = dialysate; countercurrent flow preserves the gradient.

Osmosis, UF, and organ risk in HD

Study guide

In HD, the blood pump and dialysate-side pressure control create a transmembrane pressure difference that removes water by UF.
Target weight (TW/“dry weight”/EDW): prescribed post-treatment weight representing minimal excess salt and water. Pre-treatment weight minus target is a starting estimate, but food, clothing, muscle, fat, and drinks must be considered.
UF goal: excess water to remove plus fluid expected during treatment. UFR: UF goal divided by treatment time; a faster rate can exceed plasma refill and cause intradialytic hypotension (IDH).
TMP expresses the pressure difference across the membrane and is related to UFR and the dialyzer UF coefficient: TMP = UFR ÷ KUF. Display sign depends on whether the machine references the blood or dialysate side.
IDH may present with cramps, headache, nausea/vomiting, dizziness, or shock. A high UFR can remove vascular water faster than other compartments refill.
Organ stunning: repeated or severe low blood flow/oxygen delivery can injure organs. Symptoms can include severe cramping, faintness, or prolonged recovery.
Protective strategies are prescriber-directed: lower UFR, longer treatments, and cooler dialysate for suitable patients. The module cautions that routine sodium modeling is not evidence-supported for preventing IDH and may cause harm.
Dialysis disequilibrium syndrome (DDS): rapid blood-solute reduction can leave a gradient between blood and brain. Water shifts into brain tissue, causing swelling. Risk is higher with very high initial waste levels, missed treatments, and in children or older adults.
DDS can cause headache, nausea, vomiting, dizziness, confusion, cramps, tremors, altered mental status, seizures, coma, or death. Prevention may involve slower solute clearance, longer treatment, or another osmotic agent as prescribed.

Why answers are correct

UF is not the same as diffusion: diffusion removes solutes; UF removes water using pressure. Removing water too quickly can produce circulatory and organ injury even when the total goal is reasonable.

Clinical pearl / safety

Report cramps, symptomatic BP decline, confusion, severe headache, nausea/vomiting, or prolonged post-treatment recovery promptly. Never “push through” a suspected DDS or organ-stunning event.

Exam cue

Safe UF is limited by refill. High UFR + symptoms = immediate team attention.

Convection, adsorption, and fluid dynamics in HD

Study guide

Convection drags solutes through membrane pores with moving water. It can improve middle-molecule clearance. Porous/high-flux membranes support this; the sieving coefficient estimates the fraction of a solute that can pass by convection.
Adsorption forms a protein film on the dialyzer. The film can improve biocompatibility and reduce inflammation, but it may also slow diffusion/osmosis and lower efficiency.
In the extracorporeal circuit, the pump pulls blood from the access: pre-pump pressure is usually negative. It then pushes blood through tubing, dialyzer fibers, and the venous return, creating positive post-pump pressure.
Components include blood pump, tubing, heparin pump, dialyzer, and monitors for air, access pressure, and blood flow. Resistance and pressure change along the circuit; pressure is highest near the arterial dialyzer header and falls as blood moves through fibers.
Dialysate pressure is controlled separately. Its gradient relative to blood pressure drives water across the membrane; machine control aims to remove the prescribed amount.

Limitations / traps

A pressure number is meaningful only with its location, sign convention, and trend. Do not infer a treatment problem from one number without checking the machine and clinical context.

Exam cue

Pump pulls before the pump (negative) and pushes after it (positive); resistance changes pressure.

Hemodiafiltration (HDF)

Study guide

HDF increases convective clearance by combining high UF with sterile replacement fluid. More fluid is removed than the patient’s net goal, and replacement fluid is infused to maintain safe volume while washing out additional solute.
Replacement fluid may enter before the dialyzer (pre-dilution), after it (post-dilution), or within it (mid-dilution). Pre-dilution requires more replacement/UF because blood is diluted before entering the dialyzer.
HD dialysate is treated but not sterile; HDF replacement fluid enters the bloodstream and therefore must be sterile. It may be bagged or generated online with additional ultrafiltration safeguards. Mid-dilution can involve backfiltration across the membrane.

Limitations / traps

HDF is not a routine technician-selected upgrade. Prescription, replacement fluid, sterility, and machine safeguards are essential.

Clinical pearl / safety

Treat any concern about replacement-fluid sterility or online preparation as urgent and follow the facility’s escalation procedure.

Appendix: metric and temperature cues

Study guide

High-yield conversions: 1 kg ≈ 2.2 lb; 1 oz ≈ 28.35 g; 1 fl oz ≈ 29.57 mL; 1 mL = 1 cc; 1 gal ≈ 3.785 L; 1 L ≈ 1.057 U.S. qt; 1 m ≈ 39.37 in; 1 in = 2.54 cm.
Volume: 1 dL = 0.1 L; 1 L = 1,000 mL. Mass: 1 mg = 0.001 g; 1 g = 1,000 mg; 1 kg = 1,000 g. Length: 1 mm = 0.001 m; 1 cm = 0.01 m; 1 m = 100 cm; 1 km = 1,000 m.
Temperature recognition points: water freezes at 0°C/32°F, normal body temperature is about 37°C/98.6°F, and water boils at 100°C/212°F.

Exam cue

Track units before calculating; a misplaced prefix can turn a safe dose or UF goal into a dangerous one.

Vocabulary

Uncommon words from this chapter. Plain definitions + a short dialysis-related example.

semipermeable: A barrier that lets selected particles and water through, often based on size.; Example. HD keeps blood and dialysate on opposite sides of a semipermeable membrane.
solute: The dissolved substance in a fluid.; Example. Urea is a solute that dialysis tries to remove from blood.
solvent: The fluid that dissolves other substances.; Example. In blood, plasma water is the solvent carrying electrolytes and wastes.
solubility: How completely a substance dissolves in a fluid.; Example. Temperature and pH change solubility and can create crystals in dialysate.
precipitate: Solid particles that form when dissolved substances come out of solution.; Example. Treat a visible precipitate in a concentrate as a safety problem.
diffusion: Solutes moving from higher to lower concentration.; Example. Diffusion clears small wastes down a concentration gradient.
osmosis: Water moving across a membrane toward the side with more effective solute.; Example. PD uses osmosis so dextrose in dialysate pulls water from blood.
convection: Moving water carrying dissolved solutes with it.; Example. Convection helps clear larger molecules that diffuse poorly.
adsorption: Molecules sticking to a surface and forming a film.; Example. Adsorption on a dialyzer can remove some wastes but may also slow transport.
equilibrium: No net concentration difference left, even though particles may still move.; Example. As a PD dwell nears equilibrium, diffusion slows until fresh fluid restores the gradient.
viscosity: How thick a fluid is and how much it resists flow.; Example. Higher viscosity can slow diffusion across the membrane.
interstitial: The fluid space between cells, outside the blood vessels.; Example. After UF removes vascular water, interstitial fluid slowly refills the blood.
hypotonic: Having a lower solute concentration than blood.; Example. A hypotonic fluid relative to blood can shift water into cells.
isotonic: Having a similar solute concentration to blood.; Example. Dialysate osmolality is prescribed to stay near an isotonic, physiologic range.
hypertonic: Having a higher solute concentration than blood.; Example. Hypertonic PD fluid pulls more water but is not a technician-chosen upgrade.
countercurrent: Two fluids traveling in opposite directions to keep a useful gradient.; Example. Countercurrent flow keeps blood meeting fresher dialysate along the fiber.
transmembrane: Across the dialyzer or peritoneal membrane.; Example. The blood pump and dialysate pressure create a transmembrane pressure for UF.
UFR: Ultrafiltration rate: how fast water is removed during treatment.; Example. A high UFR can exceed plasma refill and cause intradialytic hypotension.
intradialytic: During the dialysis treatment itself.; Example. Intradialytic hypotension can present with cramps, nausea, or dizziness.
disequilibrium: A dangerous imbalance when blood wastes fall faster than brain wastes.; Example. Dialysis disequilibrium syndrome can cause headache, confusion, or seizures.
stunning: Organ injury from repeated low blood flow or oxygen delivery.; Example. High UFR plus symptoms can be a warning of organ stunning.
hemodiafiltration: A treatment that adds high convection plus sterile replacement fluid to HD.; Example. HDF replacement fluid enters blood, so it must be sterile.

Must-know

Diffusion moves solutes high → low; osmosis moves water toward higher effective solute; convection carries solute with water; adsorption sticks molecules to a surface.
PD: fill, dwell, drain, exchange. Dextrose creates the osmotic gradient; fresh dialysate restores the diffusion gradient.
HD: blood inside hollow fibers, dialysate outside; countercurrent flow preserves clearance.
UF removes water by pressure; UFR must not exceed safe plasma refill. High UFR can cause IDH and organ stunning.
DDS follows rapid blood-solute reduction with brain swelling risk; watch for neurologic symptoms.
TMP is tied to UFR and KUF; machine sign depends on the monitored side.
HDF uses high convection plus sterile replacement fluid; infused fluid must be sterile.
Technician role: deliver the prescribed treatment, watch patient and machine, recognize danger, and escalate—do not independently alter the prescription.