Kidney Functions and ESRD Basics
Key Takeaways
- Each kidney holds about one million nephrons; the glomerulus filters ~180 L of plasma per day, and tubules reabsorb almost all of it so only 1-2 L becomes urine.
- Healthy kidneys perform six core jobs: clear nitrogenous wastes, balance fluid and electrolytes, regulate acid-base by excreting acid and making bicarbonate, control blood pressure (renin), make erythropoietin for red cells, and activate vitamin D for calcium-phosphorus and bone health.
- CKD is staged by eGFR: Stage 1 ≥90, Stage 2 60-89, Stage 3a 45-59, Stage 3b 30-44, Stage 4 15-29, and Stage 5 (ESRD/kidney failure) <15 mL/min/1.73 m2.
- Diabetes and hypertension cause the majority of U.S. ESRD; hemodialysis replaces only clearance and fluid removal—it does NOT replace the endocrine functions, so patients still need EPO/ESA, active vitamin D, and phosphate binders.
- On the CCHT exam the safe action is verify, report, document, and follow facility protocol—never diagnose or independently change the prescription.
The Nephron and What Healthy Kidneys Do
The two kidneys sit retroperitoneally on either side of the spine and together receive roughly 20-25% of cardiac output. Each kidney contains about one million nephrons, the microscopic functional units. A nephron has two parts: the glomerulus (a tuft of capillaries inside Bowman's capsule that filters plasma) and the renal tubule (which reabsorbs and secretes to fine-tune the filtrate).
Filtration is enormous. The glomeruli filter about 180 liters of plasma every day (the glomerular filtration rate, GFR). The tubules then reabsorb roughly 99% of that water and most solutes, so only 1-2 liters leaves as urine. This is why a small drop in tubular function produces large changes in fluid and electrolyte balance.
Healthy kidneys perform six core functions the CCHT must be able to name, because dialysis replaces only some of them.
The six functions (and which ones dialysis cannot replace)
| Function | Mechanism | Replaced by hemodialysis? |
|---|---|---|
| Waste clearance | Filter urea, creatinine, uric acid, drugs | Yes—diffusion across the dialyzer |
| Fluid balance | Adjust water excretion | Yes—ultrafiltration removes excess water |
| Electrolyte balance | Handle Na+, K+, Ca2+, PO4, Mg2+ | Partly—dialysate sets the gradient |
| Acid-base balance | Excrete H+, regenerate bicarbonate | Partly—bicarbonate dialysate buffers acid |
| Blood pressure control | Renin → angiotensin → aldosterone; fluid volume | Indirectly (via fluid removal) |
| Red cell production | Erythropoietin (EPO) stimulates marrow | No—needs ESA injections |
| Bone-mineral health | Activates vitamin D (calcitriol) for Ca/PO4 | No—needs active vitamin D + binders |
The last three—blood pressure hormones, EPO, and vitamin D activation—are endocrine jobs. A dialyzer is a passive filter; it cannot make a hormone. That is why a dialysis patient still receives erythropoiesis-stimulating agents (ESAs) for anemia, active vitamin D and phosphate binders for bone-mineral disease, and blood-pressure medications. A common exam distractor claims dialysis "replaces all kidney functions"—it does not.
From CKD to ESRD: Causes and Stages
Chronic kidney disease (CKD) is the gradual, usually irreversible loss of nephrons over months to years. The leading causes in the United States are diabetes mellitus (the single most common) and hypertension, which together account for most cases; glomerulonephritis and polycystic kidney disease follow. CKD is staged by the estimated glomerular filtration rate (eGFR) in mL/min/1.73 m2:
- Stage 1 — eGFR ≥90 with evidence of kidney damage (e.g., protein in urine).
- Stage 2 — eGFR 60-89, mild loss.
- Stage 3a — eGFR 45-59; Stage 3b — 30-44, moderate loss.
- Stage 4 — eGFR 15-29, severe loss; this is when access placement and modality education usually begin.
- Stage 5 / ESRD — eGFR <15 mL/min/1.73 m2, kidney failure requiring kidney replacement therapy (dialysis or transplant).
End-stage renal disease (ESRD) means the kidneys can no longer keep the internal environment stable on their own. The earlier term uremia describes the toxic syndrome that results, in which retained wastes poison nearly every organ system.
Uremia: How Failed Clearance Shows Up
As nephrons are lost, urea, creatinine, and dozens of other waste molecules accumulate in the blood. The resulting illness, uremia, is what dialysis is designed to relieve. Its signs span many systems and are mostly nonspecific, which is why the CCHT watches for change from a patient's baseline rather than a single textbook sign.
- Gastrointestinal: nausea, vomiting, loss of appetite (anorexia), a metallic taste.
- Neurologic: fatigue, difficulty concentrating, restless legs, and—when advanced—confusion or drowsiness (uremic encephalopathy).
- Skin: generalized itching (pruritus), dry skin, and a sallow color.
- Cardiopulmonary: with fluid retention, shortness of breath, edema, and high blood pressure.
Two lab values track this: the blood urea nitrogen (BUN) and serum creatinine, both of which rise as clearance falls. Because dialysis lowers BUN with each session, the before-and-after BUN also becomes the basis for measuring how well a treatment worked—a link explored in the adequacy section. The CCHT's job is to recognize uremic patterns, report changes, and reinforce attendance, since missed or shortened treatments let wastes climb back up between sessions.
Linking findings to physiology (without diagnosing)
Before treatment, the CCHT connects what they observe to a likely physiologic cause, then reports—they do not label a diagnosis or change the order.
| Patient finding | Likely physiology | CCHT-safe action |
|---|---|---|
| New shortness of breath, edema, big weight gain | Fluid overload from lost water excretion | Verify weight vs. target, take vitals, notify RN |
| Pale, fatigued, low hematocrit | Loss of erythropoietin → anemia | Report; reinforce ESA care plan |
| Bone pain, high phosphorus, itching | Lost vitamin D activation → mineral-bone disease | Reinforce binders/diet; report symptoms |
| Confusion, severe nausea, metallic taste | Uremia (waste accumulation) | Notify licensed staff before routine start |
| Muscle weakness, palpitations | Electrolyte problem (e.g., high K+) | Report immediately; follow protocol |
The exam rewards answers that verify, report, document, and follow facility policy. Starting or changing treatment because a symptom "is common in dialysis patients" is the classic wrong answer—common symptoms can still signal an emergency, and interpreting or treating them is outside the technician's scope.
A patient new to your unit asks why she still needs an injection "to build blood" and a pill with meals to "hold down phosphorus" if dialysis "cleans everything." What is the best basis for your answer?
A pre-dialysis patient's chart lists an eGFR of 12 mL/min/1.73 m2. Which CKD stage does this represent?
Which pair represents the two most common causes of end-stage renal disease in the United States?
Approximately how much plasma do the glomeruli filter each day, and how much normally leaves the body as urine?