1.2 Renal Physiology & Chronic Kidney Disease
Key Takeaways
- The nephron filters blood at the glomerulus and reabsorbs electrolytes, glucose, and water in specialized segments like the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
- Estimated GFR is calculated using creatinine-based equations like Cockcroft-Gault, MDRD, and CKD-EPI, though clinicians must consider body surface area, age, sex, and muscle mass limitations.
- Chronic kidney disease progresses through stages one to five based on estimated GFR and the presence of kidney damage.
- Loss of renal erythropoietin secretion causes normochromic normocytic renal anemia, requiring monitoring of hemoglobin levels and iron optimization prior to initiating erythropoiesis-stimulating agents.
- Mineral and bone disorders in renal failure arise from hyperphosphatemia, hypocalcemia, secondary hyperparathyroidism, and calcitriol deficiency, manifesting as osteitis fibrosa cystica or adynamic bone disease.
Functional Anatomy of the Nephron and Segmental Transport
The nephron is the functional unit of the kidney. Filtration begins at the glomerulus, where hydrostatic pressure drives water and solutes across the glomerular filtration barrier into Bowman's capsule. This three-layered barrier consists of fenestrated capillary endothelium, the glomerular basement membrane, and podocyte filtration slits. Large proteins and cellular elements are restricted from entering the filtrate.
The ultrafiltrate enters the proximal convoluted tubule (PCT) for bulk reabsorption. Here, roughly 65% of filtered water, sodium, chloride, and potassium, and 100% of glucose and amino acids are reabsorbed. The PCT also regulates acid-base balance by reabsorbing bicarbonate ($HCO_3^-$) and secreting hydrogen ions ($H^+$).
Next, the filtrate enters the loop of Henle, operating as a countercurrent multiplier to create a hypertonic medullary interstitium. The descending limb is highly permeability to water but impermeable to solutes, causing water to leave the tubule. The ascending limb is impermeable to water but active in solute transport. The thick ascending limb contains the $Na^+-K^+-2Cl^-$ cotransporter, pumping sodium, potassium, and chloride into the medullary interstitium to establish the osmotic gradient.
The filtrate then passes into the distal convoluted tubule (DCT), where thiazide-sensitive cotransporters reabsorb sodium and chloride. Calcium reabsorption here is regulated by parathyroid hormone (PTH). Finally, the collecting duct regulates water and acid-base excretion. Antidiuretic hormone (ADH) stimulates principal cells to insert aquaporin-2 water channels, enabling water reabsorption. Aldosterone acts on principal cells to promote sodium reabsorption and potassium excretion, while intercalated cells regulate hydrogen and bicarbonate secretion.
Glomerular Filtration Rate (GFR) Calculations and Staging
The Glomerular Filtration Rate (GFR) is the gold standard clinical index of kidney function, representing the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time. GFR is typically indexed to a standard body surface area of 1.73 square meters. Because direct measurement is difficult, clinicians calculate estimated GFR (eGFR) using endogenous markers, primarily serum creatinine.
Common eGFR formulas include:
- Cockcroft-Gault Equation: Estimates creatinine clearance ($CrCl$) based on age, weight, serum creatinine, and sex:
- MDRD Equation: Estimates GFR adjusting for serum creatinine, age, sex, and race.
- CKD-EPI Equation: The current clinical standard, providing more accurate GFR estimates at higher ranges than MDRD.
Creatinine-based equations have limitations. Patients with low muscle mass (due to amputation or muscle wasting) will have low creatinine levels, overestimating their GFR, whereas high muscle mass or high protein intake can underestimate it.
Chronic Kidney Disease (CKD) is defined as kidney abnormalities present for more than 3 months, classified into 5 stages based on GFR and albuminuria:
| CKD Stage | GFR Range ($mL/min/1.73m^2$) | Clinical Description |
|---|---|---|
| Stage 1 | $\ge 90$ | Kidney damage (e.g., microalbuminuria) with normal GFR. |
| Stage 2 | $60 - 89$ | Kidney damage with mild decrease in GFR. |
| Stage 3a | $45 - 59$ | Mild-to-moderate GFR decrease; complications begin. |
| Stage 3b | $30 - 44$ | Moderate-to-severe GFR decrease. |
| Stage 4 | $15 - 29$ | Severe GFR decrease; initiate renal replacement prep. |
| Stage 5 | $< 15$ | Kidney failure (End-Stage Renal Disease); requires dialysis or transplant. |
Systemic Complications of Chronic Kidney Disease
As GFR declines, loss of excretory and endocrine functions causes severe systemic complications.
Renal Anemia
The primary cause of renal anemia is erythropoietin (EPO) deficiency. Erythropoietin is a glycoprotein hormone produced by renal peritubular cells that stimulates red blood cell production. As renal tissue fibroses, EPO synthesis falls, leading to normochromic, normocytic anemia. Secondary causes include iron deficiency and chronic inflammation. Management involves erythropoiesis-stimulating agents (ESAs) like epoetin alfa. The target hemoglobin range for CKD patients on ESAs is 10 to 11 g/dL; higher targets increase cardiovascular risks. Iron stores must be optimized (transferrin saturation $\ge 20%$ and ferritin $\ge 200\text{ ng/mL}$) before initiating ESAs.
Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD)
CKD-MBD is a systemic syndrome characterized by mineral, bone, and calcification abnormalities. As GFR falls, phosphorus excretion declines, causing hyperphosphatemia. Elevated phosphorus stimulates fibroblast growth factor-23 (FGF-23) secretion and suppresses renal active vitamin D (calcitriol) synthesis. Calcitriol deficiency impairs intestinal calcium absorption, causing hypocalcemia.
The resulting hypocalcemia, hyperphosphatemia, and calcitriol deficiency stimulate parathyroid glands, leading to secondary hyperparathyroidism (sHPTH). Persistent PTH elevations cause renal osteodystrophy, manifesting as high-turnover bone disease (osteitis fibrosa cystica) with rapid bone resorption. Conversely, aggressive calcitriol therapy can over-suppress PTH, causing low-turnover adynamic bone disease, increasing fracture and vascular calcification risks.
Electrolyte and Acid-Base Alterations
- Hyperkalemia: Reduced filtration and aldosterone responsiveness cause potassium retention. Severe hyperkalemia ($K^+ > 6.0\text{ mEq/L}$) alters myocardial membrane potentials, causing ECG changes (peaked T-waves, PR prolongation, widened QRS, or ventricular fibrillation). Treatments include calcium gluconate to stabilize the cardiac membrane, insulin and glucose to shift potassium intracellularly, and sodium bicarbonate. Oral potassium binders are used for chronic management.
- Metabolic Acidosis: Reduced renal excretion of ammonium ($NH_4^+$) and impaired bicarbonate reabsorption lead to metabolic acidosis. This is managed with oral sodium bicarbonate when serum bicarbonate falls below 22 mEq/L, slowing CKD progression and reducing bone buffering-induced osteopenia.
A patient with stage 4 chronic kidney disease presents with a serum potassium of six point four milliequivalents per liter. The nurse notes peaked T-waves and a prolonged PR interval on the electrocardiogram. Which of the following interventions should the nurse expect to implement first to protect the patient from life-threatening arrhythmias?
A patient with stage 5 chronic kidney disease is being evaluated for renal anemia. The laboratory results show a hemoglobin of eight point two grams per deciliter, a transferrin saturation of fifteen percent, and a ferritin level of one hundred nanograms per milliliter. Which of the following is the most appropriate initial nursing intervention based on these findings?
During a clinical assessment, the nurse reviews the laboratory results of a patient with advanced chronic kidney disease and notes elevated serum phosphorus, low serum calcium, and an elevated parathyroid hormone level. Which of the following sequences best describes the physiological progression leading to this condition?