Table of Contents
- 1 Overview of Kidney Function
- 1.1 Kidney Anatomy & Filtration Structure
- 1.2 The Filtration Process
- 1.3 Filtration Rates and Normal Values
- 1.4 Substances Filtered and Their Fate
- 1.5 Hormonal Regulation of Filtration
- 1.6 Factors Affecting Filtration
- 1.7 Clinical Assessment of Kidney Function
- 1.8 Signs of Impaired Kidney Filtration
- 1.9 Maintaining Healthy Kidney Filtration
Overview of Kidney Function
The kidneys are vital organs that filter approximately 120-180 liters of blood daily, producing about 1-2 liters of urine. They maintain fluid balance, electrolyte levels, blood pressure, and remove waste products from the body.
Kidney Anatomy & Filtration Structure
Major Components
- Renal Cortex: Outer layer containing glomeruli and proximal tubules
- Renal Medulla: Inner layer with loops of Henle and collecting ducts
- Renal Pelvis: Central area where urine collects before entering ureter
- Nephrons: Functional units (approximately 1 million per kidney)
Nephron Structure
- Glomerulus: Cluster of tiny blood vessels where filtration begins
- Bowman’s Capsule: Surrounds glomerulus, collects filtered fluid
- Proximal Tubule: Reabsorbs most nutrients and water
- Loop of Henle: Concentrates urine and maintains salt balance
- Distal Tubule: Fine-tunes electrolyte balance
- Collecting Duct: Final concentration of urine
The Filtration Process
Step 1: Glomerular Filtration
Location: Glomerulus and Bowman’s Capsule Process: Blood pressure forces water and small molecules through glomerular membrane Rate: Normal GFR (Glomerular Filtration Rate) = 90-120 mL/min/1.73m²
What Gets Filtered:
- Water
- Glucose
- Amino acids
- Urea and creatinine
- Electrolytes (sodium, potassium, chloride)
- Small proteins
What Stays in Blood:
- Red blood cells
- White blood cells
- Platelets
- Large proteins (albumin)
- Large molecules
Step 2: Tubular Reabsorption
Location: Throughout the tubule system Process: Selective recovery of useful substances back into bloodstream
Proximal Tubule Reabsorption
- 65-70% of filtered sodium and water
- 100% of filtered glucose (under normal conditions)
- 90% of filtered bicarbonate
- Most amino acids and proteins
- Phosphate and calcium
Loop of Henle Reabsorption
- 20-25% of filtered sodium
- 15% of filtered water
- Creates concentration gradient for urine concentration
Distal Tubule & Collecting Duct
- 5-10% of filtered sodium (regulated by aldosterone)
- Variable water reabsorption (regulated by ADH)
- Potassium secretion (regulated by aldosterone)
- Acid-base balance (hydrogen ion secretion)
Step 3: Tubular Secretion
Process: Active transport of substances from blood into urine Purpose: Elimination of toxins and maintenance of pH balance
Substances Secreted:
- Hydrogen ions (H+)
- Potassium ions (K+)
- Ammonia (NH₃)
- Creatinine
- Para-aminohippuric acid (PAH)
- Medications and toxins
Filtration Rates and Normal Values
Glomerular Filtration Rate (GFR) by Age
| Age Group | Normal GFR (mL/min/1.73m²) |
| 20-29 years | 116 ± 20 |
| 30-39 years | 107 ± 17 |
| 40-49 years | 99 ± 17 |
| 50-59 years | 93 ± 17 |
| 60-69 years | 85 ± 17 |
| 70+ years | 75 ± 17 |
GFR Categories (CKD Staging)
| Stage | GFR Range | Kidney Function | Description |
| 1 | ≥90 | Normal/High | Normal with kidney damage |
| 2 | 60-89 | Mild decrease | Mild decrease with kidney damage |
| 3a | 45-59 | Moderate decrease | Mild to moderate decrease |
| 3b | 30-44 | Moderate decrease | Moderate to severe decrease |
| 4 | 15-29 | Severe decrease | Severe decrease |
| 5 | <15 | Kidney failure | End-stage renal disease |
Substances Filtered and Their Fate
Completely Reabsorbed (Normal Conditions)
- Glucose: 100% reabsorbed in proximal tubule
- Amino acids: 95-100% reabsorbed
- Bicarbonate: 85-90% reabsorbed
- Phosphate: 85% reabsorbed
Partially Reabsorbed
- Sodium: 99% reabsorbed (65% proximal, 25% loop, 8% distal, 1% excreted)
- Water: 99% reabsorbed (follows sodium reabsorption)
- Chloride: 99% reabsorbed (follows sodium)
- Calcium: 98% reabsorbed
Minimally Reabsorbed
- Urea: 50% reabsorbed (concentrated in urine)
- Creatinine: 0% reabsorbed (freely filtered, slightly secreted)
- Inulin: 0% reabsorbed (used to measure GFR)
Hormonal Regulation of Filtration
Antidiuretic Hormone (ADH)
- Source: Posterior pituitary
- Function: Increases water reabsorption in collecting duct
- Trigger: High blood osmolality or low blood volume
- Effect: Concentrated urine, water retention
Aldosterone
- Source: Adrenal cortex
- Function: Increases sodium reabsorption in distal tubule
- Trigger: Low blood pressure, high potassium, angiotensin II
- Effect: Sodium retention, potassium excretion
Angiotensin II
- Source: Conversion from angiotensin I
- Function: Vasoconstriction, aldosterone release
- Trigger: Low blood pressure (via renin-angiotensin system)
- Effect: Increased blood pressure, sodium retention
Parathyroid Hormone (PTH)
- Source: Parathyroid glands
- Function: Increases calcium reabsorption, decreases phosphate reabsorption
- Trigger: Low blood calcium
- Effect: Maintains calcium homeostasis
Factors Affecting Filtration
Factors That Increase GFR
- High blood pressure
- Increased cardiac output
- Vasodilation of afferent arteriole
- Vasoconstriction of efferent arteriole
- High protein diet (short-term)
Factors That Decrease GFR
- Low blood pressure
- Dehydration
- Kidney disease
- Medications (NSAIDs, ACE inhibitors)
- Diabetes (long-term damage)
- Age (natural decline)
Clinical Assessment of Kidney Function
Laboratory Tests
- Serum Creatinine: Waste product filtered by kidneys
- Blood Urea Nitrogen (BUN): Another waste product
- BUN/Creatinine Ratio: Helps distinguish types of kidney problems
- Estimated GFR (eGFR): Calculated from creatinine, age, sex, race
- Cystatin C: Alternative marker less affected by muscle mass
Normal Laboratory Values
| Test | Normal Range |
| Serum Creatinine (Men) | 0.7-1.3 mg/dL |
| Serum Creatinine (Women) | 0.6-1.1 mg/dL |
| BUN | 7-20 mg/dL |
| BUN/Creatinine Ratio | 10:1 to 20:1 |
Urine Tests
- Urinalysis: Checks for protein, blood, glucose, bacteria
- 24-hour Urine Collection: Measures creatinine clearance
- Microalbumin: Early detection of kidney damage
- Protein/Creatinine Ratio: Quantifies protein loss
Signs of Impaired Kidney Filtration
Early Signs
- Elevated creatinine levels
- Reduced eGFR
- Protein in urine (proteinuria)
- Blood in urine (hematuria)
- High blood pressure
Advanced Signs
- Swelling (edema) in legs, ankles, feet
- Shortness of breath
- Fatigue and weakness
- Loss of appetite
- Sleep problems
- Changes in urination frequency
Maintaining Healthy Kidney Filtration
Lifestyle Factors
- Stay hydrated: Drink adequate water (8-10 glasses daily)
- Maintain healthy blood pressure: <130/80 mmHg
- Control blood sugar: If diabetic, keep HbA1c <7%
- Exercise regularly: 150 minutes moderate activity weekly
- Maintain healthy weight: BMI 18.5-24.9
- Don’t smoke: Smoking reduces kidney blood flow
Dietary Considerations
- Limit sodium: <2,300 mg daily (1,500 mg if hypertensive)
- Monitor protein: Avoid excessive protein intake
- Limit processed foods: High in sodium and phosphorus
- Control potassium: If kidney function is impaired
- Limit phosphorus: In advanced kidney disease
Medications to Avoid/Monitor
- NSAIDs: Can reduce kidney blood flow
- Certain antibiotics: May be toxic to kidneys
- Contrast dye: Can cause acute kidney injury
- High doses of vitamin C: May form kidney stones
- Herbal supplements: Some can be harmful to kidneys
Important Note: This chart is for educational purposes only. Always consult healthcare professionals for kidney health concerns, interpretation of lab results, or medical advice. Regular kidney function monitoring is recommended for people with diabetes, high blood pressure, or family history of kidney disease.
