The nephron is the functional unit of the kidney, and its operation can be divided into three distinct processes: filtration, reabsorption, and secretion.
- Filtration: Occurs in the glomerulus. Pressure pushes plasma out of the glomerular capillaries into Bowman's capsule. This process filters by size: small molecules like water, Na+, $K^+$, glucose, urea, and creatinine pass through, while large proteins and blood cells remain in the blood. The amount filtered is measured as the Glomerular Filtration Rate (GFR), with a normal range of 90–120 mL/min.
- Reabsorption: Occurs in the tubules. The tubules "sort" the filtrate, returning approximately 99% of the essential solutes back into the blood, including water, Na+, $K^+$, glucose, and bicarbonate (HCO3−).
- Secretion: The tubules add additional substances to the urine that the body needs to discard, such as $H^+$ (acid), $K^+$, and certain drugs.
Exam Tip: Think of the glomerulus as a "dump truck" that unloads everything small, and the tubules as "workers on a belt" who pick back up the valuables and toss out the extra trash.
Homeostasis: Fluid, Electrolytes, and Acid-Base Balance
The kidneys maintain homeostasis through precise hormonal and chemical mechanisms.
Fluid and Electrolyte Balance
The kidneys control extracellular fluid volume primarily by regulating sodium (Na+), as "water follows sodium".
- Osmolality: Normal serum osmolality is 275–295 mOsm/kg. When osmolality rises (dehydration), Antidiuretic Hormone (ADH) is released, prompting the kidneys to conserve water, resulting in dark, concentrated urine. When osmolality is low (overhydration), ADH is suppressed, and the kidneys excrete large amounts of pale, dilute urine.
- Aldosterone: This hormone regulates the balance of Na+ and $K^+$. Aldosterone triggers the reabsorption of Na+ (and water follows) and the excretion of $K^+$. In kidney failure, the inability to excrete $K^+$ can lead to life-threatening hyperkalemia.
Acid-Base Balance
The kidneys are the most effective long-term regulators of blood pH (normal: $7.35–7.45$). While the lungs respond in minutes by adjusting CO2, the kidneys take hours to days to fully compensate by excreting $H^+$ and reabsorbing HCO3−.
- Metabolic Acidosis (pH < 7.35): Often caused by renal failure (inability to excrete $H^+$) or diabetic ketoacidosis. The kidneys attempt to compensate by excreting more $H^+$ and retaining HCO3−.
- Metabolic Alkalosis (pH > 7.45): Can be caused by diuretic use or hyperaldosteronism. The kidneys compensate by excreting excess HCO3−.
Endocrine Functions of the Kidney
Beyond filtration, the kidneys serve as endocrine organs by producing three critical substances:
- Renin: Triggered by decreased renal blood flow or low Na+, renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), leading to vasoconstriction and increased blood pressure.
- Erythropoietin (EPO): Stimulated by hypoxia, EPO signals the bone marrow to produce red blood cells (RBCs). Consequently, patients with chronic kidney disease (CKD) often suffer from anemia.
- Calcitriol: The kidneys convert Vitamin D into its active form, calcitriol, which is essential for calcium absorption and bone integrity.
Clinical Implications: Fluid Volume Imbalances
Clinicians must distinguish between fluid volume deficit and excess using laboratory and physical cues.
| Feature | Fluid Volume Deficit (Hypovolemia) | Fluid Volume Excess (Hypervolemia) |
|---|
| Urine Appearance | Dark, concentrated | Pale, dilute |
| Specific Gravity | >1.030 | <1.005 |
| Key Labs | BUN:Cr ratio >20:1 | ↓ BUN, ↑ BNP |
| Physical Cues | Tachycardia, hypotension, dry mouth | Edema, crackles, JVD, weight gain |
The Daily Weight Rule: A weight change of >1 kg/day or >2 kg/week is clinically significant, as 1 kg of weight is roughly equivalent to 1 L of fluid.
Obstruction and Acute Kidney Injury (AKI)
Obstructions—such as kidney stones, BPH, or tumors—can cause urine to back up, leading to hydronephrosis (swelling of the kidney) and decreased GFR. This can result in Acute Kidney Injury (AKI), which is categorized by its location:
- Prerenal: Occurs before the kidney due to decreased blood flow (e.g., dehydration, hemorrhage, heart failure).
- Intrarenal: Damage within the kidney parenchyma (e.g., acute tubular necrosis, glomerulonephritis).
- Postrenal: Occurs after the kidney due to an outflow blockage (e.g., stones, BPH).
AKI often progresses through three phases: Oliguric (low urine output, buildup of toxins), Diuretic (high urine output, but poor filtration/loss of electrolytes), and Recovery.
