Renal Physiology: Glomerular Filtration and Tubular Function Summary & Study Notes

🩺 Overview

Glomerular filtration rate (GFR) is the volume of plasma filtered through the glomeruli into renal tubules per minute. In a healthy adult GFR ≈ 125 mL/min (≈180 L/day). GFR is a key index of kidney function and falls in chronic kidney disease, diabetes, hypertension, aging, and urinary tract obstruction.

⚙️ Determinants of GFR

GFR is determined by three major factors: filtration surface area, permeability (Kf), and the hydrostatic and oncotic pressure gradients across the glomerular capillary wall. Changes in any of these alter the net filtration pressure and hence GFR.

🧪 Glomerular ultrafiltration coefficient (Kf) and permeability

Kf = glomerular capillary permeability × filtration surface area. The glomerular capillary bed is more permeable than most systemic capillaries. Small molecules (<4 nm) are freely filtered, while intermediate-size molecules are filtered depending on charge. The glomerular basement membrane (GBM) carries a negative charge, so positively charged particles are filtered more readily than equally sized negatively charged particles. Large plasma proteins such as albumin are largely excluded from filtration due to size and negative charge.

🔬 Pathology affecting permeability

Loss of the negative charge or enlargement of filtration pores (for example in glomerulonephritis) increases permeability and leads to proteinuria. Altered permeability also changes the renal handling of drugs: only small, unbound molecules are readily filtered.

🧭 Hydrostatic and oncotic pressures

Glomerular capillary hydrostatic pressure is relatively high because the efferent arteriole has substantial resistance. The balance between glomerular capillary hydrostatic pressure, Bowman's capsule hydrostatic pressure, and plasma oncotic pressure determines net filtration pressure and thus GFR. Constriction or dilation of afferent or efferent arterioles changes intraglomerular pressure and GFR.

🔁 Autoregulation and Tubuloglomerular Feedback (TGF)

The kidney autoregulates renal blood flow and GFR across a wide range of systemic pressures. Two linked mechanisms are important: myogenic autoregulation of afferent arteriolar tone and tubuloglomerular feedback, where the macula densa senses tubular $NaCl$ concentration and modulates arteriolar resistance to stabilize GFR.

💊 Clinical and pharmacological implications

Drugs that alter renal hemodynamics or are nephrotoxic must be used cautiously in renal impairment. Examples:

• NSAIDs reduce prostaglandin-mediated afferent arteriolar dilation and can reduce GFR in compromised patients.
• ACE inhibitors preferentially dilate the efferent arteriole, lowering intraglomerular pressure, reducing proteinuria, and slowing CKD progression but may transiently reduce GFR on initiation. Monitor renal function and electrolytes when starting or changing doses of such agents.

📏 Measuring GFR and renal clearance

Clearance of a substance is the plasma volume completely cleared of that substance per unit time. For a substance: Clearance = (Urine concentration × Urine flow) / Plasma concentration. Substances used to measure GFR should be freely filtered, not reabsorbed or secreted, non-toxic, and not metabolized. Examples:

• Inulin: an exogenous gold standard; clearance = GFR.
• Creatinine: endogenous, convenient; slightly secreted so creatinine clearance modestly overestimates true GFR. Plasma creatinine is inversely related to GFR and used in estimation equations. If Clearance > GFR → net tubular secretion. If Clearance < GFR → net tubular reabsorption.

🔁 Tubular reabsorption mechanisms and transport maximum

Tubular reabsorption uses active transport, facilitated diffusion, passive diffusion, and osmosis. For many substances there is a transport maximum (Tm), which limits reabsorption rate. When filtered load exceeds Tm, the substance appears in urine. Renal threshold is the plasma concentration at which the substance begins to appear in urine (often lower than Tm due to nephron heterogeneity).

🧭 Major nephron segments and their functions

• Proximal tubule: reabsorbs >60% filtered $Na^+$ and $H_2O$, most glucose and amino acids, and $HCO_3^-$. Fluid leaving is essentially iso-osmotic.
• Loop of Henle: descending thin limb is highly permeable to $H_2O$; thin ascending limb is impermeable to $H_2O$; thick ascending limb (TAL) reabsorbs $Na^+$, $K^+$, $2Cl^-$ via the NKCC2 cotransporter and is impermeable to water (diluting segment).
• Early distal tubule: reabsorbs ions, impermeable to water (diluting segment).
• Collecting duct and late distal tubule: contain principal cells (regulated $K^+$ secretion, principal for water reabsorption under ADH) and intercalated cells (type A excrete $H^+$ and reclaim $HCO_3^-$). Medullary collecting ducts fine-tune final urine concentration under control of ADH.

⚖️ Regulation of tubular reabsorption

Important regulators include glomerulotubular balance (proportional increase in proximal reabsorption when GFR rises), and hormones such as aldosterone, angiotensin II, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP).

🧾 Clinical case connections

In CKD patients, dietary modifications such as limiting salt and high-protein intake help reduce hyperfiltration and fluid overload. ACE inhibitors are beneficial for reducing proteinuria and slowing progression of CKD, but require monitoring of GFR and $K^+$ due to effects on intraglomerular pressure and potassium handling.

✅ Key takeaways

• GFR depends on Kf, surface area, and pressure gradients.
• Autoregulation and TGF keep GFR stable across pressures.
• Clearance concepts allow indirect measurement of GFR; creatinine is practical but imperfect.
• Tubular transport has finite capacity (Tm) and is segment-specific; hormonal control adapts reabsorption and secretion to physiological needs.