Renal Physiology: Glomerular Filtration and Fluid/Electrolyte Homeostasis

Renal Physiology

Glomerular Filtration Rate (GFR) and Its Regulation

The glomerular filtration rate (GFR) is a key indicator of kidney function, reflecting the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time. Homeostatic mechanisms tightly regulate GFR to maintain fluid and electrolyte balance.

• Location of Glomerular Filtration: Filtration occurs in the renal corpuscle, which consists of the glomerulus (a capillary network) and Bowman's capsule. Fluid moves from the glomerular capillaries into the capsular space, forming the filtrate.

• Forces Determining Net Filtration Pressure (NFP): Three main forces determine the direction and magnitude of filtration:

  • Glomerular Capillary Hydrostatic Pressure (PGC): Pushes fluid out of the capillaries into Bowman's capsule (typically ~55 mmHg).

  • Bowman's Capsule Hydrostatic Pressure (PBC): Opposes filtration by pushing fluid back into the capillaries (~15 mmHg).

  • Blood Colloid Osmotic Pressure (πGC): Due to plasma proteins, pulls water back into the capillaries (~30 mmHg).

• Net Filtration Pressure (NFP) Equation: $NFP = P_{GC} - P_{BC} - \pi_{GC}$

• Relationship Between NFP and GFR: GFR is directly proportional to NFP; as NFP increases, GFR increases, and vice versa.

• Predicting GFR Changes:

  • Decrease in plasma proteins: Lowers πGC, increases NFP and GFR.

  • Blocked urine drainage: Raises PBC, decreases NFP and GFR.

  • Drop in mean arterial pressure (MAP): Lowers PGC, decreases NFP and GFR.

• Baroreceptor Reflex and GFR: Baroreceptors detect changes in blood pressure. A drop in MAP triggers sympathetic activation, constricting afferent arterioles, reducing GFR to conserve fluid.

Fluid and Electrolyte Balance: Homeostatic Mechanisms

The kidneys regulate the composition and volume of body fluids by adjusting the reabsorption and secretion of water and electrolytes along the nephron.

• Osmolarity in the Nephron:

  • Capsule and Proximal Convoluted Tubule (PCT): Filtrate is isosmotic (~300 mOsm/L) to plasma.

• Proximal Tubule Function:

  • Reabsorbs most filtered sodium (Na+), chloride (Cl-), potassium (K+), and water.

  • Active transport of Na+ drives reabsorption of other solutes and water (obligatory water reabsorption).

• Glucose Handling:

  • Filtration: Glucose is freely filtered at the glomerulus.

  • Reabsorption: Nearly all glucose is reabsorbed in the PCT via sodium-glucose cotransporters.

  • Excretion: If plasma glucose exceeds the transport maximum (Tm), glucose appears in urine (glycosuria).

• Loop of Henle Permeability:

  • Descending limb: Permeable to water, not solutes; filtrate becomes more concentrated.

  • Ascending limb: Impermeable to water, actively reabsorbs Na+ and Cl-; filtrate becomes more dilute.

• Osmolarity Changes Along the Nephron:

  • Filtrate osmolarity increases in the descending limb, decreases in the ascending limb, and is lowest at the start of the distal tubule.

• Osmolarity Comparisons:

  • Renal cortex: Interstitial fluid is isosmotic to plasma (~300 mOsm/L).

  • Renal medulla: Interstitial fluid osmolarity increases with depth (up to ~1200 mOsm/L).

  • Collecting duct filtrate osmolarity varies depending on ADH levels.

Hormonal Regulation of Renal Function

Several hormones act on different nephron segments to regulate water and electrolyte balance.

Hormone	Site of Action	Main Effect
ADH (Antidiuretic Hormone)	Late distal convoluted tubule (DCT), collecting ducts	Increases water reabsorption (inserts aquaporins)
Aldosterone	Late DCT, cortical collecting ducts	Increases Na+ reabsorption, K+ secretion
Angiotensin II (ANG II)	Proximal tubule, efferent arteriole, adrenal cortex	Increases Na+ reabsorption, stimulates aldosterone, constricts arterioles
ANP (Atrial Natriuretic Peptide)	Collecting ducts	Decreases Na+ reabsorption, increases natriuresis
PTH (Parathyroid Hormone)	Distal tubule	Increases Ca2+ reabsorption

• ADH Effects: Increases water permeability in the late DCT and collecting ducts, concentrating urine and reducing plasma osmolarity.

• ADH Regulation: Negative feedback via osmoreceptors (detect plasma osmolarity) and baroreceptors (detect blood volume/pressure).

• Aldosterone Effects: Increases Na+ reabsorption and K+ secretion in the late DCT and cortical collecting ducts.

• RAAS (Renin-Angiotensin-Aldosterone System): Decreased plasma volume or blood pressure stimulates renin release, leading to ANG II and aldosterone secretion, which restore volume and pressure.

• ANG II Effects: Vasoconstriction, increased Na+ reabsorption, stimulates aldosterone release.

• ANP Effects: Inhibits Na+ reabsorption, increases natriuresis and diuresis, lowers plasma volume.

• Sympathetic Nervous System: Directly constricts renal arterioles (reducing GFR), indirectly stimulates renin release and Na+ reabsorption.

• Potassium Secretion Regulation: Increased plasma K+ stimulates aldosterone, enhancing K+ secretion (negative feedback).

• Calcium Regulation: PTH increases Ca2+ reabsorption in the distal tubule; calcitonin and vitamin D also play roles.

• Long-term Blood Pressure Regulation: Kidneys adjust blood volume via Na+ and water excretion; RAAS and ANP are key hormonal mechanisms.

Summary Table: Hormonal Effects on the Nephron

Hormone	Stimulus	Effect on Nephron	Effect on Body
ADH	Increased plasma osmolarity, decreased blood volume	Increases water reabsorption in collecting ducts	Decreases plasma osmolarity, increases blood volume
Aldosterone	Increased ANG II, increased plasma K+	Increases Na+ reabsorption, K+ secretion	Increases plasma volume, decreases plasma K+
ANP	Increased atrial stretch (high blood volume)	Decreases Na+ reabsorption	Decreases plasma volume
PTH	Decreased plasma Ca2+	Increases Ca2+ reabsorption	Increases plasma Ca2+

Example Calculation: Net Filtration Pressure

Given: $P_{GC} = 55$ mmHg, $P_{BC} = 15$ mmHg, $\pi_{GC} = 30$ mmHg

Calculate NFP:

$NFP = 55 - 15 - 30 = 10$ mmHg

Example Application

• If plasma proteins decrease (e.g., in liver disease): $\pi_{GC}$ decreases, NFP and GFR increase.

• If urine drainage is blocked (e.g., kidney stone): $P_{BC}$ increases, NFP and GFR decrease.

• If MAP drops (e.g., hemorrhage): $P_{GC}$ decreases, NFP and GFR decrease.

Additional info:

• Understanding these mechanisms is essential for diagnosing and treating renal and systemic disorders affecting fluid, electrolyte, and acid-base balance.