Urinary System Physiology: Filtration, Reabsorption, and Regulation of Urine Formation

Urinary System Physiology

Introduction

The urinary system plays a crucial role in maintaining homeostasis by regulating the composition and volume of blood, removing metabolic wastes, and controlling water and electrolyte balance. This section focuses on the physiology of urine formation, the mechanisms of glomerular filtration, tubular reabsorption and secretion, and the regulation of urine concentration and volume.

I. Urine Formation

Overview of Urine Formation

• Renal blood flow rate (RBF): About 20-25% of total cardiac output enters the renal arteries per minute (~1 L/min).

• Filtrate: The fluid filtered from blood into the nephron, similar to plasma but lacking large proteins and blood cells.

• Urine: Only about 1-2 L of urine is produced daily from 180-200 L of filtrate.

Urine formation involves three main processes:

1. Glomerular Filtration: Production of cell-free, protein-free filtrate from blood in the glomerulus.

2. Tubular Reabsorption: Selective movement of substances from filtrate back into blood.

3. Tubular Secretion: Selective movement of substances from blood into filtrate.

II. Glomerular Filtration and Net Filtration Pressure (NFP)

Glomerular Filtration

• Nonselective process driven by hydrostatic pressure.

• Filtration membrane allows passage of water and small solutes, but not proteins or blood cells.

Pressures Affecting Glomerular Filtration

• Glomerular Blood Hydrostatic Pressure (GBHP): Main force pushing water and solutes out of blood (50-55 mmHg).

• Capsular Hydrostatic Pressure (CHP): Opposes filtration (10-15 mmHg).

• Blood Colloid Osmotic Pressure (BCOP): Pulls water back into plasma (30 mmHg).

Net Filtration Pressure (NFP): The sum of these pressures determines the amount of filtrate formed.

Equation:

$\text{NFP} = \text{GBHP} - (\text{CHP} + \text{BCOP})$

Glomerular Filtration Rate (GFR)

• Definition: Volume of filtrate produced per minute.

• Normal values: Adult male: 90-140 mL/min; Adult female: 80-125 mL/min.

• GFR is regulated by:

  • Intrinsic (renal autoregulation) mechanisms

  • Extrinsic (neural and hormonal) mechanisms

Regulation of GFR

• Intrinsic Controls: Maintain constant GFR despite changes in blood pressure (autoregulation).

• Extrinsic Controls: Adjust GFR to maintain systemic blood pressure (e.g., during stress or blood loss).

Intrinsic Controls: Renal Autoregulation

• Myogenic Mechanism: Afferent arteriole constricts in response to increased blood pressure, maintaining GFR.

• Tubuloglomerular Feedback: Macula densa cells sense NaCl concentration and adjust afferent arteriole diameter accordingly.

Extrinsic Controls: Neural and Hormonal Regulation

• Sympathetic Nervous System: Releases norepinephrine and epinephrine, causing vasoconstriction and reduced GFR during stress.

• Renin-Angiotensin-Aldosterone System (RAAS): Increases blood pressure and blood volume by promoting Na+ and water reabsorption.

III. Tubular Reabsorption and Secretion

Tubular Reabsorption

Reabsorption is the process by which water and solutes are moved from the filtrate back into the blood.

• Transcellular Route: Through the tubule cell, across apical and basolateral membranes.

• Paracellular Route: Between tubule cells, limited by tight junctions.

Mechanisms of Reabsorption

• Secondary Active Transport: Uses symporters and antiporters (e.g., Na+-glucose symporter).

• Osmosis: Water reabsorption via aquaporins.

• Facilitated Diffusion: Carrier proteins assist movement of substances.

• Active Transport: ATPase pumps (e.g., Na+/K+ pump).

Transport Maximum (Tm)

• The maximum amount of a substance that can be reabsorbed or secreted per minute.

• When exceeded, the excess appears in urine (e.g., glucose in diabetes mellitus).

Tubular Secretion

• Occurs mainly in the proximal and distal convoluted tubules and collecting ducts.

• Removes substances such as H+, K+, NH4+, creatinine, and certain drugs from blood into filtrate.

• Important for acid-base balance and elimination of wastes.

Nitrogenous Wastes

• Urea: From amino acid catabolism.

• Uric acid: From nucleic acid catabolism.

• Creatinine: From creatine phosphate catabolism.

Blood Urea Nitrogen (BUN): Measures urea concentration in blood; elevated BUN may indicate renal dysfunction.

IV. Renal Tubules and Collecting Ducts

A. Proximal Convoluted Tubule (PCT)

• Site of most reabsorption (about 65-70%).

• Reabsorbs all glucose and amino acids, most Na+, Cl-, K+, HCO3-, and water.

• Secretes H+ and some drugs/metabolites.

B. Nephron Loop (Loop of Henle)

• Descending limb: Permeable to water, not solutes.

• Ascending limb: Permeable to solutes, not water.

• Establishes medullary osmotic gradient for urine concentration.

C. Distal Convoluted Tubule (DCT)

• Reabsorbs Na+, Cl-, and Ca2+ (stimulated by parathyroid hormone).

• Secretes K+ and H+.

D. Late DCT, Collecting Tubule (CT), and Collecting Duct (CD)

• Reabsorption is hormonally regulated by:

  • Anti-diuretic Hormone (ADH): Increases water reabsorption by inserting aquaporins.

  • Aldosterone: Increases Na+ reabsorption and K+ secretion.

V. Regulation of Urine Concentration and Volume

Countercurrent Mechanism

The kidney uses countercurrent mechanisms to concentrate or dilute urine and maintain extracellular fluid osmolality.

• Countercurrent Multiplier: The nephron loop creates a gradient in the medulla by differential permeability to water and solutes.

• Countercurrent Exchanger: The vasa recta preserves the medullary gradient by exchanging water and solutes with the interstitial fluid.

Hormonal Regulation

• ADH: Released in response to increased plasma osmolality or decreased blood volume; increases water reabsorption.

• Aldosterone: Released in response to low Na+ or high K+; increases Na+ reabsorption and K+ secretion.

• Atrial Natriuretic Peptide (ANP): Released in response to increased blood volume; decreases Na+ and water reabsorption.

VI. Summary Table: Hormonal Regulation of Urine Formation

Hormone	Stimulus for Release	Main Effect on Kidney
ADH (Vasopressin)	Increased plasma osmolality, decreased blood volume	Increases water reabsorption in late DCT and collecting duct
Aldosterone	Low Na+, high K+, angiotensin II	Increases Na+ reabsorption, K+ secretion
ANP	Increased blood volume/pressure	Decreases Na+ and water reabsorption

VII. Physical Characteristics of Urine

• Color: Clear to deep yellow (urochrome pigment).

• Odor: Slightly aromatic; may change with diet or disease.

• pH: Slightly acidic (pH 4.5-8.0).

• Specific Gravity: 1.001-1.035 (measures solute concentration).

Key Terms

• Glomerular Filtration Rate (GFR): Rate at which filtrate is formed in the kidneys.

• Net Filtration Pressure (NFP): The pressure driving filtration at the glomerulus.

• Renin-Angiotensin-Aldosterone System (RAAS): Hormonal system regulating blood pressure and fluid balance.

• Countercurrent Mechanism: Process that establishes and maintains the medullary osmotic gradient.