Chapter 25: The Urinary System – Structure, Function, and Regulation

Chapter 25: The Urinary System

Introduction

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood, removing metabolic wastes, and balancing fluids and electrolytes. The kidney's functional unit, the nephron, plays a central role in these processes through filtration, reabsorption, and secretion.

The Functional Anatomy of the Kidney

The Nephron: Structural and Functional Unit

• Nephron: The microscopic structural and functional unit of the kidney; each kidney contains about 1 million nephrons.

• Vascular components:

  • Afferent arteriole

  • Glomerulus (capillary bed)

  • Efferent arteriole

  • Peritubular capillaries and vasa recta

• Tubular components:

  • Bowman's capsule

  • Proximal convoluted tubule (PCT)

  • Loop of Henle (descending and ascending limbs)

  • Distal convoluted tubule (DCT)

  • Collecting duct

The nephron drains into the collecting duct, which leads to the renal pelvis and ultimately the bladder.

Types of Nephrons

• Cortical nephrons: (~80%) Located primarily in the renal cortex; have shorter loops of Henle.

• Juxtamedullary nephrons: (~20%) Located near the cortex-medulla junction; have long loops of Henle that extend deep into the medulla, crucial for concentrating urine.

Renal System Functions in Homeostasis

• Regulating total volume of water and solute concentration

• Regulating concentrations of specific ions (e.g., Na+, K+, Cl-) in extracellular fluid

• Ensuring long-term acid-base balance

• Excreting metabolic wastes, drugs, and toxins

• Producing hormones (e.g., erythropoietin for RBC production, renin for blood pressure regulation)

• Converting vitamin D to its active form

• Carrying out gluconeogenesis during prolonged fasting

Urine Formation: The Three Basic Processes

Overview

• Glomerular filtration: Movement of fluid and solutes from blood into the nephron (except cells and proteins).

• Tubular reabsorption: Selective movement of substances from filtrate back into the blood.

• Tubular secretion: Selective addition of substances from blood into the filtrate.

Approximately 180 L of blood-derived fluid is filtered daily, but most is reabsorbed.

Step 1: Glomerular Filtration

• Driven by hydrostatic capillary pressure across the filtration membrane (fenestrated capillaries, basement membrane, podocyte foot processes).

• Filtrate is cell- and protein-free; contains water, ions, glucose, amino acids, and small molecules.

• Net filtration pressure (NFP): Determined by the balance of:

  • Glomerular capillary blood pressure (favors filtration)

  • Plasma colloid osmotic pressure (opposes filtration)

  • Bowman's capsule hydrostatic pressure (opposes filtration)

Equation:

$\text{NFP} = \text{Glomerular hydrostatic pressure} - (\text{Plasma colloid osmotic pressure} + \text{Bowman's capsule hydrostatic pressure})$

Typical values: 55 mm Hg (glomerular) - (30 mm Hg + 15 mm Hg) = 10 mm Hg net outward pressure.

Glomerular Filtration Rate (GFR)

• GFR: The volume of filtrate formed per minute by both kidneys (normal: 120–125 mL/min).

• GFR is regulated to ensure proper filtration and homeostasis.

Regulation of GFR

Intrinsic Controls (Renal Autoregulation)

• Maintain GFR despite changes in systemic blood pressure (MAP 80–180 mm Hg).

• Myogenic mechanism: Afferent arteriole constricts or dilates in response to pressure changes.

• Tubuloglomerular feedback: Macula densa cells sense NaCl in filtrate; adjust afferent arteriole diameter accordingly.

Extrinsic Controls

• Maintain systemic blood pressure when MAP is outside normal range.

• Neural (sympathetic) control: Norepinephrine and epinephrine cause vasoconstriction, reducing GFR and conserving water/Na+.

• Hormonal control (RAAS): Renin-angiotensin-aldosterone system increases blood pressure and Na+ reabsorption.

	Intrinsic Control (Renal Autoregulation)	Extrinsic Control
Purpose	Maintain GFR in spite of changes in blood pressure	Maintain systemic blood pressure
Mechanisms	Myogenic, Tubuloglomerular feedback	Hormonal (RAAS), Neural (sympathetic NS)
Operating Conditions	MAP 80–180 mm Hg	MAP outside 80–180 mm Hg

Natriuretic Peptides

• Atrial natriuretic peptide (ANP): Inhibits Na+ reabsorption, increasing Na+ and water excretion, lowering blood volume and pressure.

Step 2: Tubular Reabsorption

• Most reabsorption occurs in the proximal convoluted tubule (PCT):

  • 100% glucose

  • 100% amino acids

  • 65% water and ions (hormonally regulated)

  • Almost all uric acid and 50% urea

• Some reabsorption is active (requires ATP), some is passive.

Mechanisms of Reabsorption

• Transcellular route: Through tubular cells (active or passive; e.g., Na+, glucose, amino acids, phosphate, vitamins).

• Paracellular route: Between tubular cells (passive; e.g., water, ions).

Reabsorption of Sodium and Glucose

• Sodium (Na+): Most abundant cation in filtrate.

• Enters tubular cells via secondary active transport or facilitated diffusion.

• Na+/K+ ATPase pumps Na+ into interstitial space (primary active transport).

• Glucose: Reabsorbed via sodium-glucose cotransporters (SGLT2) in the PCT; exits via GLUT2 transporters.

Transport Maximum (Tm)

• Each substance has a maximum rate of reabsorption, determined by the number of available carrier proteins.

• When Tm is exceeded (e.g., in hyperglycemia), excess solute is excreted in urine.

Loop of Henle and Medullary Osmotic Gradient

• Descending limb: Permeable to water, not salt.

• Ascending limb: Impermeable to water, actively pumps out salt.

• This countercurrent mechanism creates a medullary osmotic gradient, allowing concentration of urine.

Step 3: Tubular Secretion

• Selective addition of substances from blood into the filtrate.

• Important for:

  • Eliminating drugs, toxins, and metabolic wastes

  • Regulating K+ and H+ concentrations (acid-base balance)

  • Removing substances bound to plasma proteins

Summary Table: Regulation of GFR

	Intrinsic Control (Renal Autoregulation)	Extrinsic Control
Purpose	Maintain GFR in spite of changes in blood pressure	Maintain systemic blood pressure
Mechanisms	Myogenic, Tubuloglomerular feedback	Hormonal (RAAS), Neural (sympathetic NS)
Operating Conditions	MAP 80–180 mm Hg	MAP outside 80–180 mm Hg

Key Terms and Concepts

• Nephron: Functional unit of the kidney

• Glomerular filtration: First step in urine formation; non-selective filtration of blood plasma

• Tubular reabsorption: Recovery of essential substances from filtrate

• Tubular secretion: Addition of substances to filtrate for excretion

• GFR: Glomerular filtration rate; indicator of kidney function

• RAAS: Renin-angiotensin-aldosterone system; regulates blood pressure and fluid balance

• ANP: Atrial natriuretic peptide; decreases blood volume and pressure by increasing Na+ excretion

Example: Hyperglycemia and Glucose Reabsorption

In diabetes mellitus, blood glucose levels may exceed the transport maximum (Tm) for glucose reabsorption in the PCT, resulting in glucose appearing in the urine (glycosuria).

Additional info: The kidney's ability to concentrate urine is vital for water conservation, especially in dehydration. The countercurrent multiplier system in juxtamedullary nephrons and the action of antidiuretic hormone (ADH) are essential for this process.