Urinary System: Key Structures, Blood Supply, Nephron Anatomy, and Related Physiology

Unit 4: The Urinary System

Overview

The urinary system is essential for filtering blood, removing waste products, regulating fluid and electrolyte balance, and maintaining homeostasis. This unit covers the anatomy of the kidney, renal blood supply, nephron structure, urinary tract, and related physiological processes.

Kidney Anatomy

External and Internal Structures

• Renal fascia: Outermost connective tissue anchoring the kidney.

• Perirenal fat capsule: Protective layer of adipose tissue surrounding the kidney.

• Fibrous capsule: Thin, tough layer directly covering the kidney.

• Renal hilum: Indentation where vessels, nerves, and ureter enter/exit.

• Renal cortex: Outer region containing most nephrons.

• Renal medulla: Inner region with renal pyramids.

• Renal pyramids: Cone-shaped tissues in the medulla.

• Renal papilla: Tip of each pyramid, releases urine into minor calyces.

• Renal columns: Extensions of cortex between pyramids.

• Renal pelvis: Funnel-shaped cavity collecting urine from calyces.

• Major and minor calyces: Chambers that collect urine from papillae and funnel it to the renal pelvis.

Example: The renal cortex contains the majority of glomeruli, which are essential for blood filtration.

Renal Blood Supply

Arterial and Venous Pathways

• Renal artery: Supplies blood to the kidney.

• Segmental artery: Branches from the renal artery.

• Interlobar artery: Passes through renal columns.

• Arcuate artery: Arches over the base of pyramids.

• Cortical radiate artery: Extends into cortex.

• Afferent arteriole: Brings blood to the glomerulus.

• Glomerulus: Capillary network for filtration.

• Efferent arteriole: Carries blood away from glomerulus.

• Peritubular capillaries: Surround renal tubules for reabsorption/secretion.

• Vasa recta: Capillaries associated with juxtamedullary nephrons, important for urine concentration.

• Venous return: Follows similar path: cortical radiate vein → arcuate vein → interlobar vein → renal vein.

Additional info: The unique arrangement of afferent and efferent arterioles regulates glomerular filtration pressure.

Nephron Structure

Cortical and Juxtamedullary Nephrons

• Renal corpuscle: Consists of the glomerulus and Bowman's capsule.

• Glomerulus: Capillary tuft for filtration.

• Bowman's capsule: Surrounds the glomerulus, collects filtrate.

• Glomerular space: Area within Bowman's capsule where filtrate collects.

• Visceral and parietal layers: Visceral layer contains podocytes; parietal layer forms the outer wall.

• Podocytes: Specialized cells with foot processes forming filtration slits.

• Filtration slits: Gaps between podocyte foot processes, allowing selective filtration.

Example: Juxtamedullary nephrons have long loops of Henle that extend deep into the medulla, crucial for concentrating urine.

Renal Tubule Segments

• Proximal convoluted tubule (PCT): Reabsorbs water, ions, and nutrients.

• Nephron loop (Loop of Henle): Descending limb reabsorbs water; ascending limb reabsorbs ions.

• Distal convoluted tubule (DCT): Further reabsorption and secretion.

• Collecting duct: Final site for water reabsorption, regulated by hormones.

Additional info: The arrangement of tubule segments allows for precise control of urine composition.

Juxtaglomerular Complex

• Macula densa: Senses sodium concentration in the distal tubule.

• Granular cells: Secrete renin in response to low blood pressure.

• Extraglomerular mesangial cells: Support and regulate glomerular filtration.

Urinary Tract

Ureter

• Ureter: Muscular tube transporting urine from kidney to bladder.

• Peristalsis: Wave-like contractions moving urine.

Urinary Bladder

• Bladder: Hollow organ for urine storage.

• Detrusor muscle: Smooth muscle layer enabling contraction.

• Trigone: Triangular area between ureteral openings and urethra.

• Internal urethral sphincter: Involuntary control of urine release.

• External urethral sphincter: Voluntary control of urination.

Urethra

• Urethra: Tube carrying urine from bladder to exterior.

• Prostatic, membranous, and spongy urethra: Male urethra segments.

• External urethral orifice: Opening to the outside.

Renin-Angiotensin-Aldosterone System (RAAS)

Hormonal Regulation of Blood Pressure

• Renin: Enzyme released by granular cells in response to low blood pressure.

• Angiotensinogen: Plasma protein converted by renin to angiotensin I.

• Angiotensin I: Converted to angiotensin II by angiotensin-converting enzyme (ACE).

• Angiotensin II: Potent vasoconstrictor, stimulates aldosterone release.

• Aldosterone: Hormone increasing sodium reabsorption in the distal tubule.

• Antidiuretic hormone (ADH): Promotes water reabsorption in the collecting duct.

Equation:

$\text{Angiotensinogen} \xrightarrow{\text{Renin}} \text{Angiotensin I} \xrightarrow{\text{ACE}} \text{Angiotensin II}$

Urine Formation and Analysis

Processes

• Urine: Fluid containing metabolic wastes and excess substances.

• Urea: Major nitrogenous waste from protein metabolism.

• Uric acid: Waste from nucleic acid metabolism.

• Creatinine: Waste from muscle metabolism.

• Ammonia: Byproduct of amino acid deamination.

Additional info: Urinalysis can detect abnormal levels of these substances, indicating kidney function or disease.

Histology of the Urinary System

Bladder Epithelium

• Transitional epithelium: Specialized tissue lining the bladder, allowing stretching and recoiling.

• Distended vs. relaxed: Cells appear flattened when the bladder is full (distended) and rounded when empty (relaxed).

Example: Transitional epithelium prevents urine leakage while accommodating volume changes.

Kidney Histology

• Renal corpuscle: Contains glomerulus and Bowman's capsule.

• Renal tubules: Lined by cuboidal or columnar epithelial cells specialized for absorption and secretion.

Summary Table: Key Structures and Functions

Structure	Location	Main Function
Renal corpuscle	Cortex	Filtration of blood
Loop of Henle	Medulla	Concentration of urine
Collecting duct	Cortex & Medulla	Final water reabsorption
Ureter	Between kidney & bladder	Transport of urine
Bladder	Pelvic cavity	Storage of urine
Transitional epithelium	Bladder	Allows stretching