Urinary System: Structure, Blood Supply, and Nephron Anatomy

Urinary System

Kidney Anatomy

The kidneys are vital organs responsible for filtering blood, removing waste, and regulating fluid balance. Their structure is divided into several regions and contains distinct internal features.

• Retroperitoneal: Kidneys are located behind the peritoneum of the abdominal cavity.

• External Features:

  • Hilum: The concave medial border where vessels, nerves, and ureter enter/exit.

  • Renal capsule: The fibrous covering of the kidney.

• Regions:

  • Renal cortex: The outer region containing most nephrons.

  • Renal medulla: The inner region, organized into renal pyramids.

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

  • Renal columns: Extensions of cortex between pyramids.

  • Minor calyx: Collects urine from renal papillae.

  • Major calyx: Formed by the convergence of minor calyces.

  • Renal pelvis: Funnel-shaped reservoir for urine before it enters the ureter.

  • Ureter: Tube that carries urine from the kidney to the bladder.

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

Blood Supply to the Kidney

The kidneys receive a rich blood supply to facilitate filtration and waste removal. Blood flow follows a specific pathway through various arteries and veins.

• Renal artery & vein: Main vessels entering and leaving the kidney.

• Segmental arteries: Branches of the renal artery.

• Interlobar arteries & veins: Run between renal pyramids.

• Arcuate arteries & veins: Arch over the base of pyramids.

• Interlobular (cortical radiate) arteries & veins: Supply the cortex.

• Afferent arteriole: Brings blood to the glomerulus.

• Glomerulus: Capillary network for filtration.

• Efferent arteriole: Drains blood from the glomerulus.

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

Additional info: The glomerulus is a specialized capillary bed where filtration of blood plasma occurs.

Nephron Structure

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Each nephron consists of a renal corpuscle and a renal tubule.

• Renal Corpuscle:

  • Glomerulus: Capillary tuft for filtration.

  • Bowman's (glomerular) capsule: Surrounds the glomerulus and collects filtrate.

• Renal Tubule:

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

  2. Loop of Henle:

     • Descending limb: Permeable to water.

     • Ascending limb: Permeable to ions.

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

  4. Collecting duct: Final site for water reabsorption and urine concentration.

Example: The loop of Henle creates a concentration gradient in the medulla, essential for water reabsorption.

Blood & Urine Flow through the Kidney

Blood enters the kidney via the renal artery, is filtered in the glomerulus, and exits through the renal vein. Urine formed in the nephrons flows through the collecting ducts to the renal pelvis and then to the ureter.

• Blood flow: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → renal vein.

• Urine flow: Nephron → collecting duct → minor calyx → major calyx → renal pelvis → ureter.

Bladder Anatomy

The bladder is a muscular organ that stores urine before excretion. It contains specialized structures for urine retention and release.

• Trigone: Triangular area at the base of the bladder.

• Ureteric (ureteral) orifices: Openings where ureters enter the bladder.

• Internal urethral orifice: Opening to the urethra.

Urethra Structure

The urethra is the tube that carries urine from the bladder to the outside of the body. It contains sphincters that control the release of urine.

• Internal urethral sphincter: Involuntary muscle at the bladder-urethra junction.

• External urethral sphincter: Voluntary muscle controlling urination.

• Urethral orifice: External opening of the urethra.

• Meatus: The external opening of the urethra.

Summary Table: Kidney Regions and Functions

Region	Main Function
Renal Cortex	Filtration of blood, houses most nephrons
Renal Medulla	Concentration of urine, contains pyramids
Renal Pelvis	Collection and funneling of urine to ureter

Key Equations

• Glomerular Filtration Rate (GFR): $GFR = K_f \times [(P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})]$ Where $K_f$ is the filtration coefficient, $P_{GC}$ is glomerular capillary pressure, $P_{BS}$ is Bowman's space pressure, $\pi_{GC}$ is glomerular capillary oncotic pressure, and $\pi_{BS}$ is Bowman's space oncotic pressure.