Urinary System Renal Anatomy Study Guide

Urinary System Renal Anatomy

  • The kidneys are positioned as retroperitoneal organs.
  • The right kidney is lower than the left kidney due to the presence of the liver.
  • Blood vessels enter and leave the kidney at the renal hilum.
  • The supportive structures of the kidneys include: renal fascia, perirenal fat, and renal capsule.
  • The regions of the kidney are: renal cortex, renal medulla, and renal sinus. The outermost layer is the renal capsule.
  • Renal pyramids are the triangular structures in the renal medulla, separated by extensions of cortical tissue known as renal columns.
  • The renal papilla is the narrow apex of a pyramid that projects into the calyces of the renal sinus.
  • There are two types of calyces: minor calyx and major calyx, which converge to form the renal pelvis.

Nephrons

  • The functional units of the kidney are the nephrons.
  • Nephrons are classified as cortical nephrons if located mainly in the cortex and juxtamedullary nephrons if located in both the cortex and medulla.
  • A nephron comprises two main regions: renal corpuscle and renal tubule.
  • The glomerular capsule and glomerulus make up the renal corpuscle.
  • The three segments of the renal tubules are:
    • Proximal convoluted tubule (PCT)
    • Loop of Henle
    • Distal convoluted tubule (DCT)
  • The glomerulus consists of a network of fenestrated capillaries where blood filtration occurs. Most substances filter with the blood except RBCs and proteins. The filtered fluid is called filtrate and collects in the glomerular capsule (Bowman's capsule).
  • Blood to the glomerulus is supplied by the afferent arteriole and exits through the efferent arteriole. This design creates a pressure gradient necessary for filtration.

Glomerular Capsule Layers

  • Bowman's capsule surrounds the glomerulus and consists of two layers:
    • Parietal layer: made of simple squamous epithelium.
    • Visceral layer: consists of podocytes with foot-like processes that create filtration slits.

Filtration Membrane

  • The three components of the filtration membrane are:
    • Fenestrated endothelium
    • Glomerular basement membrane
    • Podocyte layer
  • The PCT contains numerous microvilli that increase surface area for reabsorption.

Processes of Urine Formation

  1. Filtration

    • Occurs in the renal corpuscle.
    • Fluid moves from the glomerulus into the capsular space.
    • The fluid is called filtrate. It is a selective process.
    • Filtered substances include salts, glucose, urea, and amino acids. Proteins are not filtered.

  2. Reabsorption

    • Occurs in the renal tubules.
    • Fluid moves from the tubules into the peritubular capillaries allowing the body to retain substances like glucose.
    • This is a selective process.

  3. Secretion

    • Also occurs in the renal tubules.
    • Fluid moves from the peritubular capillaries into the tubules to excrete substances not filtered initially.
    • The final fluid is called urine.

Blood Flow Through the Kidney

  • The renal artery brings blood into the kidneys, its branches supply the nephron through:
    • Afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → vasa recta.
  • Path of blood through the kidney:
    • Aorta → Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Interlobular artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries → Interlobular vein → Arcuate vein → Interlobar vein → Renal vein → Inferior vena cava.

Ureters

  • Function: To transport urine from kidney to bladder.
  • Ureter wall layers (innermost to outermost):
    • Mucosa: Lined by transitional epithelium, with lamina propria.
    • Muscularis: Initially has 2 layers of smooth muscle, increasing to 3 near the bladder.
    • Adventitia: Composed of areolar connective tissue.

Urinary Bladder

  • A collapsible muscular sac that stores approximately 500mL of urine. Positioned posterior to the pubic symphysis.
  • Trigone: Area between the openings of the ureters and the urethra.
  • Detrusor muscle: Smooth muscle on the bladder wall that controls contraction.
  • Histology (inside to outside):
    • Mucosa: Lined by transitional epithelium, forming rugae when full.
    • Submucosa: Composed of dense irregular connective tissue.
    • Muscularis: Contains 3 layers of smooth muscle (detrusor).
    • Adventitia: Composed of areolar connective tissue.

Urethra

  • A thin muscular tube draining urine from the bladder.
  • Histology (inside to outside):
    • Mucosa: Varies between males and females; males have pseudostratified columnar epithelium transitioning to stratified squamous epithelium.
    • Muscularis: Composed of smooth muscle.
    • Adventitia: Composed of areolar connective tissue.
  • Comparison:
    • Female urethra: Approx. 3-4cm long.
    • Male urethra: Approx. 18cm, consisting of 3 regions: prostatic urethra, membranous urethra, and penile urethra.