Compendium 7 Notes: The Anatomy of the Renal System

Specific Learning Objectives

  • List the organs of the urinary system.
  • Describe the main functions of the kidneys.
  • Describe the location and external anatomy of the kidneys.
  • Describe the structure of the nephron.
  • Describe the three processes necessary for urine formation.
  • Describe the components that make up the filtration membrane.
  • Describe the role of various regions of the nephron in terms of reabsorption and secretion.
  • Explain how substances are able to move across the cell membrane of the nephron.
  • Describe the specific movement of glucose across the cell membrane of the nephron.
  • List the waste products excreted in urine.

Gross Anatomy of the Renal System

  • Components:
    • 2 kidneys: responsible for urine formation.
    • 2 ureters: act as passages for urine.
    • Urinary bladder: stores urine.
    • Urethra: serves as a passage for urine.
  • Textbook readings:
    • VanPutte, C. L., Regan, J. L. & Russo, A. F. (2020). Seeley’s Anatomy and Physiology (12th edition). New York, USA: McGraw-Hill
      • Chapter 26: pages 964-975, 977-982 and 997-1001
    • VanPutte, C. L., Regan, J. L. & Russo, A. F. (2017). Seeley’s Anatomy and Physiology (11th edition). New York, USA: McGraw-Hill
      • Chapter 26: pages 958-969, 971-976 and 989-992
    • VanPutte, C. L., Regan, J. L. & Russo, A. F. (2014). Seeley’s Anatomy and Physiology (10th edition). New York, USA: McGraw-Hill
      • Chapter 26: pages 946-957, 959-965 and 977-980
    • VanPutte, C. L., Regan, J. L. & Russo, A. F. (2010). Seeley’s Anatomy and Physiology (9th edition). New York, USA: McGraw-Hill
      • Chapter 26: pages 957-968, 970-976 and 988-990

Location of the Kidneys

  • Located posterior to the parietal peritoneum, on the posterior abdominal wall, lateral to the spine.
  • The right kidney is slightly inferior to the left kidney.
  • Partially protected by lumbar vertebrae and ribs.
  • Size: approximately 1111 cm long, 55 cm wide, and weighs around 130130g.

Kidney External Anatomy

  • Renal capsule: connective tissue surrounding each kidney.
  • Adipose tissue: surrounds the outside of the capsule for protection.
  • Renal fascia: a thin layer of connective tissue that surrounds the adipose tissue and anchors the kidneys to the abdominal wall.
  • Hilum: where the renal artery and nerves enter, and the renal vein, ureter, and lymphatics exit.

Kidney Internal Anatomy

  • Hilum: Located on the concave (medial) side, serves as the entry point for the renal artery and nerves, and the exit for the renal vein, ureter, and lymphatics.
  • Renal Sinus: The hilum opens into the renal sinus, which is filled with fat and loose connective tissue.
  • Two Major Regions:
    • Outer Cortex
    • Inner Medulla
  • Renal Pyramids: Bases project into the cortex. Cone-shaped, base is the boundary between cortex and medulla, apex is the renal papilla.
  • Renal Columns: Extensions of cortical tissue into the medulla.
  • Renal Papilla: Apex of pyramid.
  • Minor Calyces: Papillae extend into minor calyces, which are funnel-shaped chambers.
  • Major Calyces: Minor calyces funnel into a larger chamber called major calyces.
  • Renal Pelvis: A single large funnel-shaped chamber embedded in the renal sinus; it narrows at the hilum, forming the ureter.

The Nephron

  • The functional unit of the kidney.
  • Four separate regions:
    • Renal corpuscle
    • Proximal convoluted tubule
    • Loop of Henle
    • Distal convoluted tubule
  • Blood enters the nephron for filtration, producing filtrate/urine.
  • Urine Flow: nephron → papillary ducts → minor calyces → major calyces → renal pelvis → ureter.

Types of Nephrons

  • Approximately 1.3 million nephrons in each kidney, each about 50-55 mm in length.
  • Juxtamedullary Nephrons:
    • The renal corpuscle is deep in the cortex near the medulla.
    • Have a long loop of Henle extending deep into the medulla.
    • Account for 15% of nephrons.
  • Cortical Nephrons:
    • Renal corpuscles are located near the periphery/cortex.
    • Have a shorter loop of Henle.
    • Account for 85% of nephrons.

Renal Corpuscle

  • The filtration portion of the nephron.
  • Consists of the glomerulus and the Bowman capsule.
  • Glomerulus: A network/ball of capillaries.
  • Bowman Capsule: An enlarged, double-walled chamber that filters blood/fluid, which then enters the proximal convoluted tubule.
    • Blood enters glomerulus through the afferent arteriole, and filtered blood exits through the efferent arteriole.
    • Size difference between afferent and efferent arterioles creates a pressure difference.
  • Bowman capsule:
    • Parietal Layer: Outer layer of simple squamous epithelium, which becomes cuboidal in the PCT.
    • Visceral Layer: Inner layer constructed of specialized cells called podocytes that wrap around the glomerular capillaries.

The Filtration Membrane

  • Fenestrae: Highly permeable glomerular capillaries containing small windows.
  • Basement Membrane: Sandwiched between the endothelial cells of the glomerular capillaries and the podocytes.
  • Filtration Slits: Gaps between the cell processes of the podocytes.
  • The filtration membrane is specialized for filtration.

The Renal Tubules

  • Proximal Convoluted Tubule (PCT): Filtrate drains here from the Bowman capsule.
  • Loop of Henle: Contains a descending and an ascending limb.
  • Distal Convoluted Tubule (DCT): Shorter than the PCT.
  • Collecting Duct: Several DCTs connect to a single collecting duct, which has a large diameter and extends through the medulla towards the renal papilla and then to the ureter.

Nephron Histology

  • Proximal Convoluted Tubule: Simple cuboidal epithelium with many microvilli and mitochondria for active reabsorption of Na+Na^+, K+K^+ & Cl−Cl^-.
  • Loop of Henle: Thick parts consist of simple cuboidal epithelium, while thin parts consist of simple squamous epithelium for osmosis/diffusion.
  • Distal Convoluted Tubule: Simple cuboidal epithelium with few microvilli and numerous mitochondria for active reabsorption.
  • Collecting Duct: Simple cuboidal epithelium.

Major Renal Veins and Arteries

  • Includes the right and left renal veins, inferior vena cava, right and left renal arteries, and the abdominal aorta.

Ureters

  • Passageway for urine from the renal pelvis to the urinary bladder.
  • Lined with transitional epithelium.
  • Peristalsis moves urine from the renal pelvis in the kidneys to the ureters and then to the urinary bladder.
  • Ureters enter the bladder obliquely through the trigone, and pressure in the bladder compresses the ureter to prevent backflow.

Urinary Bladder

  • A hollow muscular container located in the pelvic cavity posterior to the symphysis pubis.
  • Trigone: A histologically unique triangular area on the posterior wall between the entry of the two ureters and the exit of the urethra.

Urethra

  • Transports urine from the urinary bladder to the outside of the body.
  • Lined with transitional epithelium at the top, with the remainder being stratified columnar.
  • Internal Urinary Sphincter: Located at the junction of the urinary bladder and the urethra, composed of elastic connective tissue and smooth muscle to prevent urine leakage.
  • External Urinary Sphincter: Skeletal muscle surrounds the urethra as it extends through the pelvic floor, allowing voluntary start/stop of urine flow.
  • Male Urethra: Extends from the inferior part of the urinary bladder through to the tip of the penis.
  • Female Urethra: Shorter and opens into the vestibule anterior to the vaginal opening.

Compendium 7 questions

  1. What are the main organs of the urinary system?
  2. Which of these organs produce urine and which transports urine?
  3. What are the internal structures of the kidney?
  4. What is a nephron and what are its parts?
  5. How does each part of a nephron contribute to urine formation and excretion of wastes?
  6. What do we reabsorb from urine during its production?
  7. What do we secrete into the urine as it is being produced?