Renal system

• The urinary system (or renal system)

  • Removal of metabolic waste (e.g. urea, creatinine)
  • Removal of other toxins (e.g. drugs)
  • Water balance
  • Electrolyte balance
  • Acid-base balance
  • Production of urine
  • Excretion of urine
  • Hormone and enzyme production (Erythropoietin, Renin)
  • Hormone modification (Activation of vitamin D)
• Blood pH regulation – bicarbonate buffer system

  1. Kidneys: slow regulation

  2. Lungs: fast regulation
• The kidneys

  • Bean-shaped organs
  • Located between T12 and L3 vertebrae, retroperitoneal
  • ~150 g, 9 -11 cm long (size of a fist)
  • Right kidney slightly lower to accommodate the liver
  • Renal hilum: indentation, exit and entry point for ureter, renal artery, renal vein, lymphatics, nerves
• Minor/Major calyces:

  urine draining structures that push urine into the renal pelvis, a large collecting chamber that leads into the ureter

  
• The nephron

  • Functional unit of the kidney (ca. one million per kidney)
  • Function: Glomerular filtration, Tubular reabsorption, Tubular secretion

  
• Glomerular filtration

  1. Blood in the glomerular capillaries is filtered through the membrane of the glomerular capillaries

  2. This filter is selective, based on size and charge

  3. Cells and most proteins do not pass but smaller substances (ca 7 nm) do, including water, electrolytes, protons and bicarbonate ions, small organic molecules, metabolic nitrogenous wastes

  4. These (water + solutes) enter the glomerular capsule as filtrate

  5. The kidneys produce 180 l per day of filtrate (125 ml/min)

  
• Tubular reabsorption

  The nephron modifies the filtrate as it flows through the tubules, reclaiming substances from the filtrate and returning them to the blood, most reabsorption takes place in the proximal tubule and nephron loop, the nephron reabsorbs 99% of the filtrate

  
• Tubular secretion

  Substances are moved from the peritubular capillary blood into the filtrate, helps maintain electrolyte and acid-base homeostasis, removes toxins from the blood that did not enter the filtrate via filtration, only 1.8 l per day leave the kidneys as urine (1% of filtrate)

  
• The glomerulus

  • Endothelium: fenestrated, solutes, plasma proteins and fluid can pass through, but not blood cells
  • Basement membrane: prevents plasma proteins from being filtered out of the bloodstream
  • Epithelium: consists of specialized cells called podocytes, which are attached to the basement membrane by foot processes (pedicels) and wrap around the capillaries, leaving filtration slits
• Glomerular filtration- Starling forces

  • Glomerular filtration is a passive process that does not directly consume metabolic energy
  • the driving force is the hydrostatic blood pressure
  • at entrance: glomerular hydrostatic pressure (GHP) is partially counteracted by the capsular hydrostatic pressure (CHP) and by the colloid osmotic pressure (GCOP) in the glomerulus
  • the positive net outward pressure (net filtration pressure, NFP) makes sure that there is still urine production, even in case of severe dehydration
  • NFP= GHP-(GOP+CHP)
• Glomerular hydrostatic pressure (GHP)

  • largely determined by the systemic blood pressure,
  • efferent arteriole diameter < afferent arteriole diameter -> blood pushes against glomerular capillaries walls ->favors its movement through the filtration membrane
  • can be changed by changing diameter of afferent+efferent arterioles -> changes NFP
• Glomerular colloid osmotic pressure (GOP)

  • created by the presence of proteins
  • opposes filtration, holds water in the glomerular capillaries
• Capsular hydrostatic pressure (CHP)


  • The rapidly accumulating filtrate inside the capsular space of a nephron builds up a hydrostatic pressure
  • pushes water into the glomerular capillaries and so opposes filtration
• Glomerular filtration rate (GFR)

  • volume of filtrate formed each minute by the combined activity of all 2 million glomeruli of the kidneys

  directly proportional to:

  • net filtration pressure
  • total surface area available for filtration
  • filtration membrane permeability
• Glomerular filtration rate (GFR)

  The 180 L of filtrate per day that translate to the normal GFR of 125 ml/min
• Intrinsic regulation of GFR

  • Blood pressure fluctuations are counteracted by adjusting the dilation or constriction of the afferent arterioles
  • Increase in systemic blood pressure → constriction of afferent arterioles
  • Decrease in systemic blood pressure → dilatation of afferent arterioles
• Extrinsic regulation of GFR


  • Nervous system can stimulate contraction of the afferent arteriole, reducing urine production -> (hypovolemic shock-> kidney failure)
  • Nervous system can activate the renin-angiotensin-aldosterone system, regulating blood pressure(vasoconstriction) and fluid balance (reabsorption in kidneys)
  • Atrial natriuretic peptide (hormone released by heart) acts to lower blood volume and pressure, increasing GFR
• Kidney stones

  • Small: up to 5mm
  • Medium: 5-20 mm
  • Large: > 20 mm

  
• Kidney stone treatment


  1. Observation only

  2. Extracorporeal shock wave lithotripsy (ESWL)

  3. Ureteropyeloscopy

  4. Percutaneous nephrolithotomy

  5. Laparoscopic surgery (medium to large stones)

  6. Open renal surgery (large stones)

  
• Extracorporeal shock wave lithotripsy (ESWL)

  Non surgical technique using ultrasound shock waves to break up stones
• Ureteropyeloscopy

  • Thin fiber-optic telescope introduced into the kidney from the bladder via the urethra
  • allows visualization of the entire kidney drainage system
  • through instrument port:introduction of laser fibers to fragment stones and micro-baskets to retrieve larger stone fragments
• Percutaneous nephrolithotomy

  1. Rigid tube positioned in the kidney under X-ray guidance
  2. nephroscope (kidney telescope) used to visualize stones
  3. stones are broken into small enough pieces to be removed through nephrostomy tube via laser or ultrasound energy
• Surgery

  Rarely required when other methods cannot access or fragment stones, using large incisions or laparoscopy
• nephrostomy tube


  A tube inserted through the skin into the kidney to drain urine
• Ureteral stents


  Small tubes inserted minimally invasively into the ureter to preserve urinary drainage whenever ureteral patency is deteriorated or the ureter is under a significant risk of occlusion
• Placement and removal of ureteral stents

  1. Urinary tract is accessed using a cystoscope

  2. A guidewire is placed into the ureter under fluoroscopic guidance

  3. Stent is then advanced over the guidewire and into the ureter

  4. For removal: the distal end of the stent is grasped and gently pulled out through the urethra
• Ureteral stentsrequirements

  • Optimal flow characteristics
  • Ease of insertion and removal
  • Biocompatibility and radiopacity
  • Resistance to infection, corrosion and encrustation
• Ureteral stent design

  Openings in the stent wall (side holes) to facilitate urine flux in and out of the stent lumen

  Materials: Silicone, polyurethane
• Ureteral stent coatings

  Anti-adhesive coatings or Antimicrobial compounds
• Urethral stents


  Restore urinary flow of the inferior urinary apparatus by mounting a tubular structure (the stent) from the urethral segment overlying the obstacle to the segment underlying it
• Urethral stent requirements

  • Easy to insert (blind, without visual or radiologic control) and easy to extract
  • Stabilized at both ends for preventing migration
  • Thin walls, for a minimal urodynamic resistance
  • Soft and flexible for the patient's comfort
  • Resistant to incrusting
  • Biocompatibility and radiopacity
• Urethral stent materials

  Nitinol, Polyurethane, Silicone
• Prostatic stents


  Stents used to keep open the male urethra and allow the passing of urine
• Prostatic stent indications

  • Prostatic obstruction
  • Enlarged prostate (benign prostatic hyperplasia)
  • Prostate cancer
• Prostatic stent materials

  Nitinol stents with silicone sleeve
• Spanner Prostatic Stent


  A temporary stent inserted into the urethra at the neck of the bladder to maintain urine flow. The stent extends from the bladder to just above the external sphincter. The device tethers traverse the external sphincter to allow normal sphincter function, thus providing patients the ability to volitionally void (allow voluntary urination).
• Cystoscopy


  A procedure that uses a cystoscope to look inside the urethra and bladder
• Ureteroscopy


  A procedure that uses a ureteroscope to look inside the ureters and kidneys
• Flexible cystoscopy


  Tends to be done if the reason for the procedure is just to look inside the bladder. The thin and soft surface of the flexible cystoscope enables a virtually painless and more comfortable cystoscopy.
• Rigid cystoscopy


  The advantages include a large visual field, superior telescopes, wide working channel for auxiliary instruments, irrigation channel with a large caliber and the possibility to evacuate blood clots or other bladder debris. The procedure is usually performed under a general or spinal anaesthetic.