10.3 Artificial Kidneys and Dialysis Systems

The kidneys are vital organs that filter blood, regulate blood pressure, and produce hormones. They're composed of nephrons that remove waste and maintain electrolyte balance. When kidneys fail, waste builds up, causing uremia, fluid overload, and other complications.

Dialysis is a life-saving treatment for kidney failure. It uses diffusion and ultrafiltration to remove waste and excess fluid. Hemodialysis filters blood through an external machine, while peritoneal dialysis uses the body's own abdominal lining as a filter.

Kidney Anatomy, Physiology, and Failure

Anatomy and physiology of kidneys

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• Two bean-shaped organs located in the retroperitoneal space of the abdomen, one on each side of the spine
• Composed of functional units called nephrons that filter blood and produce urine
  • Glomerulus is a cluster of capillaries surrounded by Bowman's capsule where blood filtration occurs
  • Tubules include the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct, which reabsorb essential nutrients (glucose, amino acids) and secrete waste products (urea, excess electrolytes)
• Regulate blood pressure via the renin-angiotensin-aldosterone system (RAAS) and maintain electrolyte balance (sodium, potassium, calcium, magnesium)
• Remove metabolic waste products such as urea, creatinine, and uric acid from the blood
• Produce hormones including erythropoietin (stimulates red blood cell production), renin (regulates blood pressure), and calcitriol (active form of vitamin D)

Consequences of kidney failure

• Accumulation of waste products in the blood leads to uremia, causing symptoms such as nausea, vomiting, fatigue, and cognitive impairment
• Fluid overload and electrolyte imbalances (hyperkalemia, hypocalcemia, hyperphosphatemia) can lead to edema, hypertension, and arrhythmias
• Anemia due to decreased erythropoietin production, resulting in fatigue and reduced oxygen delivery to tissues
• Bone disorders (renal osteodystrophy) from impaired vitamin D activation and calcium-phosphorus imbalance, leading to fractures and bone pain
• Cardiovascular complications such as hypertension, left ventricular hypertrophy, and heart failure due to fluid overload and RAAS activation

Dialysis Systems and Treatment

Principles of dialysis systems

• Diffusion is the movement of solutes from an area of high concentration to an area of low concentration across a semipermeable membrane, driven by a concentration gradient
• Ultrafiltration is the removal of excess fluid using a hydrostatic pressure gradient across a semipermeable membrane, which allows water and small solutes to pass through while retaining larger molecules (proteins)
• Dialysate is a solution containing electrolytes and buffers that is used to remove waste products and correct electrolyte imbalances during dialysis

Hemodialysis vs peritoneal dialysis

• Hemodialysis
  • Blood is pumped through an external dialyzer containing a semipermeable membrane, which filters out waste products and excess fluid
  • Requires vascular access via an arteriovenous fistula (surgical connection between an artery and vein), graft (synthetic tube connecting an artery and vein), or catheter (temporary or permanent)
  • Typically performed 3 times per week for 3-4 hours per session in a dialysis center or at home with proper training and equipment
  • Advantages: rapid removal of waste products and excess fluid, performed by trained professionals
  • Limitations: requires vascular access and anticoagulation, time-consuming, increased risk of bloodstream infections (catheter-related) and cardiovascular stress (hypotension, arrhythmias)
• Peritoneal dialysis
  • Utilizes the patient's peritoneal membrane (lining of the abdominal cavity) as a natural filter
  • Dialysate is infused into the peritoneal cavity through a permanent catheter, allowing waste removal and fluid exchange
  • Can be performed continuously (CAPD: continuous ambulatory peritoneal dialysis) or intermittently (APD: automated peritoneal dialysis) at home
  • Advantages: greater flexibility and independence, preserves residual kidney function, lower risk of bloodstream infections and hemodynamic instability
  • Limitations: requires a permanent peritoneal catheter, increased risk of peritonitis (infection of the peritoneal cavity), less efficient waste removal compared to hemodialysis

Challenges in artificial kidney development

• Miniaturization of dialysis components (dialyzer, pumps, sensors) to create a compact, implantable device
• Ensuring long-term biocompatibility of materials and preventing blood clotting and foreign body reactions
• Achieving efficient waste removal and maintaining electrolyte balance with limited dialysate volume and flow rate
• Developing a reliable, miniaturized power supply and wireless communication for device monitoring and control
• Addressing ethical and regulatory challenges in clinical trials and commercialization of implantable devices
• Ongoing research focuses on microfluidic devices, bioartificial kidneys with living cells, wearable and implantable dialysis systems, and tissue engineering approaches to create functional kidney replacements
• Potential benefits include improved patient quality of life, reduced infections and complications, continuous waste removal, and decreased healthcare costs and burden