Urine Formation Processes

Urine Formation

• Urine formation occurs through three processes: filtration, reabsorption, and secretion.

Step 1: Filtration

• Fluids and solutes move from the blood into Bowman's capsule.
• Approximately 1400 liters of blood pass through the kidneys daily.
• Bowman's capsules filter about 180 liters of fluid daily.
• This fluid is 65 times the volume of blood plasma.
• Only 1.5 liters of daily filtrate is excreted as urine. The rest is reabsorbed.

Step 2: Reabsorption

• Essential solutes and most water are reabsorbed back into interstitial fluid.
• For every 120 mL of filtered fluid, approximately 1 mL of urine is formed.
• This process mainly happens in the proximal tubule, loop of Henle, and distal tubule.
• It's driven by active and passive transport.

Step 3: Secretion

• Wastes move from blood and interstitial fluid into the nephron.
• The purpose is to eliminate toxins, drugs, and help maintain electrolyte and acid-base balance.
• This includes secretion of H+ ions, K+, and other substances.

Further Considerations

• Wastes are secreted in several points of the nephron, from the interstitial fluid.
• Proximal tubule secretes H+, toxins, water-soluble drugs, and small amounts of ammonia.
• Distal tubule secretes in response to hormones, including potassium (K+) and hydrogen (H+).
• Collecting ducts secrete excess H+ in response to blood/body fluid pH levels.
• Bottom of collecting ducts concentrates urine.
• Urine flows from the ducts to the renal pelvis, then through ureters to the bladder, and finally exits through the urethra.
• Kidney disease can be diagnosed through urinalysis - any metabolite is a sign of a potential disease in the kidney.
• Loss of kidney function leads to the need for dialysis or a transplant.

Additional Notes

• Water, sodium chloride, glucose, amino acids, etc. are reabsorbed.
• Blood cells, platelets, and large proteins are not reabsorbed.
• Some substances are actively reabsorbed or secreted, while others are passively transported.
• The processes of reabsorption and secretion are crucial for maintaining electrolyte and fluid balance in the body.
• The kidneys also help regulate blood pH levels.
• Kidney stones can be a problem - they form from minerals combining with calcium forming crystals. These block the flow of urine through the ureters, possibly requiring surgical removal.