Urine

Waste disposal — and recycling

Urine is a fluid by-product of human metabolism that is discarded through the process of urination, the primary method for excreting non-usable, water-soluble waste substances from the body.

Formation and composition

Urine formation is a multi-step process consisting of filtration, reabsorption and secretion stages:

  1. The kidneys contain millions of blood-filtering structures called nephrons, each with a glomerulus (a network of capillaries within a collecting capsule) that is the active site of filtration. Blood flowing through the glomerulus passes through a filtration membrane to begin the urine-creation process.
  2. The glomerulus’ filtration membrane is a layer of specialized cells that keeps blood cells and larger proteins in the bloodstream while allowing a filtrate of water and smaller solutes to continue into the nephron.
  3. The post-glomerulus filtrate represents a mixture of waste substances along with other materials the body needs, such as glucose, amino acids, various small proteins and essential ions. These substances, along with some water, are reabsorbed into the blood via renal tubules in the nephrons.
  4. At the same time these reclaimed nutrients are moving from renal tubule to capillary, ionic waste products are moving across capillary walls in the opposite direction, from bloodstream to renal tubule. This secretion process adds waste ions to the filtrate to form urine, which eventually passes through ureters into the bladder where it can be expelled.
  5. The final urine product is about 95 percent water and five percent waste products, including nitrogen-rich compounds (urea, ammonia, creatinine, uric acid) and excess ions (sodium, potassium, calcium, hydrogen).

Analysis of urine

A urine sample analysis, or urinalysis, is a frequently performed is a frequently performed procedure useful for health monitoring and diagnosis of various diseases or disorders. Common reasons for a urinalysis include:

  • Routine medical checkup
  • Pregnancy determination
  • Hospital admission screening
  • Drug screening
  • Investigating the cause of back pain, abdominal pain, urination pain or blood in the urine
  • Monitoring of existing diseases or medical conditions

References

Filtration, Reabsorption, Secretion: The Three Steps of Urine Formation. Visible Body. Available from URL: https://www.visiblebody.com/.

Urinalysis. Mayo Clinic. Available from URL: https://www.mayoclinic.org/.

Urine. Encyclopedia Britannica. Available from URL: https://www.britannica.com/.

Additional Body Fluids

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