Urea, a waste product formed during protein metabolism, is synthesized in the liver and excreted through the urinary system. The journey of urea from its site of synthesis to the point of its exit involves several steps and organs. Here is a detailed explanation of the possible route of urea:

Synthesis of Urea:
Urea is synthesized in the liver from ammonia, which is produced as a byproduct of protein breakdown in various tissues. The urea cycle, also known as the ornithine cycle, takes place in the liver hepatocytes.

Transport from the Liver:
Once synthesized, urea is transported from the liver hepatocytes into the bloodstream. This process occurs through the sinusoidal membrane of hepatocytes.

Circulation through the Bloodstream:
Urea enters the bloodstream and gets carried to various organs and tissues through the circulatory system. It is dissolved in plasma and circulates throughout the body.

Filtering in the Kidneys:
The kidneys play a crucial role in the excretion of urea. As blood flows through the renal arteries, it enters the kidneys where the filtration process takes place. The glomerulus, a network of capillaries, filters waste products, including urea, from the blood.

Reabsorption and Concentration:
After filtration, urea enters the renal tubules. The majority of urea is reabsorbed back into the bloodstream, primarily in the proximal convoluted tubule. However, a small amount of urea is excreted in the urine.

Concentration in the Collecting Ducts:
As the filtrate moves through the renal tubules, it gradually concentrates in the collecting ducts. Urea is actively transported from the tubules into the interstitial fluid and then diffuses into the medullary collecting ducts.

Excretion through the Urinary System:
Finally, the concentrated urine containing urea is transported from the collecting ducts to the renal pelvis and then to the ureters. The ureters carry the urine to the urinary bladder, where it is stored until it is eliminated from the body through the urethra during urination.

In summary, urea is synthesized in the liver, transported through the bloodstream, filtered in the kidneys, reabsorbed to some extent, concentrated in the collecting ducts, and ultimately excreted through the urinary system. This process ensures the removal of waste urea from the body, maintaining a healthy nitrogen balance.