Can you trace the path taken by a molecule of urea from time it is produced in liver to leaves body urine?

The path of urea from its production in the liver to its excretion from the body in urine can be traced as follows:

1. Production in the Liver: Urea is produced in the liver as a byproduct of protein metabolism, particularly the deamination of amino acids. Ammonia, formed during deamination, is converted to urea through a series of reactions involving carbamoyl phosphate synthetase I (CPS I), ornithine transcarbamoylase (OTC), argininosuccinate synthetase (ASS), and argininosuccinate lyase (ASL).

2. Transportation to the Kidneys: Once produced, urea is transported from the liver into the bloodstream. The hepatic veins carry urea-containing blood from the liver to the inferior vena cava, which then delivers it to the right atrium of the heart. From there, the blood is pumped to the lungs via the pulmonary artery, where it undergoes gas exchange. The oxygenated blood, now containing urea, returns to the heart through the pulmonary veins and enters the left atrium. Finally, the left ventricle pumps the blood containing urea into the systemic circulation through the aorta.

3. Filtration in the Kidneys: The blood containing urea reaches the kidneys, where the renal arteries supply blood to the nephrons, which are the functional units of the kidneys responsible for filtering waste products. Within the nephrons, the glomerulus, a cluster of tiny blood vessels, allows for the filtration of substances from the blood. Urea, along with other waste products, electrolytes, and water, is filtered out of the blood and enters the renal tubules.

4. Reabsorption and Secretion: As the filtrate progresses through the renal tubules, essential substances like glucose, amino acids, and electrolytes are reabsorbed back into the bloodstream. At the same time, active secretion of certain substances, including drugs, toxins, and excess ions, occurs. Urea undergoes both passive reabsorption and active secretion, with the majority of filtered urea being reabsorbed passively.

5. Concentration in the Collecting Ducts: The reabsorbed urea, along with other waste products and excess water, is concentrated as the filtrate moves through the collecting ducts. The collecting ducts merge together, and the concentrated urine, now containing a high concentration of urea, enters the renal pelvis.

6. Storage in the Urinary Bladder: The urine produced in the kidneys flows through the ureters and into the urinary bladder, where it is stored until it reaches a certain volume. The urinary bladder acts as a temporary reservoir for urine.

7. Excretion through Urination: When the urinary bladder is full, it contracts, and the stored urine is expelled through the urethra during urination. The urine, along with the urea it carries, is eliminated from the body, completing the path from urea production in the liver to excretion from the body through urine.