Question

The principal nitrogenous excretory compound in humans is synthesized (a) in kidneys but eliminated mostly through liver (b) as well as eliminated by kidneys (c) in liver and also eliminated by the same through bile (d) in the liver, but eliminated mostly through kidneys.

Short answer

Urea is synthesized in the liver and eliminated mostly through the kidneys.

Step-by-step solution

Identify the principal nitrogenous excretory compound

The principal nitrogenous excretory compound in humans is urea, which needs to be identified to solve this exercise.

Determine the site of synthesis of urea

Urea is synthesized in the liver through the urea cycle, where ammonia, a toxic byproduct of protein metabolism, is converted into urea.

Determine the route of elimination of urea

Once synthesized, urea is then transported to the kidneys through the bloodstream, where it is filtered out of the blood and eliminated from the body in the urine.

Key concepts

Urea Synthesis

Understanding the synthesis of urea is crucial for grasping how our body handles the toxic byproducts of protein metabolism. The process begins in the liver, where ammonia, a substance that would be harmful if allowed to accumulate, is converted into a less toxic compound called urea. This transformation is part of a vital detoxification pathway.

Ammonia is produced when amino acids, the building blocks of proteins, are broken down. Because ammonia is highly toxic, even at low concentrations, its quick conversion to urea is essential for our health. Enzymes in the liver, through a series of reactions known as the urea cycle, mediate this conversion. The urea cycle involves ornithine and other intermediates that facilitate the incorporation of ammonia into urea. This soluble compound is then released into the bloodstream to be transported to the kidneys for excretion.

It's important to note that improper function of the urea synthesis process can lead to a buildup of ammonia in the bloodstream, which can result in severe health issues, including brain damage and other metabolic disturbances.

Urea Cycle

The urea cycle, also known as the ornithine cycle, is a series of biochemical reactions that converts the toxic ammonia into urea within the liver cells. The cycle consists of five main steps, catalyzed by different enzymes, to ensure the safe disposal of nitrogen from the body. It effectively eliminates excess nitrogen, which originates from amino acid breakdown, from our system.

The cycle begins with the combination of ammonia and carbon dioxide to form carbamoyl phosphate. Subsequently, the compound ornithine combines with carbamoyl phosphate to form citrulline, which then enters the cytosol from the mitochondria. Further reactions involve the synthesis of argininosuccinate, its conversion to arginine, and the cleavage of arginine to produce urea and regenerate ornithine. This elegant biochemical arrangement not only produces urea but also recycles ornithine to keep the cycle in motion.

Without the efficient functioning of the urea cycle, individuals would suffer from hyperammonemia, a condition characterized by high levels of ammonia in the blood. The urea cycle is therefore a life-sustaining pathway that serves as a bridge between ammonia detoxification and nitrogen excretion.

Kidney Function in Excretion

Once urea is synthesized in the liver, it must be eliminated from the body. The kidneys play the pivotal role in this excretory process. Acting like sophisticated filtration units, they remove urea and other waste products from the bloodstream.

The process begins in the glomeruli, tiny clusters of blood vessels within the kidneys, where blood is filtered. This filtration includes the separation of urea, excess salts, and water from the blood. After filtration, the fluid enters the renal tubules, where selective reabsorption occurs. During this phase, the kidneys reclaim substances that the body still needs, such as certain ions and water, while leaving urea and other waste products in the fluid.

As the final composition of this fluid—now urine—is determined, it flows down to the bladder through the ureters, ready for excretion. The delicate balance maintained by the kidneys is crucial for homeostasis. They not only remove urea but also regulate water balance, electrolyte levels, and blood pressure, showcasing their integral role in overall health and bodily function.