Definition
Urea is an organic compound that is formed in the human body as the end product of the urea cycle and is then excreted mainly through the kidneys, but also through sweat. Urea contains the substance “ammonia”, which is toxic to humans. This accumulates in various metabolic pathways of amino acids in the body and is then packaged in urea and excreted. It is important not to confuse urea with uric acid.
Urea cycle
The urea cycle is a chain of metabolic reactions in the human body, which ensures that the toxic substance “ammonia” is packaged as urea and then excreted in a safe manner. In the human body, organic substances are continuously built up, broken down or converted into each other. The amino acids play an important role in this process.
They form the basis for proteins and are therefore the body’s most important building material. In other words, amino acids are created when proteins are broken down. When amino acids are broken down, a wide variety of substances are produced.
Some of them have a carbon skeleton and can therefore provide energy. Another large proportion of degradation products, however, is nitrogen, which in combination with hydrogen is converted to ammonia (NH3) or ammonium (NH4+). Ammonium is the variant of ammonia that occurs in the human body.
Under the conditions prevailing in the body, ammonia is usually immediately converted to ammonium. To prevent ammonium from accumulating in the body, it must be constantly excreted. Since the substance is to be excreted via the kidneys and is toxic there, it must first be well packaged.
This happens within the urea cycle. The urea cycle takes place partly in the mitochondria and partly in the cell plasma of a liver cell and consumes a lot of energy. Ammonium is transferred from one substance to another in a continuous process.
The first step is one of the most important ones, since the ammonium is converted together with “bicarbonate” and under the consumption of energy into a substance that is no longer toxic for humans. The further reactions still change the substance, but the toxic ammonium is already packaged. The urea cycle is also connected with other important reaction pathways of the human body, such as the citrate cycle.
In the last step, urea is split off. This is then transported via the blood to the kidneys, filtered and added to the urine. In this process, urea shows its second major strength: it helps the kidneys to produce urine by creating a concentration gradient. It is therefore not only constantly excreted via the kidney, but also plays a decisive role in the function of the kidney.
