Urea in the Blood and Urine, in Skin Creams and in Fertilizers

Urea is a small but important compound in the living world. It’s found naturally in the body and can also be made artificially. The liver produces urea as a waste substance when it breaks down amino acids. The urea then travels through the bloodstream to the kidneys, which excrete it in the urine. Doctors measure the concentration of urea in the blood to help them determine how well someone's kidneys are working.

Urea is added to skin creams to remove thickened or scaly areas and to moisturize the skin, and is a useful soil fertilizer because it's a good source of nitrogen, an important nutrient for plants. Since human urine contains urea some people fertilize soil with urine to improve plant growth.

In its solid form urea exists as white or colorless crystals which have no odor and are highly soluble in water. Urea is also known as carbamide and has very low toxicity.

A protein is a chain of amino acids, which are separated from each other when the protein is digested. Excess amino acids are broken down in a process called deamination. In this process the amino group of an amino acid (-NH₂) is removed and converted to an ammonia molecule (NH₃). Deamination takes place mainly in the liver.

Ammonia is very toxic to cells, but ammonia molecules react with carbon dioxide to make urea, which is much safer. The conversion of ammonia to urea takes place in the liver in a process known as the urea cycle. Blood vessels transport the urea to the kidneys, which remove it from the blood and send it into the urine.

A BUN test (Blood Urea Nitrogen) detects the concentration of urea in the blood. If the kidneys aren’t doing their job the amount of urea in the blood will increase. However, there are other possible reasons for this increase besides kidney problems. Eating a lot of foods that are rich in protein will cause the liver to produce a large amount of urea. Dehydration will also increase the blood's urea concentration, since this depends on the amount of water in the blood. If there is less water but the same amount of urea, the concentration of the urea will be higher.

It’s also possible to have a lower than normal urea concentration in the blood. This can be caused by drinking too much water and diluting the blood, not eating much protein or being unable to absorb enough amino acids through the wall of the small intestine, such as in celiac disease. In this condition the villi, the tiny projections of the small intestine which absorb digested food, are damaged or destroyed.

Urea is added to skin creams and is useful in the treatment of conditions such as corns, calluses, eczema, psoriasis and ichthyosis. Depending on its concentration, urea either removes thickened or scaly areas of skin or makes the skin soft and supple.

The tough outermost layer of the skin is called the stratum corneum. This layer is made of dead cells and contains a fibrous protein called keratin. When a cream containing a high concentration of urea is applied to a thickened area on the skin, the urea weakens the attachment between the cells of the stratum corneum and dissolves keratin, allowing the area to be shed. Urea is said to be a "keratolytic" substance - one that causes the stratum corneum to soften and peel. The removal of the thickened skin surface is called debridement.

Urea is also hygroscopic, which means that it absorbs water from the air. Therefore skin creams containing low concentrations of urea act as emollients. The softened skin can absorb substances better, which helps medications such as corticosteroids enter the skin. At concentrations of 2% to under 20% urea is used as a humectant - a substance that retains moisture in the skin. At a concentration of 20% or higher urea is keratolytic.

Urea contains forty six percent nitrogen by weight and is an excellent fertilizer. It’s cheaper and safer to transport and store than other nitrogen-containing products. When urea is applied to soil, bacteria in the soil use an enzyme called urease to react the urea with water, producing ammonia and carbon dioxide. The ammonia then reacts with water to make ammonium ions, which are absorbed by plant roots.

Urine contains urea, so it could be used as a natural fertilizer. In fact, some scientists in Finland have found that urine is a very good fertilizer for soil in which beets and other vegetables are planted. In a controlled experiment they found that the beets grown in urine-fertilized soil grew significantly bigger than the beets grown in soil treated with a mineral fertilizer, while still looking attractive and tasting good.

Unlike feces, which may contain dangerous bacteria, urine is virtually sterile (unless someone has an infection of the urinary tract). It's rich in nitrogen, potassium and phosphorus, which are all elements that plants need. However, practitioners of urine fertilization say that the urine must be diluted before use, since the chemicals in undiluted urine are far too concentrated for the health of most plants. Anywhere from a 1:3 to a 1:10 mixture of urine and water is suggested. In addition, they say that urine should be applied to the soil and not placed directly on plants. Urine is reportedly a good compost activator as well as a good fertilizer!