What is Nitrogenous Waste?

Laura Langford, Christopher Muscato
  • Author
    Laura Langford

    Laura Langford is a nationally Certified Health Education Specialist. She has a Bachelor's degree in Health Education and Health Promotion from Arizona State University, where she also works as an Academic Associate.

  • Instructor
    Christopher Muscato

    Chris has a master's degree in history and teaches at the University of Northern Colorado.

Learn about nitrogenous waste. Understand what nitrogenous wastes are, explore their types, and discover which type of nitrogenous waste is in the urine. Updated: 03/16/2022

Table of Contents


What Is Nitrogenous Waste?

Nitrogenous waste is any waste product that is nitrogen-based. Nitrogenous wastes are formed when proteins are broken down into amino acids for energy. Ammonia is the most basic form of nitrogenous waste and is formed from the remaining amino acids that occur in the breakdown of proteins. Ammonia is highly toxic, especially to the brain, and it requires mass amounts of water to remove it from the body.

Not all organisms can efficiently remove ammonia from the body, requiring the chemical formula of ammonia to be modified into a less toxic form. An organism must live in a marine environment to dispose of nitrogen through ammonia. These marine organisms are constantly surrounded by water, which is necessary to excrete this nitrogenous waste. Organisms that excrete ammonia are referred to as ammonotelic.

Types of Nitrogenous Wastes

The types and forms of nitrogenous waste that are excreted vary between different species. As mentioned above, ammonia is the most basic form of nitrogenous waste. The molecular formula of ammonia is {eq}NH_3 {/eq}, which means that it consists of one nitrogen atom and three hydrogen atoms. Ammonia is produced by the cells within the intestines, liver, and kidneys. In the body, the liver uses ammonia to form urea, which is not as toxic.

Increased levels of ammonia can be very dangerous to the body and can cause low energy levels, confusion, and in severe cases, coma. The normal level of ammonia in the blood is 10 to 80 milligrams per deciliter for adults. Although ammonia can be excreted by marine organisms, it cannot be excreted in its basic form by mammals, birds, or reptiles. The additional types of nitrogenous wastes include:

  • Urea — Molecular formula is {eq}NH_2CONH_2 {/eq}. Less toxic than ammonia.
  • Uric Acid — Molecular formula is {eq}C_5H_4N_4O_3 {/eq}. Less toxic than both ammonia and urea.

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Nitrogenous Waste Excreted in Urine: Urea

In mammals, nitrogenous waste is excreted in urine as urea. Urea is the byproduct of ammonia; however, it is less toxic and does not require as much water to remove it from the body. Mammals that excrete urea are referred to as ureotelic. Examples of mammals that excrete nitrogenous wastes as urea are humans, dogs, and cats. The urea cycle refers to the process of removing ammonia from the blood by forming urea. To convert ammonia into urea, five different enzymes are utilized in the process. The chemical equation of ammonia to urea is as follows:

{eq}2 NH_3 (ammonia) + CO_2 + 3 ATP + H_2O = H_2N-CO-NH_2 (urea) + 2 ADP + 4 \pi + AMP {/eq}

The first and second chemical reactions of the urea cycle begin in the mitochondria of a cell, whereas the last three steps occur in the cytoplasm. Urea is made by the cells in the liver and is excreted through urine. If the urea cycle does not function properly, hyperammonemia can result as a buildup of ammonia. Blood concentration of urea is referred to as blood urea nitrogen, or BUN, and is measured to evaluate kidney function. A normal level of blood urea nitrogen is between 7 to 21 milligrams per deciliter for adults. High levels of nitrogenous waste in the blood, such as urea, can be associated with impaired kidney function.

Mammals, such as humans and dogs, excrete nitrogenous waste as urea.

An image of a man and a dog, which are examples of mammals that excrete nitrogenous waste as urea

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Frequently Asked Questions

What is nitrogenous waste in urine?

Urea is a nitrogenous waste that is excreted in urine. Ammonia is converted into urea, which is less toxic and requires less water to remove from the body. Urea is created by cells within the liver.

What are the two types of nitrogenous waste?

The two types of nitrogenous waste are urea and uric acid. Urea is the byproduct of ammonia in mammals, whereas uric acid is the byproduct of ammonia in birds and reptiles. Urea is less toxic than ammonia and requires less water to remove it from the body; however, uric acid is less toxic than both ammonia and urea and requires minimal water to remove it from the body.

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