Metabolic Waste and the Excretory System

Metabolic waste consists of the substances produced by chemical reactions in cells that the body must remove, and excretion is the removal of these wastes by organs such as the kidneys, lungs and skin. In brief: the main metabolic wastes in humans are carbon dioxide (removed by the lungs), urea (removed by the kidneys in urine) and excess water and salts (removed by the kidneys and skin). Excretion differs from egestion, which is the removal of undigested food from the gut.

Waste products can be toxic if they accumulate, so excretion is essential for homeostasis. This guide explains what metabolic wastes are and where they come from, how the liver makes urea, which organs excrete which wastes, the structure of the urinary system, compares nitrogenous wastes in different animals, works through a urea calculation and corrects common mistakes.

What is metabolic waste?

Metabolism is the sum of chemical reactions in the body. Many reactions produce substances that are not needed or that are harmful in large amounts. Excretion is the removal of these wastes from the body. The main metabolic wastes are:

• Carbon dioxide: produced by respiration in all cells. A resting adult produces about 200 to 250 mL per minute. It forms carbonic acid in water and lowers the blood pH if it accumulates.
• Nitrogenous wastes: from the breakdown of proteins and nucleic acids. The amino group (–NH₂) removed from amino acids first becomes ammonia, which is very toxic. The main product in humans is urea.
• Bile pigments: from the breakdown of hemoglobin in old red blood cells. The liver converts them to bilirubin, which is excreted in bile. They give feces their brown color.
• Excess water and salts: removed to keep the correct water balance and ion concentrations.

Excretion is not the same as egestion (defecation). Egestion removes undigested food, which has never been part of the body’s cells. Feces are not mainly an excretory product, except for bile pigments.

How is urea made?

The body cannot store amino acids, so extra amino acids from the diet are broken down in the liver. In deamination, the amino group is removed from the amino acid and converted into ammonia. The remainder of the molecule, a keto acid, is used in respiration to release energy or converted into carbohydrate or fat. Ammonia is toxic, particularly to the brain, so the liver quickly converts it into urea in a series of reactions called the ornithine cycle (urea cycle). The equation for the overall process is: 2NH₃ + CO₂ → CO(NH₂)₂ + H₂O.

Urea is less toxic than ammonia and highly soluble. It is carried in the blood to the kidneys and excreted in the urine. Normal blood urea is about 2.5 to 7.5 millimoles per liter, and high levels may indicate kidney problems. Each molecule of urea contains two nitrogen atoms, so it is an efficient way to remove nitrogen, and it needs less water than the removal of ammonia.

Which organs excrete waste?

• Lungs: excrete carbon dioxide and some water vapor with each breath.
• Kidneys: the main organs of excretion and water balance. They filter blood and remove urea, excess water, salts and many drugs and toxins in urine.
• Skin: sweat contains water, salts and a small amount of urea. Its main function, however, is cooling, so sweating is mainly regulation of temperature and only partly excretion.
• Liver: does not excrete directly but makes urea, breaks down toxins and hormones and makes bile containing bile pigments.

What are the parts of the urinary system?

The urinary (renal) system has the following organs.

1. Kidneys: two bean-shaped organs, each about 11 centimeters long, on either side of the spine at the back of the abdomen. They receive about 20 to 25 percent of the heart’s output, around 1.2 liters of blood per minute.
2. Ureters: two muscular tubes, each about 25 centimeters long, which carry urine from the kidneys to the bladder by peristalsis.
3. Bladder: a muscular sac that stores urine. An adult bladder can hold about 400 to 600 mL, and the urge to urinate begins at around 200 to 300 mL.
4. Urethra: the tube that carries urine from the bladder out of the body. Two sphincters, one involuntary and one under voluntary control, regulate its opening.

The renal artery brings blood to each kidney, and the renal vein takes filtered blood away. The structure of the kidney and the way it forms urine are explained in the next article.

Excretory products and organs

How do other animals excrete nitrogen?

The form of nitrogenous waste is related to the habitat and to the availability of water.

• Ammonia (very toxic, needs a lot of water to dilute it): excreted by most aquatic animals such as fish and tadpoles, where it diffuses away across the gills or skin.
• Urea (moderately toxic, soluble): mammals, adult amphibians and sharks.
• Uric acid (almost insoluble, non-toxic, forms a white paste): birds, reptiles and insects. It requires very little water and is also useful for eggs with shells, which cannot release waste. The cost is that more energy is needed to make it.

Worked example: nitrogen balance and urea in urine

A person eats 90 g of protein in a day. Protein is about 16 percent nitrogen by mass. Assume that the person is in nitrogen balance (the nitrogen eaten equals the nitrogen excreted) and that 85 percent of the nitrogen is excreted as urea. The molar mass of urea, CO(NH₂)₂, is 60 g/mol, which contains 28 g of nitrogen.

1. Nitrogen eaten: 90 g × 0.16 = 14.4 g.
2. Nitrogen excreted as urea: 14.4 × 0.85 = 12.24 g.
3. Mass of urea: each 60 g of urea contains 28 g of nitrogen, so urea = 12.24 × (60 ÷ 28) = 26.2 g.
4. Concentration in urine: if the person makes 1.5 liters of urine in a day, the concentration is 26.2 ÷ 1.5 ≈ 17.5 g/L, or about 1.7 percent.

A typical value for urea excretion in urine is 20 to 35 g per day, which is consistent with this estimate. The calculation shows how protein intake affects urea production: a high-protein diet increases the amount of urea that the kidneys must remove. A person with reduced kidney function may be advised to limit protein.

How was excretion studied?

Urea was the first organic compound to be synthesized from inorganic material, by Friedrich Wöhler in 1828, an achievement that challenged the idea that organic molecules could only be made by living things. The ornithine cycle was worked out by Hans Krebs and Kurt Henseleit in 1932. Willem Kolff built the first working kidney dialysis machine in the Netherlands in the 1940s, which allowed the artificial removal of wastes in patients with kidney failure.

Which mistakes are common?

Mistake 1: Confusing excretion with egestion

Excretion removes metabolic waste made by cells. Egestion removes undigested food.

Mistake 2: Saying that the liver excretes urea

The liver makes urea. The kidneys excrete it.

Mistake 3: Thinking that sweating mainly excretes waste

Sweat is mostly water and salt, and its main role is cooling. Only a small amount of urea is excreted.

Mistake 4: Forgetting the lungs as excretory organs

The lungs excrete carbon dioxide, a waste product of respiration.