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Urine and Urinalysis (The Composition of Urine)

The Structure of the Urinary System. Diagram of the Urinary System.

This page is about urine. Recognition and analysis of abnormalities of this substance expelled from the body can provide information about the condition of the body - both concerning general health, and also specific medical conditions.

Biochemical analysis of urine is called "urinalysis", and is commonly used to diagnose a wide range of diseases. Examples include high levels of urinary glucose in diabetics, and high levels of urinary ketone bodies in cases of ketonuria. Immunological analysis of urine is the basis of most pregancy tests.

What is a typical normal volume of urine ?

1-2 litres / 24 hours per normal adult.
However, the amount per day varies considerably.

The actual quantity per person per day is affected by factors such as: * recent fluid intake (water, and other food/drinks that include water) * diet * temperature * blood pressure * general health (some disease states may affect urine volume/time) * mental state.

What are the physical characteristics of normal urine ?

Volume (as mentioned above) is one of the physical characteristics of urine. Other physical characteristics that can apply to urine include colour, turbidity (transparency), smell (odour), pH (acidity - alkalinity), and density.

  • Colour: Typically yellow-amber but varies according to recent diet and the concentration of the urine. Drinking more water generally tends to reduce the concentration of urine, and therefore cause it to have a lighter colour. (The converse is also true.)
  • Smell: The smell (or "odour", which is the more clinical term, American spelling "odor") of urine may provide health information. For example, urine of diabetics may have a sweet or fruity odour due to the presence of ketones (organic molecules of a particular structure). Generally fresh urine has a mild smell but aged urine has a stronger odour, similar to that of ammonia.
  • Acidity: pH is a measure of the acidity (or alkalinity) of a solution. The pH of a substance (solution) is usually represented as a number in the range 0 (strong acid) to 14 (strong alkali, also known as a "base"). Pure water is "neutal" in the sense that it is neither neither acid nor alkali, it therefore has a pH of 7. The real significance of pH in terms of physical chemistry is that pH is a measure of the activity of hydrogen ions (H+) in a solution.
    The pH of normal urine is generally in the range 4.6 - 8, a typical average being around 6.0. Much of the variation is due to diet. For example, high protein diets result in more acidic urine, but vegetarian diets generally result in more alkaline urine (both within the typical range 4.6 - 8).
  • Density: Density is also known as "specific gravity". This is the ratio of the weight of a volume of a substance compared with the weight of the same volume of distilled water.
    Given that urine is mostly water, but also contains some other substances dissolved in the "water", its density is expected to be close to, but slightly greater than, 1.0. This is true - the density of normal urine is in the range 0.001 to 0.035.


What is contained in normal urine ?

  • Approx. 95% of the volume of normal urine is due to water.
  • The other 5% consists of solutes (chemicals that are dissolved in the water).
    Some of these solutes are the results of normal biochemical activity within the cells of the body. Other solutes may be due to chemicals that originated outside of the body, such as pharmaceutical drugs.

    Solutes found in urine may be classified as ions (e.g. single elements that are positively or negatively charged due to loss or acquisition of one or more electrons from/to the outer-levels of the atom), or organic molecules (i.e. several, sometimes many, atoms that have joined together to form a group of atoms called a "molecule". "Organic" molecules are formed from groups, rings, or chains of carbon atoms and are the "building blocks" of the living things on the earth - i.e. plants and animals).

Identify substances found in normal urine ...
Short Answer:

The biochemicals found in urine are predominately the end-products of the nitrogen metabolism process.
These include urea, uric acid, and creatinine. Other components of urine include sodium chloride (common salt), and over 100 other substances that are usually present, but only in trace (i.e. very small) quantities.

Identify substances found in normal urine ...
Longer Answer:

Urine is aprox. 95% water.
The other components of normal urine are the solutes that are dissolved in the water component of the urine. These solutes can be divided into two categories according to their chemical structure (e.g. size and electrical charge).

Organic molecules are electrically neutral and can be relatively large (compared with the 'simpler' ions - below).
These include:

  • Urea - Urea is an organic (i.e. carbon-based) compound whose chemical formula is: CON2H4 or (NH2)2CO. It is also known as carbamide. Urea is derived from ammonia and produced by the deamination of amino acids. The amount of urea in urine is related to quantity of dietary protein.
  • Creatinine - Creatinine is a normal (healthy) constituent of blood. It is produced mainly as a result of the breakdown of creatine phosphate in muscle tissue. It is usually produced by the body at a fairly constant rate (which depends on the muscle mass of the body).
  • Uric acid - Uric acid is an organic (i.e. carbon-based) compound whose chemical formula is: C5H4N4O3.
    Due to its insolubility, uric acid has a tendency to crystallize, and is a common part of kidney stones.
  • Other substances/molecules - Example of other substances that may be found in small amounts in normal urine include carbohydrates, enzymes, fatty acids, hormones, pigments, and mucins (a group of large, heavily glycosylated proteins found in the body).

Ions are atoms or groups of atoms that have either, lost some outer electrons, hence have a positive electric charge, or have gained some outer electrons (to the atom or group of atoms), and hence have a negative electric charge. Even in the cases of ions formed by groups of atoms (they are ions due to the few lost or gained electrons), the groups are formed from only a small number of particles and therefore tend to be relatively small.
These include:

Individual elements:

  • Sodium (Na+) : Amount in urine varies with diet and the amount of aldosterone (a steroid hormone) in the body.
  • Potassium (K+) : Amount in urine varies with diet and the amount of aldosterone (a steroid hormone) in the body.
  • Chloride (Cl-) : Amount in urine varies with dietart intake (chloride is a part of common salt, NaCl).
  • Magnesium (Mg2+) : Amount in urine varies with diet and the amount of parathyroid hormone in the body. (Parathyroid hormone increases the reabsorption of magnesium by the body, which therefore decreases the quantity of magnesium in urine.)
  • Calcium (Ca2+) : Amount in urine varies with diet and the amount of parathyroid hormone in the body. (Parathyroid hormone increases the reabsorption of calcium by the body, which therefore decreases the quantity of calcium in urine.)

Small groups formed from a few different elements:

  • Ammonium (NH4+) : The amount of ammonia produced by the kidneys may vary according to the pH of the blood and tissues in the body.
  • Sulphates (SO42-) : Sulphates are derived from amino acids. The quantity of sulphates excreted in urine varies according to the quantity and type of protein in the person's diet.
  • Phosphates (H2PO4-, HPO42-, PO43-) : Amount in urine varies with the amount of parathyroid hormone in the body - parathyroid hormone increases the quantity of phosphates in urine.

Note: This page is about the normal characteristics and constituents of urine. Abnormal components of urine is a different topic.

Quick Summary: The Properties and Composition of normal human urine

  1. Recognition and analysis of abnormalities of urine can provide information about the condition of the body.
    Biochemical analysis of urine is called "urinalysis", and is commonly used to diagnose a wide range of diseases
  2. Typical volume of urine: 1-2 litres / 24 hours per day (normal adult).
    Variations due to :
    recent fluid intake, diet, temperature, blood pressure, general health, mental state.
  3. Physical characteristics include: volume, colour, smell, acidity, density.
  4. Constituents of urine include: approx. 95% water, approx. 5% other solutes (incl. organic molecules such as urea, creatinine and uric acid, and ions derived from amino acids, hormones, and other biochemicals).
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