Urine is a liquid by-product of metabolism in humans and in many animals. It is a collection of substances that have not been reabsorbed during glomerular filtration or tubular secretion and flows from kidneys to urinary bladder through the ureters. Urination results in urine being excreted from body through urethra.
Normal urine is a transparent solution ranging from colorless to deep amber but is usually a pale yellow. In a healthy individual the color comes primarily from the presence of pigment urochrome, also termed as urobilin. Pigments and other compounds in certain foods and medications can change its color. Abnormal color may be caused by infection, disease, medicines, or food consumed.
Cellular metabolism generates many by-products which are rich in nitrogen such as urea, uric acid, and creatinine etc. and must be cleared from bloodstream. These by-products are excreted through urination, which is primary method for excreting water-soluble chemicals from the body.
pH of urine is normally within range of 5.5 to 7 with an average of 6.2 and is often influenced by consumed food. E.g. high protein diets results more acidic urine while vegetarian diets generally results in more alkaline urine.
Average urine production in adult humans is around 1.4 L of urine per person per day with a normal range of 0.6 to 2.6 L per person per day, produced in around 6 to 8 urinations per day.
The normal range for 24-hour urine volume is 800 to 2000 milliliters per day (with a normal fluid intake of about 2 liters per day). Oliguria is urine output < 500 mL in 24 h in an adult and output < 1 L in 24 h in a child.
Composition of urine
Urine is mainly made of water, salt, and chemicals called urea and uric acid. The nephrons of kidneys process blood and create urine through a process of filtration, reabsorption, and secretion. Urine is about 95% water and 5% waste products. Nitrogenous wastes excreted in urine include urea, creatinine, ammonia, and uric acid. Ions such as sodium, potassium, hydrogen, and calcium are also excreted.
Common constituents of urine includes:
- Water- About 95%
- Urea- 9.3 g/L.
- Chloride- 1.87 g/L.
- Sodium- 1.17 g/L.
- Potassium- 0.750 g/L.
- Creatinine- 0.670 g/L.
- Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).
It is formed in kidneys through a filtration of blood where unwanted substances are filtered from the blood and urine is produced to excrete.
There are three major steps of urine formation:
- Glomerular filtration.
- Selective Re-absorption
- Tubular Secretion
These processes ensure only waste and excess water are removed from body.
Download Link: Urine and its Composition.pdf