We all know that urine is made of water, certain electrolytes and the colour of urine i.e. yellow is due to a chemical which is called urobilins. Its original name was urochrome. It was assumed that this pigment has given colour to urine but later on it was found that many pigments were responsible for the colour of urine, so the name has been changed.

Urine is formed mainly by the breakdown of product in the blood cells. Kidney’s functional units are called nephrons. They are found in millions of numbers in kidneys. These nephrons are responsible for the formation of urine by way of filtration, re-absorption and secretion. A normal 170 litres of waste should be excreted out of our bodies in the form of urine. But we never drink this much of water or secrete out. Some amount of water is also re-absorbed into our blood through the process of passive re-absorption.

The nutrition that we consume never gets wasted. Some ions like sodium and urobilins are not re-absorbed into the blood stream. As we drink, large amount urine starts looking clear as urobilin dilutes into water.

When a person drinks less amount of water, the urobilin content decreases in urine and urine looks dark yellow. It looks dark yellow as body tries to maintain some amount of water in the body.

So, when a person takes less amount of water the posterior pituitary gland secretes antiduretic hormone {ADH}. This secretion of hormone causes the urobilin concentration to increase and makes urine dark yellow. This is a sign of dehydration. When a person takes lot amount of water, ADH decreases and the water is eliminated in order to maintain water content.