Theory:

The process of excretion in animals:
The cells of our body perform a wide range of functions due to which certain waste products are released. These wastes are generally toxic and hence should be removed from the bodies of the living organisms.
The process of removal of wastes that are produces by the cells of the living organisms is known as excretion.
Human excretory system:
The organs that are involved in the removal of waste products from the body is known as excretory system. All the nitrogenous wastes of the body are removed by the excretory system.
 
The various organs which form the excretory system are as follows:
 
Kidneys:
  
All the wastes that are thrown out by the cells are transported to the excretory system by the blood. Hence, the blood is filtered to remove these wastes.
This filtration is performed by blood capillaries present in the two bean-shaped organs known as the kidneys.
The functional units of the kidneys are known as nephrons
When the blood reaches the kidneys, the nephrons filter the blood. During this the useful substances are absorbed back into the blood. The wastes that are dissolved in the water are removed as urine.
 
Important!
The kidneys receive the blood rich in oxygen from the heart through the renal arteries.
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Excretory system
 
Urinary bladder:
The pouch-like structure which stores the urine produced in the kidneys is known as urinary bladder.
 
Ureters: 
The two tube-like structures which connect the Kidneys and the urinary bladder are known as the ureters.
 
Urethra:
The urine which is stored in the bladder is sent out through the opening found at the end of a muscular tube known as the urethra.
 
Renal artery:
It is a blood vessel that brings the blood containing oxygen and urea from the aorta to the kidneys.
 
Renal vein:
It is the blood vessel that brings the filtered blood from the kidneys to the inferior vena cava.
 
Normally an adult human being passes about \(1-1.8\)L of urine in \(24\)hrs. The urine comprises of  \(95\)% of water, \(2.5\)%urea and  \(2.5\)% of other waste products.
The importance of drinking water:
The human body consists of \(70%\) of water. Some other parts of the body such as the grey matter of the brain has about 85% and parts like the fat cell has about \(15%\)
On an average humans consume \(1.5 \)to \(3.5 \)litres of water everyday in the form of food and water.
 
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Illustration of importance of water
Reference:
https://upload.wikimedia.org/wikipedia/commons/9/9b/Man_Drinking_Water_Cartoon.svg