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Excretion in Humans

Chapter: Excretion in Humans


During the process of energy generation where carbon source and oxygen are utilized, a lot of by- products are generated which are not useful to the body while some could be harmful to the body.  So, these by-products need to be eliminated out of the body by a process called excretion. These waste products can be very toxic to the body. The waste products include urea, carbon dioxide and salts.

Carbon dioxide:

Carbon dioxide is the waste product produced from the process of respiration. This has to be eliminated out of the body else it can dissolve with plasma and tissue fluid to form a weak acid (carbonic acid), which can denature proteins and enzymes at high concentrations.


 Urea is produced in the liver during deamination of excess amino acids. This has to be eliminated from the body else it can denature the enzymes


Salts which are excess in the diet can have an effect on water potential of the blood.


It is the removal of waste products of metabolism and substances in excess requirements.

The role of kidney

  1. The excretory system of a human being includes a pair of kidneys, ureters, urinary bladder and urethra.
  2. Kidneys are located in the abdomen, on either side of the backbone.
  3. Urine produced in the kidneys passes through the ureters and from the ureters it is passed into the urinary bladder where it is stored until it is released through urethra.
  4. Urine is produced in the body for the elimination of nitrogenous wastes like urea or uric acid from the kidneys.
  5. A tuft of capillaries called glomerulus present in cup shaped structure called Bowman’s capsule collect the filtered urine.
  6. As the urine passes along the tube some substances such as glucose, amino acids, salts and major amount of water are reabsorbed. The amount of water reabsorbed depends on the how much amount of excess water is present in the body.
  7. Urine forming in each kidney enters a long tube, called the ureter which connects kidney with the urinary bladder.
  8. Until the pressure of the expanded bladder leads to the urge to pass urine out through urethra the urine is stored in the urinary bladder.
  9. We can control the urge to urinate as the muscular urinary bladder is under nervous control.


Ureters are long tubes that arise from the pelvis of each kidney. These tubes are made of smooth muscle fibres. Ureters carry urine from kidney to urinary bladder.

Urinary bladder:

It is a hollow, muscular and elastic organ that sits on the pelvic floor. It collects urine from the ureter and stores up to about 500 cm3 of urine before expelling it from the body through urethra.


Urethra is a tube which connects urinary bladder to the outside of the body. It carries urine from bladder to the outside.

How is urea formed ?

Urea is produced in the body through urea cycle. Urea cycle takes place in the liver. Excess amino acids that are present in the body undergo deamination reaction in the liver to produce urea.

Role of liver in excretion

Liver plays a very important role in the body. One of the functions of the liver is excretion. Substances like alcohol, drugs, hormones and steroids are broken down in liver and excreted through bile.