The removal of waste substances from the body is called excretion.

Excretion is the process by which waste substances, produced as a result of various metabolic activities, are removed from the body. These waste substances can be harmful if they accumulate in the body, so excretion is necessary to maintain a healthy internal environment.

Importance of excretion:
Excretion plays a vital role in maintaining homeostasis, which is the body's ability to regulate and maintain a stable internal environment. It helps in the removal of harmful metabolic waste products, toxins, and excess substances from the body. Some of the key waste substances that are eliminated through excretion include:

1. Carbon dioxide: During cellular respiration, carbon dioxide is produced as a waste product. It is transported through the bloodstream to the lungs, where it is eliminated by exhalation.

2. Nitrogenous wastes: These are produced by the breakdown of proteins and nucleic acids. The main nitrogenous waste in humans is urea, which is formed in the liver and excreted by the kidneys in the form of urine.

3. Excess water and salts: The kidneys play a crucial role in maintaining the balance of water and electrolytes in the body. They filter the blood, reabsorb necessary substances, and excrete excess water and salts through urine.

Organs involved in excretion:
Several organs are involved in the excretory process, each performing specific functions:

1. Skin: The skin helps in the excretion of sweat, which contains water, salts, and small amounts of urea. Sweating helps regulate body temperature and eliminates certain waste products.

2. Lungs: Carbon dioxide, a waste product of cellular respiration, is eliminated through exhalation. The respiratory system plays a crucial role in gas exchange, ensuring the removal of carbon dioxide and the intake of oxygen.

3. Liver: The liver is responsible for the breakdown of toxins and the synthesis of urea. It detoxifies harmful substances and converts them into less toxic forms that can be eliminated from the body.

4. Kidneys: The kidneys are the primary excretory organs in the body. They filter waste products, excess water, and salts from the blood to produce urine. The urine is then transported to the bladder for storage and eventually eliminated from the body through the urethra.

Conclusion:
Excretion is a vital process for maintaining a healthy internal environment. It involves the removal of waste substances from the body, which are produced as a result of various metabolic activities. The organs involved in excretion, such as the skin, lungs, liver, and kidneys, play specific roles in eliminating waste products and maintaining homeostasis.