Excretion in Human Beings: Organs and Processes

Excretion is a biological process in which an organism remove the waste material from their body. The body removes obnoxious by-products of metabolism and poisonous substances, controls the water level of the body, controls the pH of the blood, and the ionic concentration of blood fluid. Some of the waste products removed from the human body contain carbon dioxide from cellular respiration, urea from protein, ammonia, and uric acid from nucleic acid. Different entities lead to different modes of excretion.

In Human Beings, there is a particular system for excretion called the human excretion system. In the Human body, the liver, skin, Lungs, Large Intestine, and Kidneys are the excretory organs. Together combined, these organs form the human excretory system. Based on their environment and lifestyle, different organisms have different products of excretion. For example- Human Beings fabricate urea as the main excretory product while Aquatic animals excrete waste in the form of ammonia. This chapter is a part of CBSE Science syllbus, unit 2 world of living which carries a total of 50 periods and 23 marks.

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Excretory System Organs

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• Skin: The skin is usually part of the integumentary system i.e the body’s outermost layer, but it also takes part in excretion by the production of sweat through sweat glands in the dermis. Even though the main objective of sweat glands is to maintain the temperature of the human body, sweating also removes excess water and salt as well as a high amount of urea. 

• Large Intestine – The Large Intestine plays a significant role in human digestion. It is an important part of the digestive system and the last organ in the Gastrointestinal Tract. The main function of disposal of solid waste of the body remains after the digestion of food and separation of water from indigestible waste.

• Liver - The liver performs various functions involving the discharge of bile for digestion of lipids, proteins, and many other components, keeping glycogen and other substances separating the endocrine hormones. The liver performs the main function of excretion by dividing various substances in blood including toxins.

•  Lungs - Although lungs are part of the respiratory system, they also perform a significant role in the process of excretion. The lungs remove gaseous waste from the human body. The waste gas removed by the lungs is in the form of carbon dioxide from cellular respiration. Carbon dioxide is dispersed from the blood in tiny air sacs known as alveoli.

• Kidney - The two kidneys in the human body are considered the most important organ of excretion. The primary function of the kidneys is to remove or get rid of excessive water and waste from blood circulation. Kidneys remove excessive waste in the form of a liquid called urine. In the kidney, there is a tiny structure called a nephron. Nephron filters the blood in the human body, by removing the toxic material from the blood in the form of urine.

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Process

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The process of excretion in the human body is as follows- 

Urine formation- The urine is produced in the kidney with the help of nephron filters in the following steps - 

• Glomerular Filtration- It is the first step in the production of urine. In this, the excess fluid and waste material are filtered out from the blood through the kidney into the urine collection tubules and discharged out The number of filtrates generated by the body each minute is known as Glomerular Filtration Rate. 

• Tubular Reabsorption - It is the process of absorbing solutes and water from the filtrate blood to put them back in the blood. Water causes passive absorption whereas sodium and ions include active processes.

• Secretion - This process involves the transfer of drugs, hydrogen ions, and creatinine from human blood into the assembling duct and it's basically made of water. Glucose and blood are not found in urine.

Work of different tubules involved in the process are - 

• Glomerulus - Helps in the filtration of blood.
• Proximal Convoluted Tubules (PCT) - Remove unwanted material and help in maintaining the ionic balance pH of the body fluids by eliminating potassium, ammonia, and hydrogen.
• Descending loop of Henle - This portion of Henle is greatly permeable to water and less permeable to ions, so it easily absorbs all water.
• Ascending loop of Henle - These are less permeable to water and easily reabsorb solutes from the luminal fluid.
• Distal Convoluted Tubules (DCT) & Collecting Duct - Distal Convoluted Tubules and Collecting duct are two final divisions of the kidneys nephron. They absorb ions and water and can further subdivide them with their own function.

Micturition - It's also called urination. It can be understood as the act of discharging the waste from the urinary bladder. When the bladder is full of urine, the receptor triggers the wall by slowly stretching through the micturition reflex.