Excretion in Humans – iGCSE Biology CAIE Revision

Excretion in Humans - Fundamentals

Excretion is the process by which toxic waste products and substances in excess are removed from your body. This is a necessary function for maintaining the body’s internal balance, or homeostasis.
Your body has several key organs involved in the excretion process, such as the skin, lungs, and kidneys.
Skin helps to excrete sweat, which contains water, salt, and urea. This not only assists in waste disposal but also helps in body temperature regulation.
Lungs excrete carbon dioxide, a by-product from the process of cellular respiration and water vapour as waste products during exhalation.
The primary organs of excretion are kidneys, which eliminate waste from the bloodstream in the form of urine.

The kidneys are bean-shaped organs found at the back of the abdominal cavity.
Their primary role is to filter the blood and remove wastes, control the body’s fluid balance and maintain the balance of electrolytes.
As blood passes through the kidneys, wastes such as urea, excess water and salts are filtered out and form urine.
Urine travels from the kidneys down tubes called ureters and is stored in a muscular sac called the bladder. From here, it is expelled from the body through a tube called the urethra.

The process of urine formation involves three main steps: filtration, reabsorption, and secretion.
Filtration: The kidney filters out small solutes from the bloodstream, including urea, via a component known as a nephron.
Reabsorption: Essential nutrients, water, and valuable salts are absorbed back into the bloodstream in the proximal and distal convoluted tubule of the nephron.
Secretion: Certain ions and urea are removed from the blood and into the tubule of the nephron. This is the body’s final chance to remove any unwanted substances from the blood.

Cleanliness: Through excretion, your body can get rid of harmful and excess substances, thereby preventing them from causing damage or reaching toxic levels.
Water balance: Excretion processes help maintain the correct water balance and concentration of salts in your body. This is key to avoiding dehydration or overhydration.
Healthy body function: By maintaining the balance of substances in your body, excretion processes ensure that your organs can function correctly and efficiently.