Excretion in Humans

Excretion is a vital biological process through which the body eliminates waste products. The key waste products that humans excrete include:

Waste Products

Urea

• Urea is formed in the liver as a result of an excess of amino acids.
• Unlike glucose and other nutrients, amino acids cannot be stored in the body; therefore, excess amino acids that cannot be converted to proteins must be removed from the body.
• The process of removing excess amino acids is known as de-amination.
• De-amination occurs in the liver and involves the removal of the nitrogen-containing section of the amino acids, leading to the formation of urea.
• Urea is then filtered from the blood by the kidneys and excreted as urine.
  

Carbon Dioxide

• Carbon dioxide is produced as a waste product during cellular respiration.
• It exits the cells and dissolves in the blood, where it is transported to the lungs for excretion.
  

Excess Water and Ions

• Excess water and ions are also filtered from the blood by the kidneys and excreted as urine.

The Kidney

The kidneys play a crucial role in filtering waste and excess substances from the blood, facilitating their excretion as urine. This filtering process is essential because a buildup of toxins could be detrimental to the organism.

The Renal System

• Blood enters the kidney through the renal artery and exits through the renal vein.
• Cortex: The outer region of the kidney that contains nephrons, which are the functional units of the kidney.
• Nephron: The nephron is responsible for filtering blood. It includes the glomerulus, where substances such as glucose, urea, water, and salts are filtered out.
• Medulla: The inner region of the kidney.
• Ureter: The tubes that transport urine from the kidneys to the bladder, where urine is stored before being excreted from the body.
  

Structure and Function of a Nephron

The kidney consists of millions of functional units, known as nephrons, which are essential for filtration.

• The cortex is the outermost area of the kidney, while the medulla represents the inner area.
• Blood is supplied to the kidneys through the renal artery.
• The glomerulus is located at the beginning of each nephron and is encapsulated by capillaries that branch from the renal artery.
• In the glomerulus, blood is filtered, and substances such as glucose, water, urea, and ions are separated from the blood.
• As the filtrate moves along the nephron, useful substances are reabsorbed back into the bloodstream; notably, all of the glucose and a portion of the ions are reabsorbed.
• The nephron tubule also reabsorbs most of the water back into the capillaries, which then returns the purified blood to the heart via the renal vein.
• The remaining fluid that persists in the nephron is primarily composed of excess water that could not be filtered or reabsorbed, along with urea and surplus ions, ultimately forming urine.
• Urea is toxic at elevated concentrations; hence, its removal through excretion is crucial for preventing it from reaching harmful levels within the body.