14.04 Excretion in Mammals

Definition of Excretion

• Excretion: The process of removing toxic or waste products of metabolism from the body.
• Importance: Excretion prevents the accumulation of harmful substances like carbon dioxide (CO₂) and nitrogenous wastes, which could disrupt cellular processes.

Major Excretory Products in Humans

1. Carbon Dioxide (CO₂)
   • Produced by cells during aerobic respiration.
   • Transported via the bloodstream to the lungs, where it diffuses into alveoli and is excreted in exhaled air.
2. Urea
   • Produced in the liver through the breakdown of excess amino acids.
   • Carried in the blood to the kidneys, where it is filtered and excreted in urine.

Deamination and Urea Formation

1. Deamination
   • Purpose: To process excess amino acids for energy rather than wasting them.
   • Process:
     • The amine group (-NH₂) is removed from the amino acid along with a hydrogen atom, forming ammonia (NH₃).
     • The remaining keto acid may enter the Krebs cycle for respiration or be converted into glucose, glycogen, or fat for storage.
2. Ammonia Conversion
   • Ammonia is highly toxic and increases pH in the cytoplasm, disrupting metabolism and cell signaling, particularly in the brain.
   • Urea Cycle: Ammonia is converted into urea in the liver through a series of reactions combining it with CO₂.
   • Urea is less toxic and less soluble than ammonia, making it safer for transport in the blood.
   • An average adult human produces around 25–30 grams of urea daily.
3. Other Nitrogenous Waste Products
   • Creatinine: Produced from creatine (an energy store in muscles), which is synthesized in the liver from certain amino acids.
   • Uric Acid: Formed from the breakdown of purines (components of nucleotides, such as adenine and guanine, found in DNA and RNA).

Kidney Function in Excretion

• Urea diffuses from liver cells into the blood plasma and is transported to the kidneys.
• The kidneys filter urea out of the blood, allowing it to be excreted in urine.
• Continuous excretion is essential to prevent dangerous urea build-up in the bloodstream.

Key Terms

Purines: Nitrogenous bases in nucleotides (e.g., adenine and guanine) that break down to form uric acid.

Urea: A nitrogenous waste product formed in the liver from the deamination of amino acids.

Deamination: The removal of the amine group from amino acids in the liver, resulting in ammonia and eventually urea.

Keto Acid: A compound that remains after deamination and may be used for respiration or storage.