Excretion vs egestion; waste products

Excretion is the removal of metabolic waste products from the body. These waste products are substances produced by the body's cells during various metabolic processes (e.g., respiration, protein metabolism).

Key characteristics of excreted substances:

• They are toxic or harmful if allowed to accumulate.
• They are by-products of cellular reactions.
• They are removed by specific excretory organs.

Examples of Excreted Waste Products and their Origins:

• Carbon dioxide (CO2): Produced during cellular respiration. Removed primarily by the lungs.
• Urea: Produced in the liver from the breakdown of excess amino acids (deamination). Removed by the kidneys in urine.
• Excess water: A by-product of many metabolic reactions and consumed in excess. Removed by the kidneys (urine) and skin (sweat).
• Excess mineral salts: Consumed in excess. Removed by the kidneys (urine) and skin (sweat).
• Bile pigments (e.g., bilirubin): Formed from the breakdown of haemoglobin in old red blood cells, primarily in the liver. Excreted in faeces (via bile) and to a small extent in urine.

Egestion, also known as defecation, is the removal of undigested food material from the body. This material has passed through the digestive tract but has not been absorbed into the bloodstream.

Key characteristics of egested substances:

• They are not metabolic waste products; they were never part of the body's internal metabolism.
• They are primarily dietary fibre, dead cells from the digestive tract lining, and bacteria.
• They are removed via the anus as faeces.

Examples of Egested Material:

• Dietary fibre (cellulose): Indigestible plant material.
• Undigested food particles: Any food that was not fully broken down by digestive enzymes.
• Dead cells: Cells shed from the lining of the digestive tract.
• Bacteria: A significant portion of faeces consists of bacteria from the gut microbiome.
• Water: A substantial component of faeces, though much is reabsorbed in the large intestine.

Understanding the distinction between excretion and egestion is fundamental. Here's a table summarising the key differences:

Different metabolic pathways generate specific waste products that need to be efficiently removed to maintain homeostasis.

• Carbon Dioxide (CO2):

  • Formation: Produced during aerobic respiration when glucose and other organic molecules are broken down to release energy.
  • Equation (simplified): Glucose + Oxygen $\rightarrow$ Carbon Dioxide + Water + Energy
  • Removal: Transported in the blood (mostly as hydrogen carbonate ions) to the lungs, where it diffuses into the alveoli and is exhaled.

• Urea:

  • Formation: Proteins are broken down into amino acids. Excess amino acids cannot be stored and are deaminated in the liver. Deamination involves removing the amino group (-NH2), which forms ammonia (NH3). Ammonia is highly toxic and is immediately converted into less toxic urea in the liver (ornithine cycle).
  • Removal: Transported in the blood to the kidneys, filtered out, and excreted in urine.

• Excess Water and Salts:

  • Formation: Water is a by-product of respiration and is also ingested. Salts are ingested. The body maintains a precise water and salt balance (osmoregulation).
  • Removal: Excess water and salts are removed by the kidneys in urine. Some water and salts are also lost through the skin as sweat, which also plays a role in thermoregulation.

• Bile Pigments (e.g., Bilirubin):

  • Formation: When old red blood cells are broken down, primarily in the liver and spleen, haemoglobin is released. The haem group is converted into bile pigments like bilirubin.
  • Removal: These pigments are secreted by the liver into the bile, which then enters the small intestine. They are eventually eliminated from the body in faeces, giving faeces its characteristic brown colour.