Animal Waste Elimination: Nitrogenous and Non-Nitrogenous Products

Waste Products

Non-Nitrogenous Products

These include CO2, H2O, mineral salts, and bile pigments. CO2 is removed by respiration. H2O is eliminated through the excretory apparatus and perspiration. Minerals are excreted in urine and sweat, and bile pigments pass out through the digestive system in the feces.

Nitrogenous Products

These are highly toxic compounds eliminated in various forms, such as ammonia, uric acid, and urea. They are categorized into three groups based on the primary excretion product:

• Ammoniotelics: The excretion product is ammonia. Due to its high toxicity, only organisms with abundant water supply can excrete nitrogen this way.
• Uricotelics: The excretion product is uric acid, which is ejected in crystal form. Its toxicity is low, allowing it to accumulate temporarily in the body. Examples include birds and reptiles.
• Ureotelics: The excretion product is urea. Urea is less toxic than ammonia but still requires dilution for excretion, though it demands less water than ammonia excretion. Examples include amphibians, reptiles, and mammals.

Process of Excretion / Urine Formation

Urine formation involves three main processes:

1. Filtration: This is the passage of substances from the internal environment into the excretory tubules via diffusion.
2. Reabsorption: Essential substances for the organism return to the internal environment. This occurs in the tubules that form the nephron.
3. Secretion: This is the passage of substances, such as potassium and hydrogen ions, from the blood into the filtrate within the nephron. This secretion primarily occurs in the distal tubule.

Vertebrate Excretory Organs

Kidneys

The structure of the kidney includes the renal capsule, cortex, medulla, and renal pelvis.

Nephron

The nephron consists of a renal corpuscle, where filtration of blood plasma fluid occurs, and a convoluted tubule of variable length. This tubule has different morphological and functional specializations where reabsorption and active elimination of solutes take place. The tubules of several nephrons meet in collecting ducts that empty into the ureter. In birds, the ducts empty through the cloaca. In reptiles and mammals, they empty into the urinary bladder.

The mammalian nephron is formed by the Malpighian corpuscle, which comprises the glomerulus and Bowman's capsule, and a tubule.