Ausscheidungs

Ausscheidungs, typically translated as excretion or excretory processes, refers to the biological act of eliminating metabolic wastes and toxins from an organism. Excretion is distinct from secretion, although both involve substance transfer; excretion focuses on removal from the body.

Across animals, excretory systems vary but share the function of maintaining internal homeostasis by regulating water, electrolytes, pH, and nitrogenous wastes. Vertebrates rely mainly on kidneys, which filter blood through nephrons, reabsorb essential substances, and secrete others to form urine. The liver converts toxic substances into more soluble forms for elimination, and the lungs expel carbon dioxide and water vapor. The skin can excrete salts and small amounts of waste via sweat. Invertebrates show a range of specialized structures: flatworms have protonephridia, annelids use metanephridia, and many arthropods possess Malpighian tubules.

Major waste products include ammonia (or urea or uric acid, depending on the species), creatinine, bile pigments, carbon dioxide, and excess minerals. Nitrogenous wastes are a key focus of excretion, with different strategies to balance energy costs and water use: ammonotelic species excrete ammonia, ureotelic species convert ammonia to urea, and uricotelic species excrete uric acid, which conserves water.

Regulation of excretion intersects with osmoregulation and acid-base balance. Hormonal signals, notably antidiuretic hormone in mammals, adjust kidney water reabsorption. In many animals, excretion adapts to environmental conditions, diet, and activity.

In humans, kidney disease, impaired urinary flow, or infections can disrupt excretion, leading to dehydration, electrolyte imbalances, or toxin buildup. Treatments include diuretics, dialysis, and transplant.

Historically, excretion was studied in physiology and comparative anatomy, informing our understanding of metabolism, ecology, and evolution.