Excretion and Osmoregulation: Maintaining Homeostasis

Excretion is the elimination of waste products (such as carbon dioxide and nitrogenous wastes) and is a crucial part of the broader processes aimed at maintaining homeostasis, which is essential for an organism's survival. Homeostasis is the process of maintaining constant internal conditions. The two pillars that maintain homeostasis in animals are excretion and osmoregulation.

Excretory Structures

Excretory systems use tubes to collect fluid from the blood, hemolymph, or extracellular fluid to adjust its composition through the reabsorption of useful substances and the active secretion of waste products.

• Sponges, Cnidarians, Nematodes, and Echinoderms: These organisms lack specialized excretory structures and eliminate waste through diffusion across their body surface, without significant energy expenditure.
• Platyhelminthes (Flatworms): These are among the first animals to possess a specialized excretory system: protonephridia. These consist of a network of blind tubules, each ending in flame cells.
• Annelids and Mollusks: These organisms have metanephridia, a more effective system than protonephridia. The tubules are open at both ends, facilitating fluid entry through nephrostomes, which are ciliated funnel-shaped openings.
• Insects and Spiders: These possess a unique excretory system consisting of Malpighian tubules, which are extensions of the intestinal wall.
• Crustaceans: These have an excretory system consisting of a pair of antennal glands, tubular structures located in the ventral region of the head.

The Vertebrate Kidney

The vertebrate kidney is composed of thousands of nephrons, which are tiny filters and the functional units of the excretory system. Each nephron consists of three parts:

• The Glomerulus: A network of blood capillaries where blood is filtered at high pressure.
• Bowman's Capsule: A cup-shaped sac that completely surrounds the glomerulus and collects the ultrafiltrate (a mixture of wastes and nutrients), transferring it to the next region.
• The Renal Tubule: A long tube that modifies the composition of the ultrafiltrate through tubular reabsorption and secretion.

Osmoregulation in Vertebrates

Vertebrates are characterized by their ability to colonize diverse ecosystems. However, each habitat presents a challenge to osmoregulation, which these animals have solved using different strategies:

• Freshwater fish have blood that is hypertonic compared to their surrounding environment.
• Marine teleosts face the opposite situation: their internal environment is hypotonic to the sea, causing them to lose water and gain salt.
• Chondrichthyans (cartilaginous fish) achieve an internal environment that is hypertonic to seawater because their kidneys retain urea, allowing it to accumulate in the blood.
• Terrestrial vertebrates have a higher concentration of water in their bodies than the surrounding air. Water is lost through evaporation from the skin and lungs, and through the elimination of feces and urine. To prevent dehydration, reptiles and birds produce semisolid excreta. Mammals excrete urine with increased urea concentration, which implies some water loss through the urine.