Excretion and Osmoregulation

• Metabolic activity results in the production of waste byproducts which draw water from their environment through osmosis
• Osmosis is the spontaneous diffusion of water through a partially-permeable membrane in the direction that tends to equalize the solute concentrations on the two sides
• When a cell is in a hypotonic solution, it draws in water
• When a cell is in a hypertonic solution, water is drawn out of the cell
• Isotonic solutions have an equal concentration of solutes as the cell's cytoplasm
• Desiccation is extreme dehydration and is usually fatal to living organisms
• The aim of excretion is to remove metabolic waste, specifically nitrogenous wastes
• Ammonia is extremely toxic, urea is less toxic, and uric acid is non-toxic
• Simple animals in aquatic environments can excrete ammonia directly from their cells to surrounding water
• An osmoconformer does not regulate its internal osmolarity but takes on the same internal osmolarity as its surroundings
• Osmoregulators tightly regulate their body osmolarity, maintaining constant internal conditions; homeostasis
• Osmoregulation is where organisms regulate solute concentrations and balance water loss or gain
• A selectively permeable membrane allows free movement of water but not solute molecules or ions
• Organisms regulate solute concentration between extracellular and intracellular spaces with pumps using active transport
• In fish, ammonia in the form of ammonium ions is excreted via the gills
• Marine fish have internal osmotic concentrations lower than that of the surrounding seawater so it tends to lose water and gain salt which it actively excretes from the gills, producing little liquid urine
• Freshwater fish excrete large amounts of hypotonic urine to expel all the excess water. Salts are regained via food intake and active uptake of salt from the environment by the use of mitochondria rich cells
• Excretory systems involve filtration through a series of selectively permeable membranes followed by selective reabsorption. They include protonephridia, metanephridia, Malpighian tubules, and kidneys
• Protonephridia are found in platyhelminthes, rotifera, and nemertea. They are networks of dead end tubules lacking internal openings. The tubules branch throughout the body and the smallest branches are capped by a cellular unit called a flame bulb. The tubules excrete dilute urine and function in osmoregulation
• Metanephridia are found in annelida, mollusca, and some arthropoda. The internal opening is surrounded by a ciliated funnel called the nephrostome. As fluid passes along the tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns them to the blood. Metanephridia produce dilute urine, balancing the water influx.
• Malphigian tubules are mainly found in terrestrial arthropods and tardigrades. The transport epithelium secretes solutes, including nitrogenous wastes, from the haemolymph into the lumen of the tubule. Water follows the solutes by osmosis and the fluid then passes into the rectum, where most solutes are pumped back into the haemolymph. Water again follows, and the nitrogenous wastes are eliminated as nearly dry matter along with the faeces.
• The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus