Biology Chapter 44 II

Biology Chapter 44 II - Concept 44.3 Diverse excretory...

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Concept 44.3 Diverse excretory systems are variations on a tubular theme Although the problems of water balance on land or in salt water or fresh water are very different, the solutions all depend on the regulation of solute movements between internal fluids and the external environment. ° Much of this is handled by excretory systems, which are central to homeostasis because they dispose of metabolic wastes and control body fluid composition by adjusting the rates of loss of particular solutes. Most excretory systems produce urine by refining a filtrate derived from body fluids. While excretory systems are diverse, nearly all produce urine in a process that involves several steps. ° First, body fluid (blood, coelomic fluid, or hemolymph) is collected. The initial fluid collection usually involves filtration through selectively permeable membranes consisting of a single layer of transport epithelium. Hydrostatic pressure forces water and small solutes into the excretory system. l This fluid is called the filtrate. ° Filtration is largely nonselective. It is important to recover small molecules from the filtrate and return them to the body fluids. Excretory systems use active transport to reabsorb valuable solutes in a process of selective reabsorption. Nonessential solutes and wastes are left in the filtrate or added to it by selective secretion, which also uses active transport. ° The pumping of various solutes also adjusts the osmotic movement of water into or out of the filtrate. The processed filtrate is excreted as urine. The urine in the tubules exits through openings called nephridiopores. Metanephridia, another tubular excretory system, consist of internal openings that collect body fluids from the coelom through a ciliated funnel, the nephrostome, and release the fluid to the outside through the nephridiopore. ° As urine moves along the tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns them to the blood in the capillaries. ° Nitrogenous wastes remain in the tubule and are dumped outside. ° Because earthworms experience a net uptake of water from damp soil, their metanephridia balance water influx by producing dilute urine. Insects and other terrestrial arthropods have organs called Malpighian tubules that remove nitrogenous wastes and also function in osmoregulation. ° These open into the digestive system and dead-end at tips that are immersed in the hemolymph. The transport epithelium lining the tubules secretes certain solutes, including nitrogenous wastes, from the hemolymph into the lumen of the tubule. ° Water follows the solutes into the tubule by osmosis, and the fluid then passes back to the rectum, where most of the solutes are pumped back into the hemolymph. ° Water again follows the solutes, and the nitrogenous wastes, primarily insoluble uric acid, are
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Biology Chapter 44 II - Concept 44.3 Diverse excretory...

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