BIOL 1911 Chapter Notes - Chapter 44.3: Osmoregulation, Renal Pelvis, Nephron
29 Apr 2018
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44.3 – Diverse excretory systems are variations on a
tubular theme
• Excretory system
o regulate solute movement between internal fluids and external environment
o dispose of metabolic wastes
o control body fluid composition
o therefore, central to homeostasis
Excretory Processes
• Urine
o Fluid waste
o Produced by wide range of species
o Steps
▪ Body fluid brought in contact with membrane of transport epithelium
▪ Hydrostatic (blood) pressure drives filtration
▪ Water and small solutes cross membrane and form filtrate
▪ Filtrate converted to waste fluid via specific transport of materials into or out of filtrate
▪ Selective reabsorption recovers useful molecules and water into body fluid via active transport
▪ Selective secretion – via active transport – leave nonessential solute and wastes in filtrate
▪ Water moves by osmosis into or out of filtrate
▪ Processed filtrate of nitrogenous waste released as urine
Survey of Excretory Systems
• Vary among animal groups
• Complex network of tubules that provide large surface area for exchange of water and solutes (ex: nitrogenous wastes)
• Basic key features
o Membrane acting as a filter
o Tubule to collect filtrate
o Tubule continues to accept other wastes and allow for of reabsorption of key nutrients
o Transport epithelium facilitates transfer
o Filtrate leaves the body through specialized pores
Protonephridia
• Excretory systems in flatworms, which lack a coelom
• Also in rotifers, some annelids, mollusc larvae and lancelets
• Serve chiefly in osmoregulation in freshwater flatworms
• Function in disposing nitrogenous wastes in parasitic flatworms
• Network of dead-end tubules that branch throughout the body
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