recap

1096

51.1 recap

Excretory systems control water and salt balance and the excretion of nitrogenous waste products through three mechanisms: filtration of body fluids to form urine, active secretion of substances into the urine, and active reabsorption of substances from the urine.

learning outcomes

You should be able to:

  • Describe the roles of filtration, secretion, and reabsorption in the excretory systems of animals with closed circulatory systems.

  • Given data about the extracellular composition and environment of a particular species, determine its osmoregulatory characteristics.

Question 1

What is the significance of a closed circulatory system for osmoregulation in a freshwater vertebrate?

A freshwater vertebrate has to osmoregulate by conserving salts and excreting water. Having a closed circulatory system makes it possible to use pressure to filter the extracellular fluid to excrete water without the loss of salts and other solutes.

Question 2

Why is it important for an animal like Artemia that lives in the intertidal zone to be able to osmoregulate, and how does Artemia do that at the extremes of salinities in its environment?

An animal that lives in the intertidal may be isolated in tidal pools that can be highly diluted by rain or highly concentrated by evaporation. Artemia may survive by osmoconforming over a wide range of salinities, but it cannot survive at the extremes of fresh water or saturated salt solutions without being able to osmoregulate. Artemia uses active transport of ions across its gills to regulate hypertonically in very dilute environments and to regulate hypotonically in highly concentrated environments.

Question 3

How does water get into the nasal salt gland ducts of marine birds?

Bundles of secretory tubules drain into the salt gland ducts. The cells of those tubules actively transport Cl– ions into the lumen. Na+ follows, then water flushes the contents of the tubules into the duct.

In addition to maintaining salt and water balance, animals must eliminate the waste products of metabolism from their extracellular fluids. The major problem is nitrogen. When nitrogen-containing molecules are broken down by metabolism, the end product can be toxic.