OSMOREGULATORY SYSTEMS

            In first term, we examined the effects of the external environment on individual cells and saw that changes in the external environment could have devastating effects of cells.  However, most organisms can survive fluctuations in their external environment that are more extreme than even the hardiest of cells could tolerate.  Animals survive changes in their external environment by implementing physiological systems designed to maintain a relatively constant internal environment -- a concept referred to as homeostasis.  A variety of regulatory systems and feedback mechanisms operate in order to maintain homeostasis.  Osmoregulation and excretion refer to the maintenance of solute and water balance and the elimination of the nitrogen-containing waste products of metabolism respectively. 

            To maintain water balance, the amount of water entering and leaving the body must be equal.  We acquire water through the ingestion of foods and liquids, as well as water that is produced by cellular respiration.  We lose water in urine and feces, as well as the evaporative loss due to sweating and breathing.  For all animals, individual cells cannot tolerate a net gain or loss of water.  There are two basic approaches to maintaining water balance.

            1.         Osmoconformers are animals that do not adjust their internal osmolarity and are isosmotic with their environment (ex. most marine invertebrates).

            2.         Osmoregulators are animals that are not isosmotic with their environment and have developed mechanisms to regulate their internal solute and water concentrations (ex. most saltwater vertebrates, freshwater and terrestrial animals).

            In Phylum Chordata, the kidneys eliminate the nitrogenous wastes of cellular metabolism as well as a variety of other materials that may be present in the blood in excess of the body's need.  In addition, they conserve materials not in excess.  Thus, the kidneys have a vital function both in excretion and in maintaining an internal environment that is nearly constant in the water and salt content of body fluids, in pH, and in the content of sugar and other substances in the blood.

            Perch urinary system

            The urinary system consists of the kidneys, which lie dorsal to the swim bladder.  At the posterior ends of the kidneys, two small ducts unite to form one common ureter.   The ureter and the gonoduct (which passes genital products from the gonad to the exterior) empty via the common urogenital opening (Figures 7). The urinary bladder is a small sac at the most posterior, ventral end of the body cavity.  It is a separate structure in the male, but in the female, it is incorporated into the oviducts to form the urogenital sinus.

            In freshwater fish, such as the perch, the elimination of nitrogenous waste occurs primarily across the gill epithelium.  In these animals, the kidney functions are osmoregulatory.  The body fluids of freshwater fish are hyperosmotic (hypertonic) to their environment.  Although the skin of the perch (with its mucous coating) is virtually impermeable, the lining of the oropharyngeal cavity and the gill epithelium are permeable to water and salts.  Thus, there is a constant osmotic entry of water into the body fluids and diffusional loss of salts (e.g. NaC1) from body fluids, across these surfaces.

            The kidneys of freshwater fish continually produce large volumes of dilute urine.  In these animals, the kidneys are quite efficient in reabsorbing ions (salts) from the urine being produced.  Freshwater fish also actively take up ions at the gills to replace those lost in urine and feces.  The urinary bladder in the perch is small as urine is voided almost as fast as it is produced.

Figure 7: Diagram of the internal anatomy of the perch (female).

   

     Figure 8:  Pictures of the internal anatomy of the perch.