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Lecture 19

Organismal Physiology_Lecture 19.docx

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Biology 2601A/B
Tamsen Taylor

Organismal Physiology Lecture No. 19: Water, Salt & Fish th Thursday November 15 , 2012 Introduction: -The fact that human beings are approximately 75% water means that it is important for us to discourage any form of water loss. Major Constituents Found In Seawater: - - 2- + -Referred to in parts per thousand, Chloride (Cl) , Sodium (Na), Sulfate (SO ), Magn4sium (Mg ), Calcium (Ca ), and Potassium (K ) are the primary constituents found in seawater and make up most of the ions found there (in that order). (Note that minor elements are not considered in this course). Various Animal Physiologies Adapted To Seawater: -Many invertebrates such as the Cnidarians and Annelids possess physiological membranes that are very close or similar to the salt content in sea water. They are said to be isosmotic to sea water salinity (their osmolarity is very similar to sea water). Sharks and cartilaginous fish are very much like the mentioned invertebrates as they are, for the most part, isosmotic with sea water. The hagfish is a primitive, jawless (no paired fins) vertebrate that is quite isosmotic to its saltwater environment. -Bony fish however, are very different from sharks, rays and invertebrates because of their being hypoosomotic to salty seawater. In essence, they are bags of very dilute water living in a saline environment. This presents a physiological problem in maintaining adequate water balance. We are strangely much like these fish in that we need to keep from dehydrating. The Issues Of Bony Fish In Saltwater: -Salts are ingested and absorbed across the gill surface, which results in a major increase in the body’s concentration of salts. The issue lies in how to get rid of the excess salt. Essentially salts must be excreted in a higher concentration than the concentration of salt that is ingested. The fish kidney cannot solve the problem since the urine of saltwater fish is limited (they pee very little) and mostly get rid of divalent ions. The fish therefore requires the assistance of another organ to solve the problem. Gills As Exchange Organs In Bony Fish: -Although gills are usually known for gas exchange (CO and O 2, they 2re also used to remove the salts that are ingested with food and water (gill surfaces are the major site for absorption in marine teleosts). Gills create the problem because they are a permeable membrane that lets in salt, however they have also evolved to be part of the solution (expelling salt concentration). Chloride Cells & The Export Of Chloride: -In the longitudinal cross-section of a lamella, a chloride cell is present at the base. Also known as mitochondrion-rich cells (MRC), these crucial cells are embedded in the epithelium
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