Freshwater fish vs seawater fish ion regulation-Animal Physiology

Marine water fish blood has a higher water concentration than their surrounding sea water. causing water molecules to diffuse out of the blood into their surrounding water by osmosis. These fish must replace the lost water, by the drinking their surrounding concentrated water, intaking salts which is then removed by active transport through their gills Na+-Cl- ATPase molecules, back to the seawater.

Ion exchange in saltwater fish:

The Na+ K+/2Cl- (NKCC) co-transporter brings in potassium ion (K+) and chloride ion (Cl-) from the extracellular fluid in the fish gill epithelial cells using the Na+ electrochemical gradient. The K+ and the Na+ separate from the 2Cl- in the the cell. The 2Cl- separate out and exit the cell through the apical surface via the Chloride channel down their concentration gradient. the K+ goes back to the extracellular fluid via the K+ channel from the cell. Na+ exit the cell that had entered through the NKCC via the Na+/K+ ATPase channel in exchange for K+ causing small potential difference (outside more negative due to the Cl- ions exiting than compared to the extracellular fluid) it drives the outward movement Na+ outside the cell. In freshwater fish water is absorbed through gills and skin by diffusion into the blood. While water moves in towards the higher osmotic pressure of the blood, Na+ and Cl- ions diffuse out of the fish, moving down their concentration gradients to the external environment. The fish then can actively take up the ions across their gills epithelial membranes from the surrounding environment to replace the constant loss of ions. Food is another way by which to replace lost Na+ and Cl-.

Ion regulation is freshwater fish:

The mechanism for Na+ uptake and Cl- are independent. From the dilute freshwater to the concentrated blood Na+ is actively transported against the concentration gradient. H+ is excreted out in exchange for Na+. The Cl-/HCO3 – exchange secretes the bicarbonate ion (HCO3-) in exchange for Cl- ions. HCO3- and H+ are metabolic wastes that are derived from the conversion carbon-dioxide(CO2) using carbonate anhydrase (CA). These metabolic wastes (H+ and HCO3) keeps it from generating large potential differences across the gill epithelium in freshwater fish. A protein that is responsible for sodium uptake uses ATP to exchange Na+ for K+.

Text citation:

– Animation: (2:00-250),(3:28-4:13): Dr.Gregg Dearbom,Greg Goss.(2002).Ion and Acid-Base Regulation in Fish Gill. Retrieved from

– (1:03-1:19) Am J Physiol.(1980).Morphology of gill epithelia in fish. Retrieved from

– (2:00-250),(3:28-4:13) Gene Helfman, Bruce B. Collette, Douglas E. Facey, Brian W. Bowen.(2009). The Diversity of Fishes: Biology, Evolution, and Ecology (2nd ed.). Wiley-Blackwell

– (2:00-250),(3:28-4:13) Richard Hill, Gordon Wyse, and Margaret Anderson. ().Animal Physiology (3rd Ed.).Sinauer Associates, Inc. Box Extension 5.2

Image citation:

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