Ion-Osmoregulation and Excretion

The Problem

All fish have large areas of permeable epithelia in contact with the water. Water is the major solvent of cells and extracellular fluids of all organisms. It is the medium in which macromolecules must function. Macromolecular machinery of cells requires an hospitable micromolecular environment to maintain function, in which water and ion composition are major contributors.

Waste products "pollute the factory". Maintaining an hospitable micromolecular environment for macromolecules requires wastes be removed from the vicinity of the cellular machinery, and usually removed from the animal.

The energy costs of ion-osmo-regulation are high, the major cost being in ion transport.

Ion and water movement

Ions and water move down their electro-chemical gradients, from regions of higher charge and/or high concentration. Water is not charged, so it moves strictly along its concentration gradient. The major biological mechanism for moving ions is by active transport and/or locally setting up favorable electrical gradients, while the resultant ionic concentration gradients are used to move water. Water is not actively transported.

Evolutionary patterns in Ion-osmoregulation

Some agnathan groups invaded FW in the upper Silurian, about 350 million years BP. Radiation of bony fishes occurred in FW, but some actinopterygians returned to SW. Some lost glomeruli, and some of these then returned to FW. A descendant of the FW agnathans may have returned to SW giving rise to the elasmobranchs, some of which radiated into FW after adapting to SW. However, it is unclear whether cartilaginous fishes evolved in fresh or seawater.

Summary of Problems:

• ion-osmoregulation regulation with blood isosmotic relative to sea water.
• ion-osmoregulation with blood hyper-osmotic relative to the (fresh water) medium.
• ion-osmoregulation with blood hypo-osmotic relative to sea water medium (usually).

Excretion of Nitrogenous Wastes

All fish couple excretion, pH regulation and other ion exchanges. Thus fish have Na⁺/NH₄⁺/H⁺ and Cl^-/HCO₃^- exchangers. All fishes have to regulate NH₄⁺ and H⁺ exchange, whether in SW or FW. These exchangers are present in marine fish, for example, where their role in pH regulation and excretion exacerbate the NaCl influx. These exchangers become especially important in FW in taking up ions to make up for the diffusional loss.

Kidney Tubules and Extra-renal Organs

Key: glom = glomerulus, B cap = Bowman’s capsule, neck = neck, 1st prox = 1st proximal tubule, 2nd prox = 2nd proximal tubule, 3rd prox = 3 rd proximal tubule, int seg = Intermediate segment, distal = Distal tubule, coll duct = Collecting duct

Integration of Mechanisms for Ion and Water Balance

In general, the kidneys serve to excrete water (elasmobranchs and FW teleosts) and bivalent ion, and to conserve water (SW teleosts) through isosmotic water uptake from tubules. Monovalent NaCl is regulated extrarenally. Water uptake in SW teleosts is extrarenal.

Hagfish

Blood is isosmotic with SW, but ion concentrations differ. Regulate water and ions.

Lampreys

More like teleosts.

Stenohaline FW bony fish.

Gain water and lose ions. Ions are replenished from food and by extrarenal uptake. Kidneys excrete excess water, and minimize ion loss.

Stenohaline SW bony fish.

Lose water and gain ions. Bivalent ions are excreted in the kidney with a little wtaer as possible. Water replenished by drinking SW with isosmotic water uptake in the intestine. The resultant monovalent ion load and ions gained by diffusion are excreted extrarenally by the gills.

Aglomerular bony fish.

Filtrate produced via secretion of ions into the lumen.

Elasmobranchs.

Blood slightly hyperosmotic to SW due to retention of urea and TMAO. Gain water and ions, lose urea and TMAO. The kidney excretes bivalent ions and excess water, retaining urea and TMAO. Monovalent ions are excreted extrarenally at the rectal gland. TMAO and urea losses made up by synthesis and/or from food.