Physiology of Finfish and Shellfish (2+1)

2.3.6. Mechanisms of excretion/ function

The primary function of the kidney tubule is to remove excess of water, salts, waste material and foreign substances from the blood. For this blood, is filtered under high pressure through the glomerulus and the Bowman’s capsule which work as ultra filters. Glomerulus is composed of a leaky capillary endothelium, a fine porous basement membrane and the epithelium of the Bowman’s capsule. These 3 layers form a fine sieve like filter. During ultrafilteration under arterial blood pressure, inorganic ions, organic molecules, glucose, urea, amino acids are filtered into the tubule. The filtrate flows through the tubule by the filtration pressure and the movements of the cilia of neck segment and of other segments. During this, glucose and salts (Na⁺, cl^-) are reabsorbed from the filtrate. Water is also absorbed so that the urine becomes reduced in volume.

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but that of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Kidney in freshwater fishes also serves as an excretory organ for nitrogenous compounds such as creatine and uric acid. For this the kidney has a large number of well developed glomeruli and removes large quantity of water from the body. Thus copius urine, 5-12% of body weight/day is produced in freshwater fishes. The urine is dilute, 20 milli osmoles/ltr and a urinary bladder is often present for its storage.

In marine teleosts, the kidney performs the function of removing magnesium and sulphate ions along with Na, Cl, K and Ca. Trimethyl amine oxide (TMAO) is also excreted but only traces of ammonia and urea. Very little amount of urine (3 ml/kg/day) is produced, as water has to be conserved to avoid dehydration.

The gills perform the function of main organs of excretion removing ammonia and urea along with carbon dioxide. Marine fishes excrete common marine salts via the salt excreting glands located on the lamellae of the gills while in the freshwater fishes it is through kidney.

(Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable (like those of the kidney) of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water in their food.)