TeleostsExcretory and osmo-regulatory system
Renal excretory system
The renal excretory system consists of the kidney and the collecting ductsIt plays a role in osmoregulation (maintaining the stability of water and salt content in the inner medium), and excretion (compounds that the kidney withdraws from the bloodstream and evacuate as urine), hence it complements the gills regarding these two functions. Certain catabolites, toxic metabolic residues such as ammonium, urea, creatine and creatinine are eliminated in urine, although greater amounts are excreted through the gills. The kidney also has a haematopoietic function (Haemopoietic system).
Although the kidney arises embryologically as paired organs, in the adult fish there are different degrees of fusion depending on the species, and in cases it may look and is referred to as a single organ. It is located dorsally in the abdominal cavity, in a retroperitoneal position, and ventrally to the vertebral column (Fig. 1). The kidney is covered by capsule of connective tissue and has paired urinary ducts that join up outside the kidney into a common urinary duct. The urinary duct may form a thin-walled, sac-shaped bladder (Fig. 2 ) efore opening into the urinary pore or urogenital pore.
The kidney is divided functionally into an anterior and a posterior part. The richly vascularized anterior region, also called head kidney, is composed of haematopoietic tissue with endocrine elements: chromaffin cells (suprarenal) and interrenal tissue (Haemopoietic system, Endocrine system). The posterior kidney is made up of nephrons with haematopoietic tissue in between. At the middle portion are the corpuscles of Stannius (see Fig 1), which have an endocrine function (Endocrine system), this region represents the transition between haematopoietic and excretory kidney.
The functional unit for excretion is the nephron. Each nephron consists of a renal corpuscle which includes Bowman’s capsule and glomerulus and a renal tubule (Fig. 3), which has distinct portions according to function, differing mainly in diameter of the lumen and type of epithelium. Each nephron continues into a collecting duct that opens into the urinary duct.
The kidney receives blood from two sources: arterial blood through the renal artery, and venous blood through the renal portal system. The glomerulus is a cluster of vessels (afferent and efferent glomerular arterioles and intermediate capillaries) which come from the renal arteries (see Fig 3). The high blood pressure allows the glomerulus to act as an ultra-filtering device. Plasma containing the smallest molecules filters through the capillary walls, through which red blood cells and large molecules such as proteins cannot pass. Water, salts and waste products from the bloodstream enter the Bowman’s capsule and then flow through the renal tubule and the collecting duct to the urinary duct to be expelled. The fluid to be excreted undergoes changes as it passes through the length of the tubule; e.g., glucose, several minerals and in some cases, water, are reabsorbed. Filtering and reabsorption are controlled by hormonal action (Endocrine system).
The structure of nephrons are different for freshwater and marine fish. As fish live in a fluid environment, excess salt or lack of salt in the water has a direct effect by causing either dehydration or loss of salts. In freshwater teleosts, renal corpuscles are large and numerous, tubules are short and urine is abundant and contains a low electrolyte concentration. The kidney in freshwater fish is often bigger in relation to body weight than the kidney in marine fish. In contrast, marine teleosts need to retain water and the volume of urine needs to be reduced. Renal corpuscles are smaller and fewer, and tubules are longer and produce little urine with a high concentration of electrolytes and nitrogenous waste products. Some highly evolved marine teleosts have completely aglomerular kidneys; nevertheless, the kidney cannot produce urine which is more concentrated than the inner medium.
Among the most common alterations of the kidney are hypertrophy, deformation, rupture of the capsule, oedema, nodules, cysts, inflammation of urinary ducts and bladder, compression and obstruction of tubules and changes in colour due to granulomas, mineral deposits in the ducts and tubules (nephrocalcinosis).