Renal System

The kidneys are important in filtering undesirable materials out of the blood as well as serving a role in water and ion balance. Active transport, together with epithelial permeability are adjusted in order that ions and other materials vital to the body can be conserved and control over body water content can be exercized.

Basic Anatomy and Physiology

The kidney in salmonidae is made up of two parts running along the anterior-posterior axis between the body cavity and vertebral column. Anterior and posterior portions, also known as head and trunk, respectively, have slightly different functions but there is no visual distinction between them. The anterior kidney is associated with inter-renal tissue and chromaffin cells, which are involved in blood cell and hormone production; the posterior kidney is associated with filtration and urine production.

Kidney tissue is made up of individual units called nephrons (Fig. 3). Each nephron is made up of a renal corpuscle, consisting of the glomerulus and Bowman's capsule and a kidney tubule. The glomerulus is a network of afferent and efferent arterioles whose blood supply comes from

Glomerulus

Renal artery

Bowman's capsule (cross section)

Proximal tubule Renal portal vein

To posterior vena cava

To bladder

Glomerulus

Distal tubule

Renal artery

Bowman's capsule (cross section)

To posterior vena cava

To bladder

Renal vein

Proximal tubule Renal portal vein

Collecting duct

Figure 3. Kidney nephron.

Distal tubule

Renal vein

Collecting duct

Figure 3. Kidney nephron.

the dorsal aorta or a venous supply called the renal portal system. The Bowman's capsule encapsulates the glomerulus and nonselectively filters solutes and molecules less than 70,000 mol. wt. from the blood. The filtrate is carried through the renal tubule, which is made up of the proximal, intermediate, and distal segments. It is in the tubule that reabsorption of selected electrolytes, minerals, amino acids, glucose, and other plasma organics takes place via a capillary network back into the blood. The remaining fluid containing unreabsorbed constituents flows to the collecting tubule and gathers in the urinary bladder, if present, where reabsorption of more ions may take place before being excreted out the urinary duct.

The process of nonselective filtration and subsequent selective reabsorption is an efficient process, because the body reabsorbs only what it needs. It is an adaptable system, because needs may change over time and through changing circumstances. Although efficient, this system requires energy as many ions are reabsorbed by active transport. Control of kidney function is accomplished by a variety of hormonal mechanisms and includes tissues of the thyroid, kidney, gonads, hypothalamus, and possibly the pituitary. In mammals, it is well established that a renin-angiotensin system and antidiuretic hormone (ADH) regulate renal function for ionic and water regulation. While the exact role of such a system in fishes is not well established, there is evidence that similar mechanisms are found in fishes. In this model, renin is secreted under nervous and hormonal control from a juxtaglomelular apparatus, located near the glomerulus, in response to decreased Na+ concentration or blood pressure as well as under stressful conditions. Renin is then converted to angiotensin, which increases blood pressure through the constriction of blood vessels and causes aldosterone to be release from the adrenal cortex (of mammals). Aldosterone stimulates ionic uptake from the distal tubule of the nephron. ADH, from the pituitary, controls the amount of wa ter leaving the nephron by controlling the permeability of the epithelium in the collecting duct. An increase in blood osmolarity causes an increase in ADH, which increases the permeability of the epithelium, resulting in the increased absorption of water out of the filtrate, back into the blood.

Freshwater fishes are hyperosmotic regulators. That means that the concentration of ions and other solutes is greater in the blood than in their surrounding water, which in most cases is very dilute. The fish will thus absorb water osmotically from their environment through all permeable epithelia such as the gills, skin, and gut. The regulatory problem is one of getting rid of the excess water and the kidney plays that important role. A large amount of urine is produced, which is dilute and contains creatine, uric acid, and some ions. The volume of urine produced must balance the quantity of water entering the body. Sodium (Na+) and chloride (Cl~) ions passively diffuse out of the body across permeable epithelia and are actively taken up, to a large extent, across the gill epithelium.

Electrolyte reabsorption out of the urine takes place across the renal tubule. Na+ is actively extracted and it appears that Cl~ passively follows. Calcium (Ca2+), magnesium (Mg2 +), and other divalent ions must also be reabsorbed, because they are normally absent in the urine of freshwater fish. The reabsorption of these ions is usually accomplished without the osmotic absorption of water. The distal segment, collecting duct and urinary bladder appears to be relatively impermeable to water. Macromole-cule reabsorption, including glucose, amino acids, and other plasma organic constituents takes place in the first segment of the proximal tubule. Salts are reabsorbed in the distal segment, and any remaining in the filtrate may be reabsorbed from the urinary bladder. Only a small proportion of total organic nitrogen is excreted via the kidneys, although this appears to be an important excretory pathway for minor nitrogenous products, such as creatine and uric acid. Major nitrogenous products such as ammonia and urea are excreted via the gills.

The kidney in marine fishes plays a crucial role in hy-poosmotic osmoregulation. Converse to the freshwater fish, the blood concentrations of ions and solutes in fish blood relative to the concentrated ionic environment of the marine environment is rather dilute. The osmotic problem created by such a gradient is one of dessication, which the fish counteracts by actively taking in water from the environment by drinking and reducing water loss at the kidneys. Between 60 and 80% of ingested water is absorbed through the gut, along with the monovalent ions Na +, CI ~, and potassium (K+). Those excess salts are actively excreted. The gills play a major role in that function, although the kidney excretes Mg2+, sulfate, and other divalent ions. Although less than 20% of the ingested divalent ions are actually absorbed most are passively eliminated via the intestines, that which is absorbed is handled by the kidney tubules.

The kidney nephron of marine fishes often lack the distal segment of the tubule and has less glomeruli. In some cases, such as the goosefish, glomeruli are absent. Because glomeruli are suited to the excretion of large volumes of water the importance of the glomerulus in marine teleosts is greatly reduced. Urine from a marine kidney thus has high osmolality and is low in volume. Much of the water entering the glomerulus reabsorbed to help prevent dehydration, a case opposite to the freshwater kidney. Nitrogenous excretion is similar to the pathway in freshwater fish, with ammonia and urea excreted through the gills and minor nitrogenous products such as creatine and uric acid excreted by the kidney.

Anadromous and catadromous fish must be capable of adjusting their osmotic balance to survive the changes in salinities that they experience in their life cycle. They have glomerular kidneys that can adjust to changing urine volumes, and possess gills and oral membranes capable of both uptake and secretion of certain ions against the prevailing diffusion gradients. The kidneys in those animals are capable of adjusting urine volume and composition on demand. When those fish, for example, are transferred or voluntarily move from fresh water to salt water, the urine composition gradually changes after a few days. Urine flow decreases and osmolality increases as divalent ions are excreted. Sodium and chloride content in the urine decreases as the chloride cells (specialized salt excreting cells on the gills) take over this function. Glomelular filtration rate may temporarily slow down and tubular reabsorption increases.

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