Excretory Products and their Elimination Test 13

Urea, the principal nitrogenous waste product of amphibians and mammals, is synthesized in the liver from ammonia and carbon dioxide by a sequence of reactions known as the urea cycle. Like the formation of uric acid, these reactions require specific enzymes and the input of energy by the cells. Compared to the energy cost of producing ammonia (in aquatic vertebrates), that of producing urea and uric acid (in terrestrial vertebrates) is high. Urea has the advantage of being far less toxic than ammonia and can accumulate in higher concentrations without causing tissue damage; thus, it can be excreted in more concentrated form. Because urea is highly soluble, however, it is dissolved in water, and more water is needed to excrete urea than to excrete uric acid.

Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood, which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity.

The total volume of blood passing through the kidneys is about 1200 mL per minute or about one-fourth of the entire cardiac output. As plasma passes through the  glomerulus, it loses more than 10% of its volume to the glomerular filtrate. The normal glomerular filtration rate amounts to about 180 liters (about 45 gallons) each 24 hours. This is 4.5 times the amount of fluid in the entire body. So, the correct answer is option D.

The total volume of blood passing through the kidneys is about 1200 ml per minute, or about one fourth of the entire cardiac output. As plasma passes through the glomerulus, it loses more than 10% of its volume to the glomerular filtrate. The normal glomerular filtration rate amounts to about 180 litres each 24 hours. This is 4.5 times the amount of fluid in the entire body. The renal tubules reabsorb about 99% of the filtrate into the blood, leaving only about 1.5 litres to be excreted as urine during a 24-hour period.

The urea cycle or the ornithine cycle describes the conversion reactions of ammonia into urea. In urea cycle, one amine group comes from oxidative deamination of glutamic acid while the other amine group comes from aspartic acid. Aspartic acid is regenerated from fumaric acid produced by the urea cycle. The fumaric acid first undergoes reactions through a portion of the citric acid cycle to produce oxaloacetic acid, which is then changed by transamination into aspartic acid.

Ammonotelic organisms are those, which eliminate their nitrogenous metabolic wastes mainly as ammonia. Ammonia is constantly produced in the organisms by the deamination of amino acids and it is highly toxic if retained in the system. So it must be immediately removed from the body as soon as it is formed. Elimination of ammonia requires large amounts of water. This can be done only in aquatic forms of life.  In aquatic animals (like aquatic invertebrates, body fishes and aquatic amphibians etc.,), it is quickly eliminated in the surrounding water because it is highly soluble.  

The glomeruli prevent passage of globulin from the blood to the glomerular filtrate due to their large molecular weight. So normally they are not present in the nephric filtrate. whereas urea, glucose and amino acids are present in the filtrate, of which urea is excreted in the urine, while glucose and amino acids are reabsorbed into blood capillaries by active transportation.

The basic functional unit of the kidney is the uriniferous tubule. This is a continuous tubule, which can be divided into a nephron (3 to 4 cm in length) and a collecting duct (2 cm long). 

There are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial. Animals whose internal osmotic concentration is the same as the surrounding environment are considered osmoconformers, whereas those that maintain an osmotic difference between their body fluid and the surrounding environment are osmoregulators. Among marine animals, most invertebrates are osmoconformers whereas most vertebrates are osmoregulators. There is a tendency for marine fishes to lose water to the environment through the gill epithelium. The net result of combined osmotic work of the gills and kidneys in the marine teleosts is a net retention of water. They compensate for this by the use of specialized glands for the secretion of salts in a strong hyperosmotic fluid. Marine animals with these salt glands compensate for the inability of their kidney to produce urine that is strongly hypertonic relative to body fluids. 

The Bowman's capsule has an outer parietal layer composed of simple squamous epithelium. The visceral layer, composed of modified simple squamous epithelium, is lined by podocytes.