Introduction
Learning Objectives
• Importance of excretory system and physiological functions of the kidney
• General and functional ultrastructural morphology of the renal system
• The process of urine formation includes glomerular filtration, tubular reabsorption and secretion
• Urine concentration mechanisms following countercurrent multiplier and countercurrent exchange mechanisms
• Renal function tests
Maintaining an internal aqueous environment with consistent water and solute composition is vital for homeostasis.
Loss of water and electrolytes during digestion, metabolism, thermoregulation and elimination of waste products always creates imbalances in fluid and electrolyte composition in the internal environment. In animals, the urinary or renal excretory system counteracts such imbalances by effectively eliminating metabolic wastes and selective retention of water and solutes. Thus, the renal system is considered a major excretory system in the body. The other systems involved in excretion are therespiratory system, digestive system and integumentary system and have a minor role in animals.
Protein and nucleic acid metabolism always produces nitrogenous waste products in the body. Ammonia (NH3), a potentially toxic base that can bind with protons to become an ammonium ion (NH4+), constitutes the major end product of protein metabolism. If present in excess, these ions can also interfere with Na+/K+ ATPase substituting for K+, causing morphological changes in neurons, interrupted ion conduction and disrupted neurotransmitter metabolism. Thus, NH3 and NH4+ must either be highly diluted and rapidly excreted or be converted into a less toxic form. The most common of these forms are urea and uric acid.
Most aquatic animals are ammonotelic, and they rely on ammonia excretion through gills in the most diluted form as water is freely available for them.
Most terrestrial mammals used metabolic energy to convert ammonia to urea using ATP in the liver and excreted through kidneys, known as ureotelic. Urea is 10-100 times less toxic than ammonia. Thus, it can be accumulated in too much higher concentrations and has the benefit of removing two nitrogens per molecule. It takes about ten times less water to excrete a given amount of nitrogen as urea than ammonia. In reptiles, birds and insects, ammonia is converted to uric acid (uricotelic) before excretion. It requires more ATP for the production of uric acid but is less toxic as it is highly insoluble and has the added benefit of removing four nitrogens per molecule. It is converted as urates into the hindgut and excreted in a semisolid form. It takes about 50-100 times less water to excrete a given amount of nitrogen as uric acid than ammonia. Compared to urea, which is essentially infinitely soluble in solution, uric acid is highly insoluble (precipitates at concentrations greater than about 0.4 mM). Urea and uric acid are excreted through renal system, which is constituted by kidneys and other organs with functional and structural modifications in mammals and birds.9.2