Lecture_15_Comparative Anatomy_The Urogenital System

Cuboidal cells are thick and squamous cells are thin

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Unformatted text preview: perm (see notes on reproduction below). • • In amniotes, the anterior end of the nephric ridge rarely produces pronephric tubules. The predominate embryonic kidney is a mesonephros, but in all amniotes, it is supplemented in later development and completely replaced by a metanephros drained by a ureter. Metanephric ducts tend to be long with well ­ differentiated proximal, intermediate, and distal regions. o In mammals, the intermediate sections are elongated to form the loop of Henle.  Positionally, the loop includes the part of the nephron that departs from the cortex and dips into the medulla (a descending limb), makes a sharp turn, and returns to the cortex (the ascending limb).  Structurally, three regions contribute: • the straight portion of the proximal tubule • the thin ­walled intermediate region • the straight portion of the distal tubule The terms thick and thin refer to the height of the epithelial cells forming the loop. • Cuboidal cells are thick and squamous cells are thin.  The length of the loops determine how capable the organism is of concentrating urine (long in desert species, short in species where water is in abundance). In general, vertebrate kidneys contribute to the maintenance of a constant, or near constant, internal environment (termed homeostasis) such that active cells are not stressed by radical departure from optimum operating conditions. This is accomplished by two fundamental physiological functions: excretion and osmoregulation.  •  Excretion: Removing the Products of Nitrogen Metabolism • Most excreted components in urine are metabolic byproducts that collect within the organism and must be voided so that they will not interfere with the organism’s physiological balance. • • ...
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