Chap. 14 THE BY-PRODUCTS OF METABOLISM EXCRETION 247 



are filtered from the blood into the renal capsule and a dilute urine is formed. 

 In primitive animals, a funnel of the tubule also opens into the coelomic fluid. 

 Altogether each unit has a two-way access to the vital fluids and water content 

 of the body and an equipment for the selective filtering of metabolic products 

 and water. 



Adult reptiles, birds, and mammals all have the metanephric type of kidney 

 whose units are associated solely with the blood. These kidneys are provided 

 with large supplies of blood and consist of large numbers of kidney units held 

 together by connective tissue and the blood vessels. Externally they have no 

 resemblance to tubes, actually each kidney contains, in different species, from 

 a few dozen to about a million microscopic tubular units. 



Human Urinary System 



The human urinary system includes two kidneys, two long tubes, the ureters 

 which carry urine from each kidney to the bladder, a reservoir for urine, and 

 the urethra, a tube leading to the external opening (Fig. 14.4). 



Kidneys 



General Structure. The kidneys lie against the dorsal body wall beneath the 

 peritoneal lining. Although they appear to be in the coelom or body cavity 

 they are separated from it by the transparent layer of tissue which covers all 

 the other organs. All mammalian kidneys are bean-shaped and very similar in 

 structure. In the kidneys of rodents and carnivores the tubules all run toward 

 one point making a single pyramid. In the human and other mammalian kid- 

 neys the tubules come to a focus in several pyramids (Figs. 14.4, 14.5). When 

 split in half longitudinally the cut surface shows two parts: an outer finely 

 rayed band, the cortex, and a central part or medulla. The cortex contains 

 the renal bodies and the coiled parts of the tubule. The medulla contains 

 the U-shaped part of it and the collecting ducts which deliver urine through 

 pores in the tip of each pyramid. The urine flows through the minute open- 

 ings of the collecting tubules into the pelvis and drains from the pelvis into 

 the ureter. The ureter delivers it to the urinary bladder from whence it is 

 discharged through the urethra. 



Circulation of Blood. The kidneys are located on the high road of circulat- 

 ing blood. The renal arteries bring blood directly from the heart under high 

 pressure, and the renal veins turn the great part of it into an easy road back to 

 the heart. There are no renal portal veins such as those in the frog that bring 

 blood from the hind legs to the kidneys (Fig. 34.18). In frogs, these help to 

 combat the income of water through the skin by providing the blood with 

 extra access to the kidneys where more water is filtered out, a process that 

 helps to prevent drowning from inside. Mammals and other land vertebrates 

 have waterproof skins and their kidneys are less important as water pumps. 



