The Urinary System 189 



cells forming the kidney tubule, other than those of the Bowman's cap- 

 sule are thick and cuboidal in shape. 



The renal artery which enters the kidney at the hilus breaks into 

 smaller vessels, finally becoming small afferent blood vessels that enter 

 the space surrounded by each Bowman's capsule. Here the afiferent 

 vessel breaks into the capillaries of the glomerulus, and then leaves the 

 glomerulus as the efferent vessel. Each efferent vessel passes to the 

 remaining portion of the tubule where it breaks up into smaller vessels 

 closely surrounding the tulnile. Gradually these smaller vessels coalesce 

 to form the renal vein. 



Accessory Urinary Structures. — From each kidney, a slender 

 tube, the ureter, leads to the base of the bladder. Each ureter begins as 

 the expanded pelvis which partially surrounds the sinus of the kidney. 

 Urine is conducted through the ureter by means of peristaltic waves 

 which begin at the pelvis. 



Just ventral to the rectum is the pear-shaped urinary bladder. It 

 is a highly extensible organ due to its layers of smooth muscle. In the 

 adult human being it has a capacity of about one pint. The passage of 

 the urine from the bladder to the urethra is controlled by a sphincter 

 muscle. 



The length of the uretha is somewhat greater in the male than in 

 the female. Essentially its walls are continuous with those of the 

 bladder. In the male, the distal portion of the urethra serves as a 

 passageway for both urine and the sperm from the testes. 



THE PHYSIOLOGY OF THE URINARY SYSTEM 



Since much of the formation of urine is a physical process, the best 

 clues as to the method by which this occurs may be found in the struc- 

 ture of the nephron. 



The Formation of Urine. — Close examination of an individual 

 Bowman's capsule shows that the capillaries forming the glomerulus are 

 in very intimate contact with the thin epithelial cells of the capsule. 

 Further study will reveal that the efferent blood vessel is of slightly 

 smaller diameter than is the afferent; thus the blood pressure within 

 the glomerulus is slightly higher than that of the afferent vessel. This 

 arrangement facilitates the movement of much of the liquid portion 

 of the blood into the Bowman's capsule. The resulting filtrate then 

 moves through the capsule into the urinary tubule. 



