



THE EXCRETORY SYSTEM 223 



the pelvis. This latter medullary region is made up for the 

 most part of larger blood vessels, connective tissue and col- 

 lecting tubules. 



The blood that flows out of the kidney by the renal vein is 

 the purest in the body. It is arterial blood containing con- 

 siderable urea when it enters, but in flowing through the 

 kidneys the urea and some other wastes are removed, so that 

 it flows out actually purer than when it entered. 



Urine Excretion. The rate at which the kidneys excrete 

 urine is very variable, depending largely upon conditions of 

 the air one breathes. More is excreted in cold weather than 

 in warm, and more in wet weather than in dry. In hot, dry 

 weather so much of the water waste of the body leaves it in 

 the form of perspiration that less is passed off through the 

 kidneys. Certain foods and drugs, the drinking of a great 

 quantity of water or other liquids, and nervousness are all 

 agents which may alter the rate of secretion. The rate at 

 which the urea is secreted is not, however, dependent upon 

 these factors. It is dependent simply upon the rate of its 

 formation in the body. If formed rapidly, it is excreted rapid- 

 ly, even though the total amount of urine is small/ The rate 

 of urea formation is dependent upon the rate at which pro- 

 teid material is broken down in the body, and the kidneys 

 will eliminate this urea as fast as it is brought to them by the 

 blood. An average daily amount of kidney secretion is three 

 pints. 



The Ureters. The ureters, which receive the urine from 

 the kidneys, are small tubes of about the diameter of a good 

 sized quill. Internally, they are lined with a mucous mem- 

 brane, outside of which muscles are arranged, essentially 

 as in the intestinal walls. By their peristaltic contraction 

 these muscles force the urine downward into the bladder. 



The Bladder. The bladder, Fig. 108, is the reservoir for 

 the temporary storage of the urine, and is located in the 

 middle of the abdominal cavitv, in front of the rectum. It 



