78 



THE INDIVIDUAL ORGANISM 



The Excretion of Nitrogenous Wastes 



Nitrogenous wastes are formed by the continual wearing out of proto- 

 plasm and from the nitrogenous portions of such amino acids as are 

 oxidized for energy. Together with certain salts and a part of the excess 

 body water, they are eliminated by a special excretory system. 



The excretory system consists of a pair of kidneys, a urinary bladder, a 

 pair of tubes called the ureters that lead from the kidneys to the bladder, 

 and another tube called the urethra extending from the bladder to the 

 exterior of the body. Each kidney is a compact, glandular organ somewhat 

 larger than the fist. One lies on each side of the middorsal line of the 

 abdominal cavity in the region of the small of the back. The medial 

 (inner) margin of each kidney is concave, and on this concave side the 

 kidney contains a funnel-shaped space that leads into the ureter. In this 

 same region the kidney also receives a large artery and gives off a vein. 

 Internally the kidney is closely packed with a multitude of microscopic 

 tubules. Each tubule begins in an expanded and invaginated blind end 

 (Bowman' s capsule) situated near the outer surface of the kidney. Beyond 

 this expanded portion the tubule becomes truly tubular and, after several 

 loops and turnings, runs into a larger collecting duct. This, in turn, leads 

 to the funnel-shaped cavity that ends in the ureter. 



Each minute Bowman's capsule encloses a dense capillary network 

 known as a glomerulus; a capsule and glomerulus together form the struc- 

 ture known as a malpighian corpuscle. Each glomerulus is supplied by a 

 tiny artery and is drained by an even smaller one, 1 so that the blood pres- 

 sure within the capillaries of the capsule is unusually high. A some- 

 what more diffuse network of capillaries is also in close contact with the 

 walls of the long tubular portion of each tubule. 



Blood, entering the glomerulus under pressure, is filtered in large 

 quantities through the thin walls of the capillaries and the thin inner wall 

 of the capsule into the expanded upper end of the tubule. Blood cor- 



1 The outgoing vessel, like the incoming one, is an arteriole, not a vein as might 

 naturally be supposed; it carries concentrated arterial blood to the capillaries around 

 the uriniferous tubules. 



