372 - Multicellular Animals, Especially Man 



STRUCTURE AND FUNCTIONS OF 

 THE KIDNEY 



The work of the kidney is to separate urine 

 from the blood that flows through it; and by 

 extracting excess wastes and other substances 

 from the plasma and delivering these com- 

 pounds into the urine, the kidney plays a 

 major role in regulating the composition of 

 the plasma and the lymph. 



Each kidney (Fig. 20-1) receives a major 

 artery — the renal artery — directly from the 

 dorsal aorta; and the kidneys are drained by 

 large veins — the renal veins — that pass to the 

 postcaval vein. While it is being formed, 

 urine collects in a small chamber — the pelvic 

 chamber — situated along the medial border 

 of the kidney (Fig. 20-1); and from the pelvic 

 chamber the urine drains into the urinary 

 bladder, via each of the two ureters. The 

 urinary bladder distends its muscular walls 

 to leceive the urine; but finally — when a 

 critical pressure is reached — the urination 

 reflex is generated. Then the bladder con- 

 tracts — voiding the urine through the ure- 

 thra, a single duct leading to the outside 

 (Fig. 20-1). 



The Nephrons, or Functional Units of the 

 Kidney. The human kidney represents an 



aggregation of about one million excretory 

 tubules — called the nephrons (Fig. 20-4). 

 Each nephron is a highly coiled microscopic 

 tubule that is responsible for forming a small 

 part of the total quantity of urine produced 

 by the kidney as a whole. Thus the function- 

 ing of the whole kidney can be determined 

 by studying the structure and activity of the 

 single nephron. 



The first part of a kidney tubule is a 

 double-walled capsule, that surrounds a com- 

 pact cluster of blood capillaries (Fig. 20-4). 

 This peculiar tuft of capillary coils is called 

 a glomerulus; and the surrounding double- 

 walled chamber is called Bowman's capsule, 

 first observed in 1842 by a young English 

 anatomist, William Bowman. Both the outer 

 and inner walls of Bowman's capsule consist 

 of a single layer of flattened epithelial cells; 

 and the inner wall adheres very intimately to 

 the tufted glomerular capillaries. 



The cavity of Bowman's capsule leads di- 

 rectly into the long lumen of the nephron 

 that eventually leads into a collecting tu- 

 bule (Fig. 20-4). To reach a collecting tubule, 

 however, fluid in Bowman's capsule must 

 flow consecutively through (1) the proximal 

 convoluted tubule, (2) the hairpin-shaped 

 loop of Henle, and (3) the distal convoluted 



PELVIC 

 CHAMBER 



URETER — r 



GLOMERULUS 

 S 



DISTAL CONVOLUTED TUBULE 



VEIN 

 ARTERY 



CAPILLARY 

 NETWORK 



-GIOMFRULUS 



Fig. 20-4. Structure of the human kidney. Note particularly the blood supply of the nephron. About a million 

 of these excretory units are present in each kidney. 



