Excretion - 373 



tubule. The walls of the kidney tubules are 

 made up of a single layer of glandular epi- 

 thelium; and the different parts of the neph- 

 rons occupy characteristic positions in the 

 kidney (Fig. 20-4). The collecting tubules 

 serve merely to drain the urine from the 

 nephrons proper into the pelvic chamber of 

 the kidney. 



The blood supply of the nephron is very 

 important in determining kidney function. 

 As may be seen in Figure 20-4, each glomer- 

 ulus is supplied by an afferent vessel, which 

 is a branch of the renal artery, and each 

 glomerulus is drained by an efferent vessel. 

 The efferent vessels, however, return to the 

 renal vein by a very indirect route — through 

 a network of capillaries that enmeshes all 

 the tubular parts of the nephron. Thus a 

 given sample of blood in passing from the 

 renal artery to the renal vein must flow 

 through two sets of capillaries: (1) the capil- 

 lary coils of the glomerulus itself and (2) the 

 capillary network that lies in intimate con- 

 tact with the tubular parts of the nephron 

 (Fig. 20-5). 



The Formation of Urine. Kidney function 

 has been studied intensively for many years, 

 and today, although some problems remain 

 unsolved, the main issues have been defined 

 quite clearly. 



The work of the kidney involves three 

 processes: 



1. The glomerulus and capsule, working 



together, subject each sample of blood to a 

 process of pressure filtration. This process 

 forms a fluid, the nephric nitrate, that passes 

 into Bowman's capsule while a given part of 

 the blood flows through the glomerulus. 



2. The tubular parts of the nephron sal- 

 vage very large quantities of water (which 

 accounts, mainly, for a concentration of the 

 waste products) and smaller quantities of 

 glucose and other useful compounds — by re- 

 absorbing these substances from the nephric 

 filtrate while it passes through the tubules 

 toward the pelvic chamber. This process of 

 selective reabsorption demands that the 

 gland cells in the tubule wall transfer the 

 reabsorbed compounds back into the blood 

 in the capillaries surrounding the tubule. 



3. The glandular walls of the nephric tu- 

 bules secrete additional quantities of meta- 

 bolic wastes into the nephric filtrate, extract- 

 ing these wastes from the capillaries around 

 the tubule and passing them into the lumen 

 of the nephron. 



Reabsorption and secretion greatly change 

 the composition of the nephric filtrate as 

 this fluid drains through the nephron toward 

 the collecting tubules. In fact, the fluid as it 

 reaches the collecting tubule can no longer 

 be called the filtrate — it is the urine. 



Filtration: Nature of the Nephric Filtrate. 

 The glomerulus and Bowman's capsule to- 

 gether form a very effective force filter. In 

 filtering the blood the glomerular and capsu- 



-^ ,1 r 



Fig. 20-5. A single nephron, with its blood 

 supply. Arrows indicate direction of the flow 

 of blood. A number of nephrons drain into 

 each collecting tubule. Be sure to identify the 

 proximal and distal convoluted tubules 

 and the loop of Henle. The letter O at lower 

 right indicates the opening of a collecting 

 tubule into the pelvic chamber of the kidney 

 (cf. Fig. 20-4). 



ISOLATED NEPHRON 



