METABOLISM IN VERTEBRATES 



Fig. 3.25. The skin of man, 

 in section, showing char- 

 acteristic structures; dia- 

 grammatic. The bulb is the 

 so-called root of the hair, 

 where growth occurs. Blood 

 vessels and nerves associated 

 with the hair are found in 

 the papilla. Capillaries are 

 shown around the sweat 

 gland at the left but are 

 dissected away on the right. 



Epidermis 



Oil gland 



Hair shaft 



Sweat gland 



Adipose tissue 



weighing nearly five times as much as the kidneys. Each kidney in man is 

 made up of approximately a million structural and functional units, the 

 nephrons or excretory tubules (Fig. 3.12). The arrangement of the blood 

 vessels of the nephron is unique and makes possible its function of excretion. 

 A short arteriole carrying blood from the renal artery is continuous with the 

 glomerulus, which is a tuft of capillaries with few anastomoses. Blood is 

 forced through the glomerular capillaries into another arteriole of about the 

 same diameter as the entering one. Under such circumstances, pressure 

 filtration of the blood occurs for as long as it remains in the glomerulus; 

 about one-fifth of the plasma entering the glomerulus is filtered out into the 

 lumen of the nephron. All the constituents of the blood plasma except the 

 large protein molecules can pass through the capillary wall and the epithelium 

 of Bowman's capsule. 



Blood from the outgoing arteriole of the glomerulus passes into a capillary 

 bed which surrounds the excretory tubule, which is very long and convoluted 

 in the kidneys of mammals. The difTerence in concentration between the 

 glomerular filtrate and the blood in the capillary bed results in the reabsorp- 

 tion by the blood of most of the material removed in the glomerulus. About 

 99 per cent of the water is reabsorbed, thus concentrating the filtrate which 



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