RESPIRATION AND EXCRETION 



391 



tubules opens into the pelvis, the space within the kidney; the 

 inner end, in the cortex, forms a tiny closed sac. In each sac, the 

 outer wall of the tube has grown inward and carried with it a very 

 tiny artery. This artery breaks up into a mass of capillaries. 

 These capillaries, in turn, unite to form a 

 small vein as they leave the little sac. 

 Each of these sacs with its contained 

 blood vessels is called a glomerulus. 



Wastes given off by the Blood in the 

 Kidney. — In the glomerulus the blood 

 loses by osmosis, through the very thin 

 walls of the capillaries, first, a consider- 

 able amount of water (amounting to 

 nearly three pints daily) ; second, a ni- 

 trogenous waste material known as urea ; 

 third, salts and other waste organic sub- 

 stances, uric acid among them. 



Diagram of kidney circula- 

 tion, showing a glomerulus 

 and tubule : a, artery bring- 

 ing blood to part ; b, capil- 

 lary bringing blood to 

 glomerulus ; b', vessel con- 

 tinuing with blood to tu- 

 bule ; L, vein ; t, tubule ; G, 

 glomerulus. 



These waste products, together with the 

 water containing them, are known as urine. 

 The total amount of nitrogenous waste leav- 

 ing the body each day is about twenty grams ; 

 this is nearly all accounted for in the urea 

 passed oft by the kidney, as urine is secreted 

 in the kidney. It is passed through the ureter 

 to the urinary bladder; from this reservoir it 

 is passed out of the body, through a tube called the urethra. After the 

 blood has passed through the glomeruli of the kidneys it is pmrer than in 

 any other place in the body, because, before coming there, it lost a large 

 part of its burden of carbon dioxide in the lungs. After leaving the kid- 

 ney it has lost much of its nitrogenous waste. So dependent is the body 

 upon the excretion of its poisonous material that, in cases where the kid- 

 neys do not do their work properly, death may ensue within a few hours. 



Structure and Use of Sweat Glands. — If you examine the sur- 

 face of your skin with a lens, you will notice the surface is thrown 

 into little ridges. In these ridges may be found a large number 

 of very tiny pits; these are the pores or openings of the sweat- 

 secreting glands. From each opening a little tube penetrates deep 

 within the epidermis ; there, coiling around upon itself several 

 times, it forms the sweat gland. Close around this coiled tube are 



