xiv \VASTK AND EXCRETION 165 



Excretion of Urine. The blood passing through the 

 capillaries whirh form the glomerulus of a Malpighian capsule 

 is only separated from the cavity of the tubule by the thin 

 wall of the capillaries, and by the layer of very thin flattened 

 cells, the part of the wall of the capsule which covers them. 

 Water with certain salts in solution passes from the blood 

 through these two thin membranes into the cavity of the 

 capsule. The thin epithelial membranes allow the water and 

 certain salts of the blood to pass through, but will not allow 

 other substances, such as the albumin, to pass. The process 

 is, therefore, not a mere filtration, such as takes place through 

 blotting paper, through which all substances in solution can 

 pass. The epithelial membrane consists of living cells, and 

 these decide what shall pass, and what shall not. Part of the 

 urine is thus derived from the blood passing through' the 

 glomeruli. From the Malpighian capsules the fluid travels along 

 the tubules, following the winding course in the cortex, and the 

 straighter but still complicated course in the medulla, till it 

 is discharged, at the united opening of several of the tubules, 

 into the pelvis of the kidney. As it passes along the tubules, 

 the urea and some other substances are added to it by the 

 agency of the epithelial cells which form the wall of the tubules. 

 These epithelial cells separate the urea and the other substances 

 from the blood in the capillaries around them, and pass them on 

 into the interior of the tubule. The urea already exists, though 

 in minute quantity, in the blood as it flows to the kidney, so 

 that these cells do not make it, but merely let it pass out into 

 the tubules. Some of the less important substances which leave 

 the blood by the kidney are modified and changed into other 

 substances as they pass through the epithelial cells of the 

 tubules. 



The amount of urine formed depends chiefly on the quantity 

 of blood which is sent through the glomeruli. The greater 

 the amount of blood sent to the kidney, the greater is the 

 flow of urine. In cold weather more urine is passed than 

 in warm weather. This is because cold causes a constriction 

 of the hlood vessels of the skin, and so less blood reaches the 

 surface of the body, while more is sent to the internal organs 

 and so to the kidneys. Hot weather, on the contrary, cause-. .1 

 dilation of the- vessels of the skin ; more blood is sent to the 



