This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The kidneys are compound tubular glands, being composed of branched microscopic uriniferous tubules, lined by a single layer of secreting cells, supported by connective tissue, and supplied with blood-vessels, nerves, and lymphatics. The final branches of each tubule end in a dilatation which contains a knot of blood-vessels, through whose walls water and salts filter into the tubule. As the water trickles along the latter, the cells lining it pass out the nitrogenous wastes of the blood brought there by the capillaries which Wrap closely around them. The tubules unite in the pyramids to form fewer and larger ducts, which pour the secretion into the calices of the pelvis of the ureter, and this tube then conveys it to the bladder.
How does the apex of a pyramid end? Where do we find the cortical substance of the kidney?
Describe the general distribution of the renal artery and its branches in the kidney. What part of the kidney contains most capillary blood-vessels?
To what type of gland do the kidneys belong? Of what are they made up? How do the uriniferous tubules end? What lies in each dilatation? What filters from the blood-vessels of its dilatation into the cavity of the tubule? What is added to this as it trickles along the tubule? How is the nitrogenous waste of the body brought close to the kidney tubules? What becomes of the tubules in the pyramids? Where do the larger ducts convey the secretion?
The Renal Secretion is less in bulk in warm weather, when perspiration carries off a good deal of the excess water of the blood, than in cold. On an average the kidneys eliminate from the body in twenty-four hours about fifty ounces (2 1/2 pints) of water, and 500 grains (1 1/7 ounces) of urea, which contain a little more than 230 grains of nitrogen.