Urine Formation

A critical body function in living organisms involves balancing water intake and with its loss. This balance is determined, to a large measure, by the volume and composition of the urine, the composition of which varies and mirrors the water requirements of the organism. Freshwater animals excrete very dilute urine, while marine animals, seeking to conserve water, secrete highly concentrated urine. Depending upon their habitat, terrestrial animals generally retain water and secrete concentrated urine.

The kidney is responsible for filtering blood. In most mammals, blood plasma is filtered by the nephrons in the kidneys, with most of the water and useful materials returned to the blood stream and conserved by the body. The remaining excess water and the waste products of metabolism— including urea (from amino acid metabolism)—remain in the urine and are eliminated. Amphibians and fish do not retain great amounts of water and, therefore, excrete large volumes of dilute urine containing the water-soluble urea. By contrast, in most birds, reptiles, and terrestrial insects, the end product of amino acid metabolism is the water-insoluble uric acid. The urine of birds and reptiles is a white suspension of uric acid that is mixed with fecal material prior to elimination.

The nephron (shown), the basic structural and functional unit of the kidney, filters the blood and returns to the blood what is needed, excreting the remainder as urine.

Based upon microscopic examination, William Bowman, English physician and histologist, studied the structure of the kidneys. In 1842, he identified the glomerular capsule (now called “Bowman’s capsule”) as the beginning of the nephron, the function unit of the kidney. The capsule is the keystone in Bowman’s filtration theory of urine formation and the basis for our current understanding of kidney function. In 1844, Carl Ludwig proposed that blood pressure forced fluids out of the kidney capillaries into the nephrons. This fluid contained all the components of plasma but proteins, and water was returned to the bloodstream to concentrate the urine. Ludwig, one of the greatest physiologists, taught that the functions of living organisms were dictated by chemical and physical laws and not by special biological laws and divine influences. More specifically, he argued that urine was formed in the kidneys from a filtration process and not by vital forces, as suggested by Bowman.