Formation of Diluted and Concentrated Urine

Formation of Diluted and Concentrated Urine

The kidney can alter composition of urine in response to body daily needed, thereby maintaining osmolarity of fluid, eg. When necessary to conserve body water, kidney excrete urine with highly concentrated solute, and vice versa.
The principle of regulator of whether urine is concentrated or diluted is ADH. In its presence, kidney excretes large concentrated urine, while in its absence, kidney excretes large diluted urine.
Basic requirements for concentrated urine formation:
I. High ADH level increase permeability of distal tubule and collecting duct to water.
II. High osmolarity of renal medullary interstitial fluid provide osmolar gradient for water reabsorption.
The major factors contribute to build up of high solute concentration (hyperosmolarity) in renal medulla include:
A. Active transport of Na+ and cotransport of K+/Cl- out of thick portion of the ascending segment to medullary interstitium.
B. Active transport of ions from collecting duct to medullary interstitium.
C. Passive diffusion of large urea from inner medullary collecting duct to meduulary interstitum.
D. Diffusion of only small amount of water from tubule to medullary interstitum far less than reabsorption of solute to medullary interstitum.