Urine output by increasing the number of channels in

This preview shows page 19 - 21 out of 32 pages.

urine output by increasing the number of channels in cells of the collecting ducts so that water passes easily into the surrounding interstitial space making the osmolarity of the filtrate equal to that of the interstitial fluid. When ADH is not being released, dilute urine is excreted (as low as 65 mosm) because tubular filtrate, which enters the glomerulus at about 300 mosm, is finally diluted to about 100 mosm or less. All the kidney needs to do to secrete dilute urine is allow the filtrate to pass into the collecting ducts where no further water is reabsorption but sodium and some other ions are removed from the filtrate by distal and collecting tubule cells making the urine even more dilute. When ADH is released, as the filtrate passes through the collecting ducts, the osmolarity of the filtrate becomes equal to that of the interstitial fluid (as high as 1200 mosm) as water rapidly leaves the filtrate through the channels opened by the hormone. The release of ADH is linked to the degree of body hydration allowing the body to respond to dehydration conditions (such as excessive water loss through sweating or diarrhea, or reduced blood volume or blood pressure or hemorrhage) by retaining up to 99% of the water in filtrate, excreting only a very small volume of highly concentrated urine. There are substances called diuretics that act in the opposite way as ADH by increasing urinary output but by many different mechanisms. Some of these that are present in higher concentrations than able to be reabsorbed like glucose (in diabetes mellitus) cause osmotic reabsorption of water into the tubules. Most diuretic drugs (usually prescribed to decrease blood pressure or edema such as that caused by congestive heart failure) decrease Na+ ion reabsorption (and therefore water reabsorption) into the renal tubule filtrate. Caffeine causes the renal tubules
to increase in diameter which increases the flow through the tubules. Alcohol inhibits the release of ADH. All of these effects cause a greater flow of urine. Characteristics and Composition of Urine Color - Recently excreted urine is usually clear and pale to deep yellow in color depending on the body's degree of hydration. The yellow color is caused by the presence of urochrome, the principal pigment of urine that is derived from the metabolic breakdown of hemoglobin. Abnormal colors may result from drugs or foods like beets or rhubarb or be caused by the presence of bile pigments or blood in the urine. Cloudy urine (due to the presence of pus) may indicate a urinary tract infection. Odor - Recently excreted urine is slightly aromatic, but develops a stinging ammonia odor upon standing because of bacterial breakdown of the urea. Asparagus and some drugs cause abnormal odors as do some diseases like diabetes mellitus which imparts a fruity smell due to acetone which is formed.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture