The kidneys function to maintain the osmotic and electrolytic homeostasis of the body, and play an important role in adjusting the body’s pH balance. This balance must be maintained even during the ingestion of widely varying amounts of water and salts. This laboratory experiment will show how ingesting various foods and drinks can vary plasma osmolarity and alter the glomerular filtration rate to cause changes in urine formation/excretion as the
kidneys regulate urine output. Alterations in urine output will be measured and assessed by examining quantity and concentration of urine.
The kidneys process approximately 180 litres of filtrate per day, while only approximately 1% is lost as urine (the average adult urine output is 1.5 litres per day). One of the kidneys’ critical functions it to maintain plasma fluid and electrolyte balance. The nephrons are able to respond to changes in fluid and electrolyte concentrations and vary urine output. The nephrons use several mechanisms to control urine output: they have both intrinsic and extrinsic regulatory mechanisms. The intrinsic controls mainly function to quickly alter urine output in response to changes in blood volume and blood pressure. The extrinsic controls also respond to blood volume and pressure but mainly respond to blood osmolarity. The hormones ADH and aldosterone are critical hormones for regulating urine output. These hormones can be affected by diuretics, thus affecting urine output.
AFFECTS OF SODIUM, WATER, AND HORMONES ON KIDNEY OUTPUT
There a number of steps involved in kidney filtrate production. As blood flows through the kidney, it passes into glomerular capillaries located within the cortex. These glomerular capillaries are highly permeable to water and electrolytes (charged ions such as sodium, potassium and chloride). Glomerular capillary hydrostatic pressure drives water and electrolytes into Bowman’s space and into the Proximal Convoluting Tubule (PCT). The PCT, which lies within the cortex, is the site of sodium, water and bicarbonate transport from the filtrate (urine), across the tubule wall, and into the interstitium of the cortex. About 65‐70% of the filtered sodium is removed from the urine at the proximal convoluting tubule. This sodium is reabsorbed isosmotically, meaning that every molecule of sodium that is reabsorbed is accompanied by a molecule of water. As the tubule dives into the medulla, or middle
zone of the kidney, the tubule becomes narrower and forms the Loop of Henle that re‐enters the cortex as the Thick Ascending Limb (TAL) that travels back to near the glomerulus. Because the interstitium of the medulla is hyperosmotic and the Loop of Henle is permeable to water, water is reabsorbed from the Loop of Henle and into the medullary interstitium. This loss of water concentrates the urine within the Loop of Henle.
The Thick Ascending Limb, which is impermeable to water, reabsorbs sodium, potassium and chloride at a ratio of 1:1:2. Approximately 25% of the sodium of the original filtrate is reabsorbed at the TAL. From the TAL, the urine flows into the distal convoluting tubule (DCT), which is another site of sodium transport (about 5% is reabsorbed here) into the cortical interstitium. Finally, the tubule re‐enters the medulla as the collecting duct and then into
the renal pelvis where it joins with other collecting ducts to exit the kidney as the ureter. Sodium is reabsorbed here (about 1‐2%) in exchange for potassium and hydrogen ions, which are excreted into the urine. At these last sites, reabsorption occurs via transporter systems (opposed to passive diffusion). The activity of the transporter is dependent on several variables. Firstly it responds to the tubular concentration of sodium (when sodium is high, more sodium is reabsorbed and more potassium and hydrogen ions are excreted). Secondly, it is regulated by
aldosterone, which is a hormone secreted by the adrenal cortex: increased aldosterone increases the reabsorption of sodium, which also increases the loss of potassium and hydrogen ion to the urine. Finally, water is reabsorbed in the collection duct through pores that are regulated by antidiuretic hormone, which is released by the posteriorpituitary. ADH increases the permeability of the collecting duct to water, which increases water reabsorption, increasing urine concentration, and reducing urine outflow (antidiuresis). Nearly all of the sodium originally filtered is reabsorbed by the kidney, so that less than 1% of originally filtered sodium remains in the final urine.
Two major hormones regulate kidney function and fluid homeostasis: Anti‐diuretic Hormone (ADH, also known as vasopressin) and Aldosterone. When the body becomes fluid‐deficient there is an increase in the secretion of these hormones and the kidneys increase water retention, resulting in decreased urine output. When fluid levels in the body are in excess, hormone secretion is suppressed and an increase in urine volume results.
There are a number of substances that we ingest that can alter the volume and concentration of urine output. A diuretic is a compound that increases the volume of urine, when administered, and an antidiuretic reduces urine volume. As explained above, there are numerous places where water and ion absorption take place in the urinary system, and these substances often act on a very specific target. Caffeine and alcohol are examples of diuretics, which increase urine output. Sodium is an example of an antidiuretic, and can increase water absorption, resulting in decreased urine output.
Based off this information, answer the following questions:
1. What is a diuretic?
2. Of the beverages coffee, Gatorade, and tomato juice, which beverage(s) is/are diuretics? Which is/are anti-diuretics?
3. If we compared the urine output to a coffee drinker compared to a Gatorade drinker, who would produce more urine? Why?
4. If we compared the urine output of a tomato juice drinker to a coffee drinker, who would produce more urine? Why?
5. Why do athletes drink Gatorade during a sporting event?
Chemstrip Test Results:
Write a 2 page paper (typed, 1.5 spaced) summarizing your Chemstrip results. Why do medical providers test for each of the above substances? What pathology may be indicated if results are abnormal? Be sure to include the 5 questions on the first page.