Textbook Question
Drugs that treat hypertension, or high blood pressure, have the following actions. Discuss the specific effect that each drug will have on the kidneys.
a. Blocking the action of aldosterone on the kidneys
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25. The Urinary System
Renal Physiology: Regulation of Glomerular Filtration
Problem L3.A3
Textbook Question
Deana is a 4-year-old girl with a rare genetic defect that causes the Na+/glucose symporters in the proximal tubule to reabsorb fewer glucose and sodium ions than normal. Predict the effects this defect will have on the composition and volume of Deana's urine. Explain why you would expect to see increased activity of the tubuloglomerular feedback and the renin-angiotensin-aldosterone system in Deana's kidneys.
Verified step by step guidance1
Understand the role of Na+/glucose symporters in the proximal tubule: These transporters are responsible for reabsorbing sodium (Na+) and glucose from the filtrate back into the bloodstream. A defect in these symporters means less glucose and sodium will be reabsorbed, leading to higher concentrations of these substances in the filtrate and, consequently, in the urine.
Predict the effects on urine composition: With less glucose and sodium being reabsorbed, more glucose will remain in the filtrate, leading to glucosuria (glucose in the urine). Sodium retention in the filtrate will also increase, which can lead to osmotic diuresis (increased urine volume) as water follows the solutes into the urine due to osmotic gradients.
Explain the increased activity of tubuloglomerular feedback: The macula densa cells in the distal tubule detect increased sodium levels in the filtrate. This triggers tubuloglomerular feedback, which reduces the glomerular filtration rate (GFR) to prevent excessive loss of sodium and water. However, in Deana's case, the defect in the symporters may cause persistent sodium loss, leading to sustained activation of this feedback mechanism.
Discuss the activation of the renin-angiotensin-aldosterone system (RAAS): The loss of sodium and water can lead to decreased blood volume and blood pressure. This stimulates the juxtaglomerular cells in the kidney to release renin, which activates the RAAS. The system works to increase sodium reabsorption (via aldosterone) and water retention (via antidiuretic hormone, ADH) to restore blood volume and pressure.
Summarize the overall effects: Deana's urine will likely have increased glucose and sodium concentrations, as well as a higher volume due to osmotic diuresis. The persistent sodium and water loss will lead to compensatory mechanisms, including increased tubuloglomerular feedback and activation of the RAAS, in an attempt to maintain homeostasis.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Na+/Glucose Symporters
Na+/glucose symporters are transport proteins located in the proximal tubule of the nephron that facilitate the reabsorption of glucose and sodium ions from the filtrate back into the bloodstream. In Deana's case, a defect in these symporters leads to decreased reabsorption, resulting in higher levels of glucose and sodium remaining in the urine, which can affect urine composition and volume.
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Tubuloglomerular Feedback
Tubuloglomerular feedback is a mechanism by which the kidneys regulate glomerular filtration rate (GFR) based on the composition of the fluid in the distal tubule. When sodium and chloride concentrations are low due to decreased reabsorption, the macula densa cells signal the afferent arterioles to dilate, increasing GFR to enhance sodium and fluid reabsorption, which would be heightened in Deana's condition.
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Renin-Angiotensin-Aldosterone System (RAAS)
The renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and fluid balance. In response to low sodium levels and decreased blood volume from the defect in glucose and sodium reabsorption, the kidneys release renin, leading to the production of angiotensin II and aldosterone, which promote sodium reabsorption and increase blood pressure, thus compensating for the loss of sodium in Deana's urine.
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