Questions and Answers
What are the intrinsic mechanisms that the kidneys use to control blood pressure?
Autoregulation and the RAA system
What does the RAA system stand for?
Renin-Angiotensin-Aldosterone system
Which cells secrete renin?
The ascending limb of the loop of Henle is permeable to water.
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How does aldosterone affect sodium reabsorption?
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ADH is also called ______.
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What occurs when ADH is present in the collecting ducts?
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In the absence of ADH, the collecting ducts are permeable to water movement.
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What is the role of the macula densa cells?
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The loop of Henle is part of a system called the ______.
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Study Notes
Renal Control of Blood Pressure
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Kidneys regulate blood pressure by modifying extracellular fluid (ECF) volume and plasma composition.
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Intrinsic Mechanisms:
- Autoregulation: Protects kidneys from fluctuations in systemic blood pressure.
- Renin-Angiotensin-Aldosterone (RAA) system: Juxtaglomerular cells sense blood pressure in afferent arterioles and release renin to restore blood pressure, reliant on the juxtaglomerular apparatus (JGA).
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Extrinsic Mechanisms:
- Sympathetic nervous system reduces glomerular filtration rate (GFR) to elevate blood pressure when necessary.
Juxtaglomerular Apparatus
- Composed of:
- Juxtaglomerular (JG) cells: Specialized muscle cells releasing renin in response to low blood pressure or decreased sodium concentration.
- Macula densa cells: Located in the distal convoluted tubule, these cells recognize changes in sodium levels in the filtrate, stimulating renin release.
Renin-Angiotensin-Aldosterone System (RAAS)
- Monitors blood pressure in afferent arterioles; a decrease triggers renin release.
- Renin initiates a cascade involving angiotensin I and II, antidiuretic hormone (ADH), and aldosterone to modulate blood pressure and fluid balance.
Effect of Aldosterone
- Produced by adrenal cortex, it enhances sodium reabsorption in distal convoluted tubules and collecting ducts.
- Stimulates the synthesis of Na+/K+ transport proteins via DNA coding for mRNA.
- In the absence of aldosterone, significant salt loss (up to 20g/day) occurs; with aldosterone, salt loss is minimal.
Regulation of Urine Volume
- Achieved by kidneys managing urine output through Counter-Current Multiplier Mechanism (CCMM).
- The anatomical structure of juxtamedullary nephrons and vasa recta enables this mechanism, with varying permeability in different segments.
Counter-Current Multiplier Mechanism
- Counter-current: Filtrate flows in opposite directions in the loop of Henle compared to blood in vasa recta.
- Multiplier mechanism: Ion movement occurs in incremental steps, resulting in significant osmotic changes.
Osmotic Gradient Creation
- The loop of Henle establishes an osmotic gradient from cortex (300 mOsm/L) to medulla (1200 mOsm/L) via:
- Water and ion movement from filtrate to blood in the vasa recta.
- Variances in tubular cell permeabilities between descending and ascending limbs.
Mechanism of Osmotic Gradient
- Descending limb: Permeable to water but not to Na+ and Cl-; water exits by osmosis, collected by vasa recta.
- Ascending limb: Impermeable to water, permeable to Na+ and Cl-; “salt pumps” reduce filtrate osmolarity to 100 mOsm/L as salts are retained by vasa recta.
Reabsorption in Collecting Ducts
- Collecting ducts are typically impermeable to water but can change permeability through ADH modulation.
Antidiuretic Hormone (ADH)
- Controls urine concentration based on body hydration states.
- Osmoreceptors in the hypothalamus detect plasma osmolality and prompt ADH release during water deficits.
- ADH increases water reabsorption in distal convoluted tubule (DCT) and collecting ducts.
Effect of ADH on Urine Volume
- Presence of ADH: In dehydration, ADH causes collecting ducts to become permeable to water, resulting in concentrated urine (~500 ml/day, <1200 mOsm/L).
- Absence of ADH: Collecting ducts remain impermeable to water, leading to dilute urine (~>100 mOsm/L, larger volumes).
Summary of Loop of Henle Functionality
- Adjusts osmotic gradients essential for urine concentration, playing a crucial role in renal function and fluid balance.
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Description
Explore how the kidneys regulate blood pressure through various intrinsic mechanisms. Understand autoregulation and the renin-angiotensin-aldosterone (RAA) system's roles in maintaining fluid balance and blood pressure stability. Test your knowledge of these critical processes.
