Lecture 20: Renal Transport Mechanisms Flashcards
Q: What is renal threshold and what happens when it is exceeded?
Renal threshold is the plasma concentration of a substance at which it starts appearing in the urine because its reabsorption mechanism in the nephron is saturated.
When a substance (like glucose) exceeds its threshold, the carrier proteins for reabsorption become saturated, and the substance spills into the urine.
Example: In diabetes mellitus, if blood glucose exceeds the renal threshold (around 180 mg/dL), glucose appears in the urine (glycosuria).
Q: How is sodium reabsorbed along the nephron?
PCT: Majority of Na⁺ reabsorbed via cotransport with glucose.
Loop of Henle: Reabsorbed in the ascending limb.
DCT: Reabsorbed in exchange for K⁺ (regulated by aldosterone).
Collecting Duct: Regulated by aldosterone and ADH.
A: Sodium (Na+) is reabsorbed differently along various segments of the nephron:
Proximal Convoluted Tubule (PCT): Majority (~65%) of Na⁺ reabsorption occurs here through cotransport with glucose and amino acids and counter-transport with H⁺.
Loop of Henle:
Descending limb: No Na⁺ reabsorption.
Ascending limb: Na⁺ is reabsorbed via active transport, creating a gradient for water reabsorption.
Distal Convoluted Tubule (DCT): Active Na⁺ reabsorption is regulated by aldosterone. Na⁺ is reabsorbed in exchange for K⁺.
Collecting Duct: Na⁺ reabsorption here is also influenced by aldosterone and ADH, controlling water reabsorption through aquaporins.
Q: What molecules are secreted in the nephron, and how?
A:
Proximal Convoluted Tubule (PCT): Secretes H⁺, organic acids, creatinine, and drugs via counter-transport mechanisms.
Distal Convoluted Tubule (DCT): Secretes K⁺ and H⁺ in exchange for Na⁺ under the influence of aldosterone.
Collecting Duct: H⁺ and K⁺ secretion continues here; H⁺ is secreted in response to acidosis (acidic blood) to help regulate pH.
Q: What do natriuretic peptides do, and why are they secreted?
A:
Natriuretic peptides (e.g., Atrial Natriuretic Peptide, ANP) are released by the heart when blood volume is high, causing stretching of the atria.
Mechanism of Action:
Increase GFR by dilating afferent arterioles, increasing urine output.
Inhibit Na⁺ reabsorption in the DCT and collecting ducts, promoting natriuresis (excretion of sodium).
Inhibit aldosterone and renin secretion, reducing blood volume and pressure.
Q: How do transport mechanisms in the nephron respond to acidosis and alkalosis?
A:
Acidosis (Low pH):
Type A intercalated cells in the distal nephron secrete H⁺ into the urine and reabsorb HCO₃⁻ to buffer blood acidity.
H⁺ ions are actively secreted in the collecting duct and combined with buffers (like phosphate) in the urine.
Alkalosis (High pH):
Type B intercalated cells secrete HCO₃⁻ and reabsorb H⁺, lowering blood pH and correcting alkalosis.
Q: What are the three major hormones involved in fluid and electrolyte balance?
A:
Antidiuretic Hormone (ADH): Promotes water reabsorption in the collecting duct.
Aldosterone: Increases Na⁺ reabsorption and K⁺ excretion, leading to water retention.
Atrial Natriuretic Peptide (ANP): Promotes Na⁺ excretion and increases urine output, lowering blood volume.
Q: How does ANP initiate its effects in the kidneys?
A: ANP binds to the NPR-A receptor on kidney cells, activating guanylyl cyclase, which converts GTP to cGMP. Elevated cGMP levels act as a second messenger, leading to vasodilation and modulation of kidney function, including decreased sodium reabsorption.
Q: How does ANP reduce sodium reabsorption in the kidneys?
A: ANP inhibits Na⁺ channels and Na⁺-K⁺-ATPase in the proximal tubule and collecting duct, reducing sodium reabsorption. This promotes natriuresis (excretion of sodium), which lowers blood volume and blood pressure.
Q: How does ANP affect the Renin-Angiotensin-Aldosterone System (RAAS)?
A: ANP inhibits renin release from the juxtaglomerular cells and reduces aldosterone secretion from the adrenal cortex. This suppression of RAAS further decreases sodium reabsorption and reduces water retention, leading to lower blood pressure and volume.
Q: How does ANP affect glomerular filtration rate (GFR)?
A: ANP increases GFR by promoting vasodilation of the afferent arterioles (leading into the glomerulus). This enhances blood flow into the glomeruli, increasing filtration and the excretion of sodium and water.
Q: What are the overall effects of ANP on blood pressure and kidney function?
A: ANP leads to vasodilation, increased GFR, reduced Na⁺ reabsorption, and inhibition of the RAAS system. Collectively, these effects cause natriuresis and diuresis, reducing blood volume and blood pressure, while providing renal protection.
