Tubular Functions Flashcards
What is the importance of tubular functions in urine formation?
Tubular functions are crucial for maintaining homeostasis by reabsorbing essential solutes, secreting waste, and regulating electrolyte and fluid balance.
What are the basic transport mechanisms across the tubular epithelium?
Passive diffusion, facilitated diffusion, primary active transport (Na⁺-K⁺ ATPase), secondary active transport (symport/antiport), and endocytosis.
Where does most of the sodium reabsorption occur?
Proximal convoluted tubule (~65-70% of filtered Na⁺).
How is sodium reabsorbed in the proximal tubule?
Na⁺-glucose symport, Na⁺-amino acid symport, Na⁺-H⁺ antiport, and Na⁺-Cl⁻ transport.
What drives water reabsorption in the renal tubules?
Water follows sodium via osmosis, mainly in the proximal tubule and collecting duct (regulated by ADH).
How does the loop of Henle contribute to urine concentration?
Creates a hyperosmotic medullary gradient via the countercurrent multiplier system.
What is tubuloglomerular feedback?
A feedback mechanism where the macula densa detects NaCl levels and adjusts GFR via afferent/efferent arteriole constriction.
What is glomerulotubular balance?
A mechanism ensuring a constant fraction (~65%) of the filtered load is reabsorbed, despite GFR fluctuations.
What happens if tubular reabsorption is impaired?
Impaired reabsorption leads to excessive loss of solutes (e.g., glucose in diabetes) and fluid imbalances.
How do the kidneys regulate ion balance?
By adjusting Na⁺, K⁺, Cl⁻, Ca²⁺, and H⁺ reabsorption/secretion via hormones (aldosterone, PTH) and transporters.
How does ADH affect kidney tubules?
ADH increases water reabsorption by inserting aquaporins in the collecting duct, reducing urine output.
What happens if tubuloglomerular feedback is dysfunctional?
Leads to improper GFR regulation, contributing to hypertension or kidney disease.
What role do peritubular capillaries play in tubular function?
They facilitate reabsorption of water and solutes from tubules back into circulation via oncotic pressure.
How is potassium regulated in the renal tubule?
Proximal tubule reabsorbs K⁺, while the distal tubule and collecting duct adjust secretion based on aldosterone and dietary intake.
How does glucose transport occur in the renal tubule?
Reabsorbed in the proximal tubule via SGLT1 and SGLT2 symporters, then exits via GLUT transporters.
What is the importance of renal acid-base balance?
Kidneys regulate pH by reabsorbing bicarbonate and secreting H⁺ ions.
What is renal splay?
Renal splay refers to the gradual rather than abrupt saturation of transporters, seen in glucose reabsorption where some glucose appears in urine before reaching the transport maximum (Tm).
Why does renal splay occur?
It occurs due to variability in nephron transporter capacity and differences in tubular load distribution.
What is the renal threshold for glucose?
The plasma concentration at which glucose first appears in urine, typically around 180-200 mg/dL.
What is the transport maximum (Tm) for glucose?
The maximum rate at which glucose can be reabsorbed, around 375 mg/min in healthy adults.
What happens when plasma glucose exceeds the renal threshold?
Glucose spills into the urine (glucosuria) because transporters are saturated.
Why does glucosuria occur in diabetes mellitus?
Due to high plasma glucose levels exceeding the renal threshold, leading to glucose excretion in urine.
What is water reabsorption in the kidney?
The process by which water is reabsorbed from the renal tubules into the bloodstream, mainly driven by osmotic gradients.
Where does most water reabsorption occur?
In the proximal convoluted tubule (~65-70%) via aquaporins and osmotic gradients.
