Renal Lecture 3 Flashcards
Topic 4: Renal System
The correct statements in the tubuloglomerular feedback mechanism are:
A. Granular (JG) cells monitor the amount of stretch of the afferent arteriole.
B. Increased stretch prompts vasoconstriction of the afferent arteriole.
C. Macula densa cells release ATP in response to fast flowing filtrate.
However, statement D is incorrect.
D. A rise in glomerular blood pressure prompts vasoconstriction of the efferent arteriole, not vasodilation.
So, the correct choices are A, B, and C.
See calculation
Transcellular Route:
Transport across apical and basolateral membranes.
Diffusion through cytosol.
Movement into peritubular capillary via interstitial fluid.
Similar to intestinal absorption.
Paracellular Route:
Movement through leaky tight junctions (mainly in PCT).
Movement into capillary.
Mainly for H2O and some ions (Ca++, Mg++, K+, Na+).
Energy Usage:
80% ATP for Na+ reabsorption (creates gradients).
20% ATP for reabsorbing glucose, amino acids, ions, vitamins.
Passive Reabsorption:
Diffusion, facilitated diffusion, osmosis.
Moves along electrochemical gradients, no ATP needed.
Na+ reabsorption pulls Cl-.
Water follows Na+ (via aquaporins in PCT).
Water movement creates gradients for lipid-soluble substances
Substances Partially or Not Reabsorbed:
Urea: 50-60% reabsorbed; small and can diffuse through membrane pores.
Creatinine: Large, lipid-insoluble; not reabsorbed by kidneys.
Uric Acid: Some reabsorbed, some excreted. Excess uric acid can lead to gout.
Glucose would normally be completely reabsorbed during urine formation and should not be in a urine sample.
Tubular Reabsorption in Renal Tubules:
PCT:
100% of glucose & amino acids, 65% Na+, K+, H2O, 90% bicarbonate, 50% Cl-.
Loop of Henle:
Descending limb: Water reabsorbed.
Ascending limb: NaCl & K+ reabsorbed.
DCT + Collecting Duct:
10% NaCl & 20% water remain, regulated by hormones:
ADH (water), Aldosterone (Na+), ANP.
Kidney Regions:
Cortex: 65% reabsorption (H2O, Na+, glucose, amino acids).
Na+ (by aldosterone), Ca2+ (by PTH).
Outer Medulla:
Secretes H+, NH4+, K+ (by aldosterone), reabsorbs water (by ADH).
Inner Medulla:
Secretes K+ (by aldosterone), regulates blood pH.
Aldosterone from the adrenal cortex promotes active tubular secretion of potassium ions in the late distal convoluted tubule (DCT) and collecting ducts. It is secreted in indirect response to actions of the juxtaglomerular apparatus.
Antidiuretic hormone (ADH) causes the insertion of aquaporins into the membranes of cells of the collecting duct. More aquaporins causes increased water reabsorption from the filtrate, so the urine becomes more concentrated.
Functions of Tubular Secretion:
Dispose of unwanted substances (e.g., drugs and metabolites bound to plasma proteins).
Eliminate substances with passive reabsorption (e.g., urea, uric acid).
Excess K+ ion disposal (e.g., secreting excess K+).
Maintain blood pH (e.g., secreting excess H+).
Secretion occurs mainly in the PCT, and also in the late DCT and early collecting ducts.
The glomerulus is responsible for filtration, where blood plasma (containing water, ions, small molecules, waste products, etc.) is filtered into the glomerular capsule to form filtrate. The glomerulus is where the initial filtration of blood occurs, but secretion of substances into the tubules happens later, in the renal tubules (specifically in the proximal convoluted tubule (PCT), distal convoluted tubule (DCT), and collecting ducts).
Countercurrent Mechanism:
In the PCT, water and salt are absorbed, keeping filtrate at 300 mOsm.
Descending limb: Permeable to water, so water moves out by osmosis.
Ascending limb: Reabsorbs Na+ and Cl-, impermeable to water.
Osmolarity can reach 1200 mOsm in the loop of Henle (in juxtamedullary nephrons).
Fluid flows in opposite directions through adjacent segments.
Definitions:
Osmolality: Concentration of solute particles.
Osmosis: Water moves from low to high solute concentration.
Countercurrent Multiplier:
Descending limb: Permeable to water, water moves out by osmosis.
Ascending limb: Impermeable to water, actively pumps Na+, Cl-, K+ out, raising interstitial osmolarity.
Positive feedback: Increased osmolarity causes more water to leave the descending limb, concentrating the filtrate.
Net effect: Reduces filtrate volume and increases concentration by ~200 mOsm per level.
Reabsorption in the Nephron Loop:
Filtrate entering the loop is isosmotic to blood plasma and cortical fluid.
Descending limb: Water moves out, concentrating the filtrate.
Filtrate reaches its highest concentration at the bend of the loop.
Ascending limb: Na+ and Cl- are pumped out, increasing interstitial osmolarity.
Filtrate becomes most dilute (100 mOsm) as it exits the loop, hypo-osmotic to interstitial fluid.
Vasa recta preserves osmotic gradient.
Permeable, dilutes; countercurrent exchange.
Blood isosmotic to fluid.
Reabsorbs without disrupting gradient.
15% medullary nephrons.
Slow blood flow, permeable walls.
Blood maintains, doesn’t disrupt gradient.
1) Glomerular Net Filtration Pressure:
Net Filtration Pressure (NFP) = Glomerular BP - (Capsular Hydrostatic Pressure + Colloidal Osmotic Pressure)
NFP = 65 mm Hg - (12 mm Hg + 28 mm Hg) = 65 - 40 = 25 mm Hg
Answer: e) 25 mm Hg
2) Statements about the JGC:
a) The macula densa cells secrete renin - False (Granular cells secrete renin)
b) The granular cells are part of the distal convoluted tubule - False (Granular cells are in the afferent arteriole)
c) The macula densa monitors filtrate concentration - True
d) The granular cells are mechanoreceptors - True
e) C and D - True
Answer: e) C and D
3) Glucose in Urine:
Glucose is reabsorbed by the tubule cells under normal conditions, and when the renal threshold is exceeded (e.g., in diabetes), glucose may appear in the urine.
Answer: c) is reabsorbed by the tubule cells
Check other slides in case
Given healthy kidneys, virtually all organic nutrients such as glucose and amino acids are completely reabsorbed to maintain or restore normal plasma concentrations. In contrast, the reabsorption of water and many ions is continuously regulated and adjusted in response to hormonal signals.
Under normal conditions, the proximal convoluted tubule (PCT) reabsorbs nearly all of the glucose, lactate, and amino acids in the filtrate, as well as about 65% of the sodium (Na+) and water. This process is essential for maintaining the body’s electrolyte balance and fluid homeostasis.
Dehydration: ADH released → Increased water reabsorption → concentrated urine.
Overhydration: ADH release decreases → Dilute urine.
Urea Recycling: Urea helps maintain osmotic gradient in the medulla.
Mechanism: Increase urine output.
Alcohol: Inhibits ADH release.
Loop Diuretics (e.g., Furosemide): Inhibit NaCl reabsorption in nephron loop.
Thiazides: Affect DCT, less potent than loop diuretics.
Osmotic Diuretics: Non-reabsorbed substances like glucose increase water loss.
Renal Clearance & Kidney Function
Renal Clearance: Volume of plasma from which kidneys clear a substance per minute.
Formula:
C
=
U
V
P
C=
P
UV
U = Concentration in urine.
V = Urine flow rate.
P = Concentration in plasma.
Inulin: Used to measure GFR (glomerular filtration rate).
Clearance of Inulin = GFR (125 mL/min).
Less than Inulin: Reabsorption (e.g., urea, 70 mL/min).
Zero Clearance: Complete reabsorption (e.g., glucose).
Greater than Inulin: Secretion (e.g., drug metabolites).
Urine Composition
95% water, 5% solutes:
Main solutes: Urea, Na+, K+, PO43−, SO42−, Creatinine, Uric acid.
Abnormal: Glucose (diabetes), Proteins (kidney issues), Ketones (starvation/diabetes), Blood (hematuria), Leukocytes (UTI).
Urine Characteristics
Color: Yellow (urobilin), darker = more concentrated.
Cloudy: UTI.
Odor: Ammonia after standing, fruity = diabetes.
Hypernatremia (excess sodium): Causes confusion, muscle twitching, and coma.
Hyponatremia (low sodium): Causes brain swelling, confusion, and shock.
Hyperkalemia (high potassium): Leads to cardiac arrhythmias and muscle weakness.
Hypokalemia (low potassium): Causes muscle weakness, arrhythmias, and confusion.
Negative Feedback Mechanisms
RAAS System:
Renin → Angiotensin I → Angiotensin II → Vasoconstriction & aldosterone release.
Increases Na+ and H2O reabsorption → ↑ blood volume & pressure.
ANP:
Released from atria due to high BP → inhibits ADH & aldosterone → ↓ Na+ & H2O reabsorption → ↓ blood volume & pressure.
