Tubular Function Flashcards by Michaela Netto

What is the first step in blood processing?

Glomerular filtration

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What does the renal corpuscle consist of?

The glomerulus and Bowman’s capsule

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Where does reabsorption occur?

Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule

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How many litres of fluid is filtered per day?

180L/day

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What percentage of fluid is reabsorbed by the renal tubule?

99%

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Which part of the renal tubule is the main site of reabsorption?

Proximal convoluted tubule

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How much sodium (%) is reabsorbed in each part of the renal tubule?

65% reabsorbed in PCT
25% reabsorbed in ascending loop of Henle
Up to 8% reabsorbed in DCT

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How much sodium in total (%) is reabsorbed by the renal tubule?

98%

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What does the close relationship between the PCT and the tubular capillaries facilitate?

Facilitates contents being reabsorbed into bloodstream

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What is the role of each part of the nephron? (renal corpuscle, PCT, Loop of Henle, DCT, collecting duct)?

Renal corpuscle - filtration 
PCT - reabsorption and secretion 
Loop of Henle - counter-current multiplier 
DCT - reabsorption and secretion 
Collecting duct - urine collection

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What is reabsorbed by the renal tubule?

Water, glucose, amino acids, urea, sodium, chloride, calcium, phosphate, potassium, bicarbonate

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What is NOT reabsorbed by the renal tubule?

Creatinine

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What does reabsorption by the renal tubules involve ?

The movement of substances from a high concentration to a low concentration, through tubule lumen, tubule wall, interstitial space, and into the peritubular capillary

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How does the movement of substances differ in secretion compared to reabsorption?

Secretion = movement of substances in opposite direction to reabsorption

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Measuring creatinine levels in the blood gives a measure of what?

Glomerular filtration rate (GFR)

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How do substances cross the phospholipid bilayer during reabsorption by the renal tubule?

Aquaporins present in walls of cells - water channel that enable the passage of water
Co-transporters - glucose and amino acids
are co-transported with sodium
Na+/K+ pump - on basolateral membrane . Pushes Na+ against its concentration gradient

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What does Na+ facilitate the diffusion of?

Almost everything ‘follows’ sodium: water (osmosis - aquaporins), glucose and amino acids (co-transported with Na+), Cl- and negative ions follow Na+ (electrical gradient)

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Where is the Na+/K+ pump located?

On the basolateral membrane of the tubule

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What does the Na+/K+ pump do?

Uses ATP to pump Na+ against its gradient
Increases the rate at which Na+ is reabsorbed
Maintains concentration gradients in the lumen (pump is between the tubule wall and interstitial space)

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How much filtrate passes to the Loop of Henle (of the initial 180L)?

60L

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What is the role of the Loop of Henle?

Counter-current multiplication

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What is counter-current multiplication?

Process that occurs in the Loop of Henle - generates an osmotic gradient that enables the reabsorption of water and the production of concentrated urine

Which nephrons perform countercurrent multiplication?

Juxtamedullary nephrons

Which structures form the Loop of Henle?

Thin descending limb, thin ascending limb, thick ascending limb

Describe the permeability and transport properties of each limb in the Loop of Henle

Thin descending limb: contains aquaporins. Freely permeable to water Thin ascending limb: does not contain aquaporins Thick ascending limb: does not contain aquaporins. Contains NKCC2 pump (site of active Na+ reabsorption)

Where is the NKCC2 pump located?

On the apical membrane of the tubule wall

What happens to the concentration of the medulla (interstitium) as salt is actively reabsorbed into the medulla, and what effect does this have on water as it flows into the Loop of Henle?

Becomes salty/high concentration of Na+ | Water flows OUT of tubule by osmosis

Which pump is a target for diuretic drugs?

NKCC2 pump

Describe counter current multiplication

Fluid flows in opposing directions Concentration gradient multiplies the deeper into the medulla you go Difference pulls water out of filtrate - recovers as much water as possible

Why does the concentration not equalise during countercurrent multiplication?

The structure and dynamics of the Loop of Henle New fluid constantly entering the Loop of Henle, and new salt is constantly being pumped out Also due to structure of vasa recta, which also employs counter current exchange Water and salt equalise with the blood as it descends and ascends

What is the purpose of counter current multiplication?

To reabsorb water and concentrate urine | Creates and maintains high osmotic pressure in the medulla

Where is sodium actively reabsorbed in the Loop of Henle?

Thick Ascending Limb

Where is water reabsorbed in the Loop of Henle?

Thin Descending Limb

Which part of the renal tubule does 'fine-tuning' i.e. fluid volume and electrolyte regulation occur?

Distal Convoluted Tubule

What effect does ADH have on water reabsorption?

Increases water reabsorption

What effect does aldosterone have on Na+?

Increases Na+ reabsorption

What effect does atrial natriuretic hormone have on Na+?

Promotes Na+ secretion

What happens to urine in the absence of hormonal regulation?

Large volume of dilute urine is produced

If we are dehydrated, what hormone does the body release?

ADH (anti-diuretic hormone)

Which part of the nephron is the target site of ADH?

The distal convoluted tubule (DCT) and collecting duct

What effect does ADH have on aquaporins?

Allows for the insertion of aquaporin channels - allows water to be reabsorbed from filtrate Result = small volume of concentrated urine is produced

What is AVPR2?

ADH/vasopressin receptor

Which hormone works with ADH?

Aldosterone

What does aldosterone do?

Upregulates activity and insertion of Na+/K+ pumps and channels Small volume of concentrated urine produced

What type of hormone is aldosterone?

Steroid hormone | Can pass through membrane and bind directly to transcription factors nucleus

What happens upon ADH-AVPR2 receptor binding?

Binding of ADH to its receptor (AVPR2): signal delivered into cell, passes into the nucleus and tells the cell to up regulate transcription and translation of aquaporin proteins to assemble in DCT wall to increase water reabsorption

Which hormone works agains ADH and aldosterone?

ANP (atrial natriuretic peptide)

Where is ANP produced?

By cardiac muscle cells in the atrial walls of the heart

Where is aldosterone produced?

The zona glomerulosa of the adrenal cortex

Where is ADH produced?

The hypothalamus -> released by posterior pituitary gland

What happens if there is an increase in blood pressure?

ANP (inhibits action of ADH and aldosterone) removes water channels and sodium pumps Large volume of dilute urine produced

What percentage of filtrate is reabsorbed?

Up to 99%

Where does most reabsorption take place?

PCT

What percentage of the 180L filtered a day is excreted as urine?

1-2%