The Proximal Tubule and the Loop of Henle Flashcards
What do the kidneys reabsorb?
- 99% of fluid
- 99% of salt
- 100% glucose
- 100% amino acids
- 50% urea
- 0% creatinine
Compare reabsorption and filtration?
Reabsorption is specific (relies on specific transporters)
Filtration is relatively non-specific
What is glomerular filtrate?
Modified filtrate of the blood, i.e: contains ions and solutes at plasma concentrations but lacks RBCs and large plasma proteins)
Describe reabsorption of fluid in the PCT
~80 ml/min of filtered fluid is reabsorbed
The fluid reabsorbed in the PCT is iso-osmotic with the filtrate, i.e: the tubular fluid does not change in osmolarity as it passes along the PCT
Which substances are reabsorbed and secreted in the PCT?
Reabsorbed: • Sugars • Amino acids • Phosphate • Sulphate • Lactate
Secreted: • H+ • Hippurates • Neurotransmitters • Bile pigments • Uric acid • Drugs • Toxins
2 routes of tubular reabsorption?
Transcellular - from the tubular lumen across the epithelial cell and then into the interstitial fluid and then the peritubular capillary
Paracellular - between the epithelial cells
3 types of carrier-mediated transport? Describe each
Primary active transport - energy is directly required to operate the carrier and move the substrate against its conc. gradient
Secondary active transport - carrier molecule is transported coupled to the conc. gradient of an ion (usually Na+)
Facilitated diffusion - passive carrier-mediated transport of a substance down its conc. gradient
Example of diffusion through a lipid bilayer?
O2 and CO2
Example of diffusion through channels?
Na+
Example of facilitated diffusion?
Glucose
Example of primary active transport?
Na+/K+ ATPase brings Na+ out and K+ in
Example of secondary active transport?
Na+/glucose transporter (on the apical membrane)- both Na+ and glucose are transported in
How does Na+ reabsorption occur?
Via the energy-dependent Na+/K+ ATPase at the basolateral membrane
This brings Na+ out of the tubular cell and into the interstitial fluid; from here, it can diffuse into the peritubular capillary
Why is there iso-osmotic fluid reabsorption?
This occurs across a “leaky” proximal tubule epithelium due to:
- Standing osmotic gradient
- Oncotic pressure gradient
There is passive water reabsorption down the NaCl osmotic gradient, i.e: water follows Cl- ions paracellularly (AKA oncotic drag of peritubular plasma); this means that osmolarity does not change because water and salt are reabsorbed in equal proportions
Which transporters are present on the luminal end of the tubular cells?
Na+/glucose (brings both in)
Na+/amino acid (brings both in)
Na+/H+ (brings Na+ in and pumps H+ out)
How is glucose reabsorbed?
On the luminal side of the tubular cell:
• Na+/glucose
On the basolateral side of the tubular cell:
• Facilitated diffusion allows glucose to enter interstitial fluid
Normally, 100% of glucose in the filtrate is reabsorbed in the PCT
What is the transport maximum for glucose?
2 mmol/min
Transport systems inv. with reabsorption and secretion are saturable; once the Tm is reached, clearance of reabsorbed or secreted substances is not constant
Function of the loop of Henle?
Generates a cortico-medullary solute conc. gradient, which enables the formation of HYPERTONIC URINE
Fluid flow in the loop of Henle?
Opposing flow in the two limbs (ascending and descending) is termed countercurrent flow; the entire loop functions as a counter-current multiplier
Together, the loop and vasa recta establish a hyper-osmotic medullary interstitial fluid
Reabsorption in the ascending limb?
Along the entire length:
• Na+ and Cl-
However, the ascending limb is relatively impermeable to water and so little/no water follows salt reabsorption
Reabsorption in the descending limb?
Does not reabsorb NaCl and is highly permeable to water
HOW is Na+ reabsorbed in the thick ascending limb?
Via the Na+/K+/Cl- (triple) co-transporter
This region is impermeable to water
How can the triple co-transporter be blocked?
Loop diuretics
Steps of reabsorption in the loop of Henle?
- Solute removed from lumen of ascending limb (water cannot follow)
- Tubular fluid is diluted and osmolality of interstitial fluid is raised
- Interstitial solute cannot enter the descending limb
- Water leaves the descending limb by osmosis
- Fluid in the descending limb is concentrated
ADD IMAGE
This means that fluid enters the descending limb and is concentrated (becomes hyperosmotic compared to the blood) before moving into the ascending limb; dilute/hypotonic fluid them moves onto the distal tubule
Why is the loop of Henle horse-shoe/hairpin shaped?
ADD IMAGE
Why is urea important?
Urea cycle contributes ~1/2 of the medullary osmolarity
The collecting ducts absorb ~50% urea (ADH promotes this); this passively diffuses into the loop
ADD IMAGE
NOTE: the distal tubule is impermeable to urea
Function of countercurrent multiplication?
To concentrate the medullary interstitial fluid (high medullary osmolarity) in order to enable the kidney to produce urine of different volume and conc. according to the amount of circulating ADH
What is the countercurrent exchanger
Vasa recta acts as this and it runs alongside the long loop of Henle of the juxtamedullary nephrons
Capillary blood euilibriates with interstitial fluid across the “leaky” endothelium:
• Blood osmolarity rises as it dips down into the medulla, i.e: water loss, solute gained
• Blood osmolarity falls as it rises back up into the cortex, i.e: water gained, solute lost
What does the countercurrent system consist of?
Vasa recta (countercurrent exchanger)
Loop of henle (countercurrent flow)
How is washing away of NaCl and urea in the medulla, by essential blood flow, minimised?
- Vasa recta capillaries follow hairpin loops
- Vasa recta capillaries are freely permeable to NaCl and water; passive exchange across the endothelium preserves medullary gradient and ensures solute is not washed away
- Blood flow to the vasa recta is low (there are few juxtamedullary nephrons)
2 components in the creation of the medullary osmotic gradient?
Countercurrent multiplier
Urea cycle
