Physiology - The Proximal Tubule and the Loop of Henle Flashcards
Where does the vast majority of reabsorption occur in the nephron?
Proximal convoluted tubule
Roughly how much fluid per minute is reabsorbed in the proximal convoluted tubule?
80ml/min
How does osmolarity change within the proximal tubule?
It does not change
(fluid reabsorbed in the proximal tubule is iso-osmotic with filtrate)
Which substances are reabsorbed from the proximal tubule?
- Sugars
- Amino acids
- Phosphate
- Sulphate
- Lactate
- Na+ and Cl-
- Water
Useful substances are reabsorbed (out on the tubule and into the blood)
Which substances are secreted into the proximal tubule?
- H+
- Hippurates
- Neurotransmitters
- Bile pigments
- Uric acid
- Drugs
- Toxins
Which two routes can a substance take in order to be reabsorbed from the proximal tubule?
- Transcellular
- Paracellular
Which barried must a substance pass in the transcellular route on its path to reabsorption?
Apical membrane
Cytoplasm
Basolateral membrane
Lateral space and extracellular fluid
Endothelial wall of capillary
Where does a substance pass, if it exits the nephron tubule via the paracellular route?
Tight junctions
What are primary active transport proteins?
Proteins which use hydrolysis of ATP to drive their substrate across its concentration gradient
What are secondary active transport proteins?
Proteins which move a substance against its concentration gradient when coupled to another ion by utilising its concentration gradient
Which types of diffusion can take place in the nephron?
Diffusion through lipid bilayer (O2, CO2)
Diffusion through channels (Na+)
Facilitated diffusion (glucose)
What drives sodium reabsorption in the proximal tubule?
Basolateral Na/K/ATPase
This maintains a concentration gradient out of the proximal tubule and into tubular cells
At which membrane is the sodium potassium pump found in the nephron?
Basolateral
Why are chloride ions reabsorbed in the proximal tubule?
An electrochemical gradient is set up as sodium is absorbed into the blood
Why is water reabsorbed in the peritubular capillary?
- Salt (is reabsorbed, water follows)
- There is oncotic drag (due to high protein concentration in capillary)
Why does osmolarity not change in the proximal tubule?
Salt and water are reabsorbed in equal proportions
By which active transport route does
a) Sodium use
b) Chloride use
a) Sodium - Transcellular
b) Chloride - Paracellular
How does glucose become reabsorbed from the proximal tubule?
Transcellular route
Sodium-glucose cotransporter at apical membrane
Faciliated glucose diffusion at basolateral membrane
At high plasma glucose concentrations, why does the rate of glucose reabsorption plateau?
Transport mechanisms become saturated
(excess glucose will exit in urine, but under normal circumstances all glucose is reabsorbed in the proximal tubule)
What is a summary of all of the main substances reabsorbed by the proximal tubule?
67% of all water and salt
100% of glucose and amino acids
Na+ reabsorption
Cl- reabsorption
What is the function of the Loop of Henle?
Generate a cortico-medullary solute concentration gradient
(enabling formation of hypertonic urine)
What is meant by the term countercurrent flow?
The opposing directions of flow between the descending and ascending limbs of the loop of Henle
Why is a cortico-medullary solute concentration gradient required?
Formation of hypertonic urine
(osmolarity is higher deep in medulla)
The process of what sets up a cortico-medullary solute concentration gradient?
Countercurrent multiplication
The cells of the descending limb are essentially impermeable to ________ and highly permeable to __________
The cells of the descending limb are essentially impermeable to salt and highly permeable to water
What do the characteristics of the descending limb allow for in terms of permeability to water?
Easy water reabsorption (out of descending limb)
Since the descending limb is impermeable to salt, the salt stays in limb and salt outside in the medulla also cannot enter
This ensures an osmotic gradient is presernt
Along the length of the ascending limb what can be reasborbed?
Sodium ions
Chloride ions
(NOT water reabsorption)
What do the characteristics of the ascending limb allow for in terms of permeability to salts?
Increases passive diffusion of water at the descending tubule
Ascending limb has active tranporters which excrete salt into the medullary space increasing osmolarity and ensuring water will passively diffuse from the descending tubule into the medulla
Describe briefly the key points in countercurrent multiplication
Ascending limb pumps salts into medulla for reabsorption
This makes the osmolarity very high in the medulla
Since the descending limb is impermeable to salt, water will pass from the inside of the limb to the outside via passive diffusion towards the osmolarity
This process means that osmolarity increased down the descending tubule and decreases up the ascending tubule
At which value for osmolarity is a stead state reached in the process of countercurrent multiplication?
1200mmol/l
The thick ascending limb of henle contains cells which posses which transporter on the apical membrane?
Triple cotransporter
How does the triple cotransporter function?
- 1 Sodium
- 1 Potassium
- 2 Chloride ions
Moved into the thick ascending loop of Henle cell from lumen (no electrochemical gradient formed as net neutral charge)
In the thick ascending limb of Henle, which transporters can be found at the basolateral membrane?
Sodium/Chloride cotransporters
Sodium/Potassium pump
(potassium can also passively diffuse across the basolateral membrane)
The actions of the triple cotransporter, result in the net reabsorption of which ions into the interstitial fluid?
Sodium and chloride
(potassium recycling allows this to happen)
Which drug class can act on the triple cotransporter?
Loop diuretetics
What is the significance of the actions of the triple cotransporter in the cells of the thick ascending limb of Henle?
Interstitial fluid osmolarity is increased
Osmotic gradient is set up from the descending limb to the interstital fluid
Tubular fluid becomes more concentrated
Why does osmolarity progressively decrease up the ascending loop of Henle?
Sodium and chloride ions are actively removed into the interstitial space
What are the two factors which set up the corticomedullary gradient?
- Sodium/chloride
- Urea
What is the urea cycle and what is its purpose regarding the medullary osmolarity?
Collecting duct absorbs roughly 50% of urea under the action of ADH
It then functions to add solute to the interstitium
Urea can then enter the descending and ascending limbs of Henle forming a cycle
This means more water is reabsorbed as the osmolarity gradient is now more stark
This reduces urine output
What is the purpose of countercurrent multipliction?
Concentrates medullary interstitial fluid
Enables kidneys to produce urine of different volumes and concentrations depending on the amount of ADH in circulation
What does the vasa recta run alongside?
Long loop of henle of juxtamedullary nephrons
What is the purpose of the vasa rectae?
The vasa rectae ensures the solute is not washed away
(helps maintain the corticomedullary gradient)
