Tubular Reabsorption and Secretion Flashcards
What is the most prevalent cation in the body?
Sodium
What is the most prevalent anion in the body?
Chloride (also is the most secreted)
What are some examples of extremes in the disparity between the amount of a substance that is filtered opposed to the amount of a substance that is reabsorbed?
- Glucose is 100% reabsorbed so it is NOT excreted
- Creatinine is 0% reabsorbed so it is 100% excreted
- Urea is 50% reabsorbed and 50% excreted
What is the purpose of the Proximal Tubule?
Accounts for about 60% of Filtrate REABSORPTION
What is the purpose of the Loop of Henle?
- Creates the Concentration Gradient in Interstitium of the Medulla
- Small amount of reabsorption
What is the purpose of the Distal Tubule?
REGULATION of REABSORPTION
- *Reabsorbs:
1. Ca
2. Na
3. H (acid)
4. K
What is the purpose of the Collecting Duct?
H2O Reabsorption
In order to be reabsorbed, a substance must undergo…
- Transcellular Pathway
2. Paracellular Pathway (easier)
What is the Transcellular Pathway?
Harder way for a substance to be reabsorbed since it must pass through 2 cell membranes and transverse an entire cell
Utilizes transporters
What is the Paracellular Pathway?
An easier way for a substance to be reabsorbed where it just transverses the Tight Junction between cells
What is Facilitated Diffusion?
- Use of an Integrated protein to facilitate the diffusion of an ion across the cellular membrane from a high concentration to low concentration
- Can be a channel or carrier protein
What is Secondary Active Transport?
- Relies on an Na gradient (created by Na/K ATPase) to provide the energy necessary to move a substance against its concentration gradient and into the cell
- Can be Cotransporter or Countertransporter
Majority of Secondary Active Transporters are?
Cotransporters
Aside from Na/H Countertransporter
Describe the movement of a substance during reabsorption:
The substance must go from Tubular Fluid across the cell layer (either via Transcellular or Paracellular Pathway) and end up in the Peritubular (Interstitial) Fluid where it is able to move into the blood via Paratubule Capillaries
What are the names of the membranes that a substance must cross during the Transcellular Pathway of Reabsorption?
- Apical Membrane (on the same side as Tubular Fluid)
2. Basolateral Membrane (on the same side as the Peritubular/Interstitial Fluid)
What is the start of the Transcellular Pathway of Reabsorption?
Starts at the Basolateral Membrane (Peritubular/Interstitial Fluid side) where millions of Na/K ATPase creates a Na gradient of high ECF Na and low ICF Na
List the steps of the Transcellular Pathway:
- Na Gradient is established by the Na/K ATPases located on the Basolateral Membrane
- Na Gradient (low Na inside cell) causes Na/Glucose Symporter located on the Apical Membrane to move Na and Glucose into the cell
- Na moved into the cell via the symporter is moved out of the cell and the Peritubular/Interstitial Fluid by the Na/K ATPases of the Basolateral Membrane
- GLUT 2 membranes on the Basolateral Membrane pull Glucose down its concentration gradient into the Peritubular/Interstitial Fluid where it can be reabsorbed into the bloodstream
Why is the Na/K ATPase on the Basolateral Membrane?
It is the only place that an Na gradient can be established while still allowing Na to be reabsorbed back into the bloodstream
The MORE/LESS Glucose present in the Plasma, the MORE/LESS Glucose is filtered.
The MORE Glucose present in the Plasma, the MORE Glucose is filtered.
If Glucose continues to be filtered past the Tmax (Transport Max) of the Glucose Transporters, then…
Glucose will no longer be reabsorbed so as a result it will be excreted in the Urine
What is the Tmax for Glucose?
320 mg Glucose/min
What is Splay?
Term for Variability within the body and Heterogeneity of Nephrons that causes Glucose to show up in urine before the Tmax is reached
Juxtamedullary Nephrons are longer than Cortical Nephrons so they have more transporters
Proximal Tubule is lined with?
Microvilli to increase Surface Area for reabsorption
What are characteristics of Typical Transport Process of Organic Substances?
All reabsorbed in Proximal Tubule because their transporters are located only there
- Active (Secondary Transport)
- Can exhibit a Tmax well above normal ranges
- Specificity
- Inhibitable by drugs and disease
- NEUTRAL
Where in the Nephron is Na reabsorbed?
EVERYWHERE
- Proximal Tubule (65-75%)
- Loop of Henle (15-20%)
- Distal Tubule (maybe 5%)
- Excreted (5%)
What is the Formula for Filtered Load?
FL=GFR x Plasma Concentration
What is the Filtered Load for Na?
FL= 180 L/Day x 140 mEq/L
FL= 25,200 mEq/day
-since 95% is reabsorbed in Nephrons, 23,940 mEq/day are reabsorbed and 1,260 mEq/day are excreted
What transporter is used to reabsorb Bicarb in the Proximal Convoluted Tubule?
The Na/H Transporter which is the lone ANTIPORTER
What molecule always follows Na as it moves across a membrane?
- H2O
2. Cl which is dragged across the tight junctions (Paracellular Pathway)
Is Inulin made in the body?
NO, but Inulin is FREELY FILTERED because we do NOT have transporters for it
Why does Na concentration stay consistent all along the Proximal Tubule?
Because every time Na moves from Tubular Fluid to Peritubular/Interstitial Fluid, H2O follows
What is the difference between the Descending Loop of Henle and the Ascending Loop of Henle?
- Descending Loop of Henle is permeable to H2O and impermeable to solutes (NO REABSORPTION)
- Ascending Loop of Henle is permeable to solutes and impermeable to H2O (Site of Reabsorption)
What makes the Apical Membrane transporter in the Ascending Loop of Henle unique?
It moves 4 ions at the same time
1 Na down its gradient
1 K up its gradient
2 Cl down its gradient
What are the Apical Membrane transporters in the Distal Convoluted Tubule?
- Na/Cl Symporter
2. Aldosterone gated Na channel (reabsorption) and K channel (secretion)
How does Aldosterone affect Na and K concentration in the urine?
“Na Reabsorption and K secretion”
- Aldosterone is a steroid hormone that enters the Distal Tubule cells and increases the amount of Na/K ATPases on the Basolateral Membrane
- The increased concentration leads to a higher concentration gradient (even lower Na inside cell)
- Aldosterone binds to Aldosterone gated Na channels on the Apical Membrane
- Na floods into the cell and is immediately put into the Peritubular/Interstitial Fluid to be taken back up into the blood
- Aldosterone also binds to a K channel on the Apical membrane and K flows into the urine
Why are Diuretics useful?
Lower Blood Volume therefore lowering Blood Pressure
They block Na transporters/channels on the Apical Membrane from Na reabsorption
What is the definition of Osmolarity?
The Concentration of Solutes
What is the Osmolarity at the end of the Proximal Tubule and why?
300 mOsm/L because although Na reabsorption is occurring all through the Proximal Tubule, water follows it out as well leaving the concentration the same
What happens as you move down the Descending Loop of Henle?
The Descending Loop of Henle is permeable only to H2O, so H2O moves out and solutes stay inside causing the concentration to increase up to 1200 mOsm/L making it Hyperosmotic
What happens as you move down the Ascending Loop of Henle?
The Urine becomes more and more dilute due to the fact that the Ascending loop is only permeable to Solutes NOT H2O. Can go as low as 150 mOsm/L
The Descending Loop of Henle is called the CONCENTRATING or DILUTING segment
The Descending Loop of Henle is called the CONCENTRATING segment
The Ascending Loop of Henle is called the CONCENTRATING or DILUTING segment
The Ascending Loop of Henle is called the DILUTING segment
What is the difference in concentration between the start of the Distal Tubule and the end of the Distal Tubule?
NO DIFFERENCE
-It should be roughly 150 mOsm/L at both the beginning and end of the Distal Tubule
What is the effect of ADH on the Nephrons?
Anti Diuretic Hormone activates the Aquaporin Channels on the Collecting Duct of the Nephron
If ADH is present, it will CONCENTRATE URINE
Where is Potassium mostly stored?
INSIDE CELLS so K outside of cells (ECF) is pretty low
What is the effect of Insulin on K levels?
Brings K INTO cells
Decreases K in ECF
What is the Potassium transport direction in each part of the Nephron in a NORMAL or HIGH K Diet? Is there Net Reabsorption or Net Secretion of K?
- Proximal Tubule- Reabsorption
- Ascending Limb of Henle- Reabsorption
- Distal Tubule and Cortical Collecting Duct- Secretion
NET SECRETION of K
What is the Potassium transport direction in each part of the Nephron in a LOW K Diet? Is there Net Reabsorption or Net Secretion of K?
- Proximal Tubule- Reabsorption
- Ascending Limb of Henle- Reabsorption
- Distal Tubule and Cortical Collecting Duct- Reabsorption
NET REABSORPTION of K
By what mechanism is K reabsorbed in the Proximal Tubule?
Through the Tight Junctions (Paracellular Pathway) via something called “Bulk Flow”
What is Bulk Flow?
So much “stuff” is being reabsorbed in the Proximal Tubule that K gets swept away and reabsorbed along with the Cl through Tight Junctions
By what mechanism is K reabsorbed in the Ascending Limb of the Loop of Henle?
- Na/K ATPase of the Basolateral Membrane establishes a gradient of high K concentration inside of the cell
- Na/K/Cl Symporter brings 1 K into the cell against its concentration gradient
- K exits into the Peritubular/Interstitial Fluid along with Cl where it can be reabsorbed into the blood
What happens to K in the Proximal Tubule?
“Aldosterone Mediated” Transport may occur:
- Aldosterone (steroid hormone) increases the amount of Na/K ATPases present on the Basolateral Membrane
- Na concentration inside of the cell decreases AND K concentration inside of the cell increases
- Aldosterone binds to gated K channels causing them to open and K to flood out of the cell and into the Urine for excretion
How do you control K Secretion?
- Homeostatic Control (Negative Feedback)
2. Aldosterone (affected by K levels and Blood Pressure levels)
Aldosterone INCREASES or DECREASES Plasma K levels and INCREASES or DECREASES K that is Excreted in Urine
Aldosterone DECREASES Plasma K levels and INCREASES K that is Excreted in Urine
Talk about the effects of K intake on Aldosterone and Blood Volume:
- Increased K Intake
- Increases Plasma K Concentration
- Increases Aldosterone Secretion
- Increase in Plasma Aldosterone
- Aldosterone increases in Na/K ATPase on Basolateral Membrane in the Proximal Tubule and opens gated K channels on the Apical Membrane
- K Secretion into the Lumen increases which increases the amount of K that is excreted
- Aldosterone also opens Na gated channels causing an increase in Na reabsorption
- H2O follows the reabsorbed Na, SO Blood Volume is increased and so is Blood Pressure
- Lower K in the plasma causes negative feedback decreasing Aldosterone secreted in the blood
What does Ca do?
- Muscle Contraction
2. Important Secondary Messenger
What happens if blood Ca levels fall?
- Parathyroid Gland releases PTH
- Ca Resorption (breaking down of bones) is stimulated in bones to obtain Ca
- Increases Ca Reabsorption in Kidneys
- Kidneys activate Vitamin D to increase absorption of Ca in GI Tract
- Blood Ca levels rise to normal
SIDENOTE: PTH also drops Phosphate levels
What happens if blood Ca levels are high?
- Parathyroid Gland releases Calcitonin
- Ca deposition is stimulated in the bones
- Ca reabsorption in the Kidneys is reduced
- Kidney stops making Vitamin D so GI absorption is decreased
- Blood Ca levels are reduced to normal
Where is Ca reabsorbed?
- Proximal Tubule (60%)
- Loop of Henle (20%)
- Distal Tubule (10%)
Very small amount <5% excreted
Why is it bad for Ca and Phosphate to bind in the blood?
It makes the Ca inactive
How is Ca reabsorbed in the Ascending Limb of the Loop of Henle?
- Ca channels open up on the Apical Membrane and Ca floods into the cell
- Ca ATPase on the Basolateral Membrane transports Ca into the Peritubular/Interstitial Fluid where it can then be reabsorbed by the blood
PTH increases the number of Ca channels on the Apical Membrane and Ca ATPase transporters on the Basolateral Membrane
How is Ca reabsorbed in the Distal Tubule?
- Ca channels open up on the Apical Membrane and Ca floods into the cell
- Ca ATPase on the Basolateral Membrane transports Ca into the Peritubular/Interstitial Fluid where it can then be reabsorbed by the blood
PTH increases the number of Ca channels on the Apical Membrane and Ca ATPase transporters on the Basolateral Membrane
