Tubular Reabsorption and Secretion Flashcards
What are the 2 options for substances entering the nephron?
- Reabsorption- MAJORITY of the time (~178 L/day)
2. Secreted
Does Filtration have regulation? Reabsorption?
- Filtration= NO
2. Reabsorption= Yes
What 3 parts of the nephron function to do reabsorption? How much?
- Proximal tubule-60 %
- Loop of Henle - a small amount
- Distal tubule- everything else- also where we have most of our regulation of reabsorption
What 11 substances are reabsorbed by the proximal tubule?
- Na+
- Cl-
- K+
- Ca2+
- H2O
- HCO3-
- Glucose
- Amino Acids
- Vitamins
- Urea
- Choline
What 6 things are reabsorbed from the Loop of Henle? What does this contribute to?
a.
- Na+
- Cl-
- K+
- Mg 2+
- Ca2+
- NH4+
b. Helps establish the concentration gradient
Why is reabsorption in the distal tubule so significant? What 6 substances are regulated?
- This is where we have our highly regulated substances
- a. H2O
b. Ca2+
c. Na+
d. Cl-
e. H+
f. K+
How many ways can reabsorption happen?Names?
2 ways
- Transcellular pathway- 2ary active transport) goes through the cell and also crosses 2 membranes (Apical + Basolateral membranes)
- Paracellular pathway- substances can go between tight junctions without going through cells.
Process/ Passageway for reabsoption.
- Substance must cross 2 membranes and one cell.
2. They require transporters and a long process
What is facilitated diffusion?
- Use of an integrated membrane protein
- No energy is used- diffusion goes from high concentration to low concentration.
- Protein can be a channel or a carrier
What does secondary active transport rely on?
- A sodium gradient
2. Active transporter that uses ATP to being a substance out of a cell.
What 2 types of transporters do Na use?
- Cotransporters
2. Countertransporters
Explain the process of secondary active transport.
Na will always move down its concentration gradient by going from ECF to ICT. The Na will help another substance move against its concentration gradient either as a co transporter or a countertransporter
What transporters are located on the apical membrane? Its role?
- Na/Glucuse transporter (co transporter)
2. Allows Na and G from the tubular fluid to enter the cell…
Explain the process to reabsorb Na from tubular lumen to the bloodstream.
- First, we have secondary active transport - Na/K ATPase at the Basolateral membrane. This will pump Na out of the cell– into the peritubular fluid, and K will enter the cell. 2. This will create a concentration gradient
- Then we go to the apical membrane where we have Na/Glucose co transporters. These will allow Na to be absorbed from the tubular lumen and into the cell… glucose is pulled along with it.
- Once Na is in the cell, it goes to the basolateral membrane– gets pumped out into the peritubular fluid and eventually through the capillary endothelial cell to enter the blood.
- Glucose will leave the cell also on the basolateral side via GLUT2 transporters– and will pull glucose DOWN its concentration gradient into the ISF and finally can enter the blood stream.
Why can’t the NA/K ATPase pump be on the apical membrane vs only on the basolateral membrane?
Na from the tubular fluid will be pumped in and back out of the apical end–
What is important to note about transporters and electrolytes
Where we place our transporters can determine where out electrolytes end up.
What is the relationship for glucose pertaining to amount of glucose and filtration ?
The more glucose, the more filtration we have.
How much glucose is usually reabsorbed when filtered?
100 %
What is Tmax?
The highest rate at which our transporters can transport molecules - they reach saturation at this point.
What happens to whatever is filtered and not reabsorbed when transporters reach Tmax?
It gets excreted in the urine.
When do we see glucose in urine?
- Diabetes
People with high glucose levels
They filter all the glucose they can and when they reach as much glucose as possible, the portions not reabsorbed are EXCRETED.
What is the Tmax of glucose?
320 mg/min
For glucose, what happens between Filtration vs. Tmax when:
a. Filtration is BELOW Tmax
b. Filtration = Tmax
c. Filtration is ABOVE Tmax
a. Reabsorption, no glucose excretion
b. Reabsorption, no glucose excretion
c. Reabsorption, glucose excretion
What is Splay?
Refers to the idea that there is variability in the body(heterogeneity of our nephrons
We have 2 types
1. Cortical- shorter nephrons- shorter proximal tubules- less transporters- hit Tmax earlier- will see glucose in the urine earlier than 320 mg/min
2. Juxtamedullary- longer nephrons - longer proximal tubules- more transporters-
What kinds of substances do we secrete?
OPPOSITE DIRECTION of Reabsorption- essentially the same process
examples…
Creatinine
Drugs
Hydrogen
Metabolic waster products (urea, uric acid,)
Bioactive products (hormones)
What helps reabsorption in the proximal and distal tubules of a nephron?How?
- microvilli– important for increased surface area for reabsorption
What are 8 organic substances we reabsorb?
ALL ARE FREELY FILTERED
- glucose
- amino acids
- acetate
- krebs cycle intermediates
- water soluble vitamins
- lactate
- acetoacetate
- many others
What are the 4 main proceseses for transporters?
- Secondary active transport
- have a Tmax- usually way over what is normally filtered
- SPECIFICITY- only substances with transporters can be freely filtered (stereospecifity)
- can be INHIBITED by drugs and disease
How much is reabsorbed at the proximal tubule?
100 %– anything else will be excreted because there are no more transporters outside the proximal tubule
What is a filtered load?
FL= GFR x Plasma concentration