Reabsorption and Secretion Flashcards
What occurs in glomerular capillaries?
Filtration
Why is the peritubular capillary hydrostatic pressure (Ppc) low?
Because the hydrostatic pressure is lost trying to overcome the frictional forces of the long and thin efferent arterioles
Why is oncotic pressure higher in the peritubular capillaries?
Due to loss of 20% of plasma which concentrates the plasma proteins
If oncotic pressure is higher than the hydrostatic pressure in the peritubular capillaries, what process does this favours?
Reabsorption
What is reabsorbed within the tubule after filtration?
99% of H2O
100% of glucose
99.5% of Na+
50% of urea
Where is most reabsorption taking place?
In the proximal convoluted tubule
What is a mechanism of reabsorption?
Carrier mediated transport systems
Carriers have a max transport capacity Tm which is due to saturation of the carriers
If Tm is exceeded then excess substrate is released into the urine
What sort of things are reabsorbed by carrier proteins?
Glucose Amino Acids Organic acids Sulphate Phosphate
Explain briefly how carrier mediated transport systems work.
Molecule to be transported enters the carrier system and binds to a binding site.
Once bound it closes the carrier on the entry side and opens it on the exit side
Molecule is released and reaborped absorbed and carrier returns back to original shape
What is the renal threshold?
The plasma concentration at which saturation of carriers occurs
In man for plasma glucose up to BLANK - all will be reabsorbed, but beyond this level of plasma [glucose], it appears in the BLANK
10 mmoles/l
Urine
So this makes 10 mmoles/l the…
Renal plasma threshold for glucose.
So if plasma [glucose] is 15 - what will be reabsorbed and what will be excreted?
10 reabosped and 5 excreted
Does the kidney regulate blood glucose?
No - done by endocrine system
What is normal conc. of glucose ?
5 mmoles/l
Why is the renal plasma threshold for glucose set way higher than the actual normal plasma glucose levels?
Makes sure that all of the glucose is reabsorbed and not wasted as its too valuable
The appearance of glucose in the urine of a diabetic patient is due to the failure of…?
Insulin not the kidneys
What is the Tm like for amino acids? What regulates AA levels
Also set so high that urinary excretion does not occur,
Regulated by insulin and counter-regulatory hormones.
What DOES the kidney actually regulate plasma levels of?
Phosphate and sulphate ions
Tm is set at a level that causes normal plasma concentrations to cause saturation and excretion
This maintains normal levels at a tight control
What ions are most abundant in the ECF?
Na+
What % of Na ions is reabsorbed?
99.5%
What part of the kidney reabsobes the most Na ions and where does this happen?
65-75% in the proximal tubule
How is Na reabsorbed?
Not by Tm mechanism but by active transport pumps establishing a gradient across the tubule wall
Explain how Na is pumped back into the body.
Na flows from the tubule lumen into the proximal tubule cells down its electrochemical gradient
Once in the proximal tubule cell - a Na/K-ATPase on the basolateral side of the cell pumps it out into the interstitial fluid
How does Na get into the proximal tubule cell if cell membranes are NOT permeable to Na?
The brush border of the proximal tubule cells has a higher permeability to Na+ ions
This is partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels,
Why is Na reabsorption so important?
It is key to the reabsorption of other components of the filtrate
How do negative ions like Cl- get reabsorbed?
Diffuse passively across the proximal tubular membrane down the electrical gradient created by the active transport of Na+.
What does the reabsorption of these ions do water?
Creates an osmotic force which draws H2O out of the tubules
When water is drawn out of the tubules, how does this help reabsorption of further ions and solutes?
Less water = more concentrated ions which creates outgoing conc. gradients
So what does the rate of reabsorption of the non-actively reabsorbed solutes depend on?
- Amount of H2O removed determining the conc. gradents
2. Permeability of the membrane to a particular solute
Why is only 50% or urea reabsorbed?
Tubule membrane is only moderately permeable to it
Name some substances that cannot pass through the tubule at all.
Insulin
Mannitol
Recap - what one ion establishes the gradients for water and all other solutes to follow?
Na
What would a decrease in blood flow do to renal function?
Would decrease active transport which would disrupt all renal function not just Na ions
Why is Na active transport important for maintaining carrier proteins?
Substances such as glucose, amino acids etc, share the same carrier molecule as Na+ (symport).
So high Na in tubules helps transport of glucose and low inhibits
What does secretion do?
Transports substances from the peritubular capillaries after filtration - providing a 2nd route into the tubule
Why do we need secretion if we have filtration at the glomerulus?
Filtration is very restrictive so secretion is important for protein bound substances
Also for potentially harmful substances helping remove them faster
What is the secretion mechanism?
Non-specific Tm limited carrier mediated secretion
What sort of things are secreted?
Large number of endogenous substances but also exogenous like drugs
What does it mean that these carrier mechanisms are not specific?
Organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin etc
Similarly organic base mechanisms can be used for choline, creatinine or morphine and atropine
What is the major cation in the cells of the body?
K+
Normal ECF conc of K+?
4 mmoles/l
When does hyperkalaemia occur?
Over 5.5
When does hypokalaemia occur?
Under 3.5
What does hyperK cause?
Reduced resting membrane potential of excitable cells
= ventricular fibrillation and death
What does hypoK cause?
Increased RMP - hyperpolarizing muscle, and cardiac cells leading to arrhythmias and eventually death
What happens to K+ as it passes through the renal system?
Filtered at glomerulus
Reabsorbed at proximal tubule
Re-secreted at the distal tubule as needed
Changes in total excretion depends on how much is secreted back into the distal tubule = an increase in renal tubule cell K+ conc. due to increase ingestion leads to increased secretion and excretion
What else regulates K+ secretion? How?
Aldosterone
Increase in K+ conc. in ECF stimulates aldosterone release which circulates to kidneys causing stimulation of renal tubule to secrete K+
What effect does aldosterone have on Na?
stimulates Na+ reabsorption at the distal tubule
How are H+ ions secreted?
Actively from the tubule cells, not the peritubular capillaries, into the lumen
