Reabsorption and secretion Flashcards
In peritubular capillaries state if hydrostatic pressure and oncotic pressure are high or low. Why?
hydrostatic - low - overcoming resistance
oncotic - high - lost 20% fluid
If oncotic pressure is more than hydrostatic pressure what process occurs?
reabsorption
What is Tm?
maximum transport capacity of carrier due to saturation of carriers
What substances have a carrier mediated transport system?
glucose, amino acids, sulphate and phosphate, organic acids
If Tm is exceeded what happens?
excess enters the urine
What is the renal threshold?
plasma threshold at which saturation occurs
How much of plasma glucose is filtered and how much reabsorbed?
all of it is filtered
10mmoles/l
What is the renal plasma threshold for glucose?
10mmoles/l
Does the kidney regulate glucose?
no - insulin and the counter regulatory hormones regulate glucose
What is the significance of the Tm of glucose in terms of the normal plasma volume
normal plasma volume is 5 and Tm is 10
set high above non-diabetic values to ensure the valuable nutrient is reabsorbed
What is glycosuria a failure of?
insulin - not the kidney
What other substance is Tm set high above the normal plasma concentration?
amino acids
Give an example of substances the kidney does regulate by the tm mechanism. How?
phosphate and sulphate ions
Tm set close to plasma volume so any excess is excreted
Apart from tm control by the kidneys what else can regulate phosphate?
PTH
Where are sodium ions most abundant?
ECF
What percentage of sodium is reabsorbed and what percent of this occurs in the proximal tubule?
99.5
65-75
How is sodium reabsorbed?
active transport which establishes a gradient for sodium across the tubule wall
Explain briefly the reabsorption of sodium
Na enters tubule cell from lumen by following its concentration gradient
Pumped out due to Na+K+ATPase on basolateral surface
What drives the whole process of sodium reabsorption?Why?
sodium pumps
pumps sodium out of the cells to increase gradient for sodium to move passively into the cells
What is permeability a property of? ion or membrane?
membrane
How does sodium reabsorption occur due to permeability?
brush border of proximal tubule has higher permeability than most membranes in the body
Why does the brush border of proximal tubule have high permeability for sodium? (2)
large surface area - microvilli
large number of sodium ion channels facilitate passive diffusion
Why is the reabsorption of sodium key to the reabsorption of chlorine (among others)
establishes and maintains concentration gradient
Why does water follow the sodium and chloride during reabsorption?
due to osmotic force
What does water reabsorption do to the remaining substances in the tubule?
concentrates them due to loss of water
creates outgoing concentration gradient
2 factors which reabsorption rate on non-actively reabsorbed (urea, calcium) depends on
amount of water removed from lumen - extent of concentration gradient
permeability of membrane to the solute
Describe urea reabsorption
despite the concentration gradient being established the membrane is only moderately permeable so therefore only 50% is reabsorbed
Despite the concentration gradient established for reabsorption what substances are not absorbed due to the impermeable membrane?
inulin and mannitol
Does high or low sodium in the tubule facilitate glucose symport?
high
How does glucose go against its concentration gradient to move from tubule lumen to proximal tubule cell?
sodium moving down its gradient uses SGLT protein to pull glucose into cell
What protein does glucose use to exit the basolateral membrane of the proximal tubule cell?
GLUT
What direction does secretion go?
peritubular capillaries –> tubule lumen
Where is secretion NOT found?
Loop of henle
Why is secretion important?
2nd route into tubule lumen
protein bound - not filtered and harmful substances - fast excretion
Are Tm carrier mechanism specific?
no - drugs can use these which are originally made for endogenous substances
Where are the drugs secreted?
early at proximal tubule
Where is potassium found? in cells or in ECF?
in cells
Normal potassium concentration
4 mmoles/l
What happens in hyperkalaemia?
decrease resting membrane potential
VF and death
What happens in hypokalaemia?
increase resting membrane potential
arrhythmias and death
Where is potassium primarily reabsorbed?
proximal tubule
If more potassium is ingested what happens?
more is excreted - more secretion
What hormone also controls potassium?
aldosterone from adrenal cortex
Does an increase or decrease in potassium increase aldosterone? How?
increase
ECF bathing aldosterone secreting cells stimulate its release and increase secretion
What does the aldosterone do to the kidneys?
stimulate renal tubule potassium secretion
What does aldosterone do to sodium?
stimulate reabsorption at distal tubule
