Reabsorption and Secretion Flashcards
Which molecules are reabsorbed via passive carrier mediated transport?
- Glucose
- Amino Acids
- organic acids
- Sulphate
- phosphate ions
What determines the maximum amount of substrate that can be reabsorbed during carrier mediated reabsorption?
Tm: maximum transport capacity of carriers, due to saturation of all carriers
What occurs if Tm (maximum transport capacity) is met during carrier mediated reabsorption?
- The excess substrate enters the urine
What is the renal threshold of the kidneys?
The plasma [substrate] at which Tm (max. transport capacity) occurs in carrier mediated reabsorption
The renal threshold for glucose is 10mmol/L. What will happen in a person with a plasma glucose of 15mmol/L during renal filtration?
- Since glucose is freely filtered, plasma [glucose] will equal filtrate [glucose], therefore filtrate glucose 15mmol/L
- Beyond the renal threshold the excess is excreted so: 10mmol/L of filtrate [glucose] will be reabsorbed, and 5mmol/L will be excreted in urine
Why is the renal threshold for glucose and amino acids set significantly higher than the normal physiological concentration?
- To prevent the unnecessary excretion of valuable nutrients
- The concentration of glucose and AA’s is controlled by insulin and other counter regulatory hormones
What are some molecules that are regulated by their renal threshold?
- Sulphate and Phosphate ions
- The normal plasma concentration of these ions is close to their Tm
Where does most Na reabsorption occur?
The proximal tubule (65-75%)
Via which mechanism is Na reabsorbed? Why?
- Active transport
- To establish a gradient of Na across the tubule wall
How does the Na gradient across the tubule wall encourage reabsorption of Na?
- The ATP/Na pump is on the basolateral surface of the tubule epithelium, pumps Na to interstitial fluid creating low epithelium [Na]
- Concentration gradient then drives Na from [high] in the tubule lumen across the luminal membrane into the tubule epithelium
How does Na move passively across the luminal wall into the epithelium, as it is not permeable at cell membranes?
- because the luminal brush border has higher Na permeability due to the microvilli and the large amount of Na ion channels
Which molecules does Na reabsorption facilitate the reabsorption of? how?
- Cl ions and water
- negative Cl follows the electrical gradient established by Na
- the movement of Na & Cl out of the lumen creates an oncotic force that draws water out of the tubules
How does the reabsorption of Na lead to the reabsorption of other substrates in addition to Cl and water?
Water following Na and Cl out of the lumen causes fluid concentration of other substrates in the tubules to increase - creating outgoing concentration gradients
What is the rate of reabsorption for non-actively reabsorbed substrates determined by? Examples of these substrates?
Determined by:
- Amount of water that has been reabsorbed (determines substrate concentration)
- Permeability of the membrane
- K, urea, Ca
Why is only about 50% of urea reabsorbed despite a concentration gradient being established? Why is there no reabsorption of substances like inulin and mannitol?
- Urea: because the lumen membrane is only partially soluble towards urea, resulting in 50% excretion
- Inulin / mannitol: lumen membrane is not permeable to these
How does the active transport of Na also mediate the reabsorption of carrier transported molecules such as glucose, AA’s, etc.?
These molecules are often symported across the luminal membrane along with Na by the same carriers
- Therefore if the gradient bringing Na into cells that is created by active transport is large, there will be more of these other substrates reabsorbed with Na
What is the effect of high [Na] in the tubule on the reabsorption of other substrates such as glucose?
- High tubule [Na] facilitates transport across the membranes
- therefore high [Na] = high reabsorption
What is the symporter that moves glucose and Na across the luminal membrane? What protein facilitates glucose movement across the basolateral membrane?
SGLT - sodium-dependant glucose transporter
GLUT
Do the molecules that get excreted only enter the tubule via filtration at the corpuscle?
- No, secretory mechanisms also transport substances from the peritubular capillaries into the tubule lumen, providing a second route into the tubule
What substances need to enter the tubule via secretory mechanisms, as filtration is very restricted?
- Protein bound substrates
How is much of the secretion from peritubular capillaries managed? What does this mean for the rate of excretion?
- Via carrier mediated transport
- Means there is a Tm for secretion as well as reabsorption
What are some molecules that can be secreted through the same carriers due to the relative non-specific nature of secretory carrier proteins?
- The organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH
- The organic base mechanism for choline, creatinine etc, can be used for morphine and atropine
(seems to transport based on charge not shape??)
Where does most secretion occur?
Proximal tubule
What is the normal blood concentration of K? What concentration constitutes hypokalaemia / hyperkalaemia?
Normal [K]: 4mmol/L
Hyperkalaemia: >5.5mmol/L
Hypokalaemia: <3.5mmol/L
What are some consequences of hyperkalaemia? Of hypokalaemia? Why does K have this regulatory role?
- Hyper: decreased resting membrane potential, depolarization of cells: eventually VF
- Hypo: increases resting membrane potential, hyperpolarizes cells: cardiac arrhythmias
- Because K is the major cation in cells
how is plasma [K] mediated by the kidneys?
- Reabsorption of filtered K occurs mostly in the proximal tubule
- k excretion is determined by K secretion in the distal tubule, increases in tubule epithelium [K] due to ingestion stimulate K secretion. Decreases in epithelial [K] cause decreased secretion
Other than the renal secretion / reabsorption loop, what regulates K plasma concentration? How?
Aldosterone
- Increases in plasma K stimulate the release of aldosterone from the adrenal cortex, causes K to be excreted and Na to be reabsorbed
