Physiology 2 - Reabsorption & Secretion Flashcards
What are the 2 main methods of reabsorption and where does it mainly take place?
Carrier mediated transport for things like Glc/AA/Organic Acids/Sulphate and phosphate
Active Transport for Na
Mainly in the Proximal Tubule
Explain the concept of a Renal Plasma Threshold?
Carrier MEdiated Transport systems have a Maximum transport capacity (Tm) before they become saturated
This is called the Renal plasma theshold
E.g. Glc transports have a Threshold of 10mmol/l. So if your plasma Glc is higher than this (not physiological) the excess is excreted in urine
Is Tm above or below the physiological concentration?
It depends on the substance
AA & Glc Tm is above physiological levels so excretion should not occur
But Phosphate and Sulphate ions have Tm below physiological [plasma] to ensure excretion
Explain how soidum ions are reabsorped?
Active Transport
Na/KATPase pumps Na from the tubular cells into the interstitial fluid.
This generates a concentration gradient which draws more Na out the tubule into the cells
What is a side effect of Na+ being reabsorped?
It creates an electrical gradient down which Cl- and other anions can be reabsorped
What is a consequence of Na& Anion reabsorption
Movement of so many ions creates an osmotic force that causes water to be reabsorped
What happens after the water is reabsorped?
Loss of water causes the remaining substances in the tubule to be concentrated. Creatin outward conc. gradients allowing them to be reabsorped
(E.g. K+/Ca2+/Urea/Glc)
Summarize the resorption of sodium and its effects?
1) Na reabsorped by active transport usin Na/KATPase
2) Na movement generates an electrical gradient that allows anions to be reabsorped
3) Na/Anion movement generates an osmotic force that reabsorps H2O
4) H20 movement concentrates the remaining substances, creating conc. gradients that allow thier resorption
What effects rate of absorption of the remaining solutes after H2O is reabsorped?
1) Amount of H2O removed and therefore Conc gradient
2) Permeability of membrane to the particular solute
how are active transport of sodium and carrier mediated transport of substances like Glc/AA linked?
Na enters the tubular cells from the tubule through symporters with substances such as glucose (SGLT).
Hence Na+ deficiency in the tubule will affect glucose etc resorption
What is Tubular secretion?
Transport of substances from the peritubular capillaries into the tubule lumen
Opposite of reabsorption
What part of the nephron lacks secretion?
The Loop of henle (only reabsorption occurs there)
What substances use secretion most and why?
Protein bound substances which need to be actively secreted as they’re too big for filtration
Harmful substances that have to be prevented from reabsorping
By what mechanism are substances secreted?
Carrier-mediated secretory mechanisms
Like reabsorptive ones they have a Tm
Drugs are also secreted by the nephron, kinda odd we have mechanisms to remove substances we should never have in our system, why is this?
In fact our Secretory Carrier Mechanisms are just very non-specific so for instance the organic acid mechanism is also capable of secreting aspirin
