Physiology 2 - Reabsorption & Secretion Flashcards
What are the 2 main methods of reabsorption and where does it mainly take place?
- Carrier mediated transport
- Glc/AA/Organic Acids/Sulphate and phosphate ions
- Carrier proteins enable larger molecules such as glucose to cross the membrane
- Glc/AA/Organic Acids/Sulphate and phosphate ions
- Active Transport
- Na+
Mainly occurs 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 plasma [Glc] is higher than this (not physiological) the excess is excreted in urine
- If plasma [glc} = 15 mmoles/l, 15 will be filtered and 10 reabsorbed and 5 excreted
- E.g. Glc transports have a Threshold of 10mmol/l. So if 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 and Glc Tm is above physiological levels so excretion should not occur
- 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
- Generates a concentration gradient which draws more Na out the tubule into the cells
How much Na ions are reabsorbed daily?
180 L/day (filtrate) x 142 mmole/l (plasma [conc] of Na+) = 25560 mmoles/day
- 99.5% is reabsorbed
Why will Na+ not be absorbed by all cells?
Not all cells have the same permeability
- Brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes in the body
- Partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels
- Facilitate the passive diffusion of Na+
What is a side effect of Na+ being reabsorped?
Creates an electrical gradient down which Cl- and other anions can be reabsorbed
What is a consequence of N and 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
- Creating outward conc. gradients allowing them to be reabsorped (E.g. K+/Ca2+/Urea/Glc)
Summarize the resorption of sodium and its effects?
- Na reabsorped by active transport usin Na/KATPase
- Na movement generates an electrical gradient that allows anions to be reabsorped
- Na/Anion movement generates an osmotic force that reabsorps H2O
- aH20 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?
- Amount of H2O removed and therefore Conc gradient
- 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
