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Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology (view source)
Revision as of 16:34, 3 September 2008
, 16:34, 3 September 2008→[[Glucose Homeostasis - Physiology| Glucose]]
===[[Glucose Homeostasis - Physiology| Glucose]]===
===[[Glucose Homeostasis - Physiology| Glucose]]===
==Filtration and Reabsorption==
Glucose is a small molecule and so it is filtered in the same concentrations as are found in plasma which is approximately 5mmol/l. Reabsorption of glucose can only occur in the proximal tubule and occurs regardless of the concentration gradient as it is completed via [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]]. It is reabsorbed using a [[Transport Proteins - Physiology#Co-Transporters|co-transporter]] with [[Sodium Homeostasis - Physiology|sodium]]. The realisation of the [[Active Transport - Physiology#Development of Potential Energy|potential energy]] produced from sodium moving from an area of high concentration to an area of low concentration is enough energy to transport glucose across the membrane into the epithelial cells. The energy technically comes from the utilisation of ATP by the [http://w01.rvcwiki.wf.ulcc.ac.uk/images/e/ef/NaKATPaseA%2BP.jpg sodium/potassium] [[Transport Proteins - Physiology#ATPases|ATPase]] which keeps sodium concentrations within the epithelial cells low this giving the sodium in the lumen a high potential energy.
Glucose is then passively transported out of the epithelial cells across the basolateral membrane.
It is normal that reabsorption is fully completed in the first half of this tubule however as the plasma concentration of glucose increases more of the tubule will be utilised to reabsorb the glucose. Concentrations of double the normal levels are required for glucose to appear in the urine and the concentration where glucose can first be detected is termed the '''renal threshold for glucose'''.
===T Max and Splay===
At approximately three times the normal levels the kidneys begin to approach the maximum possible reabsorption levels this is termed '''T<sub>max</sub>''' and is where the entire length of all the proximal tubules of all the nephrons of the kidney are working at maximum capacity.
Prior to this point the amount of glucose excreted does not increase linearly with the amount filtered. This is because some nephrons have longer proximal tubules than others so although some may be overcome and therefore allowing glucose to be excreted others are managing to fully reabsorb the glucose along their length. This results in what is termed '''splay'''. However after '''T<sub>max</sub>''' all the nephrons are at and above full capacity and therefore after this point any increase in filtered glucose is demonstrated linearly with that excreted.
The '''T<sub>max</sub>''' for a 20Kg dog is approximately 170mg per minute or 220g per 24 hour period. The normal amount of glucose filtered in 24 hours should be 60g.
===The Kidneys Role In Glucose Regulation===
The kidneys do not really regulate plasma glucose but their main aim is to preserve this valuable nutrient. It is only at very high levels which the kidneys play a role in helping to prevent any further rises in glucose via excretion.
===[[Protein Recovery - Physiology| Protein]]===
===[[Protein Recovery - Physiology| Protein]]===