2,232
edits
Changes
Jump to navigation
Jump to search
Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology (view source)
Revision as of 15:16, 4 September 2008
, 15:16, 4 September 2008→[[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+ and the Reabsorption of HCO3- in the Proximal Tubule|H<sup>+</sup> and HCO<sub>3</sub><sup>-</sup>]]
The secretion of these compounds occurs '''only in the proximal tubules'''. These molecules are mainly bound to plasma proteins with a small amount free in an active ionised form. It is only the free ions which are able to be transported. As the ionised molecules are transported into the blood more molecules are released from the plasma proteins to take their place. These can then be secreted etc etc. This allows a large amount of the substance to be secreted at one time. The mechanisms are not very selective and so many different substances are secreted at the same time. Secretion mechanisms are responsible for the secretion of drugs, hormones and things like food additives. Many unwanted or toxic organic molecules which enter the body are unionized. They therefore cannot be secreted so it falls to the liver to alter them into ionized forms to allow them to be disposed of.
The secretion of these compounds occurs '''only in the proximal tubules'''. These molecules are mainly bound to plasma proteins with a small amount free in an active ionised form. It is only the free ions which are able to be transported. As the ionised molecules are transported into the blood more molecules are released from the plasma proteins to take their place. These can then be secreted etc etc. This allows a large amount of the substance to be secreted at one time. The mechanisms are not very selective and so many different substances are secreted at the same time. Secretion mechanisms are responsible for the secretion of drugs, hormones and things like food additives. Many unwanted or toxic organic molecules which enter the body are unionized. They therefore cannot be secreted so it falls to the liver to alter them into ionized forms to allow them to be disposed of.
===[[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+ and the Reabsorption of HCO3- in the Proximal Tubule|H<sup>+</sup> and HCO<sub>3</sub><sup>-</sup>]]===
===THe Secretion of H<sup>+</sup> and the Reabsorption of HCO<sub>3</sub><sup>-</sup>===
[[Image:acidbaseflowkidap1.jpg|right|thumb|350px|<small><center>Secretion of H<sup>+</sup> and reabsorption of HCO<sub>3</sub><sup>+</sup> in the Proximal Tubule</center></small>]]
=====Secretion of H<sup>+</sup>=====
* The secretion of H<sup>+</sup> in this section of the nephron is mainly a result of the Na<sup>+</sup>/H<sup>+</sup> exchanger
** This is an antiporter in the apical membrane
** Energy for this process is provided by the Na/K ATPase in the basolateral membrane
** Therefore it is [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]]
** The ATPase pumps sodium out of the cell into the interstitium
** This maintains a low intracellular Na which creates a gradient for the absorption of sodium by the Na<sup>+</sup>/H<sup>+</sup> antiporter
** This allows it to drive H against its concentration gradient
** Maintains a negative intracellular potential
** It is essential that HCO<sub>3</sub><sup>-</sup> is removed from the cells by the co-transporter with sodium to ensure efficient H<sup>+</sup> secretion.
=====Reabsorption of HCO<sub>3</sub><sup>-</sup>=====
* Very efficient reabsorption mechanism
* 90% in first 1-2mm of tubule
* Lots of luminal carbonic anhydrase
* Stops the accumulation of H<sub>2</sub>CO<sub>3</sub> in the lumen
* Keeps H<sup>+</sup> concentration low - helps antiporter
* Roles of '''carbonic anhydrase'''
** In the cell forms HCO<sub>3</sub><sup>-</sup> from OH and CO<sub>2</sub>
** In the tubule it works in reverse forming CO<sub>2</sub> and H<sub>2</sub>O from the intermediate H<sub>2</sub>CO<sub>3</sub> which forms from HCO<sub>3</sub><sup>-</sup> and H<sup>+</sup>
** Allows continuous H<sup>+</sup> secretion and HCO<sub>3</sub><sup>-</sup> reabsorption