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Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology (view source)
Revision as of 15:18, 4 September 2008
, 15:18, 4 September 2008→Ions and Compounds
50% of filtered urea is reabsorbed in the proximal tubule. However the concentration of urea actually increases thanks to the reabsorption of 70% of the filtered water in the same portion of the nephron. Urea is not able to be reabsorbed from this point until it reaches the lower portion of the collecting duct therefore its concentration further increases with the reabsorption of water.
50% of filtered urea is reabsorbed in the proximal tubule. However the concentration of urea actually increases thanks to the reabsorption of 70% of the filtered water in the same portion of the nephron. Urea is not able to be reabsorbed from this point until it reaches the lower portion of the collecting duct therefore its concentration further increases with the reabsorption of water.
===Glucose===
===Glucose===
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.
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.
===The Secretion of H<sup>+</sup> and the Reabsorption of HCO<sub>3</sub><sup>-</sup>===
===The Secretion of H<sup>+</sup> and the Reabsorption of HCO<sub>3</sub><sup>-</sup>===
** Maintains a negative intracellular potential
** 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.
** 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>=====
=====Reabsorption of HCO<sub>3</sub><sup>-</sup>=====
** 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>
** 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
** Allows continuous H<sup>+</sup> secretion and HCO<sub>3</sub><sup>-</sup> reabsorption
===Protein===
Peptide hormones and small amounts of albumin make it through the [[Glomerulus and Bowmans Capsule - Anatomy & Physiology#Function of Renal Corpuscle|glomerular filtration barrier]] and these need to be reabsorbed. The reabsorption occurs via '''endocytosis''' in the [[Proximal Tubule - Anatomy & Physiology| proximal tubules]]. They are then broken down to amino acids in the epithelial cell cytoplasm and move via facilitated diffusion into the interstial fluid. The reabsorption of protein is usually complete though it is normal to detect small quantities of protein in the urine of some mammals e.g. the dog
===Primary Active Secretion - Organic Acids and Bases===
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.
===Calcium===
Half the plasma calcium is bound to proteins so it is only the ionised form which is available for filtration. Reabsorption of calcium occurs in the proximal tubule however the regulation of how much is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts.