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Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology (view source)
Revision as of 11:33, 5 September 2008
, 11:33, 5 September 2008→Primary Active Secretion - Organic Acids and Bases
===Primary Active Secretion - Organic Acids and Bases===
===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.
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 out of 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===
===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.
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.