Changes

Although it is essential that the kidneys reabsorb vital ions and compounds back from the filtrate in order to maintain there levels within the body, it is also important that the kidneys are able to further secrete ions and compounds. These compounds may not be able to be filtered, such as those bound to protein, or may on the whole need to be reabsorbed but tightly regulated so some secretion takes place such as [[Potassium Homeostasis - Physiology#Absorption/Secretion Mechanisms Along The Nephron|potassium]] and [[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+|hydrogen]].  Secretion can take place via [[Active Transport - Physiology#Primary Active Transport|primary]] or [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]].

Although it is essential that the kidneys reabsorb vital ions and compounds back from the filtrate in order to maintain there levels within the body, it is also important that the kidneys are able to further secrete ions and compounds. These compounds may not be able to be filtered, such as those bound to protein, or may on the whole need to be reabsorbed but tightly regulated so some secretion takes place such as [[Potassium Homeostasis - Physiology#Absorption/Secretion Mechanisms Along The Nephron|potassium]] and [[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+|hydrogen]].  Secretion can take place via [[Active Transport - Physiology#Primary Active Transport|primary]] or [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]].

==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 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⁺ and HCO₃^-]]===

===[[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+ and the Reabsorption of HCO3- in the Proximal Tubule|H⁺ and HCO₃^-]]===