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Essential Ion and Compound Balance and Homeostasis - Anatomy & Physiology (view source)

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The diet of a herbivore contains little sodium and as such an important aspect of their sodium homeostasis is the physiological salt appetite where the animal actually craves salt if it is deficient.  This is especially apparent in sheep.  By contrast carnivores have a very poorly developed physiological salt appetite probably because their salt intake tends to outstrip their requirement.
 
The diet of a herbivore contains little sodium and as such an important aspect of their sodium homeostasis is the physiological salt appetite where the animal actually craves salt if it is deficient.  This is especially apparent in sheep.  By contrast carnivores have a very poorly developed physiological salt appetite probably because their salt intake tends to outstrip their requirement.
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==Potassium==
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===Importance of Regulation===
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====Decreased Extracellular Potassium====
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If the concentration of potassium in the ECF is reduced then the plasma membranes hyperpolarize resulting in decreased firing of action potentials.  This causes skeletal muscle weakness and cardiac abnormalities.
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====Increased Extracellular Potassium====
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In this state the membrane is depolarised and is inappropriately triggered by action potentials.  This can make the membrane insensitive to further stimulation causing cardiac abnormalities.
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===Sources===
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* Potassium is absorbed via passive diffusion from the [[Small Intestine - Anatomy & Physiology|small intestine]]
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* Also via active transport from the [[Colon - Anatomy & Physiology|colon]]
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** Affected by [[Aldosterone]]
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* Highly efficient
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* It's also recovered from cellular breakdown
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** Haemolysis
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** Tissue damage
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===Methods of Control===
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The K<sup>+</sup> in the ECF only represents a very small amount of the total K<sup>+</sup> in the body however its concentration is maintained within very strict parameters.  The homeostasis of K<sup>+</sup> is managed by three routes:
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=====Cellular translocation=====
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*Potassium is moved either into or out of the cells
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*Acute response
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* Main method of control
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=====Renal excretion=====
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* 90% of the chronic response
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* Takes 4-6 hours to respond
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* Allows fine control
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* Influenced by [[Aldosterone#Potassium|Aldosterone]]
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=====GI excretion=====
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** 10% of the chronic response
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** Also influenced by [[Aldosterone#Potassium|Aldosterone]]
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** Most important in renal failure
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===Cellular Translocation===
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* Vital for rapid control of potassium loads
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* Helps control plasma concentration
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* Moves potassium into the cell
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* Stores potassium in skeletal muscle and liver
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* Balances ECF and ICF
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* Controlled by insulin and beta2 adrenoreceptors
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** Increases the activity of Na<sup>+</sup> / K<sup>+</sup> ATPases causing sodium efflux and potassium influx
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===Renal Control===
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* Potassium ions are reabsorbed and secreted at different points along the nephron
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* Active reabsorption of potassium occurs along the [[Proximal Tubule - Anatomy & Physiology| proximal tubule]] (70%) and along the ascending limb of the [[Loop Of Henle - Anatomy & Physiology| Loop of Henle]] (10-20%)
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* This results in there only being 10% of the original amount left in the distal tubule
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* However net reabsorption / secretion of potassium occurs in the distal tubule and first part of collecting duct
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** Depends on bodies need
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* Under the influence of [[Aldosterone]]
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* This is where the amount of potassium excreted is determined
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* Reabsorption occurs in the final part of the collecting duct
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===Potassium and Aldosterone===
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* [[Aldosterone]] is the most important regulator of potassium
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* It causes increased secretion of potassium
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* Increased potassium directly stimulates [[Aldosterone]] secretion
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* Increases the activity and number of Na<sup>+</sup> / K<sup>+</sup> ATPase in basolateral membranes of the principal cells in the collecting duct and distal tubule
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* Potassium moves into the cells and is then excreted down an electro-chemical gradient
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===Factors Influencing Potassium Excretion===
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====Sodium====
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* High sodium = increased potassium excretion
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** More sodium into cells
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** Increased Na<sup>+</sup> / K<sup>+</sup> ATPase
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** Pumps sodium into peritubular renal interstitium
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** The resulting increased cellular uptake of potassium results in it moving down the electrochemical gradient into the nephron
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====Potassium====
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* High potassium = increased potassium excretion
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* Triggers aldosterone
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====Acid-Base====
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* Potassium is exchanged for H<sup>+</sup>
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* Hypokalaemia potassium to ECF and hydrogen to ICF
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** Results in alkalosis
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=====Acidosis=====
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* Potassium moves from ICF to ECF
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* pH change of 0.1 leads to potassium change by 0.6mmol/l
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* (rough guide)
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=====Alkalosis=====
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* Opposite effect
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* Potassium moves from ECF to ICF
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