Changes

Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure.  (Both of these statements are assuming a constant blood pressure). The fact that both can be altered allows independent regulation of both GFR and blood flow.

Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure.  (Both of these statements are assuming a constant blood pressure). The fact that both can be altered allows independent regulation of both GFR and blood flow.

===Physiological Regulators of GFR===

==Physiological Regulators of GFR==

The main systems which regulate renal blood flow and GFR are:

The main systems which regulate renal blood flow and GFR are:

====Autoregulation====

===Autoregulation===

<center><big>'''Q = (PA - PE) &#247; R'''</big></center>

<center><big>'''Q = (PA - PE) &#247; R'''</big></center>

Despite the efforts of the autoregulatory system an increase in blood pressure still leads to an increased secretion of salt and water.  This is because even a small percentage change in GFR leads to large percentage change in the excretion of salt and water.  This excretion is however far less drastic than would be the case without autoregulation and actually helps to restore pressure to normal.  This increase in urinary output as a result of an increase in arterial blood pressure is termed [[Kidney - Blood Pressure - Anatomy & Physiology#Pressure Diuresis|pressure diuresis]].

Despite the efforts of the autoregulatory system an increase in blood pressure still leads to an increased secretion of salt and water.  This is because even a small percentage change in GFR leads to large percentage change in the excretion of salt and water.  This excretion is however far less drastic than would be the case without autoregulation and actually helps to restore pressure to normal.  This increase in urinary output as a result of an increase in arterial blood pressure is termed [[Kidney - Blood Pressure - Anatomy & Physiology#Pressure Diuresis|pressure diuresis]].

====[[Renin Angiotensin Aldosterone System|Angiotensin 2]]====

===[[Renin Angiotensin Aldosterone System|Angiotensin 2]]===

====Sympathetic Nervous System====

===Sympathetic Nervous System===

When the animal is in a situation of crisis or stress blood flow to the kidneys is reduced for the sake of other organs such as the brain, heart and skeletal muscles.  The sympathetic nervous system and a heightened level of adrenalin in the plasma cause the contraction of both the afferant and efferant arterioles.  As the efferant arteriole is contracted alongside the afferant one there is still a pressure differance allowing for filtration to still occur and reducing the impact on filtration compared to the impact on blood flow. At times when sympathetic tone is very high the renal blood flow could be reduced to 10-30% of normal.  This practically stops filtration occuring and thus stops urine production.

When the animal is in a situation of crisis or stress blood flow to the kidneys is reduced for the sake of other organs such as the brain, heart and skeletal muscles.  The sympathetic nervous system and a heightened level of adrenalin in the plasma cause the contraction of both the afferant and efferant arterioles.  As the efferant arteriole is contracted alongside the afferant one there is still a pressure differance allowing for filtration to still occur and reducing the impact on filtration compared to the impact on blood flow. At times when sympathetic tone is very high the renal blood flow could be reduced to 10-30% of normal.  This practically stops filtration occuring and thus stops urine production.

* Increased conservation of water and sodium

* Increased conservation of water and sodium

====Nitrous Oxide and Prostaglandins====

===Nitrous Oxide and Prostaglandins===

Nitrous Oxide and Prostaglandins have an impact on arteriolar resistance. Their role in the regulation of renal blood flow and filtration is however uncertain.

Nitrous Oxide and Prostaglandins have an impact on arteriolar resistance. Their role in the regulation of renal blood flow and filtration is however uncertain.

=====Nitrous Oxide=====

====Nitrous Oxide====

* Mediates dilation in the cortical circulation  

* Mediates dilation in the cortical circulation  

=====Prostaglandins=====

====Prostaglandins====

* Mediates dilation in medullary circulation (cortical in extremes)  

* Mediates dilation in medullary circulation (cortical in extremes)  

* PGE2 is involved in the regulation of the reabsorption of sodium

* PGE2 is involved in the regulation of the reabsorption of sodium

* In healthy/hydrated individuals these compounds do not play a significant role in sodium/water homeostasis

* In healthy/hydrated individuals these compounds do not play a significant role in sodium/water homeostasis

=====Effects of Blocking Nitric Oxide or Prostaglandins=====

====Effects of Blocking Nitric Oxide or Prostaglandins====

* Loss of medullary circulation - reduced ability to concentrate water

* Loss of medullary circulation - reduced ability to concentrate water

* Loss of medullary flow - reduces the ability to preserve sodium balance and maintain normal blood pressure

* Loss of medullary flow - reduces the ability to preserve sodium balance and maintain normal blood pressure