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Low Blood Pressure Cascade
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When the blood pressure falls many physiological cascades commence in order to return the blood pressure to a more appropriate level. Two of the most poignant are below:
Blood Pressure falls and is noticed at the level of the kidneys:
A) The blood pressure fall is detected by a decrease in blood flow and thus a decrease in GFR.
B) Decrease in GFR is sensed as a decrease in Na+ levels by the Macula Densa.
C) The Macula Densa cause an increase in Na+ reabsorption, which causes water to follow in via osmosis and leads to an ultimate increase in plasma volume. Further, the macula densa releases adenosine which causes constriction of the afferent arterioles
D) At the same time the Juxtaglomerular cells sense the decrease in blood pressure and thus release Renin.
E) Renin acts on Angiotensinogen (inactive form) and converts it to Angiotensin I (active form)
F) Angiotensin I flows in the bloodstream until it reaches the capillaries of the lungs where Angiotensin Converting Enzyme (ACE) acts on it to convert it intoAngiotensin II.
G) Angiotensin II is a vasoconstrictor which will increase bloodflow to the heart and subsequently the preload, ultimately increasing the cardiac output.
H) Angiotensin II also causes an increase in the release of Aldosterone from the Adrenal glands.
I) Aldosterone further increases the Na+ and H2O reabsorption in the Distal Convoluted Tubule of the nephron.