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When blood pressure falls many physiological cascades commence in order to return the blood pressure to a more appropriate level.
- The blood pressure fall is detected by a decrease in blood flow and thus a decrease in GFR.
- Decrease in GFR is sensed as a decrease in Na+ levels by the macula densa.
- 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.
- At the same time, the juxtaglomerular cells sense the decrease in blood pressure and release renin.
- Renin converts angiotensinogen (inactive form) to angiotensin I (active form).
- 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 into angiotensin II.
- Angiotensin II is a vasoconstrictor which will increase bloodflow to the heart and subsequently the preload, ultimately increasing the cardiac output.
- Angiotensin II also causes an increase in the release of aldosterone from the adrenal glands.
- Aldosterone further increases the Na+ and H2O reabsorption in the distal convoluted tubule of the nephron.
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