Flashcards in Long term control of blood pressure Deck (21):
Features of long-term control of blood pressure
- Is probably not mediated by the arterial baroreflex
- Revolves around the control of plasma volume by the kidney
- Involves at least three hormone systems: Renin-angiotensin-aldosterone, Antidiuretic factor (ADH, vasopressin), atrial natriuretic peptide.
- Relevent to hypertension
What are basic functions of the kidney?
- Excretion of waste products
- Maintenance of ion balance
- Regulation of osmolarity
- Regulation of plasma volume
What is the control of plasma volume used to regulate?
It is used to regulate MAP
How does the kidney regulate plasma volume?
- The clever renal counter-current system creates a very high osmorality outside the collecting duct.
- Control over Na+ transport determines how big that osmotic gradient is.
- Control over the permeability of the collecting duct to water determines if water follows that osmotic gradient or not.
- Hence you can control how much water is lost in the urine, and how much is retained.
How does the kidney conserve plasma volume?
Making the collecting duct very permeable to water will result in lots of water reabsorption, little urine, and conserve plasma volume.
How does the kidney reduce plasma volume?
Making the collecting duct very impermeable to water will result in little reabsorption, lots of urine (=diuresis), and a reduction in plasma volume.
Where is Renin produced in the renin-angiotensin-aldosterone system?
From the juxtaglomerular (=granule cells) of the kidney!!
What triggers Renin production?
- Activation of sympathetic nerves to the juxtaglomerular apparatus
- Decreased distension of afferent arterioles (the "renal baroreflex")
- Decreased delivery of Na+/Cl- through the tubule
All of these are signs of low MAP
What is reduced distension a sign of?
What does Renin do?
- Converts inactive angiotensinogen to angiotensin I.
- Which is in turn converted by angiotensin converting enzyme to angiotensin II.
What does angiotensin II do?
- Stimulates release of aldosterone from the adrenal cortex.
- Increases release of ADH (antidiuretic hormone) from the pituitary
- Is a vasoconstrictor: therefore increases TPR.
What does aldosterone do?
- Increases Na+ reabsorption in the loop of Henle
- Therefore reduces diuresis and increase plasma volume.
What does increasing the release of ADH from the pituitary do?
- Increases water permeability of the collecting duct.
- Therefore reduces diuresis and increases plasma volume
- And increases sense of thirst
How is the renin-angiotensin-aldosterone system a negative feedback system?
- Multiple mechanism detect any decrease in MAP
- Stimulates release of renin
- This evokes multiple mechanisms which increase MAP.
Where is ADH produced?
- Synthesised in the hypothalamus
- Released from the posterior pituitary
What triggers ADH release?
- A decrease in blood volume (as sensed by cardiopulmonary baroreceptors and relayed via medullary cardiovascular centres).
- An increase in osmolarity of interstitial fluid (as sensed by osmoreceptors in the hypothalamus)
- Circulating angiotensin II (triggered by the renin-angiotensin-aldosterone system)
Where is ANP (atrial natriuretic peptide) produced?
- Produced in, and released from myocardial cells in the atria.
What triggers ANP release?
Increased distension of the atrium
-This is a sign of increased MAP
What does ANP do?
- Increases excretion of Na+ (natriuresis)
- Inhibits the release of renin
- Acts on medullary CV centres to reduce MAP
How is atrial natriuretic peptide a negative feedback system?
- A mechanism that detects any increase in MAP.
- Stimulates release of ANP
- This evokes multiple mechanisms which reduce MAP