Long term control of blood pressure

Question 1

Features of long-term control of blood pressure

Answer 1

• 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

Question 2

What are basic functions of the kidney?

Answer 2

• Excretion of waste products
• Maintenance of ion balance
• Regulation of osmolarity
• Regulation of plasma volume

Question 3

What is the control of plasma volume used to regulate?

Answer 3

It is used to regulate MAP

Question 4

How does the kidney regulate plasma volume?

Answer 4

• 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.

Question 5

How does the kidney conserve plasma volume?

Answer 5

Making the collecting duct very permeable to water will result in lots of water reabsorption, little urine, and conserve plasma volume.

Question 6

How does the kidney reduce plasma volume?

Answer 6

Making the collecting duct very impermeable to water will result in little reabsorption, lots of urine (=diuresis), and a reduction in plasma volume.

Question 7

Where is Renin produced in the renin-angiotensin-aldosterone system?

Answer 7

From the juxtaglomerular (=granule cells) of the kidney!!

Question 8

What triggers Renin production?

Answer 8

• 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

Question 9

What is reduced distension a sign of?

Answer 9

Reduced MAP

Question 10

What does Renin do?

Answer 10

• Converts inactive angiotensinogen to angiotensin I.

- Which is in turn converted by angiotensin converting enzyme to angiotensin II.

Question 11

What does angiotensin II do?

Answer 11

• Stimulates release of aldosterone from the adrenal cortex.
• Increases release of ADH (antidiuretic hormone) from the pituitary
• Is a vasoconstrictor: therefore increases TPR.

Question 12

What does aldosterone do?

Answer 12

• Increases Na+ reabsorption in the loop of Henle

- Therefore reduces diuresis and increase plasma volume.

Question 13

What does increasing the release of ADH from the pituitary do?

Answer 13

• Increases water permeability of the collecting duct.
• Therefore reduces diuresis and increases plasma volume
• And increases sense of thirst

Question 14

How is the renin-angiotensin-aldosterone system a negative feedback system?

Answer 14

• Multiple mechanism detect any decrease in MAP
• Stimulates release of renin
• This evokes multiple mechanisms which increase MAP.

Question 15

Where is ADH produced?

Answer 15

• Synthesised in the hypothalamus

- Released from the posterior pituitary

Question 16

What triggers ADH release?

Answer 16

• 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)

Question 17

Where is ANP (atrial natriuretic peptide) produced?

Answer 17

• Produced in, and released from myocardial cells in the atria.

Question 18

What triggers ANP release?

Answer 18

Increased distension of the atrium

-This is a sign of increased MAP

Question 19

What does ANP do?

Answer 19

• Increases excretion of Na+ (natriuresis)
• Inhibits the release of renin
• Acts on medullary CV centres to reduce MAP

Question 20

How is atrial natriuretic peptide a negative feedback system?

Answer 20

• A mechanism that detects any increase in MAP.
• Stimulates release of ANP
• This evokes multiple mechanisms which reduce MAP

Question 21

What basic drug treatments are there for hypertension?

Answer 21

• Ca2+ channel antagonists
• b-adrenoceptor antagonists
• Thiazide diuretics: affect how much water is being reabsorbed or not.
• Angiotensin converting enzyme inhibitors