Hormonal Control of Blood Pressure

Question 1

Integrated systems regulate MAP

Answer 1

• when hemorrhage causes a sudden decrease in arterial pressure, the pressure control system faces 2 challenges: 1st- respond rapidly enough to ensure survival; 2nd- gradually reestablish blood volume to normal levels
• they are accomplished by integrated actions of systems that respond rapidly (sec - min); intermediate (min-hrs); and long term (days or longer)

Question 2

Rapid acting control mechanisms

Answer 2

• typically nervous reflexes

- the baroreceptors, chemoreceptors, and CNS ischemic response

Question 3

Intermediate control mechanisms to regulate MAP

Answer 3

• include the renin-angiotensin vasoconstrictor mechanism, the stress relaxation mechanism (e.g. increased pressure for min-hrs leads to continuous stretch of the vessel to relieve the pressure
• capillary fluid shift mechanism (e.g. if capillary pressure falls too low, fluid is fluid is absorbed from the tissues through the capillary membranes thus building up blood volume and pressure
• during this time, nervous mechanisms become gradually less effective

Question 4

Long term control mechanisms to regulate MAP

Answer 4

-involves control by the kidneys, in particular the renin-angiotensin-aldosterone system

Question 5

CNS ischemic response

Answer 5

• when blood flow to the vasomotor center in the lower brain stem becomes decreased enough to cayse ischemia- the vasoconstrictor and cardioaccelerator neurons in the vasomotor center respond directly to ischemia and become strongly excited
• this arterial pressure elevation in response to cerebral ischemia is known as the CNS ischemic response
• one of the most powerful of all the activators of the sympathetic vasoconstictor system
• it does not become significant until the arterial pressure falls far below normal, down to 60 mm Hg, and to greatest degree 15-20 mm Hg

Question 6

High pressure baroreceptors

Answer 6

-decrease their firing rate leading to increased HR, cardiac contractility, and vasoconstriction

Question 7

Low pressure baroreceptors

Answer 7

• decerase their firing rate in response to decreased circulating volume
• this leads to increased SNS mediated vasocontriction, especially the renal bed (they also stimulate ADH release)

Question 8

Peripheral chemoreceptors

Answer 8

-respond to local hypoxia by increasing the firing rate of chemoreceptor afferents, leading to increased firing of SNS vasoconstrictor fibers and changes in ventilation

Question 9

Central chemoreceptors

Answer 9

-respond to brain ischemia (a fall in pH/acidosis) leading to a powerful SNS output (the kidney can actually stop producing urine)

Question 10

Angiotension II

Answer 10

-decreased arterial pressure -> renin (kidney) -> renin substrate (angiotensinogen) -> angiotensin I -> (by converting enzyme in lung ACE ) angiotensin II -> angiotensinase inactive, vasoconstriction, renal retention of salt and water -> increased arterial pressure

Question 11

Recovery from hemorrhage with angiotensin

Answer 11

• hemorrhage causes arterial pressure to drop from 100 mm Hg to 50 mm Hg, the renin-angiotensin vasoconstrictor response is powerful enough to return pressure back to ~83 mm Hg
• this response can be life-saving, especially in circulatory shock
• in the presence of renin-blocking antibody recovery was much weaker returning to 60 mm Hg

Question 12

Integrated responses to hemorrhage

Answer 12

• response to hemorrhage involves contributions of the baroreceptors, the renin-angiotensin-aldosterone system, as well as increased fluid reabsorption by the capillaries in response to a decrease in capillary hydrostatic pressure
• the unstressed volume refers to the volume of blood that the veins can hold, no pressure
• the stressed volume refers to the volume in the arteries. This volume of blood produces pressure by stretching the elastic fibers in the walls of the vessels

Question 13

Increased salt intake

Answer 13

-increased salt intake -> increased extracellular volume -> increased arterial pressure -> decreased renin and angiotensin -> decreased renal retention of salt and water -> return of extracellular volume almost to normal -> return of arterial pressure almost to normal

Question 14

Antidiuretic hormone (ADH; vasopressin)

Answer 14

• from the posterior pituitary function, synthesized in the supraoptic nuclei of the hypothalamus and stored and released in posterior pituitary
• main function is water balance; it is released in response to increase osmolarity of extracellular fluid and decreased blood pressure and has the major effect of promoting water reabsorption by the kidney.
• high ADH in plasma, a low volume of concentrated urine is produced
• vasopressin/ADH is also a vasoconstrictor
• released in response to increased body fluid osmolarity, decreased blood volume, and decreased blood pressure
• ANP (atrial naturetic peptide), ethanol, aldosterone decreases release

Question 15

Control of arteriolar tone

Answer 15

• greatest resistance to flow occurs in the small arteries and arterioles
• the state of construction of relaxation of these vessels is regulated in part by the sympathetic nervous system and release of norepinephrine
• circulating hormones, including ADH, angiotensin, may contribute to constriction through their actions on vascular smooth muscle; atrial natriuretic peptide (ANP) is a smooth muscle dilator
• endothelium plays an important role in regulating vascular tone by its release of NO and prostacycline in response to shear stress, ACh, and bradykinin
• endothelin is a potent, endothelium-derived vasoconstrictor important in some pathophysiologic states
• the endothelial cell surface also has angiotensin-converting enzyme (ACE), which forms angiotensin II by cleavage of circulating angiotensin I

Question 16

Integrated systems regulate MAP both acutely and chronically

Answer 16

• to maintain adequate blood flow to tissues, the body has a complicated system for monitoring and regulating blood pressure
• high pressure baroreceptors in the aortic arch and carotid sinus are extremely important in acute regulation of blood pressure, through their effects on the ANS
• afferent arterioles in the renal juxtaglomerular apparatus also contain high pressure baroreceptors; these are involved in regulation of renin release, and consequently, regulation of sodium and water balance, important in long term regulation of blood pressure
• low pressure baroreceptors in the heart and pulmonary circulation respond to changes in blood volume and modulate sympathetic activity and vasopressin release
• the cardiac atria also release atrail natriuretic peptide in response to elevated blood volume

Question 17

Long term response to changes in blood volume and pressure

Answer 17

• in addition to evoking mechanisms for acute adjustment of blood pressure, changes in blood volume and pressure will also activate renal mechanisms for adjusting blood volume
• reduced blood volume and therefore arterial pressure will stimulate the renin-angiotensin-aldosterone system, with the end result of sodium and water retention
• reduced BP will also activate sympathetics stimulates renin secretion and have direct effects on kidneys
• increased volume will stimulate ANP release by heart. ANP has direct renal effects (natriuresis and diuresis) and also inhibits aldosterone release by the adrenal medulla