long term regulation of arterial blood pressure and hypertension balance between fluid intake and...
TRANSCRIPT
Long Term Regulation of Arterial
Blood Pressure and Hypertension
Balance Between Fluid Intake and Output
Pressure Natriuresis. Arterial pressure is a signal for regulation of NaCl excretion.
arterial pressure NaCl reabsorbed in the proximal tubule more NaCl to the macula densa TGF autoregulation RBF, GFR.
Pressure Natriuresis. Arterial pressure is a signal for regulation of NaCl excretion.
arterial pressure NaCl reabsorbed in the proximal tubule more NaCl to the macula densa TGF autoregulation RBF, GFR.
2. Decrease PT NaCl reabsorption
3. I ncrease [NaCl] at macula
densa generates TGF signal
Natriuresis
Diuresis “Pressure Natriuresis”
1. I ncrease BP & renal
perfusion pressure
I ncrease NaCl
delivery to
distal nephron
4. I ncrease aff erent arteriole resistance
5. Autoregulate RBF, GFR
40% increase volume flow f rom PT
?
Pressure natriuresis can normalize BP by decreasing the effective circulating volume – this response connects BP and ECFV.
Renal-Body Fluid System for Arterial Pressure Control
• When the body contains too much extracellular fluid,
the blood volume and arterial pressure rise
• Pressure Diuresis and Pressure Natriuresis
• At high pressure, the kidneys excretes the excess
volume into urine and relieves the pressure
• At low pressure, the kidney excretes far less fluid than
is ingested
Renal Urinary Output Curve
or Renal function curve
Pressure Control by Renal-Body Fluid Mechanism
• Over the long period, water
and salt output must equal
intake
• Equlibrium point
• Return of the arterial
pressure always exactly
back to the equlibrium point
in the “infinite feedback
gain” principle
Two Determinants of Long-Term Arterial
Pressure Level
• The degree of pressure shift of the renal output curve
for water and salt
• The level of water and salt intake line
• It is impossible to change the long-term mAP level to a
new value without changing one or both of the two
basic determinants of long-term AP
Two Determinants of Long-Term Arterial
Pressure Level
Failure of increased TPR to elevate the long-term level
of AP if fluid intake and renal function do not change
• AP = Cardiac output x Total Peripheral Resistance
• So, increase in TPR should elevate AP
• But this acute rise in AP is not maintained if the kidneys
function properly
• Why?
• Pressure diuresis and pressure natriuresis
Failure of increased TPR to elevate the long-term level
of AP if fluid intake and renal function do not change
Increased Fluid Volume Can Elevate AP by Increasing
Cardiac Output or Total Peripheral Resistance
Importance of salt (NaCl) in the renal-body fluid
diagram for arterial pressure regulation
• An increase in salt is far more likely to elevate AP than
is an increase in water intake
• Water can be eliminated easily, but salt not
• Accummulation of salt in the body
– Stimulation of thirst center in the brain
– Increased osmotic pressure stimulates release of
vasopressin (ADH)
Hypertension
“Hypertension is defined as sustained abnormal elevation of the
arterial blood pressure”
HypertensionHypertension
•Leads to wear and tear
•is a major risk factor for cardiovascular diseases such as:
•STROKESTROKE
•HEART FAILUREHEART FAILURE
•ATHEROSCLEROSISATHEROSCLEROSIS
30% of world’s deaths
Complications
Complications as a result of hypertension include:
Stroke
Dementia
Myocardial Infarction
Congestive Heart Failure
Retinal Vasculopathy
Renal Disease or Failure
Chronic Hypertension is Caused by Impaired
Renal Function
• Mean Arterial Pressure > 110 mmHg (normal is about 90
mmHg)
• Systolic >140, diastolic >90 mmHg
• Hypertension can be lethal
– Heart failure
– Damage of a large vessel in the brain (cerebral infarct or stroke)
– Kidney failure
• Volume-loading hypertension means hypertension caused
by excess accumulation of extracellular fluid in the body
Volume-loading hypertension
• Reduced renal mass and increased salt intake
Volume-loading hypertension:
Two separate sequential stages
• The first stage: increased fluid volume causing
increased cardiac output hypertension
• The second stage: High blood pressure, high TPR but
return of the cardiac output near the normal
1) Hypertension
2) Marked increase in TPR
3) Almost complete return of the extracellular fluid volume
blood volume and cardiac output back to normal
Volume-loading hypertension in patients who have no kidneys and need for dialysis
Hypertension caused by primary Aldosteronism
• Another type of volume-loading hypertension is caused
by excess aldosterone in the body – (other steroids)
• A small tumor of adrenal glands and primary
aldosteronism
• Aldesteron increases reabsorbtion of salt and water
increased blood volume and reduced urine output
• Consequently, hypertension develops
The Renin-Angiotensin System
Pressure control and Hypertension
• Renin is an enzyme released by the kidneys when the
arterial pressure falls too low
• It is synthesized and stored in inactive form called prorenin
in juxtaglomerular cells
• JG cells are modified smooth muscle cells in the walls of
afferent arterioles
• Renin acts on angiotensinogen (a plasma globulin)
• Half life of renin is about 30 mins
• Angiotensin I, converting enzyme and Angiotensin II
The Renin-Angiotensin System
Pressure control and Hypertension
Rapidity and Intensity of Vasoconstrictor Pressure
Response to the Renin-Angiotensin System
• Renin-angiotensin vasoconstrictor system requires about
20 mins to become fully active
Effect of Angiotensin in the Kidneys to Cause
Renal Retention of Salt and Water
1) Angiotensin acts directly on the kidneys to cause salt
and water retention
- Makes the kidneys retain salt and water
- Causes vasoconstriction in renal arteries
2) Angiotensin causes the adrenal gland to secrete
aldosterone
- Aldosterone increases salt and water retention by the kidneys
Role of Renin-Angiotensin System in Maintaining a Normal
Arterial Pressure Despite Wide Variations in Salt Intake
• When the renin-
angiotensin system
functions normally,
pressure rises no
more than 4 to 6
mmHg in response
to as much as a 50-
fold increase in salt
intake
Other Types of Hypertension Caused by Combinations of
Volume-Loading and Vasoconstriction
• Aort Coarctation –
hypertension
• High BP in the upper
body, but BP is
normal in the lower
body
• Autoregulation of
hypertension
Coarctation of the Aorta
• Coarctation usually occurs
just distal to the left
subclavian artery at the
ligamentum arteriosum
– May also occur just proximal
to the left subclavian.
• Causes systemic
hypertension and secondary
LVH with heart failure.
Other Types of Hypertension Caused by Combinations of
Volume-Loading and Vasoconstriction
• Hypertension in pre-eclampsia (toxemia of pregnancy)
• Ischemia of the placenta and subsequent release of toxic
factors causing hypertension
• Endothelial dysfunction – reduction of Nitric Oxide
• Impaired renal pressure natriuresis
• Development of hypertension
• Thickening of the kidney glomerular membranes
(auotoimmune process)
Other Types of Hypertension Caused by Combinations of
Volume-Loading and Vasoconstriction
• Neurogenic Hypertension
• Acute neurogenic hypertension can be caused by
strong stimulation of the sympathetic nervous system
• Excitement, anxiety induces sympathetic activity which
causes peripheral vasoconstriction and acute
hypertension occurs
• Acute neurogenic hypertension caused by sectioning
the baroreceptor nerves
Primary (Essential) Hypertension
• 90 to 95% of hypertension cases are of primary
• It is of unknown origin
– Genetics: there is a strong hereditary tendency
– Environment: Excess weight and sedentary life style
– Neurohormonal mediators
Some Characteristics of Primary Hypertension
1) Cardiac output is increased due to additional blood flow required
for the extra adipose tissue and increased metabolism
2) Sympathetic nerve activity (especially in kidneys) is increased in
OW patients (leptin – vasomotor center ?)
3) Angiotensin II and aldosterone are increased (sympathetic
stimulation-renin-aldosterone …)
4) Renal-pressure natriuresis mechanism is impaired
• If hypertension is not treated, there may also be vascular damage
in the kidney that can reduce glomerular filtration rate
Graphical Analyses of AP Controling
Primary Hypertension
Reason for the difference between non-salt and salt-sensitive hypertension is
probably related to structural and functional differences in kidney (nephrons..)
Treatment of Primary Hypertension
• Two general classes of drugs are used
– Vasodilator drugs that increase renal blood flow
– Natriuretic and diuretic drugs that decrease
tubular reabsorption of Na and water
• Vasodilator drugs
– Inhibiting sympathetic nerve activity to kidneys
– Direct relaxation of smooth muscle of renal
vasculature
– Blocking the action of renin-angiotensin system
on renal vasculature or tubules
Summary for Arterial Pressure Regulation
• AP is regulated not by a single pressure controlling
system (several inter-related systems)
• To achieve
– Survival
– Returning the blood volume and pressure back to normal
• Mechanisms
1) Rapidly acting pressure control mechanisms
2) Intermediate mechanisms that act after several minutes – hours
3) Long-term arterial pressure regulation
Summary for Arterial Pressure Regulation
Intermediate mechanisms that act after several
minutes – hours
• Renin-Angiotensin vasoconstrictor mechanism
• Stress-relaxation of the vasculature
• Shift of fluid through capillary walls in and out of
circulation
* These mechanisms become mostly activated within
30 mins to several hours
• Role of the kidneys
• Many factors can affect pressure-regulating
level of the renal-body fluid mechanism
– Aldosterone
– Renin-Angiotensin system
– Nervous system (increased sympathetic activity)
Long term mechanisms for AP regulation