Cardiovascular System:
The Integrated System for
Blood Pressure Regulation
Mary Christenson, PT, PhD
DPT 732 – Management Applications
of Physiology II
Spring 2009
1
Kidney Facts


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
~50 gallons of blood pass through the 2
kidneys every day
~1.3 quarts of urine produced from the
50 gallons
Kidneys about the size of a computer
mouse
Several important functions including
role in maintaining BP
2
Objectives


Compare and contrast the integrated
system of both short-term and longterm (including kidney involvement)
regulation of arterial blood pressure
Compare and contrast the effects of
various physiologic stressors on the
integrated regulation of the
cardiovascular system
3
Objectives (cont.)


Describe components in measuring
cardiac output
Describe factors that contribute to the
homeostatic disruption of normal
circulatory function
4
What We Already Know

Rapidly Acting Arterial Pressure Control
Mechanisms


SNS: effect on total peripheral vascular
resistance and capacitance and cardiac
pump
Shift of fluid through the capillary walls
5
What We Are Missing

Long-term control mechanisms for arterial blood
pressure
6
Long-term mechanisms for BP
Regulation


Related to maintaining homeostasis of
body fluid volume
Based on maintaining a balance
between intake and output of body fluid


Overall regulation of kidney excretion of
H2O and Na+
Variables account for variation in blood
volume
7
Simple Concept



Increase in extracellular fluid results in
increased blood volume and arterial
pressure
Normal body response: kidneys excrete
excess extracellular fluid and returns
the pressure to normal
Mechanism reverses if reduced blood
volume
8
Terminology/General Concepts



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Pressure diuresis
Pressure natriuresis
blood volume
blood pressure
blood volume
blood pressure
Excess salt intake: increase H2O
retention – increase MAP
9
Volume X Pressure
{Guyton & Hall, 2006}
Urinary Volume
Output (x normal)
8
1
20
Arterial Pressure mmHg
200
10
Renal Output Curve and Net
Water/Salt Intake


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Over long-term, water and salt intake
must equal output
Demonstrated at equilibrium point of
curve
Two determinants of long-term arterial
pressure


Location of renal output curve (shift?)
Level of intake line
11
TPR, Arterial Pressure and
Kidney Function


Arterial Pressure = CO X TPR
If increase TPR:


Get acute rise in arterial pressure
However, normal kidney function will
respond by returning arterial pressure to
the pressure level of the equilibrium point
– Why?
12
Effect of Fluid Volume on
Arterial Pressure



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Increased extracellular fluid volume
Increases blood volume
Increased mean circulatory filling
pressure
Increased venous return
Increased CO
Increased arterial pressure
13
CO: Two Mechanisms to Increase
Arterial Pressure

Direct effect

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Increased CO increases pressure
Indirect effect

Autoregulation
14
Salt Intake



Effect of Na+ greater than effect of
H2O
Why?
Amount of salt accumulation in body is
main determinant of extracellular fluid
volume
15
Chronic Hypertension

MAP > 110 mmHg

Results of pathology

With dialysis, what happens if patient’s
body fluid level is not kept at a normal
level?
16
Renal Mechanisms for Control
of BP


Review: 1st mechanism of kidney
control of arterial pressure
2nd system: Renin-Angiotensin
17
Renin-Angiotensin System



Renin – hormone that acts as an
enzyme; released when arterial
pressure drops – i.e., when renal
perfusion is inadequate
Helps raise arterial pressure
Can be life-saving system in circulatory
shock
18
19
Renin-Angiotensin Pathway
Decreased Arterial Pressure
Angiotensin I
Renin substrate
(angiotensinogen)
Retention
Renin (kidney)
Angiotensin II
Vasoconstriction
Inactivation
(salt/H2O)
Increased Arterial Pressure
20
Angiotensin and Salt/Water
Retention


Direct effect: on kidneys to retain salt
and water
Indirect effect: causes adrenal glands to
secrete aldosterone which increases
salt/water reabsorption by kidneys
21
Renin-Angiotensin and Salt
Regulation

Allows body to deal with widely varying
Na+ intake and maintain normal BP
salt intake
arterial pressure
extracellular volume
renin and angiotensin
renal retention of Na+ and H2O
Return of extracellular volume almost to normal
Return of arterial pressure almost to normal
22
23
Primary Hypertension =
“Silent Killer”


Unknown Cause – i.e., not secondary to
a known cause
Influence of weight gain and sedentary
lifestyle

PT role?
24
Weight Gain and Obesity Role
in HTN

Cardiac output increased

SNS activity increased

Angiotensin II/Aldosterone levels
increased
25
Treatment Options in HTN


Lifestyle modifications
Pharmacological


Vasodilator drugs
Natriuretic or diuretic drugs
26
Summary of Mechanisms to
Control Arterial Pressure

Rapid (seconds)

Semi-rapid (minutes/hours)

Long-term (hours/days/months/years)
27
28
Additional Circulatory Factors

Cardiac Output

Venous Return
29
Cardiac Output and
Venous Return

Cardiac output controlled by venous
return under most normal unstressful
conditions


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Factors in the peripheral circulation
affecting venous return to the heart (not
heart itself)
Sum of local blood flows contribute to
venous return
CO inversely related to TPR
30
Heart Influence on CO

Frank-Starling Law

Receptors

Heart is limiting factor if receives more
venous return than it can handle
31
Cardiac Output

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Normal: ~5L/min
Normal CO plateaus at ~13 L/min
without any special stimulation
Hypereffective heart
Hypoeffective heart
32
How Can We Measure CO?

Fick Principle

CO (L/min) = O2 absorbed per minute by
the lungs (ml/min)/A-VO2 difference (ml/L
of blood)
33