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Blood Pressure

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Goal of the Cardiovascular
System: deliver blood to all
parts of the body
• Does so by using
different types of tubing,
attached to a pulsatile
pump
• Elastic arteries
• Muscular arteries
• Arterioles
• Capillaries
• Venuoles
• Veins
• Distribution system
broken up into areas
called vascular beds
• Skin
• Digestive (splanchnic)
• Muscle
Structure of vasculature changes in response to different needs
Why does blood flow through this closed circuit?
• Blood flows down a pressure gradient
• The absolute value of the pressure is not important to flow, but the difference in
pressure (DP or gradient) is important to determining flow.
What happens to pressure if we decrease the volume of a fluid filled compartment (i.e.
ventricles during systole)?
P directly proportional to F
The resulting pressure is called the driving pressure in the vascular system
How does the flow differ in these two vessels?
Vascular system possesses different mechanisms for promoting continuous flow of blood to the
capillaries:
Elastic recoil
smooth m. regulation of diameter
sphincters
Muscular arteries
valves
Substances causing contraction in vascular smooth muscle
Chemical
Physiologic role
Source
Type
NE (a )
Baroreceptor reflex
Sympathetic neurons
Neural
Endothelin
Paracrine
Vascular endothelium
Local
Serotonin
Platelet aggregation, smooth
muscle contraction
Neurons, digestive
tract, platelets
Local, neural
Substance P
Pain, increased capillary
permeability
Neurons, digestive
tract
Local, neural
Vasopressin
Increase blood pressure during
hemorrhage
Posterior pituitary
Hormonal
Plasma hormone
Hormonal
endothelium
local
Angiotensin II Increase blood pressure
Prostacyclin
Minimize blood loss from
damaged vessels before
coagulation
Substances that mediate vascular smooth muscle relaxation
Chemical
Physiologic role
Source
Type
Nitric oxide
Paracrine mediator
Endothelium
Local
Atrial natriuretic
peptide
Reduce blood pressure
Atrial myocardium,
brain
Hormonal
Vasoactive intestinal
peptide
Digestive secretion, relax
smooth muscle
Neurons
Neural, hormonal
Histamine
Increase blood flow
Mast cells
Local, systemic
Epinephrine (b2)
Enhance local blood flow
to skeletal muscle, heart,
liver
Adrenal medulla
Hormonal
Acetylcholine
(muscarinic)
Erection of clitoris, penis
Parasympathetic
neurons
neural
Bradykinin
Increase blood flow via
nitric oxide
Multiple tissues
Local
Adenosine
Enhance blood flow to
match metabolism
Hypoxic cells
local
Even though there are many mechanisms for altering the radius of the vascular system,
pressure still drops as blood moves further away from the heart. Why?
Resistance = tendency of the vascular system to oppose flow;
Flow =
1
R
• Influenced by: length of the tube (L), radius of the tube (r), and viscosity of the blood (h)
Poiseuille’s Law
R = Lh/r
4
• In a normal human, length of the system is fixed, so blood viscosity and radius of the blood
vessels have the largest effects on resistance
All four tubes have the same driving pressure. Which tube has the greatest flow? The
least flow? Why?
Even with a decrease in overall pressure, the pressure in the vessels is not constant. The
pressure in the vessels mirrors the pressures generated in the heart – systolic and diastolic
pressures. Systolic = ventricles contracting
Diastolic = ventricles filling
Why does the diastolic
pressure rise between the left
ventricle and the arteries?
Normal blood pressure =
120/80
High blood pressure =
140/90
What could be
happening to increase
both the diastolic and
systolic blood pressure?
Blood Pressure
• Reflects the driving pressures produced by the ventricles
• Because arterial pressure is pulsatile, a single value is used to represent the overall driving
pressure. This is called the mean arterial pressure.
MAP = diastolic P + 1/3(systolic P-diastolic P)
Why does diastolic pressure account for a greater proportion of the overall value?
MAP = Q x Rarterioles
Explain how these two equations are equivalent
SVR = systemic
vascular resistance
CO = cardiac output
SV = stroke volume
What factors influence blood pressure?
• Blood volume
• Vascular resistance
• Autoregulation
• Autonomic influences
Regulation of Blood Pressure
• Main coordinating center is in
the medulla oblongata of the
brain; medullary cardiovascular
control center
• Reflex control of blood pressure
•Baroreceptor reflex
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