Transcript Blood pressure - Lectures For UG-5

Dynamics of Blood Flow
11.12.12
Forces acting on blood during
circulation
Circulatory System
A closed double-pump system:
Left side of heart
Lung
Circulation
Right side of heart
Systemic
Circulation
Circulatory System
Branching of blood vessels
– Arteries branch into arterioles, veins into
venules
Arteries
Arterioles
Heart
Capillaries
Veins
Venules
Forces acting on blood during
circulation
The main forces acting on blood during circulation
Viscous force (Fv)
Pressure gradient force FP (force produced by heart pump)
Gravitational force FG
According to Newton’s law of motion which also governs
the motion of blood
F = FV + FP+ FG
Viscous force for Newtonian Fluids
Viscosity is the property of flowing fluid
(liquid/gas) by virtue of which relative motion
between layers in contact is opposed
Relative motion causes internal friction between
layers in contact. This internal friction is called
viscous force
For Newtonian fluids, the viscous force is
proportional to the surface area (A) of the layer
and the velocity gradient between layers (in the
direction perpendicular to the layer)
Viscous force and viscosity of
Newtonian Fluids
η is the constant of proportionality and is the viscosity.
Negative sign shows that F acts in a direction opposite to
the one in which the layers move
Viscosity can be expressed as the ratio of shear stress
and time rate of shear strain (or shear rate)
Shear stress produced by a ball
falling in a stationary liquid
A spherical ball falling in a
viscous fluid produces a
shear stress
It reaches a terminal
speed when the sum of
the forces acting on it is
zero
Laminar flow
Blood Pressure
Blood pressure (BP) is the pressure exerted by
circulating blood upon the walls of blood vessels
The blood pressure in the circulation is
principally due to the pumping action of the
heart. Differences in blood pressure are
responsible for blood flow from one location to
another in the circulation
Cardiac Cycle - Filling of Heart Chambers
 Heart is two pumps that work together, right and left half
 Repetitive contraction (systole) and relaxation (diastole)
of heart chambers
 Blood moves through circulatory system from areas of
higher to lower pressure.
› Contraction of heart produces the pressure
Cardiac Cycle - Filling of Heart
Chambers
Mechanical systole and
diastole
Systolic and Diastolic
Pressure
The force of contraction of the ventricles
raises the pressure to about 120mmHg
(systolic pressure) and the elastic recoil of
the arteries maintain the pressure to about
80mmHg during ventricular diastole
(diastolic pressure)
This pressure is enough to keep the blood
flowing continuously to all parts of the
body
Mean arterial pressure
 It is a term used in medicine to describe an average
blood pressure in an individual. It is defined as the
average arterial pressure during a single cardiac
cycle
 Mean arterial blood pressure is not the arithmetic
mean of systolic and diastolic pressure but is
instead about 93mmHg
 This is because the time the heart spends relaxing
is longer than the time it spends contracting and
ejecting blood into aorta
Blood Pressure Profile
Blood pressure is highest in the arteries. It decreases as the circulating
blood moves away from the heart through arterioles, capillaries and then
to veins due to viscous losses of energy
Although blood pressure drops over the whole circulation, most of the
fall occurs along the arterioles
Reference points for measuring blood
pressure
 While measuring pressures in cardiovascular system,
ambient atmospheric pressure is used as zero
reference point. Thus a blood pressure of 90mmHg
means that pressure is 90mmHg above atmospheric
pressure
 The second reference point for measuring blood
pressure is anatomical and is the position of heart. For
example, the usual convention is to measure blood
pressure in the brachial artery above elbow i.e.
approximately at hearts level when patient is seated
 If the blood pressure measurements are to be made in
the legs, the patient is brought to lying down position.
In this position vessel is approximately at cardiac level
Blood pressure measurement
Direct method
This is an invasive method in which artery or vein is
cannulated or catheterised. Pressure measured by direct
method is known as “end pressure” Here the kinetic
energy of blood flow is measured in terms of pressure.
Direct method is used in patients of ‘shock’ where indirect
measurements may be inaccurate or indeed impossible
Indirect methods of blood
pressure measurement
Indirect
method
(non-invasive,
measures lateral/side pressure)
Auscultatory
Oscillometric
Auscultatory Method
 The
auscultatory method uses a
stethoscope and a sphygmomanometer
 An inflatable cuff encircles the arm.
Pressure in the cuff is transmitted
through the tissue to compress brachial
artery and can be viewed on a
manometer
 A stethoscope is used to listen to
sounds in the artery distal to the cuff.
The sounds heard during measurement
of blood pressure are not the same as
the heart sounds 'lub' and 'dub' that are
due to vibrations inside the ventricles
that are associated with the snapping
shut of the valves
Auscultatory Method
 If a stethoscope is placed over the brachial artery in a
normal person no sound should be audible. As the
heart beats, pulses (pressure waves) are transmitted
smoothly via laminar (non-turbulent) blood flow
throughout the arteries, and no sound is produced
 Similarly, if the cuff of a sphygmomanometer is placed
around a patient's upper arm and inflated to a pressure
above the patient's systolic blood pressure, there will
be no sound audible. This is because the pressure in
the cuff is high enough such that it completely occludes
the blood flow
Oscillometric method
 The
oscillometric
method
was
first
demonstrated in 1876 and involves the
observation
of
oscillations
in
the
sphygmomanometer cuff pressure[ which are
caused by the oscillations of blood flow, i.e.
the pulse
 It uses a sphygmomanometer cuff, like the
auscultatory method, but with an electronic
pressure sensor (transducer) to observe cuff
pressure
oscillations,
electronics
to
automatically interpret them, and automatic
inflation and deflation of the cuff.
Oscillometric method
The cuff is inflated to a pressure initially in excess of the
systolic arterial pressure and then reduced to below
diastolic pressure over a period of about 30 seconds.
When blood flow is nil (cuff pressure exceeding systolic
pressure) or unimpeded (cuff pressure below diastolic
pressure), cuff pressure will be essentially constant. It is
essential that the cuff size is correct: undersized cuffs
may yield too high a pressure; oversized cuffs yield too
low a pressure
Oscillometric method
When blood flow is present, but restricted, the cuff
pressure, which is monitored by the pressure sensor, will
vary periodically in synchrony with the cyclic expansion
and contraction of the brachial artery, i.e., it will oscillate.
The values of systolic and diastolic pressure are
computed, results are displayed
Blood Pressure
Venous pressures
-35 mm Hg
Effect of gravity on pressure
– Distance heart-head~ 0.4 m
– Heart-feet ~ 1.4 m
– DP = rgh
1 mm Hg
105 mm Hg
Arterial pressures
55 mm Hg
95 mmHg
100 mmHg
95 mm Hg
100 mm Hg
195 mm Hg
The pressure in any vessel above heart
level is decreased by the effect of gravity
The arterial pressure is increased by
0.77mmHg for every centimeter below the
right atrium and similarly decreased for
each cm above the right atrium