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

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
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




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 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
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


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
method (non-invasive,
measures lateral/side pressure)
Auscultatory
Oscillometric



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
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 This is similar to a
flexible tube or pipe with fluid in it that is being pinched
shut




If the pressure is dropped to a level equal to that of
the patient's systolic blood pressure, the first Korotkoff
sound will be heard.
As the pressure in the cuff is the same as the pressure
produced by the heart, some blood will be able to
pass through the upper arm when the pressure in the
artery rises during systole. This blood flows in spurts as
the pressure in the artery rises above the pressure in
the cuff and then drops back down beyond the
cuffed region, resulting in turbulence that produces
an audible sound
As the pressure in the cuff is allowed to fall further,
thumping sounds continue to be heard as long as
the pressure in the cuff is between the systolic and
diastolic pressures, as the arterial pressure keeps on
rising above and dropping back below the pressure
in the cuff.
 Eventually, as the pressure in the cuff drops further,
the sounds change in quality, then become muted,
and finally disappear altogether. This occurs
because, as the pressure in the cuff drops below the
diastolic blood pressure, the cuff no longer provides
any restriction to blood flow allowing the blood flow
to become smooth again with no turbulence and
thus produce no further audible sound. The pressure
where sound just disappears is the diastolic pressure



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.


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

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

In fluid dynamics, the Hagen–Poiseuille equation is
a physical law that states that for steady laminar
flow of a Newtonian fluid through a cylindrical
tube, the pressure drop in the tube is directly
proportional to the volume flow rate Q, length of
the tube, viscosity of fluid and inversely
proportional to fourth power of radius of tube
Where ∆P is the pressure drop
L is the length of pipe,
ƞ is the dynamic viscosity,
Q is the volumetric flow rate
and
r is the radius of the pipe
r
P1
P2
L
DP= P1 - P2

Analogous to Ohms law for electrical
circuits (V=IR) Poiseuille law can be
written as
∆P = Q R
Where R is the resistance to blood flow
R= 8 L η / π r4
The volume of blood flow from heart is
called the cardiac output and is the
stroke volume (the volume of blood
ejected in each beat) multiplied by hear
rate (number of beats per minute).
 This is ~ 60 (ml/beat) x 80 (beats/min) =
4800 ml/min
 Volume flow rate of blood is Q = 5L/min

Vascular resistance is a term used to define
the resistance to flow that must be
overcome to push blood through the
circulatory system.
 The resistance offered by the peripheral
circulation is known as the systemic vascular
resistance (SVR)
 The systemic vascular resistance may also be
referred to as the total peripheral resistance





Resistance is dependent on the vessel’s dimensions
and the viscosity of blood according to Poiseuille
law
A narrowing of an artery leads to a large increase
in the resistance to blood flow because of 1/ r4
term
Vasoconstriction (i.e., decrease in blood vessel
diameter) increases SVR, whereas vasodilation
(increase in diameter) decreases SVR
Peripheral resistance can be equated to DC
resistance in electrical circuits



Arrangement of vessels also determines resistance.
When the vessels are arranged in series, the total
resistance to flow through all the vessels is the sum
of individual resistances, whereas when they are
arranged in parallel the reciprocal of the total
resistance is the sum of all the reciprocals of the
individual resistance
Less resistance is offered to blood flow when vessels
are arranged in parallel rather than in series
Series
Parallel
R1
R2
R3
DP1
DP2
DP3
DP= DP1 + DP2 + DP3
=QR1+QR2+QR3
=QR
\R=R1+R2+R3
R1,Q1
R2,Q2
Q=Q1+Q2
=DP/R1+DP/R2
=DP/R
\1/R=1/R1+1/R2


Resistances in series add
directly while resistances
in
parallel
add
in
reciprocals
Arteries,
arterioles,
capillaries, venules and
veins are in general
arranged in series with
respect to each other.
However, the vascular
supply to the various
organs and the vessels
e.g. capillaries within an
organ are arranged in
parallel
Right and left sides of the heart
which are connected in series. Also
seen are the various systemic organs
receiving blood through parallel
arrangement of vessels
Pressure drop across a vessel is greatest
when resistance offered to flow is greatest
 Most pressure drop occurs across arterioles
although it should occur in capillaries with
much smaller diameter
 Within a network of arterioles or capillaries
each arteriole/capillary is parallel to other.
We calculate equivalent resistance for
network of arterioles as well as for capillaries
to check which offers greater resistance to
blood flow

L art= 2 . Lcap
[r cap/rart]4 =0.0256
Rart=2.5 Rcap
For a network of arterioles:
Rart(eq) = Rart / nart
 For a network of capillaries
Rcap(eq) = Rcap / ncap
Plugging ncap =30nart and Rart=2.5 Rcap we
get
Rarteq=75 Rcapeq




Blood leaves heart at ~ 30 cm/s
 In capillaries, flow slows to ~ 1mm/s

› Surprising - continuity should imply higher
flow
a1 and a2 are areas of cross
section and v1 and v2 are
velocities
 If cross sectional area is
large, velocity is low and
pressure is high
 If cross sectional area of
pipe is small, velocity is high
and pressure is low





With cross sectional area of 2.5 cm2 ,linear velocity
of blood in aorta is 22.5cm/s
On the other hand, in capillaries with cross
sectional area of 2500 cm2, linear velocity of blood
is simply 0.05cm/s
Hence aorta has smallest cross sectional area but
the mean flow velocity is highest
Each capillary is tiny, but since the overall capillary
bed contains many billions of vessel, it has total
cross sectional area several hundred times that of
the aortaand hence the mean blood flow velocity
falls several folds




To understand the effect of cross sectional area on
flow velocity, a mechanical model has been
suggested
Here a series of 1cm diameter balls are depicted as
being pushed down a single tube. The tube branches
into narrower tubes. Each tributary tube has a area of
cross section much smaller than that of the wider tube
Suppose in wide tube each ball moves at 3cm/min .
This means 6 balls leave the wide tube per minute and
enter narrower tubes
Obviously then these 6 six balls must leave the
narrower tubes per minute. This means each ball is
moving at a slower speed of 1cm/min
Biophysics by P.S. Mishra
 An introduction to Med. Biophysics by
Parveen Parkash
 Medical Physiology: Principles for
Clinical Medicine By Rodney A.
Rhoades, David R. Bell
