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Blood Pressure
Clinical Science Applied to
Nursing
CopyrightCSAN2005CardiffUniversity
Learning Outcomes
• Define blood pressure
• State the function of blood pressure
• Explain how blood pressure is commonly
measured
• Define cardiac output and state how it is
calculated
• Discuss the factors that are involved in cardiac
output
• List the factors that regulate arterial blood
pressure
What is Blood Pressure?
• Blood pressure is the force exerted on a blood
vessel wall by the blood.
• Blood must circulate through the body and
organs to maintain life
• The Heart is the pump that circulates the blood
• Pressure difference in the vascular system
ensures that blood flows around the body
Effects of High Blood Pressure on
Your Body
• Artery Damage
Effects of High Blood Pressure on
Your Body
• Hardening of the
arteries
• Stroke
• Heart attack
• Kidney damage
• Blindness
Blood pressure
• Blood pressure is different in blood vessels and
varies from minute to minute dependant on
factors such as stress
Blood Pressure
• A blood pressure reading consists of two
numbers:
• Systolic pressure- the first, highest number
– Indicates pressure when the ventricles contract to
push blood out to the body
• Diastolic pressure- the second, lower number
– Indicates when the heart relaxes between beats
Function of blood pressure
• Systemic BP maintains the essential flow of
substances into and out of organs
• Control of BP is essential for the maintenance
of homeostasis
Blood flow through the vessels
Blood flow through
vessels is directly
proportional to the
difference in pressure
between the ends
of the tube
Arterial Blood Pressure the driving force for
blood in the circulatory system
healthy young adults: Resting Systolic pressure = 120
mmHg
Diastolic is about 80 mmHg
healthy children values vary (see Whaley & Wong 2000):
Rough Guide
Systolic pressure
• 1 –7years: age in years + 90:
• 8-18 (2 x age in years) + 83 &
Diastolic pressure:
• 1-5 years 56mmHg,
• 6-8years: age in years + 52
•
How can we measure Blood
Pressure?
Blood Pressure can be measured in two
ways:
1. Indirectly, using a sphygmomanometer
and ausculating (listening) with a
stethoscope, the sounds you hear are
called Korotkoff sounds
2. Directly, using an arterial cannula (only
used in Critical Care Areas, Operating
Theatres)
Measuring BLOOD PRESSURE
External blood pressure measurements:
We apply a cuff to the arm &apply pressure to
the cuff to form a constricting band around
the arm and around internal arteries.
The pressure in the cuff is initially above Blood
pressure in the arteries– as the cuff pressure
falls, it ‘meets’ the blood pressure & 1st Korotkof
sound is heard.
When the cuff pressure falls below the blood
pressure, the Korotkof sound disappears
Making sense of what we hear
Factors which influence blood
pressure
• Cardiac output (CO)
• Total Peripheral Resistance (TPR)
Or
BP = CO x TPR
Factors which influence BP
• Cardiac output (and venous
return)
• Blood volume and viscosity
• Peripheral resistance
• Elasticity of the blood vessels
Cardiac output
Cardiac Output = Heart Rate x Stroke Volume
l/min
bpm
mL
In a healthy adult this is 70 x 75mL= 5L/min
Factors that influence cardiac
output
• Stroke volume – the amount of
blood ejected from each ventricle at
each heartbeat.
• Heart Rate – regulated by the
autonomic nervous system (ANS)
Stroke Volume
Stroke volume is
determined by three factors:
Preload
Afterload
Contractility
Preload
- related to the volume of blood in the
ventricle
at the end of diastole:
- End
Diastolic Volume
Factors affecting preload are:
• Blood volume
• Venous return
• The vasomotor tone
Preload: Factors which influence
Venous return to the heart
•
Muscle pump
•
Respiratory Pump
•
Valves in the vein
‘Starling’s law of the heart’
• If, for any reason, the blood returning to
the heart increases, more ventricular
filling will take place during diastole; The
ventricular muscle fibers will be stretched
and on the next systole (contraction) it will
give more powerful contraction, resulting
in stroke volume.
• more fibers are stretched, the greater
the force of the contraction (within
reason)
Afterload
• Described as the resistance against
which the ventricle must work.
• Whilst Preload is a major
determinant of myocardial contractile
power
• Afterload is mainly mechanical
factor that affects performance
Total Peripheral Resistance
(TPR)
• Peripheral Vascular Resistance
– resistance exerted by the action of the walls of
the resistance vessels impeding blood flow
• most resistance is provided systemically by the
arterioles,and small and medium sized arteries –
– exert a powerful influence in the control of
blood pressure.
Factors which influence TPR
The length of the vessel the blood
The diameter of the lumen of the vessel
The viscosity of the blood
The tone of blood vessels
depends on the relative structure
Control of Blood Pressure
Short term control - mainly involves:
• Baroreceptor reflex
• Chemoreceptor reflex
• Circulating hormones
Long term control involves regulation
Of blood Pressure by the kidneys.
MECHANISMS OF B/P CONTROL
Higher brain stimulus
Proprioreceptors
Baroreceptors
Chemoreceptors
Renin - Angiotensin - Aldosterone Mechanism
Blood Volume
Renal Perfusion
Angiotenisogen
+ Reduced B/P & GFR 
JGA
RENIN
Low Na+Cl –
at macula densa.
Low afferent
arterial Pressure. Angiotensin 1
 B/P
 blood volume
negative feedback
(ACE)
Angiotensin 2
Renal Na+ & H2O
reabsorption
Aldosterone
Stimulates Adrenal Cortex
arterioles
&
thirst
Produces vaso
constriction of
& small arteries
ADH (Anti Diuretic Hormone)
Released from Posterior Pituitary in
response to Low Blood Volume & B/P
• EFFECTS - vasoconstriction of mainly
Splanchnic circulation & fluid
retention
Fluid Exchange
• 85% of fluid that leaves blood is returned
at venous end
• What about the other 15%?