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 directlyproportional to thedifference in pressurebetween the endsof 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 GuideSystolic 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 KorotkofKorotkof sound is heard.

When the cuff pressure falls below the blood pressure, the KorotkofKorotkof 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 VolumeStroke volume is determined by three factors:

Preload 

AfterloadContractility

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 CONTROLHigher brain stimulus

Proprioreceptors

Baroreceptors

Chemoreceptors

Blood Volume Renal Perfusion JGA RENINAngiotenisogen

+ Reduced B/P & GFR Low Na+Cl –

at macula densa.

Low afferent arterial Pressure. Angiotensin 1

B/P negative feedback

blood volume (ACE)

Angiotensin 2

Renal Na+ & H2O

reabsorption

Produces vaso

Aldosterone Stimulates Adrenal Cortex constriction of arterioles & small arteries &

thirst

Renin - Angiotensin - Aldosterone Mechanism

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%?