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Blood Pressure Regulation

The force exerted by blood against vessel wall

Arterial blood pressure fluctuates in relation to ventricular systole and diastole

Normal arterial blood pressure

120/80 mm Hg (100-140/60-90 mm Hg)

Mean Arterial Blood Pressure (MAP)

Average pressure in arteries through out cardiac cycle

Mean driving force propelling the blood through microvessels

MAP = DBP + 1/3 of pulse pressure

MAP = 80 + 13 = 93 mm Hg

Why necessary??

To ensure sufficient driving force

To avoid extra work load on heart

To avoid vascular damage

Determinants of blood pressure


    1. CO (cardiac out put)


    1. TPR (total peripheral resistance)

Sympathetic nervous system activity

Distensibility of the vessels

    1. Blood Volume

By controlling CO, TPR and blood volume blood pressure is regulated

Short term regulation

Rapid regulation of MAP – within seconds

Neurally mediated – autonomic reflexes

Acts by altering CO and TPR

Long term regulation

Slow regulation of MAP – within minutes, hours, days or months

Hormonally mediated

Through regulation of blood volume by kidneys

Short term regulation of BP

Regulates minute to minute variations in BP

Responds within seconds

Accomplished by

Alterations in CO and TPR by

Autonomic sympathetic reflexes

Organs affected are




Autonomic sympathetic reflexes

Baroreceptor reflex

Chemoreceptor reflex

CNS ischaemic response

Baroreceptor reflex

Baroreceptors (high pressure receptors)

Carotid sinus baroreceptors

Aortic arch baroreceptors

Control BP by -ve feed back manner

Responds more to rapidly changing pressure

Inhibitory to vasomotor center

BP buffering function

Buffers minute to minute variations in BP related to body posture

Resetting of baroreceptors

Inability of baroreceptors to regulate persistent change in BP

Disorders of baroreceptor reflex

Orthostatic hypotension

Vasovagal syncope

Various stimuli (emotional disturbance)

Inhibition of sympathetic system

Stimulation of parasympathetic system

Carotid sinus message

Supraventricular tachycardia

Carotid sinus syncope

Tight collar syndrome

Cardiovascular Center

located bilaterally in medulla and lower pons

§ Vasomotor center (sympathetic system)

Vasoconstrictor area (rostral ventrolateral medulla)

q Sympathetic stimulation

Vasodilator area (caudal ventrolateral medulla)

q Sympathetic inhibition (inhibitory to RVLM)

§ Cardiostimulatory center (sympathetic & vagal)

§ Cardioinhibitory center (sympathetic & vagal)

§ Sensory area – nucleus of tractus solitarius (terminus of IX & X cranial nerves)


§ From higher centers

Cerebral cortex, limbic system, hypothalamus

§ From periphery




§ To heart, arteries and veins through autonomic nervous system

Chemoreceptor reflex


In carotid and aortic bodies

Sensitive to ¯ PO2, ­CO2, ­ H+ (¯ pH)

Stimulate at BP < 80 mm Hg

Stimulatory to vasomotor center

Share the pathway with baroreceptor reflex

CNS ischaemic response

Intense response of vasomotor center to cerebral ischaemia

Stimulation of vasomotor center at BP < 60 mm Hg (especially < 15 – 20 mm Hg)

Last ditch stand – an attempt to rise BP

Cushing reaction

When CSF pressure ≥ arterial pressure

Blood supply to brain depressed

Stimulation of vasomotor center

Protects vital centers in brain if CSF pressure rises high enough

Low pressure receptors

Present in both atria, pulmonary arteries, pulmonary veins and vena cavae – cardiopulmonary receptors

Detect change in blood volume

Share the pathway with aortic baroreceptors

Affect vasomotor center accordingly

Bainbridge reflex

­ heart rate due to rise in atrial pressure

Low pressure receptors in atria ® vagus ®

Vasomotor center ® ­sympathetic & ¯ vagal activity ® increased heart rate

Prevents damming of blood

Long term regulation of arterial pressure

Blood Volume is the determinant to be regulated to control BP on long term basis

Water intake = Water output

Salt (osmolarity) affects body water

Salt & water intake = Salt & water output

Renal function curve

More the mean arterial pressure more will be the urinary volume output

Pressure diuresis

Increased urine volume output due to rise in arterial pressure

Pressure natriuresis

Increased Na+ output due to rise in arterial pressure

Water and salt output must equal intake over a long period

Infinite gain of renal BP regulating mechanism

Correction/error (0)

Two basic factors affecting long term regulation of BP

Renal function curve

Water and salt line

Arterial pressure determines the balance between salt and water intake and output

Renin angiotensin system

Augments renal function by enhancing water & salt reabsorption on long term basis

Causes vasoconstriction


Secreted from juxtaglomerular cells in response to ¯ arterial pressure

Angiotensin II (indirect actions)

Causes release of aldosterone from adrenal glands

Causes release of ADH from hypothalamus

Stimulates thirst mechanism


Causes reabsorption of salt and water

Other slow adjustments

Capillary fluid shift

­ BP ­®Pc ® fluid shift out of capillary

Stress relaxation

­ stretch on vessel wall ® relaxation of vascular smooth muscle


Persistently elevated mean arterial pressure of more than 110 mm Hg (>140/90 mm Hg)

Secondary hypertension

Secondary to some primary problem

Renal, cardiovascular, endocrine , neurogenic etc

Primary (essential/idiopathic) hypertension

Cause not known

Coarctation of aorta

Blockage in aorta distal to arterial branches to head and arms but proximal to renal arteries

Arterial pressure in

lower body is normal

Upper body high

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