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

MAP = CO x TPR

1. 1. CO (cardiac out put)

• SV x HR

1. 1. TPR (total peripheral resistance)

• Sympathetic nervous system activity

• Distensibility of the vessels

1. 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

► Heart

► Arteries

► Veins

► 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)

► Inputs

§ From higher centers

• Cerebral cortex, limbic system, hypothalamus

§ From periphery

• Baroreceptors

• Chemoreceptors

► Outputs

§ To heart, arteries and veins through autonomic nervous system

Chemoreceptor reflex

► Chemoreceptors

► 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

► Renin

► 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

► Aldosterone

► 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

Hypertension

► 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