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Blood Pressure

Prof. K. Sivapalan

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Blood pressure.

• Pressure of the blood varies in different parts of the circulatory system.

• The word “Blood Pressure” refers to the lateral pressure exerted by the blood on the arterial wall at the level of the heart.

• It is maintained by,– Cardiac output and – Peripheral resistance.

Factors that affect cardiac output.

1. Venous return.1. Blood volume.

2. Veno-motor tone.

3. Respiratory pump.

4. Muscle pump.

5. Peripheral resistance.

2. Heart rate and force of contraction (end systolic volume).

1. Cardiac nerves.

2. Circulating catecholamines.

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Factors that affect peripheral resistance.

• R = 8ηL / πr4.• Viscosity is constant [anaemia and

polycythaemia].• Radius is influenced by several factors.• Smooth muscle tone.

– Sympathetic tone.– Metabloits.– Temperature.– Hormones:

• catecholamines, angeotension, histamine, 5 hydeoxy tryptamine, kinins.

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Regulation of blood pressure.

• Physiological and pathological factors affect blood pressure.

• It has to be brought back to normal levels by regulatory mechanisms.

• Regulatory mechanisms are:– Nervous- baro receptor, vasomotor centre,

sympathetics. [Immediate]– Hormonal- renin, angiotensin. [Mid term]– Volume- aldosteron and ADH.[Long term]

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Baroreceptors.

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Baroreceptors.

• Baroreceptors are stretch receptors situated in carotid sinus and arch of the aorta.

• Increase in pressure increase stretch and increase discharge of impulse.

• Chronically changed pressure – reseting.

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Nervous regulation.• The centre for cardiovascular

regulation is in the brain stem, mainly in medulla oblongata.

• Named functionally as cardio inhibitory centre (CIC) and vaso motor centre (VMC).

• Efferents from CIC go in 10th cranial nerve, vagus, to SA and AV nodes of the heart [acetyl choline].

• Sympathetics from VMC innervate the nodes and myocardium [noradrenaline]

• Stimulation causes chronotropic and ionotropic effects.

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Nervous regulation.

• Sympathetics, in addition to heart, supply blood vessels and adrenal medulla.

• Impulses cause vasoconstriction and secretion of adrenaline from adrenal medulla.

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Feed back regulation.

Cardio Inhibitary Centre.

Vaso Motor Centre.

Baroreceptors.

+ -

Reduce heart rate. Vasodilatation.

+-

Increased blood pressure.

+ Blood pressure. reduced

-

-

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Other factors influencing CIC.

• Baro receptors: +• Thoracic volume receptors: +• Higher centers:--[excitement, anger] or ++

[fear, grief].• Respiratory centre: - Insp, and + Exp.• Chemoreceptor: +.• Pain: +• Proprioceptors: -

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Other factors influencing VMC

• Baroreceptor: -• Higher centers: + or –• Chemoreceptor: +• Hypoxia and hypercapnoea (direct); +• Respiratory centre: +[insp] or –[exp]• Pain: + or –• Proprioceptors: + [motor cortex +]• Distention of right atrium- +• Stretch of lungs: -

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Renal regulation of blood pressure.

• Reduced pressure → release of renin from JGA → conversion of angiotensinogen → angeotensin I and to II. Vasoconstriction and increase in pressure.

• Angeotensin → aldosteron → sodium retention.

• ADH → water retention.