University of Jordan 1

Cardiovascular system- L6

Faisal I. Mohammed, MD, PhD

The peripheral resistance:

It is the resistance to blood flow through a vessel caused by friction between the moving fluid and the vascular wall.

Most of the resistance to blood flow occurs in arterioles ( 50%) and capillaries ( 25%) so it is called peripheral.

Velocity of Blood Flow is Greatest in the Aorta

Velocity of Blood Flow = Blood Flow Cross sectional area

Aorta >Arterioles> Small veins >Capillaries

Hemodynamic laws

Ohm’s law: F =∆P/R F = Flow, ∆P = Change in Pressure,

R = Resistance

CO = cardiac output, MAP = mean arterial pressure, TPR = total peripheral resistance. Since Rt. Atrial pressure = 0 then

TPRAtrialPRtMAPCO .

TPR

MAPCO

Hemodynamic laws… cont

Poiseuille’s lawF = ∆Pr4/ 8LF = flow, ∆P = change in pressurer = radius of the vessel (eta) = viscosityL = length of the vesselThen Resistance, R = 8L / r4

P= 100mm Hg

How Would a Decrease in Vascular Resistance Affect Blood Flow?

FLOW = P RESISTANCE

FLOW = P RESISTANCE

Conversely,

Poiseuille’s law …cont

Poiseuille’s law …cont

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Blood Pressure Regulation

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Sphygmomanometry

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Measurement of Systolic and Diastolic Pressures

Auscultatory method is the most commonly used method for measuring systolic and diastolic pressures.

When cuff pressure reaches systolic pressure, one begins to hear tapping sounds in the antecubital artery; as the cuff pressure reaches diastolic pressure, one hears muffled sounds and then Korotkoff sounds disappear.

Mean arterial pressure can be estimated by adding 40% of systolic pressure to 60% of diastolic pressure.

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Blood Pressure Regulation

Mean Arterial Pressure (MAP) = 1/3 systolic pressure + 2/3 diastolic pressure

MAP= Diastolic Pressure+1/3 Pulse Pressure

TPR

MAPCO

TPRCOMAP

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Mean Arterial Pressure

Sympathetic Innervation of Blood Vessels

Sympathetic nerve fibers innervate all vessels except capillaries and precapillary sphincters and some metarterioles.

Innervation of small arteries and arterioles allow sympathetic nerves to increase vascular resistance.

Large veins and the heart are also sympathetically innervated.

Parasympathetic nervous system is mainly important in control of heart rate via the vagus nerve.

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Short term regulation of BP1. Baroreceptors or pressoreceptos (High pressure) Carotid and Aortic Baroreceptors

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Short term regulation of BP cont…

1. Baroreceptors or pressoreceptos (High pressure)

The Vasomotor Center (VMC)

The VMC transmits impulses downward through the cord to almost all blood vessels.

VMC is located bilaterally in the reticular substance of the medulla and the lower third of the pons.

The VMC is composed of a vasoconstrictor area, vasodilator area, and sensory area.

Arterial Baroreceptor Reflex Important in short term regulation of arterial

pressure.

Reflex is initiated by stretch receptors called baroreceptors or pressoreceptors located in the walls of the large systemic arteries.

A rise in pressure stretches baroreceptors and causes them to transmit signals to the vasomotor center (VMC) and feedback signals are sent via the automonic nervous system to the circulation to reduce arterial blood pressure back to normal.

Blood Pressure

ArterialBaroreceptors

VasomotorCenter

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Baroreceptor mechanism

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Effect of Parasympathetic and Sympathetic Nervous Systems on Factors that Influence the Mean Arterial

Pressure

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Short term regulation of BP cont…

Chemoreceptors

Carotid and Aortic Chemoreceptors

Chemoreceptors are chemosensitive cells sensitive to oxygen lack, CO2 excess, or H ion excess.

Chemoreceptors are located in carotid bodies near the carotid bifurcation and on the arch of the aorta.

Activation of chemosensitive receptors results in excitation of the vasomotor center.

Sympatheticactivity

Chemoreceptors VMCO2CO2pH

BP

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Nervous Control of the Heart

University of Jordan 26

University of Jordan 27

Cardiovascular system- L7

Faisal I. Mohammed, MD, PhD

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Blood Pressure Regulation

Mean Arterial Pressure (MAP) = 1/3 systolic pressure + 2/3 diastolic pressure

TPR

MAPCO

TPRCOMAP

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Intermediate / Long term Regulation of BP

1. Epinephrine – Adrenal medulla systemworks as intermediate term needs 10 min. to work causes vasoconstriction

2. ADH (vasopressin) system needs 30 min to work causes vasocnstriction

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Long term Regulation of BP…cont

3. Renin-Angiotensin-Aldosterone system 1 hour to be effective

Angiotensinogen (14 a.a peptide) converted into Angiotensin I (10 a.a peptide) by Renin that come from afferent arteriolar cell, the angiotensin I is converted into angiotensin II (8 a.a peptide) by Angiotensin converting enzyme mainly in the lungs.Angiotensin II (A II) is very potent vasoconstrictor. AII also stimulates aldosterone synthesis and secretion from the adrenal coretx (Zona glomerulosa), aldosterone increases Na+ reabsorption from the renal nephrone and so water.AII is also a positive inotropic agent

Renin-Angiotensin System

Renin is synthesized and stored in modified smooth muscle cells in afferent arterioles of the kidney.

Renin is released in response to a fall in pressure.

Renin acts on a substance called angiotensinogen to form a peptide called angiotensin I.

AI is converted to AII by a converting enzyme located in the endothelial cells in the pulmonary circulation.

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Long term Regulation of BP …cont

4. Atrail Natriuretic peptide (ANP): An 28 a.a peptide released mainly from the Rt. Atrium in response to stretch. It causes increase in GFR so increase Na+ and water. Its concentration decreases when BP is low and its concentration increases if BP is high, mainly due volume overload

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Determinants of Mean Arterial BP

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Negative Feedback Cycle of Elevated BP

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Cardiac Output and Venous return

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Effect of Venous Valves

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Effect Of Gravity on Venous return

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Effect of Venous Valves

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Factors that Facilitate Venous Return

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