Cardiovascular system- L6

Faisal I. Mohammed, MD, PhD University of Jordan 1

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



MAP



Rt

.

AtrialP TPR

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

CO



MAP TPR

Hemodynamic laws… cont  Poiseuille’s law F =  ∆Pr 4 / 8  L F = flow, ∆P = change in pressure r = radius of the vessel  (eta) = viscosity L = length of the vessel Then Resistance, R = 8  L /  r 4

P= 100 mm Hg

How Would a Decrease in Vascular Resistance Affect Blood Flow?

FLOW =



P RESISTANCE Conversely, FLOW =



P RESISTANCE

Poiseuille’s law …cont

Poiseuille’s law …cont

Blood Pressure Regulation

10

Shygmomanometry

11

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 sounds and then Korotkoff sounds

disappear.

muffled

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

12

Blood Pressure Regulation

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

CO



MAP TPR MAP



CO



TPR

13

Mean Arterial Pressure

14

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.

Short term regulation of BP

1. Baroreceptors or pressoreceptos (High pressure ) Carotid and Aortic Baroreceptors 16

Short term regulation of BP cont…

1. Baroreceptors or pressoreceptos (High pressure) 17

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 pressure.

short term regulation

of arterial  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 Arterial Baroreceptors Vasomotor Center

Baroreceptor mechanism

20

21

Effect of Parasympathetic and Sympathetic Nervous Systems on Factors that Influence the Mean Arterial Pressure 22

Short term regulation of BP cont

… Chemoreceptors 23

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.

O2 CO2 pH

Chemoreceptors

VMC

Sympathetic activity

BP

Nervous Control of the Heart

25

University of Jordan 26