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HEMODYNAMICS(BLOOD FLOW, PRESSURE AND RESISTANCE)

LECTURE OBJECTIVES

•  Understand the term hemodynamics•  Describe the principles governing the flow of blood in the human

body•  Know and explain the factors effecting resistance to flow and how

it effects the normal physiological environment of the body

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HEMODYNAMICS•  The part of cardiovascular physiology dealing with the forces the heart (pump) has to

develop to circulate blood through the cardiovascular system•  Cardio vascular system consists of two pumps connected in series

 –  Right ventricle- pulmonary circuit

 –  Left ventricle systemic circuit•  Cardiac output or flow through each of the circuit is equal

PATH OF BLOOD FLOW IN THE CIRCULATORY SYSTEM

PHYSICAL CHARACTERISTICS OF CIRCULATION

•  Functional organization of vessels•  Blood volume in different segments•  Cross sectional area•  Velocity of blood flow in vessels

aortaarteriesarteriolescapillariesvenulesveinsvena cava

pulmonary veinscapillariespulmonary arteries

Systemic Pulmonary

Heart (right atrium)

Heart (right ventricle)

Heart (left ventricle)

Heart (left atrium)

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FUNCTIONAL ORGANIZATION OF CIRCULATION•  ARTERIES—high pressure vessels

•  ARTERIOLES---strong muscular wall< can changeradius•  CAPILLARIES—smallest vessel for exchange of

nutrients•  VENULES—coalesce to form larger veins•  VEINS – low pressure system, major reservoir of blood

CROSS SECTIONAL AREA OF VESSELS Aorta 2.5 cm2

Small arteries 20cm2 Arterioles 40cm2Capillaries 2500 cm2Venules 250 cm2Small veins 80 cm2Venae cavae 8 cm2Largest diameter vessel aorta but smallestcross sectional areaCapillaries all together represent largest

cross section area

VELOCITY OF BLOOD FLOW•  Inversely related to total cross sectional area•  Velocity greatest in aorta•  Minimum in capillariesfavors nutrient exchange

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BLOOD VOLUME IN DIFFERENT SEGMENT•  84% in systemic circulation•  16% In heart and lungs•  Out of 84%, more than 2/3rd is in systemic veins

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PRESSURE GRADIENT IN CIRCULATION

•  Blood flows from higher pressure to lower pressure gradient,i.e., from ventricles to the rest of the body and the pulmonary

circuit, and back into atria

Systemic PulmonaryHeart (left ventricle)

Heart (left atrium)

aortaarteriesarteriolescapillarievenulesveinsvena cava

Heart (right atrium)

Heart (right ventricle)

pulmonary veinscapillariespulmonary arteries

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IMPORTANT PRESSURE GRADIENTS IN THE BODY

Left ventricle: 120/0 mmHg

Aorta: 120/80 mmHg

Systemic arteries: Mean Arterial pressure: 93mmHg

Peripheral veins: 15 mmHg

Right atrium: 0mmHg

Pressure gradient: 93-0 = 93mmHg

PULMONARY CIRCUITRight ventricle:25/0 mmHg

Pulmonary circuit: 25/8 mmHg

Mean Pulmonary Arterial pressure: 15 mmHg

Pulmonary venous pressure: 5 mmHg

Left atrium: 5-10 mmHg

Pressure gradient in pulmonary circuit: 15-5 =10 mmHg

PRINCIPLES GOVERNING HEMODYNAMICS IN THE BODY•  Flow of blood•  Mean Arterial Pressure•  Resistance

The most important factor governing flow is resistance

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RELATIONSHIP BETWEEN PRESSURE,

FLOW AND RESISTANCE

Q =P

R

Similar to Ohm’s Law I =

for electricity

 

V

Ror V = IR

 P = QR Change in Pressure = Flow x Resistance

Change in Pressure

Flow =

Resistance

Change in pressure is the difference between input (upstream) and output

(downstream) pressure

•When describing the flow of blood for an organ, the pressure difference is generallyexpressed as the difference between the arterial pressure (PA) and venouspressure (PV).

 RESISTANCE TO FLUID FLOW 

•  As fluid passes through a resistance pressure drops.

•  A resistance dissipates energy, so as the fluid works its way through the resistance it must

give up energy.

•  It gives up potential energy in the form of a drop in pressure

DETERMINANTS OF RESISTANCE IN LAMINAR FLOW The three major determinants of Resistance are:

Fluid flow

resistance

P1  P2 

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•  length•  viscosity•  Radius

• Therefore, R α L / r4

•  Or;•  R =  L / r4

TURBULENT FLOW•  Non layered flow

• Creates murmur•  Heard as bruits•  Produce more resistance than flow

•  Reynolds no.= diameter x velocity xdensityviscosity

•  > 2000 =turbulent flow•  <2000= laminar flow

POISEUILLE’S LAW •  Since,

Q =  PR

 And;R α  L / r4

Q =  P x r4

LTherefore, the most important factor effecting resistance is the radius

RESISTANCE IN SERIES•  If resistors are connected in series:•  The total resistance is the sum of all the individual resistances

i.e., R1 + R2 + R3 ….. Therefore,•  Adding a resistor in series will increase the resistance of the system•  The flow of blood in an individual organ e.g, a nephron is in series, so

adding resistance at any point in this circuit will : – Increase pressure upstream – Decrease pressure downstream

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