Dynamics of Blood FlowDynamics of Blood Flow11.12.1211.12.12

Forces acting on blood during Forces acting on blood during circulationcirculation

Circulatory SystemCirculatory System

A closed double-pump system:A closed double-pump system:

SystemicCirculation

LungCirculation

Left side of heart

Right side of heart

Circulatory SystemCirculatory System

Branching of blood vesselsBranching of blood vessels– Arteries branch into arterioles, veins into Arteries branch into arterioles, veins into

venulesvenules

ArteriesArterioles

Capillaries

Venules

Veins

Heart

Forces acting on blood during Forces acting on blood during circulationcirculation

The main forces acting on blood during circulationThe main forces acting on blood during circulation

Viscous force (FViscous force (Fvv))

Pressure gradient force FPressure gradient force FPP (force produced by heart pump) (force produced by heart pump)

Gravitational force FGravitational force FGG

According to Newton’s law of motion which also governs According to Newton’s law of motion which also governs

the motion of bloodthe motion of blood

F = FF = FV V + F+ FPP+ F+ FGG

Viscous force for Newtonian FluidsViscous force for Newtonian Fluids

Viscosity is the property of flowing fluid Viscosity is the property of flowing fluid (liquid/gas) by virtue of which relative motion (liquid/gas) by virtue of which relative motion between layers in contact is opposedbetween layers in contact is opposed

Relative motion causes internal friction between Relative motion causes internal friction between layers in contact. This internal friction is called layers in contact. This internal friction is called viscous forceviscous force

For Newtonian fluids, the viscous force is For Newtonian fluids, the viscous force is proportional to the surface area (A) of the layer proportional to the surface area (A) of the layer and the velocity gradient between layers (in the and the velocity gradient between layers (in the direction perpendicular to the layer) direction perpendicular to the layer)

Viscous force and viscosity of Viscous force and viscosity of Newtonian FluidsNewtonian Fluids

ηη is the constant of proportionality and is the viscosity. is the constant of proportionality and is the viscosity.

Negative sign shows that F acts in a direction opposite to Negative sign shows that F acts in a direction opposite to the one in which the layers movethe one in which the layers move

Viscosity can be expressed as the ratio of shear stress Viscosity can be expressed as the ratio of shear stress and time rate of shear strain (or shear rate)and time rate of shear strain (or shear rate)

Shear stress produced by a ball Shear stress produced by a ball falling in a stationary liquid falling in a stationary liquid

A spherical ball falling in a A spherical ball falling in a viscous fluid produces a viscous fluid produces a shear stress shear stress

It reaches a terminal speed It reaches a terminal speed when the sum of the forces when the sum of the forces acting on it is zeroacting on it is zero

Laminar flowLaminar flow

Blood PressureBlood Pressure

Blood pressureBlood pressure ( (BPBP) is the pressure exerted by ) is the pressure exerted by circulating blood upon the walls of blood vesselscirculating blood upon the walls of blood vessels

The blood pressure in the circulation is The blood pressure in the circulation is principally due to the pumping action of the principally due to the pumping action of the heart. Differences in blood pressure are heart. Differences in blood pressure are responsible for blood flow from one location to responsible for blood flow from one location to another in the circulationanother in the circulation

Cardiac Cycle - Filling of Heart Chambers Cardiac Cycle - Filling of Heart Chambers Heart is two pumps that work together, right and left halfHeart is two pumps that work together, right and left half Repetitive contraction (systole) and relaxation (diastole) Repetitive contraction (systole) and relaxation (diastole)

of heart chambersof heart chambers Blood moves through circulatory system from areas of Blood moves through circulatory system from areas of

higher to lower pressure.higher to lower pressure.› Contraction of heart produces the pressureContraction of heart produces the pressure

Cardiac Cycle - Filling of Heart Chambers

Mechanical systole and diastole

Systolic and Diastolic Systolic and Diastolic PressurePressure

The force of contraction of the ventricles The force of contraction of the ventricles raises the pressure to about 120mmHg raises the pressure to about 120mmHg (systolic pressure) and the elastic recoil of (systolic pressure) and the elastic recoil of the arteries maintain the pressure to about the arteries maintain the pressure to about 80mmHg during ventricular diastole 80mmHg during ventricular diastole (diastolic pressure)(diastolic pressure)

This pressure is enough to keep the blood This pressure is enough to keep the blood flowing continuously to all parts of the flowing continuously to all parts of the bodybody

Mean arterial pressureMean arterial pressure It is a term used in medicine to describe an average It is a term used in medicine to describe an average

blood pressure in an individual.blood pressure in an individual. It is defined as the It is defined as the average arterial pressure during a single cardiac average arterial pressure during a single cardiac cyclecycle

Mean arterial blood pressure is not the arithmetic Mean arterial blood pressure is not the arithmetic mean of systolic and diastolic pressure but is mean of systolic and diastolic pressure but is instead about 93mmHginstead about 93mmHg

This is because the time the heart spends relaxing This is because the time the heart spends relaxing is longer than the time it spends contracting and is longer than the time it spends contracting and ejecting blood into aortaejecting blood into aorta

Blood Pressure ProfileBlood Pressure ProfileBlood pressure is highest in the arteries. It decreases as the circulating Blood pressure is highest in the arteries. It decreases as the circulating blood moves away from the heart through arterioles, capillaries and then blood moves away from the heart through arterioles, capillaries and then to veins due to viscous losses of energyto veins due to viscous losses of energy

Although blood pressure drops over the whole circulation, most of the Although blood pressure drops over the whole circulation, most of the fall occurs along the arterioles fall occurs along the arterioles

Reference points for measuring blood Reference points for measuring blood pressurepressure

While measuring pressures in cardiovascular system, While measuring pressures in cardiovascular system, ambient atmospheric pressure is used as zero ambient atmospheric pressure is used as zero reference point. Thus a blood pressure of 90mmHg reference point. Thus a blood pressure of 90mmHg means that pressure is 90mmHg above atmospheric means that pressure is 90mmHg above atmospheric pressure pressure

The second reference point for measuring blood The second reference point for measuring blood pressure is anatomical and is the position of heart. For pressure is anatomical and is the position of heart. For example, the usual convention is to measure blood example, the usual convention is to measure blood pressure in the brachial artery above elbow i.e. pressure in the brachial artery above elbow i.e. approximately at hearts level when patient is seatedapproximately at hearts level when patient is seated

If the blood pressure measurements are to be made in If the blood pressure measurements are to be made in the legs, the patient is brought to lying down position. In the legs, the patient is brought to lying down position. In this position vessel is approximately at cardiac level this position vessel is approximately at cardiac level

Blood pressure measurementBlood pressure measurementDirect methodDirect method

This is an invasive method in which artery or vein is This is an invasive method in which artery or vein is cannulated or catheterised. Pressure measured by direct cannulated or catheterised. Pressure measured by direct method is known as “end pressure” Here the kinetic method is known as “end pressure” Here the kinetic energy of blood flow is measured in terms of pressure. energy of blood flow is measured in terms of pressure. Direct method is used in patients of ‘shock’ where indirect Direct method is used in patients of ‘shock’ where indirect measurements may be inaccurate or indeed impossiblemeasurements may be inaccurate or indeed impossible

Indirect methods of blood Indirect methods of blood pressure measurementpressure measurement

Indirect method (non-invasive, Indirect method (non-invasive, measures lateral/side pressure)measures lateral/side pressure)

AuscultatoryAuscultatory

OscillometricOscillometric

Auscultatory MethodAuscultatory Method

The auscultatory method uses a The auscultatory method uses a stethoscope and a sphygmomanometerstethoscope and a sphygmomanometer

An inflatable cuff encircles the arm. An inflatable cuff encircles the arm. Pressure in the cuff is transmitted Pressure in the cuff is transmitted through the tissue to compress brachial through the tissue to compress brachial artery and can be viewed on a artery and can be viewed on a manometermanometer

A stethoscope is used to listen to A stethoscope is used to listen to sounds in the artery distal to the cuff. sounds in the artery distal to the cuff. The sounds heard during measurement The sounds heard during measurement of blood pressure are not the same as of blood pressure are not the same as the heart sounds 'lub' and 'dub' that are the heart sounds 'lub' and 'dub' that are due to vibrations inside the ventricles due to vibrations inside the ventricles that are associated with the snapping that are associated with the snapping shut of the valvesshut of the valves

If a stethoscope is placed over the brachial artery in a If a stethoscope is placed over the brachial artery in a normal person no sound should be audible. As the normal person no sound should be audible. As the heart beats, pulses (pressure waves) are transmitted heart beats, pulses (pressure waves) are transmitted smoothly via laminar (non-turbulent) blood flow smoothly via laminar (non-turbulent) blood flow throughout the arteries, and no sound is producedthroughout the arteries, and no sound is produced

Similarly, if the cuff of a sphygmomanometer is placed Similarly, if the cuff of a sphygmomanometer is placed around a patient's upper arm and inflated to a pressure around a patient's upper arm and inflated to a pressure above the patient's systolic blood pressure, there will above the patient's systolic blood pressure, there will be no sound audible. This is because the pressure in be no sound audible. This is because the pressure in the cuff is high enough such that it completely occludes the cuff is high enough such that it completely occludes the blood flowthe blood flow

Auscultatory MethodAuscultatory Method

Oscillometric methodOscillometric method

The oscillometric method was first The oscillometric method was first demonstrated in 1876 and involves the demonstrated in 1876 and involves the observation of oscillations in the observation of oscillations in the sphygmomanometer cuff pressuresphygmomanometer cuff pressure[[ which are which are caused by the oscillations of blood flow, i.e. caused by the oscillations of blood flow, i.e. the pulse the pulse

It uses a sphygmomanometer cuff, like the It uses a sphygmomanometer cuff, like the auscultatory method, but with an electronic auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure sensor (transducer) to observe cuff pressure oscillations, electronics to pressure oscillations, electronics to automatically interpret them, and automatic automatically interpret them, and automatic inflation and deflation of the cuff. inflation and deflation of the cuff.

The cuff is inflated to a pressure initially in excess of the The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below systolic arterial pressure and then reduced to below diastolic pressure over a period of about 30 seconds. diastolic pressure over a period of about 30 seconds.

When blood flow is nil (cuff pressure exceeding systolic When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. It is pressure), cuff pressure will be essentially constant. It is essential that the cuff size is correct: undersized cuffs essential that the cuff size is correct: undersized cuffs may yield too high a pressure; oversized cuffs yield too may yield too high a pressure; oversized cuffs yield too low a pressurelow a pressure

Oscillometric methodOscillometric method

Oscillometric methodOscillometric method

When blood flow is present, but restricted, the cuff When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. and contraction of the brachial artery, i.e., it will oscillate. The values of systolic and diastolic pressure are The values of systolic and diastolic pressure are computed, results are displayedcomputed, results are displayed

Blood PressureBlood PressureEffect of gravity on pressureEffect of gravity on pressure– Distance heart-head~ 0.4 mDistance heart-head~ 0.4 m– Heart-feet ~ 1.4 mHeart-feet ~ 1.4 m– P = P = ghgh

55 mm Hg

100 mm Hg

195 mm Hg

100 mmHg95 mmHg 95 mm Hg

-35 mm Hg

1 mm Hg

105 mm Hg

Venous pressures

Arterial pressures

The pressure in any vessel above heart The pressure in any vessel above heart level is decreased by the effect of gravitylevel is decreased by the effect of gravity

The arterial pressure is increased by The arterial pressure is increased by 0.77mmHg for every centimeter below the 0.77mmHg for every centimeter below the right atrium and similarly decreased for right atrium and similarly decreased for each cm above the right atriumeach cm above the right atrium