Biomecanics of circulation

BIOMECHANICS OF CIRCULATION

BY:DR.SHILPA ( F.Y. MPT)

Dell

INTRODUCTIONThe function of the circulation is to service the needs of the tissue-

•To transport nutrients to the tissues,

•To transport waste products away,

•To conduct hormones from one part of the body to another,

•To maintain an appropriate environment in all the tissue fluids for survival and function of the cells.

PHYSICAL CHARACTERISTICSDivided into-

•Systemic circulation and

•Pulmonary circulation

PHYSICAL CHARACTERISTICSFUNCTOINAL PARTS OF THE CIRCULATION:

ARTERIES- transport blood under high pressure to the tissues

ARTERIOLS- last small branches of the arterial system

• they act as control valves

•It has a strong muscular wall that is closing the arteriole completely or allowing it to be dilated, in response to the needs of the tissue.

PHYSICAL CHARACTERISTICSCAPILLARIES-

•To exchange fluid, Nutrient, Electrolytes, hormones & other substances btw. the blood and interstitial fluid.

•For this role, the capillary walls are very thin and permeable to small molecular substances.

VENULES- collect blood from capillaries, gradually coalesce into progressively larger veins

VEINS- conduits for transport of blood from the tissues back to the heart◦ They are muscular and this allows them to

contract or expand◦ There by act as a controllable reservoir for extra

blood, either a small or large amount, depending on the needs of the body.

PHYSICAL CHARACTERISTICS

VOLUME OF BLOOD IN THE DIFFERENT PARTS OF THE CIRCULATION:

Systemic circulation-84%◦ Veins-64%◦ Arteris-13%◦ Arterioles &◦ Capillaries-7%

Pulmonary vessels-9%

The heart-7%


CROSS-SECTIONAL AREAS AND VELOCITY OF BLOOD FLOW:

• Much larger cross-sectional areas of the veins than of the arteries, averaging about four times those of corresponding arteries

• Large storage of blood in the venous system than arterial system


AORTA 2.5cm square

SMALL ARTERIES 20cm square

ARTERIOLES 40 cm square

CAPILLARIES 2500cm square

VENAE CAVAE 8cm square

VENULES 250cm square

SMALL VEINS 80cm square

PRESSURES IN THE VARIOUS PORTIONS OF THE CIRCULATION:

Pumping blood continually by the Heart into the aorta is pulsatile,

Arterial Pressure fluctuates systolic level btw 100-120 mmHg, Diastolic level of 80 mmHg


◦ Near the arteriolar end ……… 35 mmHg,◦ In most vascular bed……….. 17 mmHg,◦ Near the venous end……….. 10 mmHg,◦ Vena cava to Rt atrium …….. 0 mmHg

Pulmonary artery, systolic pressure 25 mmHg, and diastolic pressure of 8 mmHg.

Pulmonary capillary pressure averages 7 mmHg only


THE BLOOD FLOW TO EACH TISSUE OF THE BODY IS ALMOST ALWAYS PRECISELY CONTROLLED IN RELATION TO THE TISSUE NEEDS:

When tissue are active they need much more blood flow, 20-30 times than resting level.

Heart normally can’t increase c.o. more than 4-7 times.

Therefore, it is not possible to increase the blood flow everywhere in the body when a particular tissue demands.

BASIC THEORY OF CIRCULATORY FUNCTION

Instead, the microvessels of each tissue continuously monitor the tissue needs such as the

◦ availability of oxygen and nutrients,

◦ the accumulation of the CO2 and other tissue west products,

◦ and the level required for the tissue activity.

Nervous control of the circulation provides additional specific attributes to tissue blood flow control.


THE CARDIAC OUTPUT IS CONTROLLED MAINLY BY THE SUM OF ALL THE LOCAL TISSUE FLOWS:

When blood flows through the tissue, it immediately returns by way of the vein to the heart

Heart responds to increase in flow of blood back into the arteries

In this sense, heart act as an automaton, responding to the demand of tissue

Often heart needs help in the form of special nerve signals to make it pump the required amount of the blood flow.


IN GENERAL, THE ARTERIAL PRESSURE IS CONTROLLED INDEPENDENTALY OF EITHER LOCAL BLOOD FLOW CONTROL OR C.O. CONTROL.

• The circulatory system is provided with an extensive system for controlling the arterial pressure

• If pressure falls below 100 mmHg, a barrage of nervous reflexes within seconds elicits a series of circulatory changes to raise a pressure back to normal,


Including increased ◦ force of heart pumping,

◦ Contraction of the large venous reservoir to provide more blood for the heart,

◦ Constriction of most of arterioles throughout the body

Kidneys also play an additional major roll in pressure control both by hormone and by regulating blood volume


Flow determined by two factors:

1) The pressure difference btw two ends of the vessel (pressure gradient), the force which pushes the blood through the vessel

2) The impediment to blood flow through the vessel ( vascular resistance)

INTERRELATIONSHIPS AMONG PRESSURE, FLOW, AND RESISTANCE

Calculation of flow: Q= P RWhere ; Q is blood flow, P is pressure difference (p1-p2); P1 is pressure at the origin of the

pressure P2 is pressure at the other end, R is resistance to flow.

INTERRELATIONSHIPS AMONG PRESSURE, FLOW, AND RESISTANCE

DIFINATION: The quantity of blood that passes a given point in the circulations in a given period.

Expressed in milliliter or liter per minute

At rest cardiac output is about 5000 ml/min.

Method for measuring;◦ Electromagnetic Flowmeter◦ Ultrasonic Doppler Flowmeter

BLOOD FLOW

ELECTROMAGNETIC FLOWMETER:

Measuring of blood flow without opening the vessel

PRINCIPLE: Generation of electromotive force in a wire that is moved rapidly in the cross-wise direction through a magnetic field.

A blood vessel is placed on the two side of the vessels perpendicular to the magnetic lines of force,

BLOOD FLOW

When blood flows through the vessels, electrical voltage proportional to the rate of flow is generated btw the two electrodes and this is recorded using the appropriate meters

BLOOD FLOW

Ultrasonic Doppler Flowmeter:

A minute piezoelectric crystal mounted in the wall of device,

Crystal, when energized with electronic apparatus, transmits sounds at a frequency of ‘several million cycles per second along the flowing blood

A portion of sound is reflected by the red blood cells, so that reflected

wave backward toward the crystal.

BLOOD FLOW

Reflected wave have lower frequency than the transmitted wave because the red cells are moving away from the transmitted crystal is called the Doppler effect

Another portion of the apparatus determines the frequency difference btw the transmitted wave and the reflected wave,

Also determine the velocity of blood flow

It also capable of recording rapid, pulsatile changes in flow as well as steady flow

BLOOD FLOW

PARABOLIC VELOCITY PROFILE DURING LAMINAR FLOW

Laminar flow (streamlines flow) is opposite to turbulent flow

When laminar flow occurs, the velocity of flow in the center of the vessel is far greater than that toward the outer edges

LAMINAR FLOW OF BLOOD IN VESSELS

After, parabolic interface develops btw two fluids, the portion of fluid adjacent to the wall is hardly moved

Slightly away portion moved small distance and

Portion in the center of vessel has moved long distance

This effect is called the parabolic profile for the velocity of blood flow.

LAMINAR FLOW OF BLOOD IN VESSELS

TURBULENT FLOW OF BLOOD UNDER SOME CONDITIONS

Rate of blood flow become to great wheno It passes an obstruction in a vessel,oWhen it makes a sharp turn oroWhen it passes over a rough surface

Flow may become turbulent, means that the blood flows crosswise in the vessel as well as along the vessel called eddy current

TURBULENT FLOW

It increase ino Direct proportion to velocity of flow

o Direct proportion to diameter of vessels

o Inversely proportional to the viscosity of blood divided by its density.

Velocity*Diameter Turbulent flow(Re)= ── viscosity/density

When Re >2000, in some large arteries always some turbulence present, such as root of aorta and its

branches.

TURBULENT FLOW

Standards units of pressure

1) Millimeter of mercury(mm Hg)• Hg’s density allow for shortercolumn to measure given pressure

2) Centimeter of water(cmH2O)• Water is nontoxic and readily available

DEFINITION: Blood pressure means the force exerted by the blood against any unit area of the vessel wall.

BLOOD PRESSURE

BLOOD PRESSURE When one says the pressure in vessel is

50mmHg, means that the force exerted is sufficient to push

the column of mercury up to a level 50mm high.

Occasionally, pressure is measured in centimeter of water(cmH2O)

A pressure of 10 cmH2O, means a pressure sufficient to raise a column of

water to a height of 10 centimeter.

HIGH- FIDILITY METHODS FOR MEASURING BLOOD

It cannot responds to pressure changes that occurs more rapidly than 2 to 3 seconds.

Whenever, it is desired to record rapidly changing pressure, some electronic pressure transducer commonly used.

BLOOD PRESSURE

Mercury in the manometer has so much inertia that it can not rise and fall rapidly.

It used for converting pressure into electrical signals and then recording the pressure on a high-speed electrical recorder.

Each of these transducers uses a very thin, highly stretched metal membrane that forms one wall of the fluid chamber.

The fluid chamber is connected through a catheter with the vessel

BLOOD PRESSURE

It works on 3 basic principles:

i. Works on capacitance,ii. Works on inductance,iii. Works on resistance

with this high-fidelity type of recorder systems, pressureCycle up to 500 cycles/sec.

Capable of registering pressure changes that occur as rapidly as 20 to 100 cycles/sec.

BLOOD PRESSURE

Resistance is a impediment to blood flow in a vessel,

It must be calculated from measurements of blood flow and pressure difference in the vessel, in unit it is peripheral resistance unit(PRU).

mmHg Total peripheral Resistance = ------. ml/sec

RESISTANCE TO BLOOD FLOW

TOTAL PERIPHERAL RESISTANCE AND TOTAL PULMONARY RESISTANCE:

The rate of blood flow when a person is at rest is close to 100 ml/sec

And the pressure difference from the systemic artery to the systemic vein is about 100 mmHg

So, the total peripheral resistance is 1PRU.

The total peripheral resistance rises to as high as 4 PRU blood vessels become strongly constricted.


When the muscle become greatly dilated, it can fall as little as 0.2 PRU

In pulmonary system, the mean Rt arterial pressure averages 16 mmHg and the mean LT atrial pressure averages 2 mmHg

giving a pressure difference of 14 mmHg, so the total periphery resistance at rest is 0.14mmHg


“CONDUCTANCE” OF BLOOD IN A VESSELS AND ITS RELATION TO RESISTANCE :

Measure of the blood flow through a vessel for a given pressure difference.

Expressed in terms of milliliter/second per millimeter of mercury pressure.

Conductance = 1/Resistance


VERY SLIGHT CHANGES IN DIAMETER OF A VESSAL CHANGE ITS CONDUCTANCE TREMENDOUSLY

Changes in its ability to conduct blood flow is streamline

Diameter of 1,2&4 with the same pressure difference of 100mmHg


The respective flows are 1,16,256 ml/mm.

Diameter of vessel increase only 4-fold,when flows increases as 256-fold.

4

Conductance α (diameter)


POISEUILLE’S LAW : The concentric rings inside the vessels indicate that velocity of flow in each ring is

different from that in the other rings becoz of laminar flow.

Thus, the blood near the wall of the vessel flows slowly,whreas the middle of the vessel flows rapidly.


Formula :

Q=∏∆Pr4 / 8ŋl

Where Q : rate of blood flow ∆P: pressure difference r : radius of the vessel l : length of vessel ŋ: velocity of the blood flow.

RESISTANCE BLOOD FLOW

One other factor in the poiseuille’s law is viscosity of the blood.

Greater the viscosity ,less the flow

The viscosity of the normal blood is about three times as great as the viscosity of water

It is mainly due to large number of red cells

Each of which exerts frictional drag against adjacent cells and the wall of vessel.

FFECT OF BLOOD HEMATOCRIT AND VEISCOCITY

HEMATOCRIT: IS the % of the blood that is cells

If hematocrit of 40, 40% of blood volume is cells and the remainder is plasma.

average in men : about 42 women: about 38

These values vary, depending on: 1) person has anemia or not 2) degree of bodily activity 3) altitude at which person resides


EFFECT OF HEMATOCRIT ON BLOOD VISCOSITY:

The greater the percentage of hematocrit – the more friction

btw Layers of blood.

The viscosity of blood increases as the hematocrit increases


Viscosity of whole blood is 3 means,

Three times more pressure is required to force whole blood as to force water from the same tube.

If it rises > 60% - Polycythemia, blood flow through the vessels is greatly

retarded


BLOOD VISCOCITY IN MICROCIRCULATION:

Most resistance in the circulatory system occurs in the very small blood vessels.

Three factors as well as hematocrit and plasma protein affect blood viscosity in these vessels

1) blood flow in minute tubes exhibit far less viscous effect than it does in large vessels

Called FAHREAUS-LINDQUIST EFFECT


The red cells, instead of moving randomly, line up and move through the vessels as a single plug

Thus increasing the viscous resistance

2) The viscosity of blood increasing tremendously as its velocity of flow decreases.

Becoz the velocity of blood flow in the small vessels is slow, <1mm/sec,

Viscosity can increase as much as 10-folds.


3) Cells often become stuck at constriction in small vessels;

In capillaries the nuclei of endothelial cells protrude into the capillary lumen

Blood flow can become blocked for a fraction of second or for much longer period

Thus increase the viscosity


An increase in arterial pressure would cause proportionate increase in

blood flow through thevarious tissue of the body.

The effect of pressure on blood flow is far greater than one would expect

EFFECT OF PRESSURE

An increase in arterial pressure not only increase the force, to push blood through the vessels, but also distends the vessels

Which decrease their resistance

Large changes in blood flow can be change by symp. Stimulation

Inhibition of symp. stimulation greatly dilates the vessels, increase flow 2-fold or more.

EFFECT OF PRESSURE

Conversely, strong symp. Stimulation can constrict the vessels so much

That the blood flow can be decrease to as law as zero for short periods despite high arterial pressure

EFFECT OF PRESSURE

TEXT BOOK OF MEDICAL PHYSIOLOGY, 9th edition, BY GUYTON

CLAYTON’S ELECTROTHERAPY, 8th edition

REFERENCE

THANK YOU

Date post:	31-May-2015
Category:	Business
Upload:	shilpa-prajapati
View:	340 times
Download:	3 times

Biomecanics of circulation

Business