Circulation

Circulation- Dr. Chintan

Basic Theory of CirculatoryFunction

• 1. The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need

• 2. The cardiac output is controlled mainly by the sum of all the local tissue flows

• 3. In general the arterial pressure is controlled independently of either local blood flow control or cardiac output control

Pressure, Flow and Resistance• Blood flow through a blood vessel is determined by two

factors:

• (1) pressure difference of the blood between the two ends of the vessel, also sometimes called “pressure gradient” along the vessel, which is the force that pushes the blood through the vessel,

• (2) the inhibition to blood flow through the vessel, which is called vascular resistance - occurs as a result of friction between the flowing blood and the intravascular endothelium

Pressure, Flow & Resistance• Ohm’s law: F = ∆P / R• F is blood flow, • ∆P is the pressure difference between the two ends

of the vessel, • R is the resistance

Turbulent Flow• the flow may become turbulent

• When the rate of blood flow becomes too great,• when it passes by an obstruction in a vessel, • when it makes a sharp turn,• when it passes over a rough surface,

• the blood flows with much greater resistance

Turbulent Flow• Re = ρ v d / η

• where Re is Reynolds’ number and is the measure of the tendency for turbulence to occur,

• ρ is density,• v is the mean velocity of blood flow (in

centimeters/second), • d is the vessel diameter (in centimeters), • η is the viscosity (in poise).

Turbulent Flow• When Reynolds’ number rises above 200 to 400, turbulent flow will

occur at some branches of vessels but will die out along the smooth portions of the vessels.

• when Reynolds’ number rises above 2000, turbulence will usually occur even in a straight, smooth vessel

• Reynolds’ number can rise to several thousand during the rapid phase of ejection by the ventricles – causes turbulence in the proximal aorta and pulmonary artery

• (1) high velocity of blood flow, • (2) pulsatile nature of the flow, • (3) sudden change in vessel diameter, and• (4) large vessel diameter

Resistance to Blood Flow• If the pressure difference between two points is 1 mm

Hg and the flow is 1 ml/sec, the resistance is said to be 1 peripheral resistance unit (PRU)

• the resistance of the entire systemic circulation, called the total peripheral resistance, is about 1 PRU

• Widespread vasoconstriction – 4 PRU• Widespread vasodilatation – 0.2 PRU

• Total pulmonary vascular resistance - 0.14 PRU

Conductance• Conductance is a measure of the blood flow through

a vessel for a given pressure difference.

• This is generally expressed in terms of ml/sec/mmHg

• Conductance = 1 / Resistance

• Slight changes in the diameter of a vessel cause tremendous changes in the vessel’s ability to conduct blood when the blood flow is streamlined.

Conductance• Conductance α Diameter4

• the blood that is near the wall of the vessel flows extremely slowly, whereas that in the middle of the vessel flows extremely rapidly

• Poiseuille’s law: F = ∏∆Pr4 / 8ηl• F is the rate of blood flow, • ∆P is the pressure difference between the ends of the vessel, • r is the radius of the vessel, • η is viscosity of the blood, • l is length of the vessel

Fourth Power Law• fourth power law makes it possible for the

arterioles, responding with only small changes in diameter to nervous signals or local tissue chemical signals,

• either to turn off almost completely the blood flow to the tissue

• at the other extreme to cause a vast increase in flow

Hematocrit and Viscosity• The greater the viscosity, the less the flow

• the viscosity of normal blood is about 3 times as great as the viscosity of water

• large numbers of suspended red cells in the blood

• The percentage of the blood that is cells is called the hematocrit

• Male – 45, female - 40

Hematocrit and Pressure• The viscosity of blood increases considerably as the

hematocrit increases

• factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma

• increase in arterial pressure not only increases the force that pushes blood through the vessels but also distends the vessels at the same time, which decreases vascular resistance

Vascular Distensibility• the distensible nature of the arteries allows them to accommodate

the pulsatile output of the heart and to average out the pressure pulsations.

• This provides smooth, continuous flow of blood through the very small blood vessels

• The most distensible by far of all the vessels are the veins. Even slight increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra blood.

• The veins provide a reservoir function for storing large quantities of extra blood that can be called into use whenever required elsewhere

Stress-Relaxation of Vessels• a vessel exposed to increased volume at first exhibits a large

increase in pressure,

• but progressive delayed stretching of smooth muscle in the vessel wall allows the pressure to return back toward normal over a period of minutes to hours

• stress-relaxation is a valuable mechanism by which the circulation can accommodate much extra blood when necessary, such as after too large a transfusion.

• Reverse stress-relaxation is one of the ways in which the circulation automatically adjusts itself over a period of minutes or hours to diminished blood volume after serious hemorrhage

Thank You…