Lecture 10: Overview of circulation Flashcards
Velocity of blood flow is _____ proportional to vascular cross-sectional area
Inversely
Overall blood flow of an adult at rest is
5000 mL/min = Cardiac output
Three major variables that determine resistance and which one is most important
Vessel radius (most important)
Blood viscosity
Vessel length
Prime determinant of blood viscosity is the
Hematocrit
If arterial pressure falls below 100 mm Hg, nervous reflexes:
Increase force of heart pumping
Constrict large venous reservoirs
Generally constrict most of the arterioles throughout the body (increase arterial pressure)
Kidneys may later play important role in pressure control
Eddy currents
Blood flows with greater resistance when eddy currents occur
Examples of circulations arranged in parallel
Brain Kidney Muscle GI Skin Coronary circulation
% volume of blood in arteries
13
Flow is ______ proportional to pressure difference but ______ proportional to resistance
Flow is directly proportional to pressure difference but inversely proportional to resistance
For blood vessels arranged in parallel, the total resistance to blood flow is expressed as
1/Rtotal = 1/R1 + 1/R2 +1/R3 ……
Therefore, the total resistance is far less than the resistance of any single blood vessel
Amputation of limb, removal of kidney removes a parallel circuit. What is the effect
Reduces total vascular conductance
Reduces total blood flow
Increases total peripheral vascular resistance
Laminar flow
Streamline flow Blood flows at steady rate Blood vessel is long and smooth Blood flows in streamlines (layers) Velocity of blood in center is greater than on the outer edges
Two main factors controlling blood flow
Pressure difference between the two ends of a vessel (pressure gradient)
Impediment to blood flow through the vessel (resistance)
Conductance is ______ proportional to vessel diameter
Directly
When Re rises above 2000
Turbulent flow will occur even in a straight vessel
Poiseuilles law/equation
F = (Pi*(P1-P2)*r^4) / 8nl F- rate of blood flow mL/min P-pressure r- radius of vessel l- length of vessel n- blood viscosity
Turbulent flow
Non-layered flow
Creates murmurs
Produces more resistance than laminar flow
When cardiac output is normal at 100mL/sec the total pulmonary vascular resistance is
14/100 or 0.14 PRU
Functional principles of the circulatory system
Rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need
The cardiac output is controlled mainly by the sum of all the local tissue flows
Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control
When vessels are greatly dilated, total peripheral resistance may
Drop to 0.2 PRU
Ohms law (Poiseuille equation)
F=(P1 - P2)/R F- flow in mL/min P1- upstream pressure P2- pressure at end of segment R- resistance b/w P1 and P2
% volume of blood in systemic arterioles and capillaries
7
% volume of blood in heart and lungs
16
Blood viscosity
The property of blood to adhere to vessel walls and to eachother
- based on the number, shape and size of RBCs
- Viscosity ensures laminar flow of blood in vessels
The largest pressure drop in the circulatory system occurs where
In the arterioles because they have the highest resistance
Total conductance for vessels arranged in parallel is
The sum of the conductance of each parallel pathway
Rate of blood flow in entire circulatory system is
100ml/sec
% volume of blood in veins
64
Resistance of the entire systemic circulation (total peripheral resistance) is
100/100 or 1 PRU (peripheral resistance unit)
Mean left atrial pressure averages
2 mm Hg
% volume of blood in systemic circulation
84
Conductance definition and equation
The measure of blood flow through a vessel for a given pressure difference
Conductance = 1/resistance
Blood flow autoregulation
The ability of each tissue to adjust its vascular resistance and to maintain normal blood flow through changes in arterial pressure between approximately 70 and 175 mm Hg
Turbulent flow occurs when
Flow is too great
Blood passes an obstruction with the vessel
Blood has to make a sharp turn
Blood passes over rough surface
When Re rises above 200-400:
Turbulent flow will occur in some regions of a vessel
Pressure difference from systemic arteries to systemic veins is
100 mm Hg
Tendency for turbulent flow increases
In direct proportion to velocity of blood flow
In direct proportion to diameter of vessel
In direct proportion to density of the blood
Inversely to the viscosity of the blood
Mean pulmonary arterial pressure averages
16 mm Hg
A fourfold increase in vessel diameter can have what effect on blood flow
Increase flow by as much as 256x
Velocity of blood flow equation
V=F/A
F- volume of blood flow
A- vascular cross-sectional area
Resistance equation
R= 8nl / Pi x R^4
n- viscosity of blood
l- length of vessel
r^4- radius of vessel to the fourth power
In conditions when vessels are strongly constricted, total peripheral resistance may
Rise to 4 PRU
Reynolds number equation
RE=(V x D x p) / n V-mean velocity of blood flow in cm/sec D- vessel diameter in cm p- density n- viscosity (in pose)- blood viscosity is normally 1/30 poise
