Lecture 10: Circulation Flashcards
How does systemic circulation compare to pulmonary circulation?
- Systemic:
- Arterial pressure from 120 mm Hg (systolic) to 80 mm Hg (diastolic)
- Drops to 0 mm Hg by the time it reaches the termination of the vena cava.
- Systemic capillary pressure varies from 35 mm Hg to 10 mm Hg.
- Pulmonary:
- Pulmonary artery systolic pressure = 25 mm Hg.
- Pulmonary diastolic pressure = 8 mm Hg
- Briefly look at figure 14-2 on slide 5
List the 5 functional parts of circulation
- Arteries
- Transport under high pressure
- Arterioles
- Control conduits
- Capillaries
- Exchange between blood and extracellular fluid
- Venules
- Veins
How is blood distributed throughout the system?
- 84% of blood volume is in the systemic circulation.
- 64% is in the veins.
- 13% is in the arteries.
- 07% is in the systemic arterioles and capillaries.
- 16 % of blood volume is in the heart and lungs.
- Refer to Figure 14-1 (Slide 8)
Describe the velocity of blood flow
- Velocity of blood flow (V) is inversely proportional to vascular cross-sectional area (A).
- (F = volume of blood flow)
V = F/A - 33 cm/sec in aorta at rest (area = 2.5 cm(squared))
- 0.3 mm/sec in capillaries at rest(area = 2500 cm(squared))
- See Slide 10
Note: F = AV seems easier, but whatever.
Note what F and V stand for, don’t let that confuse you
Also note the units, I could see him using that as a trick question.
…also 33 cm/sec seems really fast.
Word for word list the three 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.
- Incomplete resume at slide 12
Describe the first functional principle of the circulatory system in better detail
Rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need.
- In each tissue, microvessels monitor tissue needs.
- O2, other nutrients, CO2 accumulation, tissue waste product accumulation:
- Act directly on local blood vessels and dilate or constrict accordingly.
Describe the second principle of the circulatory system in more detail
The cardiac output is controlled mainly by the sum of all the local tissue flows
- Heart responds to demands of tissues.
- Nerve signals may be needed to help the heart pump required amount of blood.
Describe the third functional principle of the circulatory system in more detail
Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control.
- 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 (increases arterial pressure).
- Kidneys may later play important role in pressure control.
What are the two major factors that determine blood flow
- Pressure difference between the two ends of a vessel (pressure gradient)
- Impediment to blood flow through the vessel (resistance)
Describe the Poiseuille Equation for Blood Flow
Flow through a vessel can be calculated by Ohm’s law (Poiseuille equation):
- F = ΔP/R
- ΔP = (P1–P2)
F = flow in mL/min P1 = upstream pressure P2 = pressure at end of segment R = resistance between P1 and P2
- Flow is directly proportional to pressure difference but inversely proportional to resistance.
- Note that Poiseuille’s equation can be applied to a single vessel, an organ, or an entire circuit.
What are other forms of the Poiseuille Equation?
Other forms:
ΔP = F X R
R = ΔP/F
- Blood flow is usually expressed as ml/min or liters/min.
Overall blood flow of an adult at rest:
= 5000 ml/min = cardiac output.
- See Slide 20
Describe the idea behind laminar flow
- Laminar blood flow = streamline flow:
- Blood flows at a steady rate.
- Blood vessel is long and smooth.
- Blood flows in streamlines (layers).
- Each layer maintains same distance from vessel wall.
- Central-most portion of the blood stays in the center.
- Each layer slips easily past surrounding layers.
- Velocity of fluid flowing in center is greater than that of fluid flowing towards the outer edges.
- See Slide 22
How does turbulent flow compare to laminar flow
- Turbulent flow:
- Turbulent flow is nonlayered flow.
- Turbulent flow creates murmurs.
- Turbulent flow produces more resistance than laminar flow.
- Occurs:
- When flow is too great
- When blood passes an obstruction within the vessel
- When blood has to make a sharp turn
- When blood passes over a rough surface
- Blood flows with greater resistance when eddy currents occur
How does the tendency for turbulent flow Increase
- In direct proportion to velocity of blood flow
- In direct proportion to the diameter of the vessel
- In direct proportion to the density of the blood
- Blood is heavier than water.
- Specific gravity = 1.055.
- Blood density depends on the proportion of its components and in particular of red blood cells and proteins
- Inversely to the viscosity of the blood
- Blood is more viscous than water.
- Relative value of blood viscosity is 4.5, compared with the viscosity of water.
- Blood viscosity is the property of blood to adhere to vessel walls and to each other and is based on the number, shape and size of red cells.
- Viscosity ensures laminar flow (in layers) of blood through the vessels.
- See Slide 27
What is the formula for laminar flow?
Re = (v · d · ρ)/η
- Re = Reynolds number = measure of the tendency for turbulence to occur
- v = mean velocity of blood flow in cm/sec
- d = vessel diameter in cm
- ρ = density (normally only slightly > 1)
- η = viscosity (in poise) (blood viscosity normally = 1/30 poise)
What happens as Reynold’s number rises?
- When Re rises above 200-400, turbulent flow will occur in some regions of a vessel.
- When Re rises above 2000, turbulence will occur even in a straight vessel.
How is blood pressure defined?
- Blood pressure means the force exerted by the blood against any unit area of the vessel wall.
- Pressure can be measured with a mercury manometer or with electronic transducers.
What is the definition of Resistance in circulation?
- Resistance is the impediment to blood flow in a vessel.
- It must be indirectly calculated from measurements of blood flow and pressure.
- If pressure difference between two points is 1 mm Hg and the flow is 1 ml/sec:
R = (1 PRU (Peripheral Resistance Movement)) = (1 mmHg/(1 mL/sec)) = (Pressure/Volume/Time)
What are the 3 largest variables that determine resistance?
- Vessel radius (Most important factor)
- Blood viscosity
- Vessel length
What happens to blood flow, upstream pressure, and downstream pressure respectively when resistance is Increased?
Decreased?
- Increased Resistance = DECREASED blood flow, INCREASED upstream pressure, and DECREASED downstream pressure
- Decreased Resistance = INCREASED blood flow, DECREASED upstream pressure, and INCREASED downstream pressure
What is the given formula for resistance in circulation?
R= ( 8ηl )/(πr4)
R = resistance η = viscosity of blood l = length of vessel r4 = radius of blood vessel to the 4th power
Review Slides 33 and 34
Know where the lowest pressure drop is, the highest pressure drop is, where the dissipation of resistance occurs, highest arterial pressure, lowest arterial pressure, and pulse pressure.
How is resistance calculated in relation to the blood flow of the body?
Resistance is the impediment to blood flow in a vessel.
- Rate of blood flow through the entire circulatory system:
= cardiac output
= 100 ml/sec.
- Pressure difference from systemic arteries to systemic veins:
= 100 mm Hg.
- Resistance of entire systemic circulation:
= 100/100 = 1 PRU
Compare the resistance of the entire circulatory system compared to the pulmonary system
- Resistance of the entire systemic circulation (= total peripheral resistance):
- = 100/100 or 1 PRU (Peripheral Resistance Unit)
- In conditions when vessels are strongly constricted, total peripheral resistance may rise to 4 PRU.
- When vessels are greatly dilated, the resistance can fall to as little as 0.2 PRU.
- Resistance in the pulmonary system:
- Mean pulmonary arterial pressure: averages 16 mm Hg
- Mean left atrial pressure: averages 2 mm Hg.
- When cardiac output is normal at 100 ml/sec, the total pulmonary vascular resistance: = 14/100 = 0.14 PRU.
Give the definition for conductance
- Conductance is the measure of blood flow through a vessel for a given pressure difference.
- Conductance is usually expressed in ml/sec per ml Hg.
- It is the exact reciprocal of resistance.
- Conductance = 1/resistance
- Conductance is directly proportional to diameter.
- A fourfold increase in vessel diameter can increase the flow by as much as 256x.
- Therefore, arterioles, with only a minor change in diameter, can cause a vast increase in blood flow.
Give Poiseuille’s Law in relation to conductance
F = πΔPr4/8ηl
- F = rate of blood flow (mL/min)
- ΔP = pressure difference between ends of vessel
- r= radius of vessel
- l = vessel length
- η= blood viscosity
Describe the arrangement of vessels in the circulatory system.
- Arterioles, capillaries, venules, and veins are arranged in series.
- Blood vessels branch extensively to form parallel circuits that supply blood to the many organs and tissues of the body.
- For blood vessels arranged in parallel the total resistance to blood flow is expressed as: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + 1/R4 …
- Therefore, the total resistance is far less than the resistance of any single blood vessel.
- Total conductance for vessels arranged in parallel is the sum of the conductance of each parallel pathway.
- Circulations arranged in parallel: Brain, Kidney, Muscle, Gastrointestinal, Skin, and Coronary circulation
- Each of the above tissues contributes to the overall conductance of the systemic circulation.
What happens to resistance when a limb is amputated?
- Amputation of a limb or removal of a kidney removes a parallel circuit:
- Reduces total vascular conductance
- Reduces total blood flow
- Increases total peripheral vascular resistance
- See Slide 44
How does viscosity relate to resistance?
- Viscosity is a measure of the fluid’s internal resistance. The greater the viscosity, the greater the resistance. The prime determinant of blood viscosity is the hematocrit. The viscosity of normal blood is about three times as great as the viscosity of water, mostly because of the large numbers of suspended red cells.
- Hematocrit of adult men ≈ 42; adult women ≈ 38;
- Viscosity of whole blood at normal hematocrit ≈ 3
- When hematocrit rises to 60-70 (polycythemia), viscosity ≈ 10
- The viscosity of blood plasma (w/o RBCs) = 1.5 that of water.
Friendly Reminder: F = π∆Pr4/8ηl
- See Slides 46, 47
What is 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.
- See Slides 49 & 50
