Lecture 2 - Pressure Flow

What are blood flow to tissues regulated by under normal conditions?

tissue needs

cardiac output

• total volume of blood your heart pumps/minute

  • sum of all local tissue blood flow

• CO = Heart rate (HR) x stroke volume (SV)

is arterial pressure related to CO and blood flow?

• no, it is highly regulated and not affected by changes in CO and local tissue blood flow

flow

• movement of substances from different points driven by energy gradients

what is the gradient that drives substances

energy gradients

what is the gradient that drives molecules

conc. gradient

what is the gradient that drives heat

temp gradient

what is the gradient that drives gases

pressure gradient

what is the gradient that drives fluids

hydrostatic pressure

How is rate of flow/mag of flow related to mag of gradient

• directly proportional

• multiple gradients can determine mag of flow

  • ex. E gradient for ions is conc. gradient and voltage gradient

what is inversely proportional to flow?

• resistance

• there’s always some resistances to flow, but some can be controlled

pressure gradient

• difference between p1 and p2 at two different points 

how is resistance created?

• when blood moves continuously through the circulatory system, primarily caused by friction between blood and the inner walls of blood vessels 

blood pressure

• force caused by blood against wall of arteries

• hydrostatic pressure (ex. mmHg, kPa)

• reflects force exerted by blood against a unit area of a vessel wall

what are the methods of measuring blood flow and pressure?

• ultrasonic doppler flowmeter = uses ultrasound to measure flow rate

• intravascular pressure transducer

• BP sphygmomanometer

How to calculate resistance?

• cannot be measured directly

• must be calculated once bp and p are known

Blood flow (F) equation

• F = delta P / R

• delta P = P1 - P2

  • difference in mean BP between aorta and right atrium (drives flow during circulation)

• R = resistance

• expressed as volume/time

resistance

• impediment to flow due to friction

• both external (vessel wall) and internal (due to blood viscosity)

conductance

• opposite of resistance

• 1/R

describe the graph

Total peripheral resistance (TPR)

• resistance of an entire systems circulation

• or total amt of force/resistance blood must overcome to flow through the circulatory system

how does CO and delta P relate to mass/body size

• CO proportional to body size

• P scales with mass fluctuations

How does TPR relate to size/mass?

TPR dec. with larger animals

How doe delta P, CO and TPR relate?

• delta P/CO = TPR

• CO x TPR = delta P

• delta P/TPR = CO

how do biological structures facilitate and constrain function?

• arrangement of elements within a tissue/organ (ex. arrangement of vessels can affect blood flow)

• physical dimensions of a structure (ex. shape and size of vessel can affect blood flow)

factors affecting resistance

• vascular arrangement: networks arranged in series or parallel

• patterns of blood flow: laminar or turbulent flow

• vascular dimension: changes in length and diameter of vessels

How do vessel networks arranged in a series affect resistance?

• series: one vessel next to the other, individual resistances (of each segment) are summed → total resistance increases

• Rtotal = R1 + R2 + R3

• Rtotal > any individual resistance

• TPR is comprised of the resistances (R) of individual vessels throughout the body

How do vessel networks arranged in parallel affect resistance?

• parallel: vessels are arranged in parallel, has multiple different resistances

• Rtotal < any single R

• Rtotal = 1/ (1/R1) + (1/R2) + (1/R3)

• TPR is comprised of the resistances (R) of individual vessels throughout the body

how are capillary networks arranged?

• usually in parallel

• blood flows via capillary beds of different systemic tissues in parallel

• based on diagram,

  • several capillary resistances are included

  • a larger body mass, more capillary vessels and more capillary beds (bigger animals) will decrease TPR

  • *recall TPR equation for parallel networks

How does laminar flow affect resistance?

• blood flows smoothly in streamline through vessels

• via relatively little mixing in radial direction

• creates lower and more stable resistance as fluid moves in smooth, parallel layers, without mixing

How does turbulent flow affect resistance?

• flow can become disorderly, whirling in a radial (crosswire) direction to form “eddy currents”

• occurs in largest vessels, with highest flowrates and around obstacles/occlusions

• increases resistance to blood flow

how does vascular dimension affect resistance?

• resistance changes directly with physical dimension of vessels

• vessel radius

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how does vessel length affect resistance?

• vessel length

  • inc. vessel length = inc. resistance to blood flow = dec. blood flow

  • longer vessels create more friction as blood travels across larger SA

  • blood flow dec. with inc. resistance

How does vessel radius affect resistance?

• small changes in r affect resistance (r is to the power of 4 according to poiseuille’s law)

• Vessel length and (internal) viscosity do not vary much over short periods 

• External vessel radius can change acutely and is thus the most important factor regulating resistance 

how does vessel radius (lumin of a rat) differ between a normal and hypertensive rat?

• normal: lumin diameter is smaller → more resistance to blood flow

• hypertensive rat: pink smooth muscle is thicker, diameter increases → less resistance

• recall: F = delta P/R

During laminar flow at a set pressure, how does vessel diameter affect blood flow?

• small changes in vessel diameter have large effects on blood flow (recall r is to the power of 4)

• Friction against vessel walls makes blood flow slower 

• In larger vessels, fewer streamlines of blood are near the vessel wall 

• In smaller vessels, greater proportion of blood along vessel walls so blood flows slower 

• Longer vessel = more resistance → P and I are inversely proportional 

vascular conductance x resistance as radius increase graph

• Bp is lower and gradually increases as vessel radius increases 

• Resistance will decrease as vessel radius increases 

• Both are exponential curves 

How is resistance regulated?

• Increased diameter = vasodilation 

• Decreased diameter = vasoconstriction 

  • Decrease in diameter (ex. Using a pharmacological vasoconstrictor)  reduces blood flow at given pressure gradient 

  • Vasoconstrictor = noreepi, epi 

  • Decrease vasoconstriction, blood flow decreases, p increases 

  • See downward shift in curve with vasoconstrictor 

What is a major site where resistance is rapidly regulated?

• Arterials are a major site where diameter, thus resistance, is regulated rapidly to meet blood flow demands

How does blood viscosity relate to resistance?

• blood viscosity is proportional to resistance

• changes in hematocrit (% of blood that’s RBC) can alter blood viscosity and resistance

• more RBC’s are suspended in blood, increases friction between adjacent cells and against vessel walls

How does anemia affect blood viscosity?

• anemic can occur during the menustral cycle in response to blood loss

• blood viscosity decreases because a low red blood cell count makes the blood thinner and causes less resistance to flow

How does polycythemia (aka. excessive erythrocytosis) affect blood viscosity?

• occurs at high elevation, in response to sleep apnea, in pulmonary diseases, etc., more resistance to blood flow 

• Orange bar shows that non EE individuals have less percentage of hematocrit 

• After putting everyone through exercise in graph 2, it found that co-increases in non-ee group highlanders have higher blood cell count and are more likely to have mor resistance to blood flow 

• Look more into graphs! 

• Decreases in hematocrit, TPR, and blood cells can cause lower blood viscosity and less resistance which increases CO 

What would be the effect of experimentally correcting haematocrit by “isovolumic hemodilution” (removing
blood and replacing it with saline)?