HEMODYNAMICS AND VENUS RETURN

Blood flows through the heart, arteries, capillaries, and veins in a 

closed, continuous circuit.

Blood flow

 the movement of a certain volume of blood through the

vasculature over a given unit of time (e.g., mL/minute).

 Fluid dynamics

 a subdiscipline of fluid mechanics that describes the flow of fluids,

liquids and gases.

Hemodynamics

the physical principles governing blood flow, which are:

• The pressure gradient between one point and another

• Resistance of the vessel

Ohm’s Law

a formula used to calculate the relationship between voltage, current and resistance in an electrical circui

Mathematically Ohm’s law

𝐼 =∆𝑉/𝑅 

 Current (I)

 flow of charged particles

Voltage (∆V)

the difference in concentration of charged particles at 2 different points

Resistance (R)

opposition to current flow

The current (flow of electrons) in a closed system and it is

• Directly proportional to voltage

• Inversely proportional to resistance within the system.

 derivation of Ohm’s law can be used to calculate

blood flow.

 Mathematically Ohm’s law in hemodynamics

𝐹 = ∆𝑃 / 𝑅

Flow (F)

 blood flow through a vessel

Pressure gradient (ΔP)

 change in pressure between 2 different points

• (i.e., ΔP = P1 ‒ P2)

Flow (F) , the output of the left heart, is quite _____ in time and depends greatly on the physiological circumstances (e.g., whether one is active or at rest)

variable

Flow: the volume of fluid passing a point per unit of time

o Caused by a ΔP between two points (there is no flow without a ΔP)

o The relationship between F and ΔP does not require any assumptions about whether the vessels are rigid or compliant, as long as R is constant.

Laminar

smooth and streamlined

Turbulent

 irregular and chaotic.

Low Reynolds number indicates while a

high Reynolds number indicates turbulent flow

laminar flow

high Reynolds number indicates

turbulent flow

renynolds number

𝑅𝑒 = 2𝑟𝑣𝜌 / η

r: radius;

p: density;

v: kinematic viscosity;

η : viscosity

flow is caused by

a ΔP between two points (there is no flow without a ΔP)

𝐹 = ∆𝑃/ 𝑅

𝐹 = ∆𝑃.(π. 𝑟4)/ 8η𝑙

Hagen-Poiseuille Law

𝐹 = ∆𝑃/ 𝑅

EX. calculate at point a

𝐹 = ∆𝑃.(π. 𝑟4)/ 8η𝑙

P= 85 mmHg

r=20 mm

η= 32 cPs at 1 c

 85 mmhg x (3.14 (2 cm)/(8 x 32 x 1)= 2.08

ΔP

the difference in pressure between one point and another

• Influences the direction of blood flow (blood flows from high pressure → low pressure)

• If the flow is constant (which the body tries to maintain), vessel resistance ↑ (e.g.,

vasoconstriction) and leads to ΔP ↑.

• Clinical relevance: narrow vessels from atherosclerotic disease = ↑ blood pressure

Types of physiologic ΔP:

o Systemic: arterial pressure > venous pressure

o Local: proximal vessel pressure > distal vessel pressure

transmural pressure (ΔP)

the distending force that tends to increase

the circumference of the vessel.

wall tension (T)

 Opposing this pressure is a force inside

the vessel wall

• Wall tension is the force that must be

  applied to bring together the two edges of

  an imaginary cut in the wall along the

  longitudinal axis of the vessel.

The equilibrium between ΔP and T

depends on the

vessel radius

Laplace’s law

𝑇 = Δ𝑃. 𝑟

Hydrostatic pressure (ΔP)

refers to the force exerted by blood within a blood

vessel against its walls.

Resistance

forces opposing flow

Arises from the friction between the moving blood and vessel walls

Equation for resistance against laminar flow:

𝑅 = (8/𝜋) x (η𝑙/𝑟^4)

• Where:

• R = resistance

• η = Viscosity (thickness of the blood)

• l = length of the vessel

• r = radius of the vessel

Viscosity

The thickness of the blood

η= 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠/𝑠ℎ𝑒𝑎𝑟 𝑟𝑎𝑡𝑒= (F/A)/ (ΔV/ΔX)

↑ Viscosity

due to polycythemia, hyperalbuminemia, and dehydration

↓ Viscosity

due to anemia, hypoalbuminemia, and adequate hydration

The body is unable to quickly regulate flow by adjusting

viscosity

factors affecting resistance

• length

• radius

length

o The longer the vessel, the greater the cumulative friction encountered

o Each vessel has a fairly fixed length (no ability for regulation).

Radius

o Significant impact on resistance

o Highly regulated by smooth muscle within the vessel walls

o Vasoconstriction: ↓ radius

o Vasodilation: ↑ radius

Capacitance

The amount a vessel can stretch without significantly increasing pressure

C = ΔV / ΔP:

• Where:

o C: capacitance

o ΔV: change in volume

o ΔP: change in pressure

Venous capacitance ? arterial capacitance

>

60%–80% of total blood volume is in the

venous circulation

Velocity

The speed blood is traveling.

Velocity is different from flow:

o Velocity is a unit of distance per unit of time.

o Flow is a unit of volume per unit time.

o Clinical relevance: The velocity of blood moving across the valve will increase with a stenotic valve (smaller diameter), but the flow will not.

 Relationship between flow and velocity:

oFlow = V x A

where V velocity and A the area of the vessel or pathway available to blood

oFlow = V x (πr2)

velocity

calculate flow at point a

o Velocity = 40 cm/sec

o r = 20 millimeters

V x (πr^2)

flow= (40 cm/sec) x (3.14 (2cm²))= 502.4 cm³/sec

velocity

calculate flow at point b where r is 2r

o Velocity = 40 cm/sec

o r = 20 millimeters

V x (πr^2)

flow= (40 cm/sec) x (3.14 (4cm²))= 2009.6 cm³/sec

Arterial blood pressure in the larger vessels

consists of several distinct components:

• Systolic pressure

• Diastolic pressure

• Pulse pressure

• Mean arterial pressure.

_________ is the peak pressure recorded in the central arterial system and occurs during ventricular ejection.

• Systolic blood pressure (SBP)

___________ is the minimum pressure recorded in the central arterial system and occurs just before the start of ventricular systole.

Diastolic blood pressure (DBP)

SBP has three determinants:

1. Stroke volume.

• Increased SV increases SBP and PP.

2. Diastolic blood pressure.

3. Aortic compliance.

If compliance is low (i.e., stiff aorta), the SV produces

a large SBP.

DBP has three determinants:

1. Vascular resistance is the main determinant of DBP.

2. Runoff of blood from the aorta.

• DBP decreases if blood flow into the circulation during diastole is reduced.

• Aortic valve insufficiency is an example where aortic pressure rapidly decreases during diastole because backflow of blood into the left ventricle reduces forward flow into the circulation.

3. Diastolic time interval.

Pulse pressure (PP) is the difference between

SBP and DBP

__ is reflected by the strength of the arterial pulse

wave palpated in the peripheral arteries

PP

Generally, a pulse pressure should be at least _____ of the systolic pressure

25 percent

A persistently high pulse pressure at or above ____ may indicate excessive resistance in the arteries and can be caused by a variety of disorders

100 mm Hg

Mean arterial pressure is the average

systemic arterial pressure.

map equation

MAP = 𝐷𝐵𝑃 + 𝑆𝐵𝑃 −𝐷𝐵𝑃/3 or (1/3) x SBP-DBP + DBP

For example, if a patient’s blood pressure is 83 mm Hg/50 mm Hg, the MAP would be

(1/3)x(83-50)+50= 61

MAP functions to

perfuse blood to all the tissues of the body to keep them

functional.

Mechanisms are in place to ensure that the MAP remains at least _____ so that blood can effectively reach all tissues.

60 mmHg

Normally, the MAP falls within the range of 

70-110 mmHg

If the value falls below 60 mm Hg for an extended time, blood pressure will not be high enough to ensure circulation to and through the tissues, which results in

ischemia, or insufficient blood flow.

ystemic vascular resistance (SVR) refers to

the resistance to blood flow offered by all the systemic vasculature, excluding the pulmonary vasculature.

sometimes referred to as total peripheral resistance (TPR).

Mechanisms that cause vasoconstriction ______ SVR

increase

mechanisms that cause vasodilation _____ SVR.

decrease

SVR can be calculated if cardiac output (CO), mean arterial pressure (MAP), and central venous pressure (CVP) are known, the equation

SVR = (MAP - CVP) ÷ CO

Blood pressure measured by cuff inflation in the upper arm of a

healthy person is 130/70 mm Hg

What is

• SBP

• DBP

• PP

• MAP

SBP= 130 mmHg

DBP= 70mmHg

PP= 130-70= 60

MAP= (1/3) x (130-70) +70= 90 mmHg

Components of Blood Pressure

• Blood pressures varies throughout the

systemic vasculature.

• Pressure is highest in the central arteries

and lowest in the central veins.

• The largest pressure decrease occurs

across the arterioles, indicating that they

are the site of highest vascular resistance.

Indices of arterial blood pressure during cardiac cycle

• During each cardiac cycle, arterial pressure

rises and falls as the heart contracts and

relaxes.

• Systolic pressure reflects ventricular

contraction, while diastolic pressure

reflects arterial recoil and resting pressure.

• Pulse pressure provides insight into heart

function and arterial health.

• MAP is the best indicator of overall tissue

perfusion.

Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as

Korotkoff sounds

Blood Pressure Measurement Steps

1. The clinician wraps an inflatable cuff tightly around the patient’s arm at about the level of the heart.

2. The clinician squeezes a rubber pump to inject air into the cuff, raising pressure around the artery and temporarily cutting off blood flow into the patient’s arm.

3. The clinician places the stethoscope on the patient’s antecubital region and, while gradually allowing air within the cuff to escape, listens for the Korotkoff sounds.

Phase I BP measurement

clear tapping sounds heard for at least two consecutive beats

• This is the systolic blood pressure

Phase II BP measurement

 the softening of the tapping sounds and the addition of a

swishing sound

Phase III BP measurement

the return of tapping sounds, as heard in phase I, but with

an increase in sharpness and intensity

Phase IV

 the abrupt muffling of sounds, exhibiting a soft and blowing

quality

Phase V of BP measurement

the complete disappearance of all sounds

• This is the diastolic blood pressure

Venous return

• It is the flow of blood from the systemic venous network towards the right heart.

• At steady state, venous return equals cardiac output, as the venous and arterial systems operate in series.

• However, unlike the arterial one, the venous network is a capacitive system with a high compliance.

Central Venous Pressure (CVP)

the pressure of venous blood in the thoracic vena cava and the right atrium.

Low CVP _______ venous return into the

central venous compartment

promotes

High CVP ________ venous return

reduces

CVP has a strong influence on cardiac preload and, through the ___________ mechanism, determines ventricular SV.

Frank-Starling

Frank-Starling

the ability of the heart to change its force of contraction and

therefore stroke volume in response to changes in venous return

equation for venous return

• 𝑉𝑅 = 𝑃𝑉 − 𝑃𝑅𝐴/ 𝑅𝑉

• Where:

• VR: venous return

• PV: venous pressure

• PRA: right atrial pressure

• RV: venous vascular resistance

Increases in cardiac output cause decrease in

right atrial pressure (RAP)

The RAP determines the extent of

ventricular filling (first of the three determinants of EDV)

The venous function curve shows how central venous pressure influences

venous return.

As RAP becomes _______, it provides a greater driving pressure (i.e., greater ΔP = CVP − RAP) for the return of blood from the periphery to the right atrium. → Higher venous return

less positive

The cardiac output steadily rises as

RAP falls

At a normal cardiac output of 5 L/min RAP is

2 mm Hg (point A)

 If VR hits 0 the central venous pressure

at that point (Usually 7 mmHg) is called

mean systemic filling pressure (No flow

state).

mean systemic filling pressure (No flow state).

Mean Systemic Filling Pressure

 happens when the heart is shocked,

and all circulation stops and pressure

throughout the body is equal

If peripheral venous pressure is at 7 mmHg

• What is venous return when

o Central venous pressure is 7 mmHg

o Central venous pressure is 6 mmHg

o Central venous pressure is 5 mmHg

o Central venous pressure is 4 mmHg

o If intrathoracic pressure is 0 mm Hg

1. 1

2. 2

3. 3

4. 4

5. 0

Why as RAP declines and eventually becomes negative there is NO further increase in venous return, even though the driving pressure, ΔP, is increasing?

due to venous collapse caused by transmural pressure dynamics.

Transmural Pressure Effect

When RAP becomes increasingly negative, it creates a situation where the pressure outside the veins (intrathoracic pressure) exceeds the pressure inside the veins. This creates a negative transmural pressure (inside pressure minus outside pressure).

Venous Collapse

When the transmural pressure becomes sufficiently negative, the thin-walled veins begin to collapse as they enter the thorax. This collapse creates a resistance to flow that effectively limits venous return.

Waterfall Effect

This phenomenon is sometimes called a "vascular waterfall" or "Starling resistor effect." Just as water flowing over a waterfall isn't affected by lowering the level at the bottom of the falls, venous return isn't increased by further decreases in RAP once the veins have collapsed.

Flow Limitation

The collapsed segment of vein acts as a bottleneck that prevents any further increase in flow, regardless of how much more negative the RAP becomes. The effective downstream pressure is no longer the RAP but rather the pressure at the point of collapse.