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Questions and Answers
What happens to blood flow as the pressure difference between the ends of a vessel increases?
What happens to blood flow as the pressure difference between the ends of a vessel increases?
In laminar flow, where is the velocity of the fluid highest?
In laminar flow, where is the velocity of the fluid highest?
How does viscosity affect the mean velocity of fluid in a tube?
How does viscosity affect the mean velocity of fluid in a tube?
What relationship exists between mean velocity and the radius of a tube?
What relationship exists between mean velocity and the radius of a tube?
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What characterizes turbulent flow compared to laminar flow?
What characterizes turbulent flow compared to laminar flow?
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What is the impact of increasing the cross-sectional area of a vessel on mean velocity?
What is the impact of increasing the cross-sectional area of a vessel on mean velocity?
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What is the primary factor affecting flow resistance in a blood vessel?
What is the primary factor affecting flow resistance in a blood vessel?
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When the flow remains constant but the radius of the tube changes, what happens to velocity?
When the flow remains constant but the radius of the tube changes, what happens to velocity?
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What happens to the average velocity of fluid as viscosity increases in laminar flow?
What happens to the average velocity of fluid as viscosity increases in laminar flow?
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What defines the term 'flow' in the context of fluid dynamics?
What defines the term 'flow' in the context of fluid dynamics?
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What structure in the heart generates action potentials at regular intervals?
What structure in the heart generates action potentials at regular intervals?
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During which phase of the cardiac cycle does the heart muscle contract and blood is ejected into the arteries?
During which phase of the cardiac cycle does the heart muscle contract and blood is ejected into the arteries?
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What causes the closure of the atrioventricular valves during the cardiac cycle?
What causes the closure of the atrioventricular valves during the cardiac cycle?
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What is the approximate duration of a single contraction during systole?
What is the approximate duration of a single contraction during systole?
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Which heart sound is associated with the closure of the atrioventricular valves?
Which heart sound is associated with the closure of the atrioventricular valves?
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What is the relationship between stroke volume and cardiac output?
What is the relationship between stroke volume and cardiac output?
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What causes a heart murmur during the rapid ejection phase of ventricular systole?
What causes a heart murmur during the rapid ejection phase of ventricular systole?
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Which phase of the cardiac cycle follows isovolumetric contraction?
Which phase of the cardiac cycle follows isovolumetric contraction?
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What occurs at the atrioventricular node during the spread of excitation in the heart?
What occurs at the atrioventricular node during the spread of excitation in the heart?
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How long does the interval from the first to second heart sound approximately last at rest?
How long does the interval from the first to second heart sound approximately last at rest?
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What happens to pressure if resistance increases while flow remains constant?
What happens to pressure if resistance increases while flow remains constant?
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Which statement accurately describes blood flow through vessels of varying sizes?
Which statement accurately describes blood flow through vessels of varying sizes?
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What is the effect of connecting blood vessels in series versus in parallel?
What is the effect of connecting blood vessels in series versus in parallel?
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What occurs to flow when pressure is constant and resistance increases?
What occurs to flow when pressure is constant and resistance increases?
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Which blood vessel type is associated with low resistance and minimal pressure drop?
Which blood vessel type is associated with low resistance and minimal pressure drop?
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What structural feature allows vessels to change resistance as pressure varies?
What structural feature allows vessels to change resistance as pressure varies?
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What phenomenon might occur in blood flow due to vessel narrowing?
What phenomenon might occur in blood flow due to vessel narrowing?
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What is the primary factor affecting pressure in the arteries?
What is the primary factor affecting pressure in the arteries?
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What happens to blood flow through distensible vessels as internal pressure rises?
What happens to blood flow through distensible vessels as internal pressure rises?
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What is the effect of blood cells congregating in the center of the flow?
What is the effect of blood cells congregating in the center of the flow?
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Study Notes
Blood Flow
- The flow of blood through blood vessels is driven by a pressure gradient.
- Flow is proportional to the pressure difference between the ends of a vessel.
- The higher the pressure difference the greater the flow.
- The flow for a given pressure gradient is determined by the resistance of the vessel.
- The flow is determined by the nature of the fluid and the vessel.
Definitions of Flow and Velocity
- Flow: The volume of fluid passing a given point per unit time.
- Velocity: The rate of movement of fluid particles along the tube.
Flow, Velocity, and Surface Area
- Flow is constant at all points along a vessel.
- Velocity can vary along the length if the radius of the tube changes.
- At a given flow, velocity is inversely proportional to surface area.
Laminar Flow
- In most blood vessels, flow is laminar.
- There is a gradient of velocity from the middle to the edge of the vessel, with the highest velocity in the center.
- Fluid is stationary at the edge.
Turbulent Flow
- As mean velocity increases, flow eventually becomes turbulent.
- The velocity gradient breaks down and fluid tumbles over.
- Flow resistance is greatly increased.
Viscosity
- In laminar flow, the fluid moves in concentric layers.
- The middle layers move faster than the edge layers.
- Viscosity is the extent to which fluid layers resist sliding over one another.
- The higher the viscosity, the slower the central layers will flow.
Radius and Flow Resistance
- Viscosity determines the slope of the gradient of velocity.
- At a constant gradient, the wider the tube the faster the middle layers move, so mean velocity is proportional to the cross-sectional area of the tube.
Poiseuille’s Law
- Mean velocity is inversely proportional to viscosity.
- Mean velocity is directly proportional to surface area.
- Flow is the product of mean velocity and surface area.
- Flow = D P x r2 x r2/(viscosity x length)
Resistance in Series and Parallel
- In series resistances combine just like electrical resistances.
- In parallel resistances combine by the formula: R=(R1xR2)/(R1+R2)
Pressure, Flow, and Resistance
- If flow is fixed, the higher the resistance the greater the change in pressure from one end of the vessel to the other.
- If pressure is fixed, the higher the resistance the lower the flow.
The Whole Circulation
- Over the whole circulation, flow is the same at all points.
- Arteries are low resistance; pressure drop over arteries is small.
- Arterioles are high resistance; pressure drop over arterioles is large.
- The overall resistance of capillaries is low, due to many connected in parallel. The pressure drop over capillaries is low.
- Venues and veins are low resistance; pressure drop over venules and veins is low.
- The pressure within arteries is high because of the high resistance of arterioles.
- If the heart pumps more blood, and the resistance of arterioles remains the same, arterial pressure will rise.
Special Problems of Flow in Blood Vessels
- Flow may become turbulent in some vessels.
- Flow becomes turbulent if a vessel is narrowed.
- Turbulent flow generates sound.
- Blood vessels have distensible walls which generate a transmural pressure between inside and outside.
Distensible Vessels
- As a vessel stretches, resistance falls.
- The higher the pressure in a vessel, the easier it is for blood to flow through it.
- Distensible vessels ‘store’ blood.
- Veins are most distensible.
The Heart
- Two pumps in series.
- Each side consists of a thin-walled atrium and a muscular ventricle.
- Flows into and out of the ventricle are controlled by valves: Atrioventricular valves (mitral & tricuspid) and outflow valves (aortic and pulmonary).
Heart Muscle
- A specialized form with discrete cells connected electrically.
- Cells contract when an action potential is in the membrane.
- An action potential causes a rise in intracellular calcium.
- Action potential is long, with a single contraction lasting 280 ms - systole.
- Action potentials are triggered by the spread of excitation from cell to cell.
Pacemakers
- Pacemakers generate an action potential at regular intervals.
- One action potential spreads over the whole heart and produces a coordinated contraction.
Phases of the Cardiac Cycle
- Systole - the interval between beats is diastole.
Spread of Excitation
- Pacemaker in the sinoatrial node (right atrium).
- Activity first spreads over the atria (atrial systole).
- The activity is delayed about 120 ms in reaching the atrioventricular node.
- From the a-v node, the activity spreads down the septum between the ventricles, then spreads through the ventricular myocardium from inner (endocardial) to outer (epicardial) surface.
- The ventricle contracts from the apex up, forcing blood towards the outflow valves
The Cardiac Cycle
- At rest, the SA node generates an action potential about once a second.
- This produces a short atrial systole, followed by a longer ventricular systole.
- Ventricular systole lasts about 280 ms, followed by a relaxation lasting about 700 ms before the next systole.
The Left Ventricle
- Inflow valve - the mitral valve.
- Opens when atrial pressure exceeds intraventricular pressure.
- Closes when ventricular pressure exceeds atrial pressure.
- Outflow valve - the aortic valve.
- Opens when intra-ventricular pressure exceeds aortic pressure.
- Closes when aortic pressure exceeds ventricular pressure.
The Cardiac Cycle
-
Start: Towards the end of ventricular systole.
- Ventricles are contracted.
- Intra-ventricular pressure is high.
- Outflow valves are open.
- Blood flows into the arteries.
- Ventricular pressure > atrial pressure, so a/v valves are closed.
-
Ventricles begin to relax: Intra-ventricular pressure falls.
- Intra-ventricular pressure becomes < arterial pressure.
- Brief backflow closes outflow valves.
- All valves are now closed: get isovolumetric relaxation.
-
During systole, blood has continued to return to the atria: Atrial pressure is relatively high.
- As intra-ventricular pressure falls, eventually atrial pressure > intra-ventricular pressure.
- So, a/v valves open.
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With a/v valves open, ventricles fill rapidly (Rapid filling phase).
- This lasts about 200-300 ms.
- Most filling of ventricles occurs in this phase.
-
As diastole continues, the ventricles fill more slowly:
- Intraventricular pressure rises as ventricular walls stretch.
- Intra-ventricular pressure matches atrial pressure and filling stops.
-
Atrial systole:
- Forces a small extra amount of blood into the ventricles.
- However, the heart pumps perfectly well without atrial systole.
-
Ventricular systole:
- Intraventricular pressure rises very rapidly.
- Intraventricular pressure > atrial pressure.
- After brief backflow, a/v valves close.
- All valves are closed: get isovolumetric contraction.
- Intraventricular pressure > arterial pressure which has been falling in diastole, so outflow valves open.
- As outflow valves open, blood is ejected rapidly into the arteries (Rapid ejection phase).
- Arterial pressure rises rapidly.
- As arterial pressure rises, the rate of ejection of blood falls.
- Both arterial and intraventricular pressures peak towards the end of systole.
- Outflow eventually ceases with blood remaining in the ventricle.
- Eventually, systole ends, and the cycle restarts.
Heart Sounds
- Two main sounds associated with the valves closing.
- First sound - ‘lup’ - closure of a/v valves.
- Second sound - ‘dup’- closure of outflow valves.
- The first sound occurs at the onset of ventricular systole.
- The second sound occurs at the end of ventricular systole.
- At rest, the interval from the 1st to the 2nd sound is about 280 ms.
- The interval from the 2nd sound to the next 1st sound is 700 ms.
Heart Murmurs
- Turbulent flow of blood generates murmurs.
- Murmurs occur when blood flow is highest.
- Narrowed valve - stenosis.
- Valve not closing properly - incompetence.
Cardiac Output
- The heart ejects a stroke volume with each beat.
- Cardiac output is stroke volume multiplied by heart rate.
- At rest: 80 ml x 60, ie, c. 5 l.min-1
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Description
Explore the essential principles of blood flow and velocity in this quiz. Learn how pressure gradients, vessel characteristics, and fluid properties affect flow and velocity. This quiz will test your understanding of laminar flow and its implications in the circulatory system.
