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Questions and Answers
What primarily drives the flow of blood through blood vessels?
Which factor does NOT affect the flow through a blood vessel?
In laminar flow within a blood vessel, where is the velocity of fluid highest?
What happens to the flow when the radius of the vessel increases, assuming a constant pressure gradient?
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How does viscosity affect the mean velocity of blood flow?
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In turbulent flow, what happens to the velocity gradient within the fluid?
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If a blood vessel's cross-sectional area decreases, what is the expected effect on mean velocity at a constant flow?
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What type of blood flow is characterized by the fluid moving in concentric layers?
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How does resistance in blood vessels affect arterial pressure during constant total flow?
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What happens to blood flow when the pressure remains fixed and resistance increases?
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What is the effect of vessel diameter on blood flow according to Poiseuille's Law?
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In a series connection of blood vessels, how is the total resistance calculated?
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What characteristic of capillaries allows for lower overall resistance despite individual high resistance?
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How does increased pressure in blood vessels influence vascular resistance?
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What is the relationship between resistance, flow, and pressure in a fixed flow condition?
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Which type of flow occurs in narrowed blood vessels such as those affected by atherosclerosis?
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How does the congregating of blood cells in the center of flow affect viscosity?
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What is a primary function of distensible blood vessels in the circulation system?
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During which phase of the cardiac cycle do the ventricles fill rapidly?
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What is the main function of the aortic valve in the cardiac cycle?
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What normally initiates the action potential that results in cardiac contraction?
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Which event occurs first during the cardiac cycle?
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Which phase of the cardiac cycle has the longest duration?
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What happens during isovolumetric relaxation in the cardiac cycle?
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Which heart sound indicates the closure of the a/v valves?
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During systole, what primarily causes the opening of the outflow valves?
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What phase follows atrial systole in the cardiac cycle?
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How is blood flow characterized during ventricular filling?
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What is the condition where valves do not close properly, causing murmurs?
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What primarily causes the rise in intraventricular pressure during ventricular systole?
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What triggers the rapid ejection phase during ventricular systole?
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Which of the following statements is true regarding diastole?
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Study Notes
Flow
- Blood flow is driven by the pressure difference between the ends of a vessel
- The higher the pressure difference, the greater the flow
Flow Resistance
- The resistance of a vessel determines the flow for a given pressure gradient.
- Resistance is determined by the nature of the fluid and the vessel.
Definitions
- Flow: the volume of fluid passing a given point per unit time
- Velocity: the rate of movement of fluid particles along the tube
Relationship between Flow and Velocity
- Flow is constant at all points along a vessel, even in a vessel with a changing radius
- Velocity is inversely proportional to surface area at a given flow
Laminar Flow
- Most blood vessels have laminar flow
- Velocity increases as the distance from the center of the vessel increases
- Flow is highest in the center of the vessel
- Fluid is stationary at the edge of the vessel
Turbulent Flow
- As mean velocity increases, the flow eventually becomes turbulent
- The velocity gradient breaks down
- Fluid tumbles
- Flow resistance is greatly increased
Now, consider a vessel with a constant pressure driving flow
- The flow will be determined by the mean velocity
- The mean velocity depends upon:
- the viscosity of the fluid
- the radius of the tube
Viscosity
- In laminar flow, fluid moves in concentric layers like layers of an onion
- The middle layers move faster than the edge layers
- Fluid layers resist sliding over one another, which is known as viscosity.
Effects of Radius
- The wider the tube, the faster the middle layers move
- Mean velocity is proportional to the cross-sectional area of the tube.
Distensible Vessels
- Distensible vessels help maintain smooth circulation by stretching and reducing resistance when pressure increases.
The Heart
- The heart is two pumps in series
- Each side has:
- a thin-walled atrium
- a muscular ventricle
- Flow into and out of ventricles is controlled by valves:
- atrioventricular valves (mitral & tricuspid)
- outflow valves (aortic and pulmonary)
Heart Muscle
- A specialized form of muscle with discrete cells that are electrically connected.
- Cells contract when an action potential occurs in the membrane.
Heart Muscle
- Action potentials cause an increase in intracellular calcium.
- Action potentials are long, allowing a contraction known as systole to last 280 ms.
- Action potentials are triggered by the spread of excitation from cell to cell.
Pacemakers
- A small group of cells generate action potentials that spread over the whole heart
- Pacemakers generate action potentials continuously at regular intervals
Phases of the Cardiac Cycle
- Each action potential produces one beat
- The interval between beats is diastole.
Spread of Excitation - 1
- The pacemaker in the sinoatrial node (right atrium) initiates the spread of excitation.
- This excitation causes atrial systole.
- The excitation is delayed for 120 ms in the atrioventricular node.
Spread of Excitation - 2
- From the atrioventricular node, the excitation spreads down the septum between the ventricles and then spreads through the ventricular myocardium.
- The contraction of the ventricle starts at the apex and moves 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 and is followed by a 700 ms long relaxation before the next systole.
The Left Ventricle
- Inflow valve: the mitral valve, which allows blood from the atrium to the ventricle
- Outflow valve: the aortic valve, which allows blood from the ventricle to the aorta
The Cardiac Cycle - 1
- The cycle begins towards the end of ventricular systole.
- Ventricular pressure is high and the outflow valves are open.
- Blood is flowing into the arteries.
- Ventricular pressure is greater than atrial pressure, so the atrioventricular valves are closed.
The Cardiac Cycle - 2
- The ventricle begins to relax.
- Ventricular pressure falls.
- Ventricular pressure becomes less than arterial pressure.
- Brief backflow closes the outflow valves.
- All valves are now closed during isovolumetric relaxation.
The Cardiac Cycle - 3
- During systole, blood continuously returns to the atria, increasing atrial pressure.
- As ventricular pressure falls, atrial pressure eventually exceeds ventricular pressure.
- This causes the atrioventricular valves to open.
The Cardiac Cycle - 4
- The rapid filling phase of the ventricle occurs after the atrioventricular valves open.
- The rapid filling phase lasts about 200-300 ms.
- Most filling of the ventricle occurs during this phase.
The Cardiac Cycle - 5
- As diastole continues, the ventricle fills more slowly.
- Intraventricular pressure increases as ventricular walls stretch until it reaches the same pressure as the atrium, which stops further filling.
The Cardiac Cycle - 6
- Atrial systole forces a small amount of blood into the ventricles.
- The heart pumps efficiently without atrial systole.
The Cardiac Cycle - 7
- Ventricular systole begins.
- Intraventricular pressure increases rapidly.
- Intraventricular pressure quickly exceeds atrial pressure, causing a brief backflow and closing of the atrioventricular valves.
- All valves are closed during isovolumetric contraction.
The Cardiac Cycle - 8
- Intraventricular pressure continues to increase rapidly, exceeding arterial pressure.
- As the aortic valve opens, blood is rapidly ejected into the arteries during rapid ejection phase.
The Cardiac Cycle - 9
- Arterial pressure rises rapidly.
The Cardiac Cycle - 10
- The rate of blood ejection falls as arterial pressure rises.
- Both arterial and intraventricular pressure peak towards the end of systole.
- Ventricular outflow ceases.
The Cardiac Cycle - 11
- Systole ends.
Heart Sounds
- There are two main heart sounds associated with the closing of the valves.
- The first sound ('lup') is the closing of the atrioventricular valves.
- The second sound ('dup') is the closing of the outflow valves.
Heart Sounds
- The first sound occurs at the start of ventricular systole.
- The second sound occurs at the end of ventricular systole.
- The interval between the first and second sounds is about 280 ms at rest.
- The interval between the second sound and the next first sound is 700 ms at rest.
Heart Sounds
- Sometimes, extra sounds are heard in normal hearts:
- third sound early in diastole
- fourth sound - atrial systole.
Heart Murmurs
- Turbulent blood flow generates murmurs.
- This can be caused by:
- valve narrowing (stenosis)
- a valve not closing properly (incompetence)
Heart Murmurs
- Murmurs are most audible when blood flow is highest.
- This means that a murmur can be used to identify when in the cardiac cycle it should occur.
Heart Murmurs
- For example, aortic stenosis produces a murmur during the rapid ejection phase.
Cardiac Output
- At each beat, the heart ejects a stroke volume.
- Cardiac output is calculated by multiplying stroke volume and heart rate.
- At rest, cardiac output is roughly 5 liters per minute (80 ml x 60).
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Description
Explore the concepts of blood flow dynamics, including the effects of pressure differences, resistance, and the relationship between flow and velocity. Understand the distinctions between laminar and turbulent flow within blood vessels through this informative quiz.
