Blood Flow and Resistance Quiz

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).