Laminar and turbulent flow

Question 1

Describe the relationship between pressure, flow and resistance

Answer 1

• Flow = pressure / resistance
• Resistance is the sum of all resistances in the vascular beds
  * Driving pressure in blood = arterial blood pressure - venous pressure
  * Driving pressure in airflow = barometric pressure - alveolar pressure (resistance is tubes in airways)
• Flow in cardiovascular and respiratory system are similar (5-6L) but driving pressure in cardiovascular system is higher (90mmHg vs 5mmHg) so resistance is also higher
• Flow is proportional to driving pressure (effort dependent)
• Main factor that determines flow which can be altered is radius of vessels/ airways
• Poiseuille’s law only directly applies to laminar flow
• Have control over airway smooth muscle, parasympathetic nerve activity leads to bronchoconstriction or adrenaline acting at β2 receptors cause bronchodilation

Question 2

Difference between turbulent and lamina flow

Answer 2

• Lamina flow describes a type of streamline flow, there is a smooth parabolic profile of velocity in the tube
• Velocity is highest in centre of tube, lowest net to tubule walls
• Parabolic profile: due to layer of fluid next to wall, experiences friction so does not move (immobile layer), each successive layer slips past immobile layer a little faster
• Features: inversely proportional to resistance, silent, energy efficient
• Turbulent flow: no parabolic flow, consists of mixed fluid streams (cross-currents and eddies), less energy efficient so requires more pressure for forward movement (energy lost as sound)
• Turbulent flow is more likely to occur if the fluid is dense, in a wider diameter tube, and when there is high flow velocity (less likely in viscous fluids)
• Airflow in trachea: turbulent, when high velocities as energy is lost, no longer directly proportional to driving force (proportional to square root), radius of airways has bigger impact on resistance
• Most airflow is transitional, turbulent flow cause by bifurcation at each generation. lamina flow between those areas
• Lamina flow in bronchioles, air arranged in concentric circles
• Most vessels have lamina flow. RBCs arranged in concentric layers
• Turbulent blood flow occurs in vessels with an obstacle/irregularity (valves/clots), flow is likely when velocity is high e.g stenosis increasing flow velocity (murmurs)
• Anaemia causes a decrease in haematocrit so decreases blood viscosity leading to functional murmurs

Question 3

How is turbulent flow used in diagnosis of disease

Answer 3

• First ‘lub’ sound occurs at ventricular systole when AV valves close
• Left ventricular pressure increases, blood moves back in direction of atria, fills leaflets of valves snapping them shut, and closure of valves generates reverberations around ventricular wall
• Second ‘dub’ sound is generated by closure of aortic and pulmonary artery valves
• Valves cause turbulence, resulting in audible murmurs
• Generating turbulent flow in blood is used to monitor ABP, when measuring blood pressure in a non-invasive way, a blood pressure cuff is used to artificially generate a turbulent flow, inflating cuff will partially occlude brachial artery causing blood flow velocity to increase through narrowed artery, generating murmurs (Korotkoff sounds) that can be heard through a stethoscope
• Korotkoff sounds are heard between systolic and diastolic blood pressures
• When cuff pressure falls below diastolic pressure, blood vessel no longer occluded, flow will become laminar so no sound and procedure gives a good approximation of systolic and diastolic pressure (systolic = sounds first heard, diastolic = sounds disappear)