CR2.6 Blood flow through the vascular

Question 1

Briefly explain the relationship of flow rate, pressure difference and resistance across the circulatory system.

Answer 1

Flow rate = pressure difference / resistance.

Pressure difference is relatively constant across the aorta and vena cava. The change in resistance is therefore the primary means by which blood flow is regulated within organs (system maintains aterial and venous BP in narrow range).

Question 2

List six determinants of resistance.

Answer 2

1. Blood vessel diameter / radius (*most important)
2. Blood vessel length
3. Blood viscosity
4. Organisation of the vascular network (e.g. series versus parallel)
5. Whether blood flow is laminar or turbulent
6. Effects of extravascular mechanical forces (e.g. compression of coronary arteries during systole)

Question 3

Explain the difference between turbulent and laminar flow and the clinical relevance.

Answer 3

Laminar flow is silent and turbulent flow is noisy. At high flow rates (beyond a critical velocity) the flow becomes turbulent due to increasing resistance. This can be heard as;

• Korotkoff sounds when measuring blood pressure
• Murmurs related to the abnormal movement of blood across valves and between cardiac chambers
• Bruits of turbulent flow outside of the heart such as indicator of carotid artery stenosis

Question 4

Briefly describe the relationship between viscosity and flow velocity.

Answer 4

Viscosity is influenced by flow velocity. A reduction in velocity results in higher viscocity and resistance. Stasis can increase the risk of blood clots.

Question 5

Describe the four determinants of pressure in blood vessels.

Answer 5

1. Driving –> the pressure difference between two points inside the vessel along the axis of the vessel due to the heart contracting and driving blood forward.
2. Hydrostatic –> the pressure between the top and bottom of a column influenced by gravity and density of blood.
3. Transmural –> the intravascular pressure - the tissue pressure. Blood vessels are compliant and distensible so transmural pressure governs vessel radius and resistance.
4. Bernoulli effect –> blood flows down an energy gradient

N.B. Functional column height IS NOT distance from heart to ground as veins have valves and this reduces the effective hydrostatic pressure.

Question 6

Describe transmural pressure and the differences between arteries and veins.

Answer 6

The effect of transmural pressure on radius depends on the specific compliance of a vessel wall. Arteries have low compliance and veins have high compliance. Compliance is determined by VSMC tone, ECM composition and blood volume. As blood vessels fill, compliance decreases.

Example:

In exercise muscle blood flow rate increases (i.e. vasodilation) while in gut the flow rate is reduced (i.e. vasoconstriction).