Haemodynamics.Understand the relationship between blood flow velocity and cross-sectional areas in different sections of the vascular system. The pressure gradient drives blood flow. By pumping blood into

the arteries, the heart increases the mean arterial pressure, which creates a difference in pressure between the arteries and veins that drives the flow of blood.

Blood flow velocity is determined the distance it travels over time (cm/s).

Volume depends upon cross-sectional area of blood vessel. Therefore, flow = velocity x area. Flow is constant in a ‘rigid’ system and the wider is the cross-sectional area, the slower is the blood flow. Velocity and cross-sectional area variables change between each other to ensure that the flow is constant, where a1v1= a2v2.

However, blood does not flow faster in narrower vessels as evident because the aorta is largest but the blood flow is very fast while the capillary is the most narrow and blood vessel there is very slow. This is because if we add up all the cross-sectional areas of the capillaries in contrast to only one cross section of the aorta, it makes capillaries the largest.

However flow is opposed by the resistance provided by the blood vessels.

Know Poiseuile’s Law and what it describes. Poiseuille’s Law describes laminar flow of homogeneous fluids

through cylindrical tubes in terms of flow, pressure, tube dimensions and viscosity of fluid.

Firstly, flow is directly proportional to the pressure gradient (the difference between inflow pressure and outflow pressure). If there are 2 reservoirs and one is filled up to h and the other is empty, then there is a certain flow rate. If the reservoir is filled up to 2h while the other is still empty, the flow rate doubles. But, if the reservoir is still at 2h while the other reservoir is at h, then the flow rate would be halved again.

Flow is inversely proportional to the length of the conducting vessel. The longer is the length of the conducting system, the less flow it is.

Flow also varies directly with the fourth power of the radius of the conducting vessel. If there is a vessel with a certain radius, if it is doubled, then there is a 16-fold increase in the flow rate. Radius of the conducting vessel is the most influential determinant.

Flow is also inversely proportional to the viscosity of the fluid. The more viscous it is, the slower is the flow rate. This is because friction (the sheering force) increases when the fluid becomes more viscous, and more friction causes less flow.

Understand the concept of resistance to blood flow and the structure and arrangement of blood vessels to counter it. Resistance (R) to blood flow is the ratio of the pressure gradient

to the flow.

Resistance to flow is greatest in arterioles relative to the cross-sectional area and also due to constriction of arterioles.

Separate elements of circulation (artery, arteriole, capillary etc.) are arranged in series while similar elements are arranged in parallel.

Resistance in series builds up and adds onto each other. Rr = R1 + R2 + R3+…Rn

The reciprocal of resistance in parallel is the sum of reciprocal of individual resistances. For any parallel arrangement, the total resistance is less than the individual resistance of any segment.

1R t

= 1R1

+ 1R2

+ 1R3

+…+ 1Rn

Total peripheral resistance. The total peripheral resistance is the resistance of the entire

circulation. The kidney has the highest resistance than any other organs.

Effect of pressure and vascular resistance on tissue blood flow. Blood vessels are not rigid tubes. The aorta is a conductance, and gets blood from the heart to the

organs as soon as possible. It is able to withstand high pressure, so it is quite fibrous. Its elasticity also allows it to adapt to changes in the pressure. Compliance is the measure of vessel elasticity. It can undergo elastic recoil after pumping blood to provide an extra pump further that drives the blood forward.

Small arteries and arterioles are muscular, resistance and they alter their diameter to control flow. When the muscle contracts, there is less blood flow due to a higher resistance.

Capillaries are thin and have endothelial cells for gaseous exchange.

Venules have thin wall, little muscle and low resistance. They are capacitance (reservoir) of blood. Most of the blood (~60%) is sitting in the veins. Venules have thin wall (low resistance) and high compliance so that they can stretch a long way.

Appreciate the concepts of laminar and turbulent flow. Blood flow that is constant has laminar flow. Due to friction with

the vessel wall, flow of blood and other fluid is at a constant velocity in a cylinder. There are layers of blood and the layer closest to the wall is slowest as the friction in the blood vessel slows it down. Meanwhile, at the centre, it is when flow is the fastest.

Increased velocity enhances the effect and there is axial streaming where cells tend to line up in the centre of the vessel where it is fastest. Increased velocity also increases shear stress of viscous drag on walls.

Irregular fluid motions within vessel cause turbulent flow. Greater pressure is required to force turbulent flow through a tube.

Turbulent is more likely if the vessel has a large diameter, flow is faster and blood viscosity is low.

Turbulent flow can be predicted in Reynold’s number.