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Understanding Blood Flow


Blood Flow in Arteries

Animation of heart pumping blood through arteriesArteries are the blood vessels taking blood away from the heart. The heart is a very powerful muscle – so the blood is ejected out at a high velocity and with high pressure. As the heart beats approximately once every second, the pressure and flow increases once per second. To withstand this regular increase of flow and pressure, the arterial walls have to be strong (to withstand the pressure), elastic and muscular (to dilate when the pressure pushes outwards, and to bounce back to normal shape when the pressure pulse passes.

The walls of the arteries have 3 main layers and a special lining that is contact with the blood. It is the Media that gives the arteries the ‘bounce’ and lets them return to their normal shape after the heart has pumped blood through them at high pressure. You can feel this ‘bounce’ in some areas of the body such as at the wrist – this is called the ‘Pulse’.


Blood Flow in Veins

Veins are the blood vessels that carry blood back to the heart from the tissues. These tissues might be in the legs, arms, brain, liver, lungs etc.

Animation of venous flow when lying down

Of course, it can easily be seen how blood returns from the brain – provided you are standing or sitting upright, gravity will do the work for you. However, to get blood back to the heart from below it, the blood needs to be “pumped up” against gravity.

It is the understanding of the venous pump in the legs – and more importantly its failure – which leads to the problems of varicose veins, venous eczema, lipodermatosclerosis and venous ulceration. However, as far as an understanding of the veins is concerned in relation to deep vein thrombosis, we can concentrate on the flow in a patient at rest, who is lying down and whose flow is steady and not being pumped.

The vein wall has the same layers as the arterial wall, but it is much thinner. It is thinner as the flow is smoother and the vein wall does not have to withstand the pressure of the heart pumping.

The endothelium, intima and adventitia layers are much the same – the difference in the wall thickness is due to the the Media layer (the muscle layer) being much thinner.

Blood flows back to the chest from the legs through the veins.

As we have said, the blood is not being pushed back by the heart and so it has Low Pressure and fairly smooth Flow when you are lying down at rest. It is for these 2 reasons that veins do not Pulsate – which is why you can’t feel a pulse in the veins – only in the arteries.

Therefore,the important things to realise at this stage are:

  • Veins contain blood flowing smoothly and at low pressure
  • Veins have thin walls but still have endothelium next to the blood which stops the blood from clotting inside the vein.

Blood Flow in Leg Veins when sitting & standing

Things change in the veins when we sit or stand up. This is particularly important when we consider sitting in an airplane. When a DVT occurs in this situation, it is called ‘Economy Class Syndrome’ or ‘Traveller’s Thrombosis’.

Animation of venous pump when moving foot at rest

However, you should be aware that research suggests that there is a difference between air travel and coach travel. It appears there is a higher risk of DVT on a flight than during the same time sitting in a coach. This is probably due to the oxygen being ‘thinner’ during a flight (most commercial aircraft are pressurised to 7,000 – 12,000 ft – so there is less oxygen in each breath than at ground level). This is obviously not the case in a coach.

Nevertheless, as we try to stop DVT and ‘Economy Class Syndrome’ or ‘Traveler’s Thrombosis’ by encouraging movement, compression socks (travel socks) and hydration, it is useful to understand the changes that occur when sitting.

Most people do not get swelling of the ankles when they sit still for long periods of time. The way they get around this problem is by using their ankles to pump blood back to the heart. By continually moving every few minutes or so, they use the muscles of the legs to pump the blood back up to the heart.

As the vein system is a Low Pressure system, whenever the heart is above the ankles, blood cannot flow smoothly back. In fact, it cannot flow up the veins at all by itself as there is nothing pushing it upwards against gravity.

Instead it collects at the ankles and, if we sit still enough for long enough, the ankles will swell. This is due to the increasing pressure in the blood at the ankles as it starts collecting here, pushing outwards against the vein walls. Eventually the pressure reaches a sufficient level that fluid is squeezed out of the blood, through the vein walls and into the tissues. This swelling due to fluid is called oedema.