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I learnt about 2 types of pressure gradients that needs to be considered in the cardio-vasculature.

  1. Transmural pressure (which refers to a pressure gradient felt across the vascular wall, at one particular point and this creates the driving pressure forward through circulation)
  2. Perfusion pressure/driving pressure (which refers to the pressure gradient between two places within the circulation.)

Intuitively, I think the vectors for the forces associated with these pressure differentials should point in the direction of blood flow. But I'm not sure what exactly are the nature of these pressures? And when we talk about the resistance to blood flow, which of the two pressure gradients is it opposing?

My attempt: transmural pressure could be colloid osmotic pressure, while perfusion pressure is hydrostatic.

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Transmural pressure is the pressure difference across the vessel wall ($P_{inside}-P_{outside}$). As such, the force vector will be pointed outward across the vessel wall, which is perpendicular to the direction of the blood flow.

Perfusion pressure is the pressure from the arterial side to the venous side of the vessel. As such, the force vector will be pointed along the vessel parallel to the direction of the blood flow.

Resistance to blood flow refers to the perfusion pressure. As the blood flow increases the required perfusion pressure also increases. This is the vascular resistance. It takes more pressure to drive a larger flow.

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