**Peripheral Resistance Unit (PTU)**

When the pressure difference between the two points of the vessels is 1 mm of Hg and the blood flow is 1 ml /sec. then the resistance is called 1 peripheral resistance unit (PRU). It is a conventional unit of vascular resistance equal to the resistance that produces a pressure difference of 1 mm Hg, corresponding to a blood flow of 1 mL/sec.

Resistance (in PRU’s) = Perfusion Pressure / flow (per sec.)

e.g. If cardiac output = 6000 ml/min. or 100 ml/sec., and mean aortic pressure – vena caval pressure = 100 mmHg

**Poiscuille’s equation:**

Q = πΔPr^{4}/8nl

Here, ‘Q’ is the rate of blood flow, Δp is the pressure difference between the ends of the vessel, ‘r’ is the radius of the vessel, ‘l’ is the length of the vessel and ‘h’ is the viscosity of the blood.

If ΔP = constant, ‘l’ is also constant, then Q α r^{4}

So, the rate of blood flow is directly proportional to the fourth power of the radius of the vessel. Which demonstrates once again that the diameter of a blood vessel.

**Factors which are related to the blood flow:** Flow through a blood vessel is determined by two factors.

- The pressure difference of the blood between the two ends of the vessel (also called pressure gradient). It is the force that pushes the blood through the vessel.
- The impediment to blood flow through the vessel, which is called vascular resistance, the flow through the vessels can be calculated by flowing formula.

**Ohm’s equation, **

Q = ΔP/R

Here, Q = Blood flow, AP = Pressure difference, and R = Resistance.

**And Poiseulle’s law: **

Q = πΔPr^{4}/8ηl

nAPr4 Q ara

So, other factor related to the blood flow –

- A diameter of the vessels,
- Length of the vessels.

Here, blood flows directly related to the pressure difference and inversely proportional to the resistance.