# PHYSIOLGY LECTURE # 6 STUDY NOTES: HEMODYNAMICS – VASCULAR RESISTANCE

Resistance can be integrated into two types of circuits; parallel and serial circuit.

### Series Circuit

The heart is placed in series with the pulmonary vasculature and as well as the rest of the body. It pumps blood to the lungs, from where the blood travels back to the left side of the heart. The left heart pumps blood to the rest of the body after which the blood is returned back to the right heart. Therefore, the pulmonary and systemic circuits are connected in series via two pumps, which are the right heart and left heart respectively.

### Calculating Resistance in Series

The total resistance in series is calculated by taking the sum of all the resistances present in the circuit.

Total Resistance = R1 + R2 +R3 … Rn

It should be noted that if resistance is added in a series circuit, the pressure of blood proximal to the new resistance will increase and the pressure downstream will decrease. The flow remains the same.

### Parallel Circuit

Various arteries branch off from the aorta in order to deliver blood to various organ systems of the body. These arterial branches are an example of parallel circuitry. The different organ systems and their visceras (CNS, GIT, Skeletal, and Renal etc.) are connected in a parallel fashion with respect to each other. Therefore, when blood is transported to these organ systems via the aorta, the total resistance offered will be the same as that offered by a parallel circuit.

### Calculating Resistance in Parallel

The total resistance in a parallel circuit is calculated by adding the reciprocals of each resistor.

(1/TR) = (1/R1) + (1/R2) + (1/R3) … (1/Rn)

As blood flows past a resistor in parallel circuit, a fraction of the blood enters the branch of blood vessel delivering blood to the resistor. Increasing the number of branches will allow blood to get distributed even more among the branches. The volume, and hence the pressure, decreases as a result. Therefore, the total resistance decreases as we add more branches to a circuit. It can be further deduced that the total resistance in a parallel circuit is always less than the value of the smallest resistor in that circuit.