Flow Equals Pressure Over Resistance

Each heart pump creates a head of pressure that drives flow through its side of the circuit. Flow, pressure and resistance in each circuit are in ohmic relationship analogous to current, voltage and resistance in electrical circuits. Flow equals the mean difference in pressure between the arterial and venous ends of each circuit divided by the total resistance of the circuit. The quotient between the pressure head and the regional flow resistance determines how much blood flows in each circuit. The body provides much more resistance to flow in the systemic circuit than the lung resists flow in the pulmonary circuit, accounting for the differences in pressures and the muscularity of the ventricles, the right ventricle being thinner and less muscular than the left.

So in review, the left heart ejects blood into the systemic vessels supplying the body. These vessels form many regional circuits serving liver, brain, muscle, bone and other organs each separately controlled, specialized in design, and arrayed in parallel. The right heart pumps blood under lower pressure into the vascular beds of the lungs. These beds, more uniform in design, are where oxygen and carbon dioxide exchange between blood and air in the alveoli across the walls of pulmonary capillaries.

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