Key Points

• The left heart pumps oxygenated blood into the systemic circulation from which the venous blood returns to the right heart; the right heart pumps deoxygenated blood into the pulmonary circulation from which the venous blood returns to the left heart.

• Hydrostatics refers to the physics of fluid at rest; hydrodynamics refers to the physics of fluid in motion.

• Pressure is a scalar quantity with units of force per unit area; pressure in a fluid system increases in proportion to depth because of the force of gravity.

• Velocity refers to the linear rate at which the molecules of a fluid are moving. In a tube the mean velocity is determined by the flow rate divided by the cross-sectional area. The pressure within the fluid is reduced as the fluid flows faster because of a kinetic energy component.

• Fluid in a tube resists motion due to viscous drag. Poiseuille's equation describes the relationship between tube dimension and this resistance to flow.

• Although blood flow in the body is approximated by Ohm's law, it is not exactly described by it because (1) the viscosity of blood which is not a newtonian fluid changes with its velocity, and (2) blood vessels are not rigid tubes and change their dimensions as a function of internal pressure.

• Pressure differs in each vessel type; arteries have the highest pressure and veins the lowest.

Essential Medical Physiology, Third Edition

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