Measurement and scaling of vascular mechanics in large and small mammals

Dr. Craig Hartley
Baylor College of Medicine

The task of the cardiovascular system is to deliver oxygen, fuel, and other substances to organs and tissues and to remove carbon dioxide and other waste products. The timing, dimensions, and mechanical properties of the cardiovascular systems of mammals are tuned to maximize transport efficiency and to minimize cardiac work. This tuning is maintained both at rest when cardiac output is low and at maximal exertion when cardiac output can increase 4-5 fold, and the mechanical characteristics involved differ by region in their magnitude and controllability. The shape and magnitude of blood pressure waveforms are similar throughout the body, but the waveforms of blood flow and velocity can be quite different. This is because flow to some organs is nearly constant (brain, and kidney), while flow to other organs (liver, gut, heart, skin, and muscles) is highly variable depending on demand. After meals flow to the liver and gut is increased, and during exercise flow to the muscles and coronary flow to the heart are increased. Thus, blood flow to organs and other tissues is under local control as are the mechanical properties of the local blood vessels. Flow to most organs and regions occurs primarily during systole when blood pressure is highest, but coronary flow is low during systole when pressure is being generated by the cardiac muscle and high during diastole when the heart is relaxed. Some believe that the vascular system is optimized to maintain a high diastolic pressure primarily to augment coronary perfusion. There are scaling laws that determine optimal values for parameters such as rates, dimensions, volumes, pressures, flows, velocities, and life span of the form: y=a*BW**b, where y is the parameter, BW is body weight, and a and b are constants. Scaling to the power (b) of BW ranges from: 0 (independent of BW) for blood pressure and velocities; 1/4 for heart period, vessel length, and life span; 3/8 for vessel diameter; 3/4 for vessel area, cardiac output, and blood flow; and 1 (proportional to BW) for heart weight, stroke volume, and blood volume. Despite large differences in body weights, the waveforms and magnitudes of pressure and blood velocity in arteries supplying the organs are similar in all mammals when normalized for heart rate. This implies that the time constants of the various parts of the cardiovascular systems are scaled to the period of the heart. Thus, the scaling laws allow us to model human cardiovascular conditions and diseases in animals as small as mice and to translate many of the measurements made in mice to humans.

Audio (MP3 File, Podcast Ready) Presentation (PowerPoint File)

Back to Workshop IV: Optimal Transport in the Human Body: Lungs and Blood