Fractals in Physiology

Some of the most visually striking examples of fractal forms are found in physiology: The respiratory, circulatory, and nervous systems are remarkable instances of fractal architecture, branches subdividing and subdividing and subdividing again.
Nice pictures are provided in Goldberger, Rigney and West.
Although no clear genetic, enzymic, or biophysical mechanism yet have been shown to be responsible for this fractal structure, few doubt this.
Careful analysis of the lungs reveal fractal scaling, and it has been noted that this fractal structure makes the lungs more fault-tolerant during growth.
Here are three pictures of lung casts, some with the circulatory system casts included. These pictures were provided by Dr. Ewald R. Weibel.

The heart is filled with fractal networks: the coronary arteries and veins, the fibers binding the valves to the heart wall, the cardiac muscles themselves, and the His-Purkinje system.

In addition to falut-tolerance during growth, fractal branching makes available much more surface area for absorption and transfer in bronchial tubes, capallaries, intestinal lining, and bile ducts.
Kalda has proposed a fractal model of the blood vessel system that achieves a homogeneous oxygen supply throughout the body.
Also, the redundancy of fractal structures make them robust against injury. For example, the heart can continue to function even after the His-Purkinje system has suffered considerable damage.

From his work on the ability of fractal drums to damp vibrations, Bernard Sapoval deduced another advantage of the fractal character of the circulatory system: "the fractal structure of the human circulatory system damps out the hammer blows that our heart generates." "The heart is a very violent pump, and if there were any resonance in blood circulation, you would die."
Fractals may save our lives every minute.

Here are some casts of animal lungs.

Finally, we note the body exhibits dynamical fractals.
For example, it is well-known that healthy heartbeats are chaotic rather than regular.
A careful plot of heart rates over several time scales reveals self-similar scaling (Goldberger, Rigney and West).