Users of the model should be aware that the model has several limitations. These stem from its' physiological age and from limitations of simulations in general. Each are discussed briefly below.
1) Physiological age - web-HUMAN is based on Dr. Colemans 1981 FORTRAN model with only very minor modification. As such its systems physiology is "frozen" at the 1980 level of physiological knowledge. For educational use this is not as serious as it may at first sound since (sadly) relatively few major advances have been made at the systems level of understanding in the intervening 20 years (of course, genetics and molecular biology of these systems is another story!). Thus while the pharmacy may be somewhat dated (see the extended variables help for more), a major controller of long term blood pressure is still understood to be blood volume. Indeed, the angiotensin converting enzyme (ACE) inhibitor in the pharmacy (and the diuretics, as has recently come out) are still among the most effective treatment strategies for hypertension.
2) An updated model - Actually, a new one. Users of web-HUMAN who wish access to a web-HUMAN like physiological model containing a more updated and expanded physiology may be interested in Dr. Coleman's QCP3 (Quantitative Circulatory Physiology 3) model. This model, which runs on the Windows platforms only, contains entire new sub-models such as a blood sugar regulation module in the which the insulin/glucagon system (not built into web-HUMAN) can be studied both in its normal operation and integrated with the ability to intervene to create pathology such as diabetes.
An example of the updating of physiology can be seen in QCP3's blood volume regulation system. The QCP3 model contains atrial natiuretic factor (ANF), a Na+ and volume regulation hormone not included in web-HUMAN. Readers interested in trying out Dr. Colemans QP3 model can access it at its own web site "biological simulators inc" (http://www.biosim.com/). There are both trial and purchasable versions available.
A feeling for how HUMAN itself was constructed can be found at Dr. Colemans simulation workshop page at (http://physiology.umc.edu/WORKSHOP/WORKSHOP.HTM).
3) Limitations of simulations in general - It is sometimes surprising to find out that users of simulations do not always realize that simulations possess an inherent limitation in that if a behavior is not built into a model it will not be there. Thus (see 2 above) one can not expect ANF responsiveness or (see 1 above) hormone-based blood sugar regulation from web-HUMAN even though the model does indeed regulate blood volume and exhibit certain metabolic functions. This limitation is often not acknowledged by those who propose the complete substitution of simulations for wet labs in courses.