Model analogues in the study of cephalic circulation.

Simple laboratory models are useful to demonstrate cardiovascular principles involving the effects of gravity on the distribution of blood flow to the heads of animals, especially tall ones like the giraffe. They show that negative pressures cannot occur in collapsible vessels of the head, unless they are protected from collapse by external structures such as the cranium and cervical vertebrae. Negative pressures in the cerebral-spinal fluid (CSF) can prevent cerebral circulation from collapsing, and the spinal veins of the venous plexus can return blood to the heart in essentially rigid vessels. However, cephalic vessels outside the cranium are collapsible, so require positive blood pressures to establish flow; CSF pressure and venous plexus flow are irrelevant in this regard. Pressures in collapsible vessels reflect pressures exerted by surrounding tissues, which may explain the observed pressure gradient in the giraffe jugular vein. Tissue pressure is distinct from interstitial fluid pressure which has little influence on pressure gradients across the walls of major vessels.