Indeed, as the authors demonstrate, the constant flux within urban undergrounds the nonstop circulation of people, substances, and energy—serves all city dwellers in myriad ways, not just with the logistics of day-to-day life but as a crucial part of a city's mythology.
In placental mammals, fetus obtains nutrients from the mother through the placenta in prenatal period, which is responsible for the transfer of the bulk of substances between maternal and fetal circulations.
The placenta is responsible for the (bidirectional) transfer of substances between the maternal and foetal circulations including carbon dioxide, oxygen, water, nutrients, hormones, vitamins, and also xenobiotics including drugs and toxic compounds (Desforges and Sibley 2010).
The transfer of substances and the amount that enters the fetal circulation are tightly controlled by at least four different mechanisms: simple diffusion, active transport, biotransformation through metabolic enzymes, and phagocytosis or pinocytosis (Syme et al. 2004).
In clinical scenario not only the mesenteric circulation affects the constitution of peritoneal fluid but also the uptake and release of substances by the parietal peritoneum.
The main misunderstanding relating to the study of CSF "circulation" has been made by (mis)interpretation of experimental results in which the two physiological processes (movement of CSF and distribution of substances) have not been differentiated, and when substance distribution has been used as a marker of "circulation".
According to Atencio et al. (2008) and Molina et al. (2005), vacuolisation might indicate an imbalance between the rate of synthesis of substances in the parenchymal cells and the rate of release of these substances into the systemic circulation.
Namely, the monitoring of certain substances in CSF system will reveal nothing about the "circulation" of CSF, but only about the distribution of that substance within the CSF.
