**4. Discussion**

The increased rate constant of lead transport through brain capillary, permeability surface area, concentration of lead in the brain, and time of exposure to lead in 20 kg weighed human as compared to 40 and 60 kg weighed humans agree with the report of Nicholson [35] indicating that young humans are more vulnerable to plumbism. The increased plasma clearance of lead in the human of 20 kg body weight as compared to 40 and 60 kg body weight human agrees with the report indicating that children have the ability to eliminate lead faster than adult [35]. Absorption of lead by bone and teeth is 94% in adult as compared to 70% in children [36]. Half-life of blood lead is 40 days in human. Renal insufficiency caused by plumbism could delay lead elimination. Lead concentration lower than 5 μg/dL is associated with reduced academic performance. Tyrosine and phenylalaine are the two main binding sites in albumin. The binding site is *Tyr*<sup>84</sup> and *Cys*<sup>34</sup> [37]. The allowable lead limit in drinking water is 15 *Ppb* ð Þ 73 *nM* [38, 39]. Serum albumin consists of 585 amino acids which have three structural domains stabilized in disulfide bonds with binding sites for fatty acids, glycerol, and metal ions. The ability of lead to bind serum albumin is used to detect micromolar concentration of lead ions in biological solution [40–42]. Centre for disease control's (CDC's) limit of 5 μg/dl in blood is contrary to revised limit of 3.5 μg/dl, but even 3 μg/dl has caused diminished cognitive function [43]. Protein kinases regulate the development of brain capillaries and expression of blood-brain barrier. Lead could stimulate protein kinase, disrupting BBB development at the regulation of neuronal development. More so, low doses of lead can stimulate protein kinase C that may enhance the release of neurotransmitters [44]. Protein kinase C is most sensitive to lead followed by calmodulin-protein kinase C and cyclic adenosine monosphosphate protein kinase [45]. Toxic action of lead is influenced by the interaction between endothelial cells and astrocytes [46]. Genetic and environmental factors could make some particular children more vulnerable to lead neurotoxicity [47]. Human brain is made up of 100 billion neurons, which consumes 15–20 W power. Brain consumes 15–20% of consumed glucose [48]. Utilization of adenosine triphosphate (ATP) in gray matter and white matter are 9.5 *μmol g*�<sup>1</sup>*S*�<sup>1</sup> and 3 *μmol g*�<sup>1</sup>*S*�<sup>1</sup> , respectively, signifying that 77% energy is consumed by cortical gray matter, which is 50% of brain volume. Components of gray matter are dendrites, neurons, glial cells, unmyelinated axons, and capillaries, but components of white matter are glial cells, myelinated axons, and capillaries. The density of brain capillary is 2–4 times more in gray matter [49]. Adult human brain weighs 1500 g and occupies 1200 cm3 , with surface area of microvessels (100–200 cm<sup>2</sup> *g*�1) [50]. Brain extracellular space constitutes 15–30% of the brain volume and brain vasculature 3% of the brain volume [51]. Increased size of human brain from 400 cm<sup>3</sup> to 1200 cm<sup>3</sup> [52] indicates increased cognition capacity of modern human. Serum creatinine 0ð Þ *:*88 � 0*:*2 *mg=dl* has been reported for human with blood lead level of 2*:*36 � 2*:*54 *μg=dl* [53]. Blood level of 24*:*43 � 5*:*31 *μg=dl* has been reported for traffic policeman in Taiwan [54]. Less than 1.5 min cerebral perfusion of labeled lead chloride was fast, providing a space of 9.7 mol/100 g of frontal cortex at 1 min. The influx was linear with concentration of 4 μM. Albumin (5%), cistern (200 Mm), and ethylene diamine tetraacetate (EDTA) (1 μm) slipped lead uptake. Potassium reduces lead 203 uptakes [55]. Concentration of lead in blood 2ð Þ *:*1 � 6*:*2 *μg=dl* has been reported for children [56] and 0.52–8.38 *μg=dl* in adults [57]. Brain weight of man decreases by 2.7 g and that of human by 2.2 g per year with increasing age. Also brain weight increase of 3.7 g is independent of sex [58]. *Application of Lead Transport through Brain Capillary for Determination of Weight, Brain… DOI: http://dx.doi.org/10.5772/intechopen.107459*

However brain weight increased by 0.78 g/year for 80 years between 1860 and 1940, amounting to 62.4 g [59]. Cognitive ability is dependent on number of neurons [60]. White matter decreases in old (75–85 years) as compared to young by 11% [61]. Hence, there is a need to revise the toxic reference value of lead in children. The reference value of 3.5 μg/dl should be revised to guide clinical and public intervention for individual children [62], because the relationship between encephalization and intelligence could be affected by low allometric value, when linear regression line was used [63]. Brain size, different regions of the brain including cerebral cortex, cortical thickness, frontal cortex, parietal cortex, cerebellum, and experience correlated positively with intelligence. Therefore, gene and environmental influence plays a key role in intelligence quotient [64], as such brain size based on cognitive equivalence is preferred to the encephalization quotient in empirical cognitive studies [65]. Environment could affect the development of neural tissues [66].
