# Defining Blood Plasma and Serum Metabolome by GC-MS
## 3 How Many Blood Metabolites Are There?  
The potential space of organic substances that make up the metabolome is truly colossal: it lies between \(10^{63}\) and \(10^{200}\) unique substances [46]. Even if these theoretical spaces are reduced by several orders or even tens of orders of magnitude, they will not cease to look astronomical. Such an impressive amplification of diverse chemical information is a blessing and a curse of metabolomics at the same time [5]. At the moment, according to the HMDB 5.0, the world's largest and most comprehensive human metabolome database [47], more than 18 thousand unique low-molecular compounds of various nature have been detected and quantified in human blood. However, the diversity of metabolites in human blood is complex and challenging to assess.  
Globally, all human blood metabolites can be divided into water-soluble and lipid-soluble groups. The share of lipid-soluble molecules accounts for 88% of the metabolome (Figure 1). Lipids and lipid-like substances represent a significant and chemically diverse fraction of metabolome (>80,000 lipid molecules exist in humans, and more than 20,000 of them are found in the blood), which play essential roles in living systems. Various functional groups of lipids make them versatile machines serving as cellular barriers (various phospho- and glycolipids [48], membrane matrices (cholesterol), signaling agents (ceramide, sphingosine), and energy reservoirs (triglycerides) [49; 50; 51].  
Non-lipid metabolites account for only about 12% of the total blood metabolome. Still, the variety of classes these substances belong to is much wider than that of the lipid-soluble fraction (Figure 1).  
Metabolites differ significantly in their physical-chemical characteristics. To illustrate this diversity, we analyzed significant experimentally established and predicted properties of blood metabolites deposited into HMDB 5.0: results are presented in Supplementary Material S1.  
This diversity inevitably leads to difficulties throughout metabolome research: from developing universal protocols suitable for a wide range of substances to post-processing and interpretation of obtained results. Even in panoramic mode, metabolomics cannot equally effectively explore and quantify the entire metabolome during one experiment, in contrast to more \"mature\" genomics, which allows sequencing the entire genome of the object under study. In the case of the proteome and metabolome, it is impossible to detect and quantify all proteins and metabolites, so researchers are forced to work under conditions of myopia, observing only part of the overall picture [52].  
The concentration range of small organic molecules in blood covers nine orders of magnitude. The most abundant organic molecules are present at the level of several mM (cholesterol, urea, amino acids), and the limit of the concentrations of quantified metabolites is pM and below (various glycerophospholipids). In general, the average concentration of metabolites in the blood of healthy donors can vary within the range of \(\pm\)100% (glucose, lactic acid, glutamine, glycine) due to several internal (age, sex, genetic patterns) and external (diet, circadian rhythms, and fitness) determinants [44].