The existence of multicellular organisms is based on the efficient capture of solar energy by plants through photosynthesis, a process by which carbon dioxide (CO2) and water are converted into glucose and oxygen (O2). These are subsequently utilized by all eukaryotic organisms to generate ATP and, as by-products, CO2 and water, thus completing the circle of life on our planet (Figure 29.1). The highly efficient recovery of the energy contained within the chemical bonds of glucose through the process of oxidative phosphorylation provides the power necessary to assemble and maintain complex multicellular machines such as Homo sapiens, in which more than 100,000,000,000,000 parts (i.e., cells) are assembled and organized into a functional unit (i.e., organism). A requirement for the efficient generation of ATP is the continuous delivery of O2 to every cell in the body. In postnatal life, this requirement is met through the concerted action of the lungs, blood, heart, and vessels. The lungs of an adult human take in 5 to 6 liters of air per minute or approximately 8,000 liters per day. Within the lungs, O2 is bound by hemoglobin present within erythrocytes, which are pumped by the heart through blood vessels that represent the transportation infrastructure for O2 and glucose delivery to the tissues and for the removal of CO2, hydrogen ions (H+), potassium ions (K+), and other toxic metabolites.

Whereas the major blood vessels are generated in a stereotypical pattern during development, the capillaries through which O2 and CO2 are exchanged in the tissues develop not through a hardwired program but rather based on physiological signals generated by individual cells.