Lubrication of Joints - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 294

limbs, and the lower limbs. All 206 bones in the human body function to support and pro-
tect the body, store minerals, produce blood cells, and act as levers for motion.
In this textbook, we will restrict ourselves to a discussion of the long bones, which are
typically elongated and thin and are located within the limbs of the body ( Figure 11.1 ). All
long bones have a tubular shaft that extends for the majority of the length of the bone.
This shaft is termed the diaphysis of the bone. The wall of the diaphysis is composed of
compact bone, which is very solid and stiff and provides much of the mechanical stiffness
of bone. The diaphysis also forms a protective layer that surrounds a central open space
termed the medullary cavity (or the marrow cavity). At the longitudinal ends of the diaph-
ysis, the bone expands to a region termed the epiphysis. The epiphysis is composed
mostly of spongy bone (or trabecular bone). Spongy bone is an open mesh of bone tissue
surrounded by a thin covering of compact bone. This thin layer of compact bone is termed
cortical bone. Spongy bone acts to transmit stresses, from many directions, in a uniform
direction to the stiffer compact bone. In between the diaphysis and the epiphysis is a small
region termed the metaphysis. The metaphysis is composed of both compact bone and
spongy bone, with about 50% of each. It is the region of bone that transitions from majorly
compact bone in a tube structure to mostly spongy bone.
Bone is a connective tissue that is composed of specialized cells and a stiff matrix that
consists of extracellular proteins and a hardened ground substance. The bone matrix is
especially stiff due to the deposition of calcium salts within the protein fibers. The major
component of bone, by weight, is calcium phosphate Ca 3 (PO 4 ) 2 , which accounts for nearly
65% of the total bone mass. In bone tissue, three calcium phosphates interact with one cal-
cium hydroxide to form hydroxyapatite crystals. The molecular formula for hydroxyapa-
tite is Ca 10 (PO 4 ) 6 (OH) 2 and the chemical reaction for hydroxyapatite formation can be
described by

Þ 2
As hydroxyapatite forms, other ions (such as sodium and fluoride) and other calcium
compounds (e.g., calcium carbonate)

ð Þ 2 1 ð Þ 2 - ð Þ 6 ð

3Ca 3 PO 4 Ca OH Ca 10 PO 4 OH

incorporate into the crystal lattice structure of

FIGURE 11.1

A representative long bone, showing the vari-
ous regions within the bone. Long bones are composed of both
compact bone and spongy bone. Regions of the bone are classi-
fied both by the bone type that composes the majority of the sec-
tion and by the load direction that the bone normally experiences
in that region. Adapted from Martini and Nath (2009).

Epiphysis

Spongy bone

Me taphysis

Diaphysis

Medullary cavity

Compact bone

Me taphysis

Epiphysis