Page 235 - Fundamentals of anatomy physiology

222 Unit 2 Support and Movement

Clinical Note limbs (Photo b). The cause is excessive cartilage formation at
the epiphyseal cartilages. These body proportions are not in
Abnormal Bone Development A variety of endocrine themselves dangerous. However, the underlying mutation
or metabolic problems can result in characteristic skeletal affects the structure of connective tissue throughout the
changes. In pituitary dwarfism, inadequate production of body and commonly causes life-threatening cardiovascular
growth hormone leads to reduced epiphyseal cartilage activ- problems.
ity and abnormally short bones. This condition is becoming
6 increasingly rare in the United States, because children can be
treated with synthetic human growth hormone.
a Gigantism b Marfan’s syndrome
Gigantism (also known as giantism) results from an over-
production of growth hormone before puberty (Photo a).
(The world record for height is 272 cm, or 8 ft, 11 in. It was
reached by Robert Wadlow, of Alton, Illinois, who died at
age 22 in 1940. Wadlow weighed 216 kg, or 475 lb.) If growth
hormone levels rise abnormally after epiphyseal cartilages
close, the skeleton does not grow longer. Instead, bones
get thicker, especially in the face, jaw, and hands. Cartilage
growth and alterations in soft-tissue structure lead to changes
in physical features, such as the contours of the face. These
physical changes take place in the disorder called acromegaly.

Several inherited metabolic conditions that affect many
systems influence the growth and development of the skel-
etal system. These conditions produce characteristic varia-
tions in body proportions. For example, many individuals
with Marfan’s syndrome are very tall and have long, slender

6-8 Calcium plays a critical role crisis. Calcium ions are particularly important to both the
plasma membranes and the intracellular activities of neurons
in bone physiology and muscle cells, especially cardiac muscle cells. If the calcium
concentration of body fluids increases by 30 percent, neurons
Learning Outcome Explain the role of calcium as it relates and muscle cells become unresponsive. If calcium levels de-
to the skeletal system. crease by 35 percent, neurons become so excitable that con-
vulsions can occur. A 50 percent reduction in calcium concen-
A dynamic relationship exists between calcium and the skel- tration generally causes death. Calcium ion concentration is
etal system, and hormones influence calcium balance in the so closely regulated, however, that daily fluctuations of more
body. than 10 percent are highly unusual.

The Skeleton as a Calcium Reserve Hormones and Calcium Balance

A chemical analysis of bone reveals its importance as a mineral Two hormones with opposing effects maintain calcium ion
reserve (Figure 6–14). For the moment, we will focus on the homeostasis. These hormones, parathyroid hormone and cal-
homeostatic regulation of calcium ion concentration in body citonin, coordinate the storage, absorption, and excretion of
fluids. (We will consider other minerals in later chapters.) Cal- calcium ions. Three target sites and functions are involved:
cium is the most abundant mineral in the human body. The (1) the bones (storage), (2) the digestive tract (absorption),
typical human body contains 1–2 kg (2.2–4.4 lb) of calcium, and (3) the kidneys (excretion).
with nearly 99 percent of it deposited in the skeleton.
Figure 6–15a indicates factors that raise calcium levels in
Calcium ions play a role in a variety of physiological pro- the blood. When calcium ion concentrations in the blood fall
cesses, so the body must tightly control calcium ion concen- below normal, cells of the parathyroid glands, embedded in
trations in order to prevent damage to essential physiological the thyroid gland in the neck, release parathyroid hormone
systems. Even small variations from the normal concentration
affect cellular operations. Larger changes can cause a clinical