hydrochloric acid and the enzyme pepsin. Both of
these secretions are part of gastric juice.
Endocrine glandsare ductless glands. The secre-
tions of endocrine glands are a group of chemicals
called hormones, which enter capillaries and are cir-
culated throughout the body. Hormones then bring
about specific effects in their target organs. These
effects include aspects of growth, use of minerals and
other nutrients, and regulation of blood pressure, and
will be covered in more detail in Chapter 10.
Examples of endocrine glands are the thyroid gland,
adrenal glands, and pituitary gland.
The pancreas is an organ that is both an exocrine
and an endocrine gland. The exocrine portions secrete
digestive enzymes that are carried by ducts to the
duodenum of the small intestine, their site of action.
The endocrine portions of the pancreas, called pan-
creatic islets or islets of Langerhans, secrete the hor-
mones insulin and glucagon directly into the blood.
CONNECTIVE TISSUE
There are several kinds of connective tissue, some of
which may at first seem more different than alike. The
types of connective tissue include areolar, adipose,
fibrous, and elastic tissue as well as blood, bone, and
cartilage; these are summarized in Table 4–2. A char-
acteristic that all connective tissues have in common is
the presence of a matrix in addition to cells. The
matrixis a structural network or solution of non-
living intercellular material. Each connective tissue
has its own specific kind of matrix. The matrix of
blood, for example, is blood plasma, which is mostly
water. The matrix of bone is made primarily of cal-
cium salts, which are hard and strong. As each type of
connective tissue is described in the following sec-
tions, mention will be made of the types of cells pres-
ent as well as the kind of matrix.BLOOD
Although bloodis the subject of Chapter 11, a brief
description will be given here. Blood consists of cells
and plasma; cells are the living portion. The matrix of
blood is plasma, which is about 52% to 62% of the
total blood volume in the body. The water of plasma
contains dissolved salts, nutrients, gases, and waste
products. As you might expect, one of the primary
functions of plasma is transport of these materials
within the body.
Blood cells are produced from stem cells in the red
bone marrow, the body’s primary hemopoietic tissue
(blood-forming tissue), which is found in flat and
irregular bones such as the hip bone and vertebrae.
The blood cells are red blood cells, platelets, and the
five kinds of white blood cells: neutrophils,
eosinophils, basophils, monocytes, and lymphocytes
(see Figs. 4–4 and 11–2). Lymphocytes mature and
divide in lymphatic tissue, which makes up the spleen,74 Tissues and Membranes
BOX4–1 CYSTIC FIBROSIS
defensin, a bacterium called Pseudomonas aerugi-
nosastimulates the lung cells to produce copious
thick mucus, an ideal growth environment for bac-
teria. Defensive white blood cells cannot get
through the thick mucus, and their activity mistak-
enly destroys lung tissue. A person with CF has
clogged bronchial tubes, frequent episodes of
pneumonia, and, ultimately, lungs that cannot carry
out gas exchange. CF is a chronic, progressive dis-
ease that is eventually fatal unless a lung transplant
is performed.
CF is one of several disorders believed to be cor-
rectable by gene therapy, but because it involves
human subjects, this kind of work proceeds very
slowly.Cystic fibrosis (CF) is a genetic disorder (there are
many forms) of certain exocrine glands including
the salivary glands, the sweat glands, the pancreas,
and the mucous glands of the respiratory tract.
In the pancreas, thick mucus clogs the ducts and
prevents pancreatic enzymes from reaching the
small intestine, thus impairing digestion, especially
of fats. But the most serious effects of CF are in the
lungs. The genetic mistake in CF often involves a
gene called CFTR, which codes for chloride ion
channels (proteins) in the membranes of epithelial
cells. In the lungs, the defective channels are
destroyed (by proteasomes), which causes a change
in the composition of the tissue fluid around the
cells. This change inactivates defensin, a natural
antibiotic produced by lung tissue. In the absence of