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.
the lymph nodes, and the thymus gland. The thymus also contains stem cells, but they produce only a subset of lymphocytes. Stem cells are present in the spleen and lymph nodes as well, though the number of lymphocytes they produce is a small fraction of the total.
The blood cells make up 38% to 48% of the total blood, and each type of cell has its specific function.
Red blood cells(RBCs) carry oxygen bonded to the iron in their hemoglobin. White blood cells(WBCs) destroy pathogens by phagocytosis, the production of antibodies, or other chemical methods, and provide us with immunity to some diseases. Platelets prevent blood loss; the process of blood clotting involves platelets.
AREOLAR CONNECTIVE TISSUE
The cells of areolar(or loose) connective tissueare called fibroblasts. A blast cell is a “producing” cell, and fibroblasts produce protein fibers. Collagenfibers are very strong; elastinfibers are elastic, that is, able to return to their original length, or recoil, after being stretched. These protein fibers and tissue fluid make up the matrix, or non-living portion, of areolar con- nective tissue (see Fig. 4–4). Also within the matrix are mast cells that release inflammatory chemicals when tissue is damaged, and many white blood cells, which are capable of self-locomotion. Their importance here is related to the locations of areolar connective tissue.
Areolar tissue is found beneath the dermis of the Blood
Areolar
Adipose White blood cell
Fibroblast
Collagen fibers
Elastin fiber
Adipocytes
(Approximately 430X)
Red blood cells White blood cell
(Approximately 300X)
(Approximately 150X)
A
B
C
Platelets
Figure 4–4. Connective tissues. (A) Blood. (B) Areolar. (C) Adipose.
QUESTION:What is the matrix of blood, and what is found in adipocytes?
skin and beneath the epithelial tissue of all the body systems that have openings to the environment. Recall that one function of white blood cells is to destroy pathogens. How do pathogens enter the body? Many do so through breaks in the skin. Bacteria and viruses also enter with the air we breathe and the food we eat, and some may get through the epithelial linings of the respiratory and digestive tracts and cause tissue dam- age. Areolar connective tissue with its mast cells and many white blood cells is strategically placed to inter- cept pathogens before they get to the blood and circu- late throughout the body.
ADIPOSE TISSUE
The cells of adipose tissueare called adipocytesand are specialized to store fat in microscopic droplets.
True fats are the chemical form of long-term energy storage. Excess nutrients have calories that are not wasted but are converted to fat to be stored for use when food intake decreases. Any form of excess calo- ries, whether in the form of fats, carbohydrates, or amino acids from protein, may be changed to triglyc-
erides and stored. The amount of matrix in adipose tissue is small and consists of tissue fluid and a few col- lagen fibers (see Fig. 4–4).
Most fat is stored subcutaneously in the areolar connective tissue between the dermis and the muscles.
This layer varies in thickness among individuals; the more excess calories consumed, the thicker the layer.
As mentioned in Chapter 2, adipose tissue also cush- ions organs such as the eyes and kidneys.
Recent research has discovered that adipose tissue does much more than provide a cushion or store energy. Adipose tissue is now considered an endocrine tissue, because it produces at least one hormone.
Leptin is an appetite-suppressing hormone secreted by adipocytes to signal the hypothalamus in the brain that fat storage is sufficient (see also Chapter 17).
When leptin secretion diminishes, appetite increases.
Adipocytes secrete at least two chemicals that help regulate the use of insulin in glucose and fat metabo- lism. Adipose tissue is also involved in inflammation, the body’s first response to injury, in that it produces cytokines, chemicals that activate white blood cells.
Our adipose tissue is not simply an inert depository of 76 Tissues and Membranes
Fibrous tissue Fibrous tissue
Collagen fibers
Fibroblasts
Example: Trachea (approximately 430X)
B
A
C
(Approximately 430X)
Haversian canal Canaliculi Matrix Osteocytes Bone
Chondrocytes Matrix Cartilage
Example: Tendons (approximately 430X)
Figure 4–5. Connective tissues. (A) Fibrous. (B) Cartilage. (C) Bone.
QUESTION:What is the matrix of fibrous tissue, and of bone?
fat, rather it is part of the complex systems that ensure we are nourished properly or that protect us from pathogens that get through the skin.
FIBROUS CONNECTIVE TISSUE
Fibrous connective tissueconsists mainly of parallel (regular) collagen fibers with a few fibroblasts scat- tered among them (Fig. 4–5). This parallel arrange- ment of collagen provides great strength, yet is flexible. The locations of this tissue are related to the need for flexible strength. The outer walls of arter-
ies are reinforced with fibrous connective tissue, because the blood in these vessels is under high pres- sure. The strong outer wall prevents rupture of the artery (see also Box 4–2: Vitamin C and Collagen).
Tendons and ligaments are made of fibrous connective tissue. Tendons connect muscle to bone; ligaments connect bone to bone. When the skeleton is moved, these structures must be able to withstand the great mechanical forces exerted upon them.
Fibrous connective tissue has a relatively poor blood supply, which makes repair a slow process. If you have ever had a severely sprained ankle (which Table 4–2 TYPES OF CONNECTIVE TISSUE
Type Structure Location and Function
Blood
Areolar (loose)
Adipose
Fibrous
Elastic
Bone
Cartilage
Within blood vessels
• Plasma—transports materials
• RBCs—carry oxygen
• WBCs—destroy pathogens
• Platelets—prevent blood loss Subcutaneous
• Connects skin to muscles; WBCs destroy pathogens Mucous membranes (digestive, respiratory, urinary,
reproductive tracts)
• WBCs destroy pathogens Subcutaneous
• Stores excess energy
• Produces chemicals that influence appetite, use of nutrients, and inflammation
Around eyes and Kidneys
• Cushions
Tendons and ligaments (regular)
• Strong to withstand forces of movement of joints Dermis (irregular)
• The strong inner layer of the skin Walls of large arteries
• Helps maintain blood pressure Around alveoli in lungs
• Promotes normal exhalation Bones
• Support the body
• Protect internal organs from mechanical injury
• Store excess calcium
• Contain and protect red bone marrow Wall of trachea
• Keeps airway open On joint surfaces of bones
• Smooth to prevent friction Tip of nose and outer ear
• Support Between vertebrae
• Absorb shock Plasma (matrix) and red blood cells,
white blood cells, and platelets
Fibroblasts and a matrix of tissue fluid, collagen, and elastin fibers
Adipocytes that store fat (little matrix)
Mostly collagen fibers (matrix) with few fibroblasts
Mostly elastin fibers (matrix) with few fibroblasts
Osteocytes in a matrix of calcium salts and collagen
Chondrocytes in a flexible protein matrix
means the ligaments have been overly stretched), you know that complete healing may take several months.
An irregular type of fibrous connective tissue forms the dermis of the skin and the fasciae (membranes) around muscles. Although the collagen fibers here are not parallel to one another, the tissue is still strong.
The dermis is different from other fibrous connective tissue in that it has a good blood supply (see also Box 4–3: Cosmetic Collagen).
ELASTIC CONNECTIVE TISSUE
As its name tells us, elastic connective tissueis pri- marily elastin fibers. One of its locations is in the walls of large arteries. These vessels are stretched when the heart contracts and pumps blood, then they recoil, or
snap back, when the heart relaxes. This recoil helps keep the blood moving away from the heart, and is important to maintain normal blood pressure.
Elastic connective tissue is also found surrounding the alveoli of the lungs. The elastic fibers are stretched during inhalation, then recoil during exhalation to squeeze air out of the lungs. If you pay attention to your breathing for a few moments, you will notice that normal exhalation does not require “work” or energy.
This is because of the normal elasticity of the lungs.
BONE
The prefix that designates bone is “osteo,” so bone cells are called osteocytes. The matrix of bone is made of calcium salts and collagen and is strong, hard, 78 Tissues and Membranes
BOX 4–2 VITAMIN C AND COLLAGEN Many people take extra vitamin C, for various rea- sons. Vitamin C has several functions, and an im- portant one is the synthesis of collagen.
Imagine the protein collagen as a ladder with three uprights and rungs that connect adjacent uprights. Vitamin C is essential for forming the
“rungs,” without which the uprights will not stay together as a strong unit. Collagen formed in the absence of vitamin C is weak, and the effects of weak collagen are dramatically seen in the disease called scurvy.
In 1753 James Lind, a Scottish surgeon, recom- mended to the British Navy that lime juice be taken on long voyages to prevent scurvy among the sailors. Scurvy is characterized by bleeding gums and loss of teeth, poor healing of wounds, fractures, and bleeding in the skin, joints, and elsewhere in the body. The lime juice did prevent this potentially fatal disease, as did consumption of fresh fruits and vegetables, although at the time no one knew why.
Vitamin C was finally isolated in the laboratory in 1928.
BOX 4–3 COSMETIC COLLAGEN
system as foreign tissue. More seriously, an autoim- mune response may be triggered in some individu- als, and the immune system may begin to destroy the person’s own connective tissue.
In an effort to avoid these problems, some cos- metic surgeons now use the person’s own collagen and fat, which may be extracted from the thigh, hip, or abdomen. The long-term consequences and outcomes of such procedures have yet to be evalu- ated. We might remember that for many years the use of silicone injections had been considered safe.
Silicone injections are now banned by the FDA, since we now know that they carry significant risk of serious tissue damage.
Collagen is the protein that makes tendons, liga- ments, and other connective tissues strong. In 1981, the Food and Drug Administration (FDA) approved the use of cattle collagen by injection for cosmetic purposes, to minimize wrinkles and scars. Indeed, collagen injected below the skin will flatten out deep facial wrinkles and make them less prominent, and many people have had this seemingly simple cosmetic surgery.
There are, however, drawbacks. Injected colla- gen lasts only a few months; the injections must be repeated several times a year, and they are expen- sive. Some people have allergic reactions to the cat- tle collagen, which is perceived by the immune
and not flexible. In the shafts of long bones such as the femur, the osteocytes, matrix, and blood vessels are in very precise arrangements called haversian sys- temsor osteons(see Fig. 4–5). Bone has a good blood supply, which enables it to serve as a storage site for calcium and to repair itself relatively rapidly after a simple fracture. Some bones, such as the sternum (breastbone) and pelvic bone, contain red bone mar- row, the primary hemopoietic tissue that produces blood cells.
Other functions of bone tissue are related to the strength of bone matrix. The skeleton supports the body, and some bones protect internal organs from mechanical injury. A more complete discussion of bone is found in Chapter 6.
CARTILAGE
The protein–carbohydrate matrix of cartilage does not contain calcium salts, and also differs from that of bone in that it contains more water, which makes it resilient. It is firm, yet smooth and flexible. Cartilage is found on the joint surfaces of bones, where its smooth surface helps prevent friction. The tip of the nose and external ear are supported by flexible carti- lage. The wall of the trachea, the airway to the lungs, contains firm rings of cartilage to maintain an open air passageway. Discs of cartilage are found between the vertebrae of the spine. Here the cartilage is a firm cushion; it absorbs shock and permits movement.
Within the cartilage matrix are the chondrocytes, or cartilage cells (see Fig. 4–5). There are no capillar- ies within the cartilage matrix, so these cells are nour- ished by diffusion through the matrix, a slow process.
This becomes clinically important when cartilage is damaged, for repair will take place very slowly or not at all. Athletes sometimes damage cartilage within the knee joint. Such damaged cartilage is usually surgically removed in order to preserve as much joint mobility as possible.