Bone And Muscle As Levers

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Compact bone

Medullary cavity


(c) A bony callus replaces fibrocartilage. (d) Osteoclasts remove excess bony tissue, restoring new bone structure much like the original.

Figure 7B

Major steps in repair of a fracture.

Forearm movement

Forearm movement

Elbow Structure Movement And Function

Triceps brachii- J (b) contracting muscle

Figure 7.14

(a) When the forearm bends at the elbow or (b) when the forearm straightens at the elbow, the bones and muscles function as a lever.

Triceps brachii- J (b) contracting muscle

Figure 7.14

(a) When the forearm bends at the elbow or (b) when the forearm straightens at the elbow, the bones and muscles function as a lever.

the resistance and the force, making the sequence of components resistance-pivot-force. Other examples of first-class levers are seesaws and hemostats (devices used to clamp blood vessels).

The parts of a second-class lever are in the sequence pivot-resistance-force, as in a wheelbarrow. The parts of a third-class lever are in the sequence resistance-forcepivot. Eyebrow tweezers or forceps used to grasp an object illustrate this type of lever.

The actions of bending and straightening the upper limb at the elbow illustrate bones and muscles functioning as levers (fig. 7.14). When the upper limb bends, the forearm bones represent the rigid bar; the elbow joint is the pivot; the hand is moved against the resistance provided by its weight; and the force is supplied by muscles on the anterior side of the arm. One of these muscles, the biceps brachii, is attached by a tendon to a projection (radial tuberosity) on the radius bone in the forearm, a short distance below the elbow. Since the parts of this lever are arranged in the sequence resistance-force-pivot, it is a third-class lever.

When the upper limb straightens at the elbow, the forearm bones again serve as the rigid bar, the hand as the resistance, and the elbow joint as the pivot. However, this time the triceps brachii, a muscle located on the posterior side of the arm, supplies the force. A tendon of this muscle is attached to a projection (olecranon process) of the ulna bone at the point of the elbow. Since the parts of the lever are arranged resistance-pivot-force, it is a first-class lever.

A second-class lever (pivot-resistance-force) is also demonstrated in the human body. The pivot is the tem-poromandibular joint, and the resistance is supplied by muscles attaching to a projection (coronoid process) and body of the mandible that resist or oppose opening the mouth. The muscles attached to the chin area of the mandible provide the force that opens the mouth.

Levers provide a range of movements. Levers that move limbs, for example, are arranged in ways that produce rapid motions, whereas others, such as those that move the head, help maintain posture with minimal effort.

Blood Cell Formation

The process of blood cell formation, called hemopoiesis (he"mo-poi-e'sis), or hematopoiesis, begins in the yolk sac, which lies outside the embryo (see chapter 23, p. 953). Later in development, blood cells are manufactured in the liver and spleen, and still later they form in bone marrow.

Marrow is a soft, netlike mass of connective tissue within the medullary cavities of long bones, in the irregular spaces of spongy bone, and in the larger osteonic canals of compact bone tissue. There are two kinds of marrow—red marrow and yellow marrow. Red marrow functions in the formation of red blood cells (erythro-cytes), white blood cells (leukocytes), and blood platelets. It is red because of the red, oxygen-carrying pigment hemoglobin contained within the red blood cells.

Red marrow occupies the cavities of most bones in an infant. With increasing age, however, yellow marrow replaces much of it. Yellow marrow stores fat and is inactive in blood cell production.

In an adult, red marrow is primarily found in the spongy bone of the skull, ribs, sternum, clavicles, vertebrae, and pelvis. If the blood cell supply is deficient, some yellow marrow may change back into red marrow and produce blood cells. Chapter 14 (p. 552) discusses blood cell formation.

Inorganic Salt Storage

Recall that the intercellular matrix of bone tissue contains collagen and inorganic mineral salts. The salts account for about 70% of the matrix by weight and are mostly tiny crystals of a type of calcium phosphate called hydroxyapatite. Clinical Application 7.2 discusses osteoporosis, a condition that results from loss of bone mineral.

The human body requires calcium for a number of vital metabolic processes, including blood clot formation, nerve impulse conduction, and muscle cell contraction. When the blood is low in calcium, parathyroid hormone stimulates osteoclasts to break down bone tissue, releasing calcium salts from the intercellular matrix into the blood. On the other hand, very high blood calcium inhibits osteoclast activity, and calcitonin from the thyroid gland stimulates osteoblasts to form bone tissue, storing excess calcium in the matrix (fig. 7.15). This mechanism is particularly important in developing bone matrix in children. The details of this homeostatic mechanism are presented in chapter 13, p. 524.

In addition to storing calcium and phosphorus (as calcium phosphate), bone tissue contains lesser amounts of magnesium, sodium, potassium, and carbonate ions. Bones also accumulate certain harmful metallic elements such as lead, radium, and strontium, which are not normally present in the body but are sometimes accidentally ingested.

Biomineralization — the combining of minerals with organic molecules, as occurs in bones — is seen in many animal species. Ancient Mayan human skulls have teeth composed of nacre, also known as "mother-of-pearl" and found on mollusk shells, but tooth roots of human bone. The Mayan dentists knew that somehow the human body recognizes a biomineral used in another species. Today, nacre is used to fill in bone lost in the upper jaw. The nacre not only does not evoke rejection by the immune system, but it also stimulates the person's osteoblasts to produce new bone tissue.

Bone Blood Vessels

Name three major functions of bones.

Explain how parts of the upper limb form a first-class lever and a third-class lever.

Figure 7.15

Hormonal regulation of bone calcium resorption and deposition.

Distinguish between the functions of red marrow and yellow marrow.

Explain regulation of the concentration of blood calcium.

List the substances normally stored in bone tissue.

Skeletal Organization

Number of Bones

The number of bones in a human skeleton is often reported to be 206, but the actual number varies from person to person. People may lack certain bones or have extra ones. For example, the flat bones of the skull usually grow together and tightly join along irregular lines called sutures. Occasionally, extra bones called sutural bones (wormian bones) develop in these sutures (fig. 7.16). Extra small, round sesamoid bones may develop in tendons, where they reduce friction in places where tendons pass over bony prominences (table 7.3).


It is an all-too-familiar scenario. The elderly woman pulls herself out of bed, reaches for the night table for support, and misses. She falls, landing on her hip. A younger woman would pull herself up and maybe ache for a few minutes and develop a black-and-blue mark by the next day. But the eighty-year-old, with weakened, brittle bones, suffers a broken hip. Each year in the United States, 200,000 senior citizens break their hips, more than 90% of the time as the result of an accident.

In osteoporosis, the skeletal system loses bone volume and mineral content. This disorder is associated with aging. Within affected bones, trabeculae are lost, and the bones develop spaces and canals, which enlarge and fill with fibrous and fatty tissues. Such bones easily fracture and may spontaneously break because they are no longer able to support body weight. For example, a person with osteoporosis may suffer a spontaneous fracture of the thigh bone (femur) at the hip or the collapse of sections of the backbone (vertebrae). Similarly, the distal portion of a forearm bone (radius) near the wrist may be fractured as a result of a minor stress.

Osteoporosis causes many fractures in persons over forty-five years of age. Although it may affect either gender, it is most common in thin, light-complexioned females after menopause (see chapter 22, p. 910).

Factors that increase the risk of osteoporosis include low intake of dietary calcium and lack of physical exercise (particularly during the early growing years). However, excessively strenuous exercise in adolescence can delay puberty, which raises the risk of developing osteoporosis later in life for both sexes.

In females, declining levels of the hormone estrogen contribute to development of osteoporosis. The ovaries produce estrogen until menopause. Evidence of the estrogen-osteoporosis link comes from studies on women who have declining estrogen levels and increased risk of osteoporosis. These include young women who have had their ovaries removed, women who have anorexia nervosa (self-starvation) that stopped their menstrual cycles, and women past menopause. Drinking alcohol, smoking cigarettes, and inheriting certain genes may also increase a person's risk of developing osteoporosis.

Osteoporosis may be prevented if steps are taken early enough. Bone mass usually peaks at about age thirty-five. Thereafter, bone loss may exceed bone formation in both males and females. To reduce such loss, doctors suggest that people in their mid-twenties and older should take in 1,000-1,500 milligrams of calcium daily. An 8-ounce glass of nonfat milk, for example, contains about 275 milligrams of calcium. It is also recommended that people engage in regular exercise, especially walking or jogging, in which the bones support body weight. Additionally, postmenopausal women may require estrogen replacement therapy. As a rule, women have about 30% less bone mass than men; after menopause, women typically lose bone mass twice as fast as men do. Also, people with osteoporosis can slow progress of the disease by taking a drug that is a form of the hormone calcitonin, if they can tolerate the side effect of throat irritation.

Confirming osteoporosis is sometimes difficult. A radiograph may not reveal a decrease in bone density until 20% to 30% of the bone tissue is lost. Noninvasive diagnostic techniques, however, can detect rapid changes in bone mass. These include a densitometer scanner, which measures the density of wrist bones, and quantitative computed tomography, which can visualize the density of other bones.

Alternatively, a physician may take a bone sample, usually from a hipbone, to directly assess the condition of the tissue. Such a biopsy may also be used to judge the effectiveness of treatment for bone disease. ■

Divisions of the Skeleton

For purposes of study, it is convenient to divide the skeleton into two major portions—an axial skeleton and an appendicular skeleton (fig. 7.17). The axial skeleton consists of the bony and cartilaginous parts that support and protect the organs of the head, neck, and trunk. These parts include the following:

1. Skull. The skull is composed of the cranium (brain case) and the facial bones.

2. Hyoid bone. The hyoid (hi'oid) bone is located in the neck between the lower jaw and the larynx (fig. 7.18). It does not articulate with any other bones but is fixed in position by muscles and ligaments. The hyoid bone supports the tongue and

The End Muscles That Does Not Move

Figure 7.16

Sutural bones are extra bones that sometimes develop in sutures between the flat bones of the skull.

is an attachment for certain muscles that help move the tongue during swallowing. It can be felt approximately a finger's width above the anterior prominence of the larynx.

3. Vertebral column. The vertebral column, or spinal column, consists of many vertebrae separated by cartilaginous intervertebral disks. This column forms the central axis of the skeleton. Near its distal end, several vertebrae fuse to form the sacrum (sa'krum), which is part of the pelvis. A small, rudimentary tailbone called the coccyx (kok'siks), is attached to the end of the sacrum.

4. Thoracic cage. The thoracic cage protects the organs of the thoracic cavity and the upper abdominal cavity. It is composed of twelve pairs of ribs, which articulate posteriorly with thoracic vertebrae. It also includes the sternum (ster'num), or breastbone, to which most of the ribs are attached anteriorly.

The appendicular skeleton consists of the bones of the upper and lower limbs and the bones that anchor the limbs to the axial skeleton. It includes the following:

1. Pectoral girdle. The pectoral girdle is formed by a scapula (scap'u-lah), or shoulder blade, and a clavicle (klav'i-k'l), or collarbone, on both sides of the body. The pectoral girdle connects the bones of the upper limbs to the axial skeleton and aids in upper limb movements.

! 7.

FA Bones of the |l skeleton


1. Axial Skeleton

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  • alice
    What is bone muscle lever system?
    5 years ago

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