Chapter 7

1. Chapter 7 Bone Function

2. Support, Protection, and Movement • Bones give shape to body structures • Ex. Bone of the skull and face • Bones provide support and protection • Ex. Bone of the lower limbs, pelvis, and vertebral column support the weight of the body • Ex. Bones of the rib cage and shoulder girdle protect the heart and lungs • Bones interact with muscles to make the body move

3. Blood Cell Formation • Hematopoiesis • Process of blood cell formation • Begins in the yolk sak (lies outside the embryo) • Later in development, blood cells are manufactured in the liver and spleen • Even later in development, blood cells are formed in the bone marrow

4. Blood Cell Formation • Marrow • Soft, netlike mass of connective tissue • Found in the medullary cavity of long bones, in the irregular spaces of spongy bone, and in the larger central canals of compact bone

5. Blood Cell Formation • Marrow • 2 types: red marrow and yellow marrow • Red marrow • Functions in the production of red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets • Has a red color due to the oxygen-carrying pigment hemoglobin that is found in the red blood cells • Occupies the cavities of most bones in infants but is mostly replaced by yellow marrow as individuals get older • In adults, red marrow is found primarily in the spongy bone of the skull, ribs, sternum, clavicles, vertebrae, and hipbones • Yellow marrow • Store fat • Is inactive in blood cell production • Can change back into red marrow to produce blood cells if the blood cell supply is deficient

6. Inorganic Salt Storage • Inorganic salts account for about 70% (by weight) of the extracellular matrix of bone tissue • Most inorganic salts are small crystals of a type of calcium phosphate called hydroxyapatite • Calcium is required for metabolic processes such as blood clot formation, nerve impulse conduction, and muscle cell contraction

7. Inorganic Salt Storage • Maintenance of calcium level is controlled through a negative feedback system: • When blood is low in calcium, parathryoid hormone stimulates osteoclasts to break down bone tissue, releasing calcium from the extracellular matrix into the blood • When blood is high in calcium, calcitonin (another hormone produced by the thyroid) stimulates osteoblasts to form bone tissue, storing excess calcium in the extracellular matrix

8. Inorganic Salt Storage

9. Inorganic Salt Storage • Bone tissue also stores smaller amounts of magnesium, sodium, potassium, and carbonate ions as well as accumulates harmful metallic elements such as lead, radium, and strontium

10. Chapter 7 Life-Span Changes

11. Life-Span Changes • Starting at about age 30, humans shrink about 1/16th of an inch each year • In later years, compression fractures in the vertebrae may contribute significantly to loss of height • Calcium levels fall, ration of mineral to protein increases, and bone material gradually vanishes leading to bones becoming brittle (and prone to fracture) • Bones thin leading to the skeleton losing strength • Osteoclasts begin to outnumber osteoblasts leading to bone breakdown occurring at a quicker rate then bone formation (which results in spaces in the bone) (By age 35, all humans loose bone mass)

12. Life-Span Changes • Trabecular (spongy) bone shows changes first as it thins, increases in porosity, and weakens structurally • The vertebrae and the upper part of the femur consist of mainly trabecular bone • Compact bone loss begins around the age 40 and continues at about half the rate of loss of trabecular bone

13. Life-Span Changes • In women, bone loss is related to hormonal changes • In the first decade following menopause, 15% to 20% of trabecular bone and 10% to 15% of compact bone is lost • This is 2 to 3 times and 3 to 4 times, respectively, the rate of bone loss in men • By about age 70, men and women are losing bone at about the same rate • Very old women may have ½ the bone mass they did at a young age whereas very old men may have 2/3’s of their bone mass