1019
Muscles can only contract and relax; to achieve organized movement, they must pull against rigid structures—
learning outcomes
You should be able to:
Describe features of hydrostatic skeletons, exoskeletons, and endoskeletons.
Explain the functional relationships among osteoblasts, osteoclasts, and osteocytes.
Explain the structural features of a lever of an endoskeletal system that give the lever its functional characteristics.
Why is molting a universal characteristic of arthropods but not of vertebrates?
Arthropods have external skeletons and therefore cannot grow without shedding (molting) their exoskeletons.
What changes in the activities of bone cells might explain the loss of bone mass in astronauts who spend long periods at the International Space Station, and what mechanism in the bone might be responsible?
Since astronauts are losing bone mass, it is reasonable to hypothesize that the activity of their osteoclasts has increased, or the activity of their osteoblasts has decreased. It is possible that these changes are being induced by altered activity of their osteocytes in response to lack of weight-
What is the significance of the fact that the ratio of the force arm to the load arm is much greater in the jaw joint than in the elbow joint?
If the joint has a large force arm relative to the load arm, it can generate great pressures. If the force arm is short relative to the load arm, the end of the load arm can move over a large distance very quickly but cannot exert much pressure. Thus a larger force arm:load arm ratio for the jaw joint enables the jaws to apply great pressure over a small distance, whereas the smaller force arm:load arm ratio for the elbow enables the lower arm to move quickly over a larger distance but does not enable it to apply great pressure.