The Structural System
Lesson 14, Page 2 of 6

Filtering the blood is not a function of the structural system.



bones There are 206 bones in the human skeleton. The different types of bone include short bones, long bones, irregular bones and flat bones. The general structure of nearly all bones consists of a dense, hard tissue surrounding spongy tissue that contains many blood vessels and nerves. Long bones, such as the femur of the thigh and the humerus of the arm, contain yellow marrow in the long spaces and red marrow in the ends near the joints. The red marrow is also found in the short bones, the vertebrae (backbones), and in the skull, sternum and ribs. The red marrow is where blood cells are formed.

When people think of bones, they usually think of the dried specimens seen in school libraries and museums. Living bone, however, is quite different, being much more pliable and quite vascular. Living bone contains many blood vessels and nerves and is constantly changing or remodeling. Osteoblasts, bone cells that lay down new bone, are continuously working in balance with osteoclasts, bone cells that break down bone. This constantly changes the shape and density of our bones to meet the changing demands of life. That is why astronauts who spend many hours in zero gravity must regularly perform exercises to keep their bones strong. Dense bones are not needed in zero gravity, so when astronauts don't perform weight-bearing exercises, their osteoclasts become more active than their osteoblasts and their bones become weaker.

Bones are made up of minerals, mostly calcium, on a protein (collagen) matrix. Calcium is the primary mineral that keeps our bones strong. Ninety-nine percent of the calcium in the body is stored in the bones and teeth. The remaining 1% is in the blood and soft tissues. This 1% is extremely important however. Without it our muscles (including the heart) wouldn't contract properly and our nerves (including the brain) wouldn't function. This calcium is also necessary for the blood to clot. This 1% in the blood must be maintained at all costs, and the body will do anything necessary to keep it, including pulling the calcium from the bones when necessary. This is what happens when we don't get enough calcium in our diet.

True or False: One of the functions of the bones is to store calcium, releasing it into the bloodstream when necessary, to keep enough calcium in the blood for vital bodily functions such as muscle contraction and nerve conduction.


(Select the best answer and click on the "Continue" button.)