 This video will cover part one of an introduction to the skeletal system. We will cover the following objectives, list and describe the functions of the skeletal system, describe how bones are classified, and provide examples for each class. One of the functions of the skeletal system is support. The bones of the skeleton provide a framework for soft tissues of the body to help hold our body up against the force of gravity. Movement is another major function of the skeletal system. The joints, which are known as articulations, can enable movement between the bones and skeletal muscles that attach to bones through tendons, contract to pull on bones, providing the force to enable motion. Protection is another major function of the skeletal system. We see here the example of the cranium protecting the brain. Another example would be the ribs and sternum that protect the lungs and heart, or the vertebral bones that protect the spinal cord. Mineral storage is another major function of the skeletal system. Calcium is stored in our bones and can be mobilized from the bone in order to maintain blood calcium concentration. The breakdown of bone is regulated by the endocrine system. The parathyroid gland detects a decrease in blood calcium concentration below the homeostatic set point of 10 milligrams per deciliter. If calcium concentration falls below the set point, the parathyroid gland will produce the hormone parathyroid hormone PTH, and parathyroid hormone will stimulate osteoclasts, which are cells in the bone that break down the matrix of bone. They break down the hydroxyapatite and release calcium from the bone into the blood. Another function of parathyroid hormone is regulating the activity of the kidneys, where parathyroid hormone stimulates calcium reabsorption in the kidneys, that is, bringing the calcium back into the blood so that it's not lost in the urine. Parathyroid hormone also stimulates the activation of vitamin D in the kidneys. Vitamin D that is first synthesized in the skin becomes activated by multiple steps of hydroxylation that occur in the liver and in the kidney. The final step of activation occurring in the kidney is stimulated by parathyroid hormone, and then vitamin D has a function of stimulating calcium absorption in the small intestine. All of these functions work together to increase blood calcium concentration. However, if calcium concentration of the blood is too high, if it is above the homeostatic set point, then the thyroid gland will respond by producing calcitonin. Calcitonin is a hormone that will inhibit the activity of the osteoclast, but stimulate the activity of cells that store calcium in the bone, the osteoblast cells, and this will also have an effect in the kidney. Calcitonin will decrease the reabsorption of calcium by the kidneys, which means more calcium will be lost in the urine, and this will work to lower blood calcium concentration back to the homeostatic set point. Blood cell production is another major function of the skeletal system. Blood cell production is also known as hemopoiesis or hematopoiesis. This occurs in red bone marrow where there are stem cells called hematopoietic stem cells that can divide and differentiate into all of the different types of blood cells, the erythrocytes, leukocytes, and thrombocytes. Red bone marrow is found deep within our bones inside medullary cavities deep inside of long bones, and there's another type of bone marrow called yellow bone marrow that is mostly adipose tissue that replaces red bone marrow as we grow older. However, in an adult there is still some red bone marrow found in the proximal end of long bones like the femur and the humerus. So here we can see a photo of the head of the femur, a cross-section through the head of the femur showing the red bone marrow and an inner core of yellow bone marrow that's within the proximal end of the femur, the head of the femur, where the joint with the hip is formed. Other places that red bone marrow occur in adult are within the ribs and sternum, in the crest of the ilium, the crest of the ilium is the superior part where if you put your hands on your hips they're resting on the crest of the ilium, and then the bodies of the vertebrae, the bodies of the vertebrae are the part of the vertebral bones that's most anterior. The skeleton consists of 206 bones in total and we categorize those bones as either axial or appendicular. Here in the illustration the lighter gray color is used for the axial skeleton which consists of 80 bones found along the central axis along the midline of the body. This includes the skull made of 29 bones, the vertebral column which consists of 26 bones, and the thoracic cage which consists of 24 ribs and one sternum. And then the bluish green color in the illustration is used to show the appendicular skeleton which is located in the limbs and also the girdles which attach the limbs to the axial skeleton. For example the pectoral girdle attaches the upper limb and the pelvic girdle attaches the lower limb. There are 126 bones in the appendicular skeleton. Bones are also classified by shape. We have long bones, flat bones, irregular bones, short bones, and sesamoid bones. All of the long bones are found in the appendicular skeleton and have the characteristic shape of a narrow long central shaft called a diaphysis and two expanded ends called the epiphysis. In the upper extremity the humerus is the long bone found in the brachial region, the radius is a long bone found in the lateral anti-brachial region, the forearm, and the ulna is a long bone found in the medial anti-brachial region. The metacarpals are found in the palmar region and the phalanges are found in the digits. Notice that the metacarpals and phalanges are are long bones because they have the shape of a long bone with a long central diaphysis and expanded epiphysis even though they're relatively small bones they still have the shape of a long bone. In contrast short bones are box shaped and the carpals and tarsals found in the carpal and tarsal regions are examples of short bones that have a boxy shape. Flat bones have a broad surface. For example the flat bone shown in the illustration here is the sternum. Other examples of flat bones include the scapula, clavicle, frontal bone, parietal bone, occipital bone, the frontal parietal and occipital bones are all bones of the cranium, the ribs are flat, the nasal bones in the face, lacrimal bones which are found in the medial portion of the orbit, the vomer which is found in the inferior of the nasal region forming a septum, the inferior part of the nasal septum that is a separation between the left and the right portions of the nasal cavity and the coxal bone, the hip bone, the coxal bones there's a left and a right are also flat bones. A sesamoid bone is a small flat bone that is embedded within a tendon. The most familiar example of a sesamoid bone is the patella or kneecap. The flat bone on the anterior of the knee is embedded within the patella or tendon. Irregular bones have a complex shape and so they don't fit into the category of long short or flat then we consider them to have an irregular shape. The vertebrae are examples of regular bones shown here in the illustration. The temporal bone of the cranium is also irregular and within the temporal bone are the incis, malius, and stapes. Three very small bones, the smallest bone in the body is the stapes and these three small bones also known as the auditory ossicles are found inside of the temporal bone. The sphenoid bone is found in the floor of the cranium and is another irregular bone and just anterior to the sphenoid bone in the cranium is the ethmoid bone forming the roof of nasal cavity another irregular shaped bone then in the face the zygomatic bone found in the in the bucle region a the cheek bone in the superior bucle region is the zygomatic bone the maxillae which are the upper jaw bones and the mandible the lower jaw bone the palatine bones which are found in the roof of the mouth the the posterior portion of the roof of the mouth and the inferior nasal conchi which are found in the lateral inferior regions of the nasal cavity and also the highway bone found in the cervical region the superior cervical region just inferior to the mandible is another irregular bone the highway bone is a unusual bone in that it's located in the inferior region of the skull but does not phonate form any bony attachments it's only held in place by ligaments and muscles