Skeletal System

A human skeleton: Image as overview of the human skeletal system.

The skeletal portion of the system serves as the main storage system for calcium and phosphorus. The importance of this storage is to help regulate mineral balance in the bloodstream. When the fluctuation of minerals is high, these minerals are stored in bone; when it is low, minerals are withdrawn from the bone.

The skeleton also contains critical components of the hematopoietic (blood production) system. Located in long bones are two distinctions of bone marrow: yellow and red. The yellow marrow has fatty connective tissue and is found in the marrow cavity. In times of starvation, the body uses the fat in yellow marrow for energy.

The red marrow of some bones is an important site for hematopoeisis or blood cell production that replaces cells that have been destroyed by the liver. Here, all erythrocytes, platelets, and most leukocytes form in bone marrow from where they migrate to the circulation.

Muscular System

Muscles contract (shorten) to move the bone attached at the joint. Skeletal muscles are attached to bones and arranged in opposing groups around joints. Muscles are innervated—the nerves conduct electrical currents from the central nervous system that cause the muscles to contract.

Three types of muscle tissue exist in the body. These are skeletal, smooth, and cardiac muscle.

• Only skeletal and smooth muscles are considered part of the musculoskeletal system.

• Skeletal muscle is involved in body locomotion.

• Examples of smooth muscles include those found in intestinal and vessel walls.

• Cardiac and smooth muscle are characterized by involuntary movement (not under conscious control).

• Cardiac muscles are found in the heart.

Tendons, Joints, Ligaments, and Bursae

A tendon is a tough, flexible band made of fibrous connective tissue, and functions to connect muscle to bone. Joints are the bone articulations allowing movement. A ligament is a dense, white band of fibrous elastic tissue.

Ligaments connect the ends of bones together in order to form a joint. These help to limit joint dislocation and restrict improper hyperextension and hyperflexion. Also made of fibrous tissue are bursae. These provide cushions between bones and tendons and/or muscles around a joint.

Musculoskeletal system: Image depicting the human muscular system (skeletal muscle)

The Axial Skeleton

The axial skeleton functions to support and protect the organs of the dorsal and ventral cavities and serves as a surface for the attachment of muscles and parts of the appendicular skeleton.

LEARNING OBJECTIVES

List the components of the axial skeleton

KEY TAKEAWAYS

Key Points

• The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate animal, including humans.

• The primary divisions of the skeleton system are the head, thorax, and vertebral column.

• The human cranium supports the structures of the face and forms the brain cavity.

• The rib cage functions as protection for the vital organs of the chest such as the heart and lungs.

• The cervical vertebrae make up the junction between the vertebral column and the cranium, and the bone makes up the junction between the vertebral column and the pelvic bones.

Key Terms

• flat bones: Thin bones (although often curved) that serve as points of attachment for muscles and protect internal organs (examples, cranium, sternum).

• cranial vault: The space in the skull occupied by the brain.

• sutures: Fibrous joints that are only found in the cranium.

The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate animal, including humans.

A diagram of the apendicular skeleton: Image depicting the human skeleton with the appendicular skeleton colored red.

The appendicular skeleton is divided into six major regions:

1. The pectoral girdles consist of 4 bones: The left and right clavicle (2) and the scapula (2).

2. The upper arms and forearms are made up of 6 bones: The left and right humerus (upper arm, 2), the ulna (2), and the radius (forearm, 2).

3. The hands have 54 bones: The left and right carpals (wrist, 16), metacarpals (10), proximal phalanges (10), intermediate phalanges (8), and the distal phalanges (10).

4. The pelvis has 2 bones: The left and right hip bone (2).

5. The thighs and legs have 8 bones: The left and right femur (thigh, 2), patella (knee, 2), tibia (2) and fibula (leg, 2).

6. The feet and ankles have 52 bones: The left and right tarsals (ankle, 14), metatarsals (10), proximal phalanges (10), intermediate phalanges (8), and distal phalanges (10).

Pectoral Girdle

The bones of the pectoral girdle consist of two bones (scapula and clavicle) and anchor the upper limb to the thoracic cage of the axial skeleton.

The three regions of the upper limb are: arm (humerus), forearm (ulna medially and radius laterally), and the hand.

The base of the hand contains eight bones (carpal bones), and the palm is formed by five bones (metacarpal bones). The fingers and thumb contain a total of 14 bones, called phalanges.

Pelvic Girdle

The pelvic girdle is formed by a single bone, the hip or coxal bone, and serves as the attachment point for each lower limb. Each hip bone is joined to the axial skeleton by its attachment to the sacrum of the vertebral column. The right and left hip bones attach to each other anteriorly.

The lower limb contains 30 bones and is divided into three regions, the thigh, leg, and foot. These consist of the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and phalanges.

• The femur is the single bone of the thigh.

• The patella (kneecap) articulates with the distal femur.

• The tibia is located on the medial side of the leg,

• The fibula is the thin bone of the lateral leg.

The bones of the foot are divided into three groups, the tarsal bones, metatarsal bones, and phalanges of the foot.

Gross Anatomy

All the bones in the body can be described as long bones or flat bones.

LEARNING OBJECTIVES

Differentiate long bones from flat bones

KEY TAKEAWAYS

Key Points

• Long bones are those that are longer than they are wide.

• The end of the long bone is the epiphysis and the shaft is the diaphysis. When a human finishes growing these parts fuse together.

• The outside of the flat bone consists of a layer of connective tissue called the periosteum.

• The interior part of the long bone is the medullary cavity with the inner core of the bone cavity being composed of marrow.

• Flat bones have broad surfaces for protection or muscular attachment.

• Flat bones are composed of two thin layers of compact bone that surround a layer of cancellous (spongy) bone. In an adult, most red blood cells are formed in the marrow in flat bones.

Key Terms

• endosteum: A thin vascular membrane of connective tissue that lines the surface of the bone tissue that forms the medullary cavity of long bones.

• medullary cavity: The medullary cavity, also known as the marrow cavity, is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored.

• diaphysis: The central shaft of any long bone.

• epiphyseal plate: A hyaline cartilage plate in the metaphysis, located at each end of a long bone where growth occurs in children and adolescents.

Bone Tissue

Bones support and protect the body and its organs. They also produce various blood cells, store minerals, and provide support for mobility in conjunction with muscle. Bone is made of bone tissue, a type of dense connective tissue.

Bone (osseous) tissue is the structural and supportive connective tissue of the body that forms the rigid part of the bones that make up the skeleton. Overall, the bones of the body are an organ made up of bone tissue, bone marrow, blood vessels, epithelium, and nerves.

There are two types of bone tissue: cortical and cancellous bone. Cortical bone is compact bone, while cancellous bone is trabecular and spongy bone.

Cortical bone forms the extremely hard exterior while cancellous bone fills the interior. The tissues are biologically identical but differ in the arrangement of their microstructure.

Bone Cells

The following are the different types of bone cells:

• Osteoblasts-involved in the creation and mineralisation of bone

• Osteocytes and osteoclasts: These are involved in the reabsorption of bone tissue. The mineralized matrix of bone tissue has an organic component—mainly made of collagen—and an inorganic component of bone mineral made up of various salts.

Bone Types

There are different types of bone. These are:

• Long bones

• Short bones

• Flat bones

• Sesamoid bones

• Irregular bones

Bone types: This image show the different bone classifications, based on shape, that are found in a human skeleton. These are flat bone, sutural bone, short bone, irregular, sesamoid bone, and long bone.

Long Bones

Long bone: A long bone is longer than it is wide. Growth occurs by a lengthening of the diaphysis. located in the center of the long bone.

Long bones grow primarily by elongation of the diaphysis (the central shaft), with an epiphysis at each end of the growing bone. The ends of epiphyses are covered with hyaline cartilage (articular cartilage). At the cessation of growth, the epiphyses fuse to the diaphysis, thus obliterating the intermediate area known as the epiphyseal plate or growth plate. The long bones in the body are as follows:

• Legs: The femur, tibia, and fibula.

• Arms: The humerus, radius, and ulna.

• The clavicles or collar bones.

• Metacarpals, metarsals, phalanges.

The outside of the bone consists of a layer of connective tissue called the periosteum. The outer shell of the long bone is compact bone, below which lies a deeper layer of cancellous bone (spongy bone), as shown in the following figure. The interior part of the long bone is called the medullary cavity; the inner core of the bone cavity is composed of marrow.

Short Bones

Short bones are about as wide as they are long. These provide support with less movement. Examples of short bones include the carpal and tarsal bones of the wrist and feet. They consist of a thin layer of cortical bone with cancellous interiorly.

Compact bone and spongy bone: The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. Its porosity is 5–30%. Inside the interior of the bone is the trabecular bone tissue, an open cell, porous network that is also called cancellous or spongy bone.

Flat Bones

Flat bones are broad bones that provide protection or muscle attachment. They are composed of two thin layers of compact bone surrounding a layer of cancellous (spongy) bone.

These bones are expanded into broad, flat plates, as in the cranium (skull), ilium (pelvis), sternum, rib cage, sacrum, and scapula. The flat bones are named:

• Occipital

• Parietal

• Frontal

• Nasal

• Lacrimal

• Vomer

• Scapula

• Os coxæ (hip bone)

• Sternum

• Ribs

Sesamoid Bone

Sesamoid bones are smaller bones that are fixed in tendons to protect them. An example is the patella (knee cap) located in the patellar tendon. Other examples include the small bones of the metatarsals and the pisiform bones of the carpus.

Irregular Bone

The irregular bones are named for their nonuniform shape. Examples include the bones of the vertebrae. These typically have a thin cortical layer with more cancellous bone in their tissue.

Supply of Blood and Nerves to Bone

The blood and nerve supply to bones are carried in Haversian canals that run along the long axis of bones.

LEARNING OBJECTIVES

Describe the blood and nerve supply of bones

KEY TAKEAWAYS

Key Points

• Haversian canals typically run parallel to the surface and along the long axis of the bone and generally contain one or two capillaries and nerve fibers.

• Volkmann’s canals are channels that assist with blood and nerve supply from the periosteum to the Haversian canal.

• The vascular supply of long bones depends on several points of inflow.

• Except for a few with double or no foramina (places in bone where capillaries enervate), 90% of long bones have a single nutrient foramen in the middle third of the shaft.

• Young periosteum is more vascular and its vessels communicate more freely with those of the shaft compared to adult periosteum.

Key Terms

• perichondrium: A layer of dense irregular connective tissue that surrounds the cartilage of developing bone.

• Volkmann’s canal: Also known as perforating holes, these are microscopic structures found in the compact bone that carry small arteries throughout the bone.

• anastomose: Joined or run together.

• Haversian canal: A hollow channel in the center of an osteon, running parallel to the length of a bone.

Blood is supplied to mature compact bone through the Haversian canal. Haversian canals are formed when individual lamellae form concentric rings around larger longitudinal canals (approx. 50 µm in diameter) within the bone tissue.

Haversian canals typically run parallel to the surface and along the long axis of the bone. The canals and the surrounding lamellae (8–15) are called a Haversian system or an osteon. A Haversian canal generally contains one or two capillaries and nerve fibers.

The Haversian canals also surround nerve cells throughout the bone and communicate with osteocytes in lacunae (spaces within the dense bone matrix that contain the living bone cells) through canaliculi. This unique arrangement is conducive to the storage of mineral salt deposits that give bone tissue its strength.

Haversian canal: The Haversian canals surround blood vessels and nerve cells throughout the bone.

The vascular supply of long bones depends on several points of inflow, which feed complex sinusoidal networks within the bone. These in turn drain to various channels through all surfaces of the bone except that covered by articular cartilage.

Epiphyseal plate: Image shows the location of the epiphyseal plates (or lines) and the articular surfaces of long bones.

Volkmann’s canals are channels that assist with blood and nerve supply from the periosteum to the Haversian canal. One or two main diaphyseal nutrient arteries enter the shaft obliquely through one or two nutrient foramina leading to nutrient canals. Their sites of entry and angulation are almost constant and characteristically directed away from the growing epiphysis.

Except for a few with double or no foramina, 90% of long bones have a single nutrient foramen in the middle third of the shaft. The nutrient arteries divide into ascending and descending branches in the medullary cavity. These approach the epiphysis dividing into smaller rami. Near the epiphysis, they anastomose with the metaphyseal and epiphyseal arteries.

The blood supply of the immature bones is similar, but the epiphysis is a discrete vascular zone separated from the metaphysis by the growth plate. Epiphyseal and metaphyseal arteries enter on both sides of the growth cartilage, with anastamoses between them being few or absent.

Growth cartilage receives its blood supply from both sources and also from an anastamotic collar in the adjoining perichondrium. Young periosteum is more vascular, has more metaphyseal branches, and its vessels communicate more freely with those of the shaft than adult periosteum.

Microscopic Anatomy of Bone

The basic microscopic unit of bone is an osteon, which can be arranged into woven bone or lamellar bone.

LEARNING OBJECTIVES

Classify woven bone and lamellar bone

KEY TAKEAWAYS

Key Points

• Woven bone is found on the growing ends of an immature skeleton or, in adults, at the site of a healing fracture.

• Woven bone is characterized by the irregular organization of collagen fibers and is mechanically weak, but forms quickly.

• Lamellar bone is much stronger than woven bone, and is highly organized in concentric sheets with a much lower proportion of osteocytes to mineralized tissue.

• When the same lamellar bone is loosely arranged, it is referred to as trabecular bone. Trabecular bone gets its name because of the spongy pattern it displays on an x-ray.

• After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as bony substitution.

Key Terms

• osteoblast: A mononucleate cell from which bone develops.

• osteocytes: A star-shaped type of bone cell that is found in the cells of mature bone.

• lamellar bone: A bone with a regular, parallel alignment of collagen into sheets (lamellae) that is mechanically strong.

• woven bone: Characterized by an irregular organization of collagen fibers, this bone is mechanically weak.

Bones are composed of bone matrix, which has both organic and inorganic components. Bone matrix is laid down by osteoblasts as collagen, also known as osteoid. Osteoid is hardened with inorganic salts, such as calcium and phosphate, and by the chemicals released from the osteoblasts through a process known as mineralization.

The basic microscopic unit of bone is an osteon (or Haversian system). Osteons are roughly cylindrical structures that can measure several millimeters long and around 0.2 mm in diameter.

Each osteon consists of a lamellae of compact bone tissue that surround a central canal (Haversian canal). The Haversian canal contains the bone’s blood supplies. The boundary of an osteon is called the cement line. Osteons can be arranged into woven bone or lamellar bone.

Osteon: A photo taken through a microscope that shows the anatomy of compact bone with a detailed view of an osteon.

Woven Bone

Woven bone: Woven bone is characterized by the irregular organization of collagen fibers and is mechanically weak.

Woven bone is found on the growing ends of an immature skeleton or, in adults, at the site of a healing fracture. Woven bone is characterized by the irregular organization of collagen fibers and is mechanically weak, but forms quickly.

The criss-cross appearance of the fibrous matrix is why it is referred to as woven. It has a high proportion of osteocytes to hard inorganic salts that leads to its mechanical weakness.

Woven bone is replaced by lamellar bone during development. In contrast to woven bone, lamellar bone is highly organized in concentric sheets with a much lower proportion of osteocytes to surrounding tissue. The regular parallel alignment of collagen into sheets, or, lamellae, causes lamellar bone to be mechanically strong.

Lamellar Bone

Femur head showing trabecular bone: A cross-section of the head of the femur showing lamellar bone on the borders and trabecular bone in the center.

Lamellar bone makes up the compact or cortical bone in the skeleton, such as the long bones of the legs and arms. In a cross-section, the fibers of lamellar bone can be seen to run in opposite directions in alternating layers, much like in plywood, assisting in the bone’s ability to resist torsion forces.

When the same lamellar bone is loosely arranged, it is referred to as trabecular bone. Trabecular bone gets its name because of the spongy pattern it displays in an x-ray. The spaces within trabecular bone are filled with active bone marrow.

After a fracture, woven bone forms initially, but it is gradually replaced by lamellar bone during a process known as bony substitution.

Chemical Composition of Bone

Acid-base imbalances, including metabolic acidosis and alkalosis, can produce severe, even life-threatening medical conditions.

LEARNING OBJECTIVES

Differentiate among the acid-base disorders

KEY TAKEAWAYS

Key Points

• Metabolic acidosis can produce, among other symptoms, chest pains, altered mental states, nausea, abdominal pain, and muscle weakness.

• Rapid, deep breathing during metabolic acidosis is an attempt to lower carbon dioxide levels and return pH to normal.

• Extreme acidemia can lead to coma, seizures, heart arrhythmias, and low blood pressure.

• Slowed breathing, which results in retaining more CO₂, is the primary method of reducing metabolic alkalosis.

• Chronic respiratory acidosis is a result of COPD, obesity hypoventilation syndrome, ALS, and thoracic deformities.

• Respiratory alkalosis can be caused by excessive mechanical ventilation, psychiatric problems, stroke, drug use, traveling to high altitude regions, lung disease, fever, and pregnancy, among other factors.

Key Terms

• metabolic alkalosis: A metabolic condition in which the pH of tissue is elevated beyond the normal range ( 7.35 to 7.45 ). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate concentration, or a direct result of increased bicarbonate concentration.

• respiratory acidosis: A medical condition in which decreased ventilation (hypoventilation) causes increased blood carbon dioxide concentration and decreased pH (a condition generally called acidosis).

• metabolic acidosis: A condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body.

EXAMPLES

Traveling to a high altitude can cause an acid-base imbalance due to reduced levels of oxygen in the atmosphere, and, therefore, in the blood. To compensate for this, the traveler begins to hyperventilate, trying to expel excess carbon dioxide and bring pH back to normal. However, if the traveler stays at high altitude, it may take several days for their pH to fully return to normal.

Acid-Base Disorders

Acid-base imbalance is an abnormality of the human body’s normal balance of acids and bases that causes the plasma pH to deviate out of normal range (7.35 to 7.45). In the fetus, the normal range differs based on which umbilical vessel is sampled (umbilical vein pH is normally 7.25 to 7.45; umbilical artery pH is normally 7.18 to 7.38). Acid-base imbalances can exist in varying levels of severity, some life-threatening.

An excess of acid is called acidosis and an excess in bases is called alkalosis. The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis).

Mixed disorders may feature an acidosis and alkalosis excess at the same time that partially counteract each other, or there can be two different conditions affecting the pH in the same direction. The phrase mixed acidosis, for example, refers to metabolic acidosis in conjunction with respiratory acidosis.

Metabolic Acidosis

In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, that is, blood pH is less than 7.35 due to increased production of hydrogen by the body, or because of the body’s inability to form bicarbonate (HCO₃-) in the kidneys.

Acidosis refers to a low pH in tissue. Acidemia refers to a low pH in the blood. Symptoms may include chest pain, palpitations, headache, altered mental status such as severe anxiety due to hypoxia, decreased visual acuity, nausea, vomiting, abdominal pain, altered appetite (either loss of or increased) and weight loss (longer term), muscle weakness, and bone pains.

Rapid deep breaths increase the amount of carbon dioxide exhaled, thus lowering the serum carbon dioxide levels, resulting in some degree of compensation. Overcompensation via respiratory alkalosis to form an alkalemia does not occur.

Neurological complications include lethargy, stupor, coma, seizures. Cardiac complications include arrhythmias (ventricular tachycardia) and decreased response to epinephrine; both lead to hypotension (low blood pressure).

Metabolic Alkalosis

Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35 to 7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate concentration, or as a direct result of increased bicarbonate concentrations. Alkalosis refers to a high pH in tissue.

Alkalemia refers to a high pH in the blood. The causes of metabolic alkalosis can be divided into two categories, depending upon urine chloride levels. Chloride-responsive causes result from the loss of hydrogen ions via vomiting or the kidneys. Vomiting results in the loss of hydrochloric acid (hydrogen and chloride ions) with the stomach contents.

The kidneys compensate for these losses by retaining sodium in the collecting ducts at the expense of hydrogen ions (sparing sodium/potassium pumps to prevent further loss of potassium), and leads to metabolic alkalosis. The excess sodium increases extracellular volume and the loss of hydrogen ions creates a metabolic alkalosis.

Later, the kidneys respond through the aldosterone escape to excrete sodium and chloride in urine. Compensation for metabolic alkalosis occurs mainly in the lungs, which retain carbon dioxide (CO₂) through slower breathing, or hypoventilation (respiratory compensation).

CO₂ is then consumed towards the formation of the carbonic acid intermediate, thus decreasing pH. Renal compensation for metabolic alkalosis, less effective than respiratory compensation, consists of increased excretion of HCO₃– (bicarbonate), as the filtered load of HCO₃– exceeds the ability of the renal tubule to reabsorb it.

Respiratory Acidosis

Respiratory acidosis is a medical condition in which decreased ventilation (hypoventilation) causes an increase in blood carbon dioxide concentration and decreased pH (a condition generally called acidosis). Carbon dioxide is produced constantly as the body’s cells respire, and this CO₂ will accumulate rapidly if the lungs do not adequately expel it through alveolar ventilation.

Acute respiratory acidosis occurs when an abrupt failure of ventilation occurs. This failure in ventilation may be caused by depression of the central respiratory center by cerebral disease or drugs, an inability to ventilate adequately due to neuromuscular disease (e.g., myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barré syndrome, muscular dystrophy), or airway obstructions related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.

Respiratory Alkalosis

Respiratory alkalosis is a medical condition in which increased respiration (hyperventilation) elevates the blood pH (a condition generally called alkalosis). There are two types of respiratory alkalosis: chronic and acute.

Acute respiratory alkalosis occurs rapidly. During acute respiratory alkalosis, the person may lose consciousness whereupon the rate of ventilation will resume to normal.

Chronic respiratory alkalosis is a more long-standing condition. Respiratory alkalosis may be produced accidentally (iatrogenically) during excessive mechanical ventilation. Other causes include: psychiatric causes, drug use, fever, and pregnancy.