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Skeletal System 1: the Anatomy and Physiology of Bones

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Skeletal System 1: the Anatomy and Physiology of Bones Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Keywords Skeletal system/Bone physiology/Musculoskeletal health Systems of life This article has been Skeletal system double-blind peer reviewed In this article... l The key functions and structure of bone l Bone formation and growth, and the process of remodelling l Diet and lifestyle factors that can affect bone structure Skeletal system 1: the anatomy and physiology of bones Key points Author Jennie Walker is principal lecturer, Nottingham Trent University. Bones are key to providing the body Abstract The skeletal system is formed of bones and cartilage, which are connected with structural by ligaments to form a framework for the remainder of the body tissues. This article, support and the first in a two-part series on the structure and function of the skeletal system, enabling movement reviews the anatomy and physiology of bone. Understanding the structure and purpose of the bone allows nurses to understand common pathophysiology and Most of the body’s consider the most-appropriate steps to improve musculoskeletal health. minerals are stored in the bones Citation Walker J (2020) Skeletal system 1: the anatomy and physiology of bones. Nursing Times [online]; 116: 2, 38-42. Diet and lifestyle can affect the quality of bone formation he skeletal system is composed of Protection bones and cartilage connected by Bones provide protective boundaries for After bones have ligaments to form a framework for soft organs: the cranium around the brain, formed they Tthe rest of the body tissues. There the vertebral column surrounding the undergo constant are two parts to the skeleton: spinal cord, the ribcage containing the remodelling l Axial skeleton – bones along the axis of heart and lungs, and the pelvis protecting the body, including the skull, vertebral the urogenital organs. Changes in the column and ribcage; remodelling process l Appendicular skeleton – appendages, Mineral homoeostasis can result in such as the upper and lower limbs, As the main reservoirs for minerals in the pathology such as pelvic girdle and shoulder girdle. body, bones contain approximately 99% of Paget’s disease of the body’s calcium, 85% of its phosphate bone or osteoporosis Function and 50% of its magnesium (Bartl and Bartl, As well as contributing to the body’s 2017). They are essential in maintaining overall shape, the skeletal system has sev- homoeostasis of minerals in the blood with eral key functions, including: minerals stored in the bone are released in l Support and movement; response to the body’s demands, with l Protection; levels maintained and regulated by hor- l Mineral homeostasis; mones, such as parathyroid hormone. l Blood-cell formation; l Triglyceride storage. Blood-cell formation (haemopoiesis) Blood cells are formed from haemopoietic Support and movement stem cells present in red bone marrow. Bones are a site of attachment for ligaments Babies are born with only red bone and tendons, providing a skeletal frame- marrow; over time this is replaced by work that can produce movement through yellow marrow due to a decrease in eryth- the coordinated use of levers, muscles, ten- ropoietin, the hormone responsible for dons and ligaments. The bones act as stimulating the production of erythro- levers, while the muscles generate the cytes (red blood cells) in the bone marrow. FRANCESCA CORRA FRANCESCA forces responsible for moving the bones. By adulthood, the amount of red marrow Nursing Times [online] February 2020 / Vol 116 Issue 2 38 www.nursingtimes.net Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Bone structure flexibility to withstand the daily forces exerted on them. This flexibility and ten- sile strength of bone is derived from the collagen fibres. Over-mineralisation of the fibres or impaired collagen production can Hyaline cartilage Epiphysis increase the brittleness of bones – as with the genetic disorder osteogenesis imper- fecta – and increase bone fragility (Ralston Epiphyseal line and McInnes, 2014). Red bone marrow Structure Bone architecture is made up of two types Marrow cavity of bone tissue: l Cortical bone; l Cancellous bone. Yellow bone marrow Cortical bone Also known as compact bone, this dense outer layer provides support and protec- tion for the inner cancellous structure. Periosteum Diaphysis Cortical bone comprises three elements: l Periosteum (Fig 1); l Intracortical area; l Endosteum (Bartl and Bartl, 2017). The periosteum is a tough, fibrous Nutrient outer membrane. It is highly vascular and foramen Compact almost completely covers the bone, except bone for the surfaces that form joints; these are Site of covered by hyaline cartilage. Tendons and endosteum ligaments attach to the outer layer of the periosteum, whereas the inner layer con- tains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) respon- sible for bone remodelling. Spongy The function of the periosteum is to: bone Epiphysis l Protect the bone; l Help with fracture repair; l Nourish bone tissue (Robson and Syndercombe Court, 2018). It also contains Volkmann’s canals, small channels running perpendicular to the diaphysis of the bone (Fig 1); these has halved, and this reduces further to and 10% other proteins, such as glycopro- convey blood vessels, lymph vessels and around 30% in older age (Robson and Syn- tein, osteocalcin, and proteoglycans (Bartl nerves from the periosteal surface through dercombe Court, 2018). and Bartl, 2017). It forms the framework for to the intracortical layer. The periosteum bones, which are hardened through the has numerous sensory fibres, so bone inju- Triglyceride storage deposit of the calcium and other minerals ries (such as fractures or tumours) can be Yellow bone marrow (Fig 1) acts as a poten- around the fibres (Robson and Synder- extremely painful (Drake et al, 2019). tial energy reserve for the body; it consists combe Court, 2018). The intracortical bone is organised into largely of adipose cells, which store triglyc- Mineral salts are first deposited between structural units, referred to as osteons or erides (a type of lipid that occurs naturally in the gaps in the collagen layers with once Haversian systems (Fig 2). These are cylin- the blood) (Tortora and Derrickson, 2009). these spaces are filled, minerals accumulate drical structures, composed of concentric around the collagen fibres, crystallising and layers of bone called lamellae, whose struc- Bone composition causing the tissue to harden; this process is ture contributes to the strength of the cor- Bone matrix has three main components: called ossification (Tortora and Derrickson, tical bone. Osteocytes (mature bone cells) l 25% organic matrix (osteoid); 2009). The hardness of the bone depends on sit in the small spaces between the concen- l 50% inorganic mineral content the type and quantity of the minerals avail- tric layers of lamellae, which are known as (mineral salts); able for the body to use; hydroxyapatite is lacunae. Canaliculi are microscopic canals l 25% water (Robson and Syndercombe one of the main minerals present in bones. between the lacunae, in which the osteo- Court, 2018). While bones need sufficient minerals to cytes are networked to each other by fila- Organic matrix (osteoid) is made up of strengthen them, they also need to prevent mentous extensions. In the centre of each FRANCESCA CORRA FRANCESCA approximately 90% type-I collagen fibres being broken by maintaining sufficient osteon is a central (Haversian) canal Nursing Times [online] February 2020 / Vol 116 Issue 2 39 www.nursingtimes.net Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 2. Anatomy of cortical bone Box 1. Types of bones l Long bones – typically longer than Canaliculi they are wide (such as humerus, radius, tibia, femur), they comprise a Osteocyte diaphysis (shaft) and epiphyses at Inner circumferential lamella Lacuna the distal and proximal ends, joining Lymphatic vessel Osteon at the metaphysis. In growing bone, Concentric lamellae Outer this is the site where growth occurs circumferential and is known as the epiphyseal lamella growth plate. Most long bones are Periosteum: Medullary located in the appendicular skeleton Inner osteogenic cavity and function as levers to produce layer movement Outer fibrous l Short bones – small and roughly layer Trabeculae cube-shaped, these contain mainly Central canal cancellous bone, with a thin outer Perforating canal layer of cortical bone (such as the bones in the hands and tarsal bones Periosteal vein in the feet) Spongy bone Periosteal artery l Flat bones – thin and usually slightly Compact bone curved, typically containing a thin layer of cancellous bone surrounded by cortical bone (examples include the skull, ribs and scapula). Most are through which the blood vessels, lymph Blood vessels in bone are necessary for located in the axial skeleton and offer vessels and nerves pass. These central canals nearly all skeletal functions, including the protection to underlying structures tend to run parallel to the axis of the bone; delivery of oxygen and nutrients, homoeo- l Irregular bones – bones that do not Volkmann’s canals connect adjacent stasis and repair (Tomlinson and Silva, fit in other categories because they osteons and the blood vessels of the central 2013). The blood supply in long bones is have a range of different canals with the periosteum. derived from the nutrient artery and the characteristics. They are formed of The endosteum consists of a thin layer of periosteal, epiphyseal and metaphyseal cancellous bone, with an outer layer connective tissue that lines the inside of the arteries (Iyer, 2019). of cortical bone (for example, the cortical surface (Bartl and Bartl, 2017) (Fig 1). Each artery is also accompanied by nerve vertebrae and the pelvis) fibres, which branch into the marrow cavi- l Sesamoid bones – round or oval Cancellous bone ties.
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