Chapter 20 Anatomy and physiology
This chapter provides an overview of the aspects of anatomy and physiology that are relevant to radiographic practice. Developmental anatomy has been excluded from this chapter. Anatomy described has been based on the normal average adult male/female, and wherever possible and appropriate normal variations have been mentioned.
Bone is hardest of all connective tissue found in the human body and is formed by a process of ossification which takes place in two ways. The first is intramembranous ossification, where connective tissue is replaced by calcium phosphate, and this occurs in the skull. The second is intracartilaginous ossification, where hyaline cartilage is replaced by calcium phosphate, and this occurs almost throughout the skeleton.
Bone tissue comprises spongy/cancellous or compact bone. Microscopic structure of bone consists of Haversian systems arranged in concentric circles of lamellae (layers), which surround the Haversian canals. Each Haversian canal contains blood and lymphatic vessels and nerves. In compact bone, these Haversian systems and lamellae are packed closely together with very little space between them. In spongy bone there are fewer Haversian systems and the Haversian canals are larger with bigger gaps between the lamellae. These spaces help to reduce the weight of the bone. Bone marrow, consisting of both yellow and red marrow, fills the spaces created by the gaps.
Bone tissue is dependent on nutrients such as calcium, phosphorous and vitamins C and D for growth and repair. Exercise affects the growth and repair of bone and muscle by stimulating blood supply and circulation.
Bone has an outer covering called the periosteum, which is a tough outer membrane made up of fibrous tissue and containing blood vessels. Thecortex is made up of compact tissue and lies directly below the periosteum. The inner layer of is made of spongy or cancellous bone, which is softer, compared to the tough outer cortex. Lastly, the innermost part of the bone is formed from ‘fatty’ yellow bone marrow, which contains a few white blood cells, and red marrow containing red blood cells.
|Shape||Description||Location in the body|
|Long bones||Length is greater than width|
|Short bones||Equal in length, breadth and thickness||Wrist (carpal bones), ankle (tarsal bones)|
|Flat bones||Usually more curved and thin than flat; e.g. the curved bones of the skull protect the brain||Skull, chest (scapula, ribs, sternum), pelvis|
|Irregular bones||These do not have any of the above-mentioned shapes, hence ‘irregular’||Axial skeleton, both shoulder and pelvic girdle and vertebrae|
|Sesamoid bones||Small bones that are found embedded in certain tendons connecting muscle to bone||Knee (the commonest sesamoid bone is the patella), but may also be seen on images of the hand, wrist and foot)|
Plain film radiography is still the first line of investigation following injury or trauma to the bones and joints. If there appears to be ligamentous or tendon involvement then CT, MRI or ultrasound may be requested for further diagnoses, management and treatment. However, successful interpretation can usually be made by carefully evaluating the images in terms of bony and soft tissue appearances, e.g. tissue swelling, size of joint spaces, position of fat pad and cortical markings.
A joint forms at a point where two bones and cartilages meet or where adjacent bones and cartilages are joined. Although bone gives the body protective structure and muscles provide the ability to move, it is actually the joints that provide the mechanism by which movement takes place. Radiographic contrast can sometimes be injected into a joint space (e.g. in the glenoid cavity of the shoulder) to visualise any underlying pathology. This procedure is known as an arthrogram.
A joint can be classified according to the range of movement it provides or by its articular surface structure. All joints in the body can be classified as shown in Table 20.2; however, certain areas of the body may have a combination of two joints; for example the temporomandibular joint (TMJ) comprises gliding and pivot joints.
The heart is a muscle that acts as a pump and provides the energy and force to keep blood circulating throughout the body. Blood is circulated via a closed transport system; that is, oxygenated blood leaves the heart via arteries, passes through a tiny network of capillaries where transfer of oxygen and nutrients take place, and then deoxygenated blood returns to the heart via the veins.
The heart is conical in shape and, under normal circumstances, about the size of its owner’s clenched fist. It is located anteriorly in the centre of the thorax, with about two-thirds of its bulk lying towards the left of the sternal margin. The tip of the ‘cone’ is called the apex and the flat portion is called the base. The base, normally found at the level of T5–T8, faces forwards and downwards to the left, ending in the apex. The apex lies at the level of the fifth intercostal space on the left midclavicular line (Fig. 20.2).
Occasionally patients may present with dextrocardia, a condition where the heart and great vessels originate with the same structure but opposite in direction (i.e. the apex lies towards the right side of the thorax rather than the left). This is a normal variant and is usually discovered as an incidental finding when the patient presents for a chest examination for an unrelated symptom.
The heart and the great vessels are surrounded and supported by a protective covering called the pericardium. The pericardium is a fibroserous sac that is attached to the sternum, diaphragm and great vessels by connective tissue.
The pericardium consists of two layers of tissue. The outer layer is a tough fibrous layer that serves to protect the heart wall and secure its position within the thorax. The inner layer is a serous layer. This serous pericardium is furtherdivided into an outer parietal layer, which forms the inner lining of the fibrous pericardium, and an inner visceral layer that forms the outer covering of the heart (also known as epicardium). Between these layers is a potential space, called the pericardial cavity. This contains serous fluid, which allows for flexibility in the movement of the heart during contraction and relaxation phases (heartbeats), thus reducing friction during these movements.
The myocardium is a thick layer of cardiac muscle lying between the pericardium and the inner endocardium. It has two layers of cardiac muscle arranged in a spiral form and it is this muscular arrangement that gives the heart its squeezing ability.
The endocardium is a thin fibrous layer made up of endothelial cells and connective tissue. It lines the inner surface of the heart walls and continues as the inner lining of the great vessels that emerge and leave from the heart.
The heart is divided into left and right halves by a muscular septum. Each half has an upper atrium and a lower ventricle (Fig. 20.4). The atria are separated from each other by an interatrial septum and the ventricles are separated from each other by the interventricular septum. The atria are linked to the ventricles by atrioventricular valves. In a normal heart, blood flows from atria to ventricles and not the reverse. Figure 20.5 shows the sequence of events during the cardiac cycle.
The heart’s own inherent autorhythmic cells act as a pacemaker to initiate and maintain the beating and pumping actions of the heart. These cells are also responsible for conducting these impulses throughout the cardiac muscle, thus creating an action in the path in which it travels (Fig. 20.6). Because the ventricles are responsible for sending blood out of the heart, the pressure in the ventricles is greater than the pressure inthe atria. The normal rhythm of a heart beat can be seen on an ECG (electrocardiograph) trace (see Fig. 20.7).
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs and after gaseous exchange takes place, oxygenated blood is then transported back to the heart via four pulmonary veins which empty into the left atrium. The pulmonary artery is the only artery in the body to carry deoxygenated blood and the pulmonary veins are the only veins in the body to carry oxygenated blood. This is known as the pulmonary circulation.
The aorta and other arteries carry oxygenated blood to the body (Fig. 20.8) and have thicker walls to withstand the high pressure at which blood is pumped into them. The closer the artery is to the heart, the thicker the walls of the artery. Arteries and veins can be demonstrated radiographically by using contrast media (e.g. in angiography and venography).
Surrounding musculature exerts positive pressure on veins and this helps to force the blood through the veins and towards the heart. In addition, veins have thin walls due to their relatively low internal pressure and small flaps that act as valves to prevent the backflow of blood.
Blood is the ‘life’ of the body. It is a viscous tissue made up of liquid plasma and a combination of formed elements (blood cells). The plasma helpsthe body to maintain its normal state of hydration by responding to change in the internal environment via osmosis.
|Erythrocytes (red blood cells)||Contain haemoglobin (iron-containing pigment attached to a globular protein) for absorption and transport of oxygen from the lungs to cells and tissues|
|Leucocytes (white blood cells)||Provides immunity for the body by protecting against harmful invasion by disease-causing microorganisms – removes these, and debris from dead or damaged cells, from the blood.|
|Thrombocytes (blood platelets)||Responsible for clotting of the blood|
The thorax is probably the most frequently imaged body part in radiology departments today because a single chest X-ray is sometimes sufficient to make a diagnosis and determine the overall health of a patient. The mechanism of breathing (inspiration and expiration) occurs within this system. It functions as a series of passages through which air travels from the outside (atmosphere) to the inside(lungs). In addition, this system contributes to wider ranging functions of voice production, coughing and sneezing.
The nose has two external nostrils through which air enters. As the air enters it is warmed, filtered and moistened by the mucous membranes lining the nasal cavity. The mucous membranes have a constant supply of blood, which provide the warmth. The nasal cavity also serves as the centre of smell and has large air chambers for sound production.
Surrounding the nasal cavity is the hard palate, which forms the floor inferiorly, and the soft palate, which separates it from the oropharynx posteriorly. Leading into the nasal cavity, through small foramina, are a series of air spaces in the facial bones called the paranasal, maxillary, ethmoid, frontal and sphenoidal sinuses. These sinuses add a rich, full-bodied tone to the voice.
Sinuses are air filled; however, once infection sets in radiographs of the sinuses may be needed to demonstrate air-fluid levels that are especially prominent in the maxillary sinuses during an infection.
The pharynx is between the nasal cavity and the trachea and oesophagus, and therefore serves as a passage for both air and food. The pharynx has three parts: the nasopharynx (superior), oropharynx (middle) and the laryngopharynx (inferior).
The nasopharynx has two auditory air passages (Eustachian tubes/pharyngotympanic tubes) that open into it. These air passages serve to balance out the air pressure on both sides of the tympanic membrane (the eardrums).