Chapter 20 Anatomy and physiology

Skeletal system 256

Cardiovascular system 258

Respiratory system 265

Digestive system 269

Urogenital system 274

Nervous system 278

Endocrine system 283

KEY POINTS

The respiratory system functions as a series of air passages through which air enters the lungs and travels to the alveoli where gaseous exchange occurs.

Respiration consists of active inspiration and passive expiration.

The cardiovascular system is a closed network of blood vessels that carry oxygenated blood to all body tissues and transport deoxygenated blood away from body tissues to the lungs for excretion, with the exception of the pulmonary vessels.

Blood is composed of a liquid plasma and formed elements consisting of red and white blood cells as well as platelets.

Blood is the ‘life’ of body cells and tissues as it carries vital life-sustaining oxygen and nutrients.

The skeletal system is made up of the axial and appendicular skeleton.

This skeleton provides a supportive and protective framework for the body, which together with muscles gives the body its ability to carry out a wide range of movements.

A joint is where two or more bones meet. Muscles acting across joints provide the mechanism by which movement takes place.

The nervous system is the body’s network of communication that receives, interprets and relays messages to and from the brain.

The functional unit of the nervous system is the nerve cell or neuron.

The spinal cord is a long tract of fibres that provides a ‘road’ through which impulses can travel.

The alimentary tract extends from the mouth to the anus.

Food taken into the mouth is swallowed (ingested); digested by mechanical movements and chemical enzymes; its nutrients are absorbed and the left over waste eliminated.

The liver, gall bladder and pancreas are accessory organs of digestion.

The nephron forms the functional unit of the kidney to filter waste material from the blood and eliminate this as urine.

The kidneys play an important role in regulating the electrolyte balance of the body.

The kidneys produce rennin which helps to regulate blood pressure levels.

The endocrine system plays a role in maintaining homeostasis by the action of chemical messengers called hormones.

Hormone release is regulated by the hypothalamus via a feedback mechanism of communication.

INTRODUCTION

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.

SKELETAL SYSTEM

The skeletal system is made up of bones and joints that work together with muscles and ligaments to provide a framework for the body (Fig. 20.1).

Figure 20.1 The human skeleton – anterior view.

BONES

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.

General structure and appearance

Bone has an outer covering called the periosteum, which is a tough outer membrane made up of fibrous tissue and containing blood vessels. The cortex 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.

Functions of bone

• Provide a framework of support and protection of the body’s soft organs.

• Enable a range of movement, together with muscles, to provide stability and mobility.

• Provide storage for certain nutrients; e.g. calcium and phosphorous.

• Produce red blood cells (erythropoiesis). Bone marrow is an important site for this.

Bones are classified into the following types (Table 20.1):

Table 20.1 Classification of bone

Shape	Description	Location in the body
Long bones	Length is greater than width	Upper limb (humerus, radius and ulna) Lower limb (femur, tibia and fibula)
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.

JOINTS

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.

Classification

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.

Table 20.2 Classification of joints

Type of joint	Structure	Description of movement, with examples
Fibrous	• Do not have joint cavities • Mostly immovable although some have slight movement	• Sutures of skull – fibrous tissue is fused in adults and immovable, but has some movement in the fetus and in children • Bones held together by ligaments (e.g. radius/ulna and tibia/fibula) have a limited twisting movement • Roots of teeth in gums (where a ‘peg fits into a socket’) are mostly immovable but may have very slight movement
Cartilaginous	• Bones are joined by a hyaline cartilage • Mostly slightly movable • Some immovable	• Cartilage forms a disc-like cover over bone (e.g. symphysis pubis and intervertebral joints), giving slight movement • Some cartilage forms temporary joints (e.g. epiphyseal plates of long bones), which are immovable so that the bones can grow well
Synovial	Have lubricated articular cartilage between bones giving smooth, free movement in a range of directions	• Hinge joints – give movement of flexion and extension; e.g. elbow; knee; ankle and interphalangeal joints • Pivot joints – give movement of supination; pronation and rotation; e.g. atlantoaxial joint; proximal radioulnar joint and distal tibiofibular joint • Condyloid (ellipsoid) joints – are modified ball and socket joints that give movement of flexion; extension; abduction; adduction and circumduction; e.g. most metacarpophalangeal joints • Gliding joints – give movement of limited gliding action; e.g. acromioclavicular joint; articular processes of vertebrae and between some carpal and tarsal bones • Saddle joints – give movement of abduction, adduction, opposition and reposition; e.g. carpometacarpal joint of thumb (between the trapezium and first metacarpal) • Ball and socket joints – give movement of flexion; extension; internal, external and lateral rotation; abduction, adduction and circumduction; e.g. hip and shoulder joints

CARDIOVASCULAR SYSTEM

This system forms the transport network for the body.

THE HEART

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.

Size, shape and location

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).

Figure 20.2 Position and location of the heart.

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.

Structure

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 heart wall is made up of three layers of tissue (Fig. 20.3):

• pericardium

• myocardium

• endocardium.

Figure 20.3 Structure of heart wall.

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 further divided 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.

Blood supply to heart wall is via the left and right coronary arteries and venous return is via the coronary sinus and cardiac veins.

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.

Figure 20.4 Chambers of the heart.

Figure 20.5 The cardiac cycle.

Pumping action of the heart

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).

Figure 20.6 The pathway of electrical impulses through the heart.

Figure 20.7 ECG trace.

BLOOD VESSELS

Blood flows in the body in a closed system of circulation. The great vessels that come from the ventricles of the heart are the pulmonary artery and the aorta.

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).

Figure 20.8 Systemic circulation.

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

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 helps the body to maintain its normal state of hydration by responding to change in the internal environment via osmosis.

The formed elements are red blood cells (erythrocytes), a combination of different types of white blood cells (leucocytes) and blood platelets (thrombocytes).

These blood cells have different functions (Table 20.3). Together they serve to:

• maintain a healthy circulation by delivering oxygen, nutrients and useful substances to cells and tissues throughout the body

• play a role in homeostasis by regulating body temperature, initiating blood clotting to stop bleeding, and maintaining the pH (acid–base) balance of body fluids

• protect against harmful invasion of microorganisms.

Table 20.3 Blood cells and their function

Blood cell	Function
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