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. 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.

Functions of bone

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)

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

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).

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):

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.

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

Chambers of 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.

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).

Mouth/buccal cavity

This is the oral cavity that is found at the very beginning of the digestive tract, through which food enters. The mouth is lined by mucus secreting stratified epithelial cells. Its anterior and lateral boundaries are the lips and cheeks, respectively.These help to keep the food in the mouth and have a role in speech.

Superiorly lies the hard and soft palate; inferiorly is the floor of the mouth and tongue. The hard palate forms a rigid surface for the tongue to press against during chewing. The soft palate lies posterior to the hard palate and during swallowing it rises to close off the nasopharynx, preventing fluid from entering into it. Beyond this the soft palate is continuous with the pharynx.

The tongue is a muscular organ attached to the hyoid bone inferiorly. The soft layer of tissue that attaches the tongue to the floor of the mouth is called the frenulum. The superior surface of the tongue contains small finger like projections called papillae which contain nerve endings for taste. Nerve supply to the tongue is via the hypoglossal nerve for voluntary movement; taste sensations are via the facial and glossopharyngeal nerves. The tongue has a very rich blood supply and injury can therefore cause extensive bleeding. Blood supply is via the lingual branch of the carotid artery and venous drainage is via the lingual vein to the internal jugular vein.

Salivary glands

Each gland (Fig. 20.13) is surrounded by a fibrous outer capsule and is lined with mucous-secreting cells. The secretions are called saliva, which is made up of water, mineral salts, a digestive enzyme called salivary amylase, mucous, lysosomes (to remove/repair/replace injured cells), immunoglobulins (protection against microorganisms) and certain blood clotting factors. Secretions are controlled by the autonomic nervous system. The presence of food in the mouth triggers a reflex secretion of saliva. In addition, sight, smell or even the thought of food may have the same effect.

Pharynx

This is a funnel-shaped structure that contributes to both the digestive and respiratory tract (Fig. 20.14). Once food leaves the mouth it enters the pharynx. As discussed in the respiratory system, the pharynx continues inferiorly as the oesophagus. The epiglottis acts as a gate allowing food to enter the oesophagus by closing off the entrance to the trachea. The digestive function of the pharynx is to aid swallowing together with the effort of the muscles associated with the tongue, hyoid and soft palate.

Oesophagus

This is a long tube connecting the pharynx to the stomach. It is located just anterior to the vertebral column and posterior to the trachea. Once food leaves the pharynx it enters the oesophagus and is moved along by wave-like movements called peristalsis.

At its lower end the oesophagus has a band of smooth muscle that acts as a sphincter, which functions to let food pass into the stomach as well as preventing gastric contents from being refluxed back into the oesophagus. If the sphincter does not function properly, it could allow the acidicgastric contents to move up the oesophagus causing a burning sensation, commonly referred to as heartburn.

Stomach

The stomach is a sac-like structure that can be located under the left hemi-diaphragm and functions to store food and aid in mechanical and chemical digestion (Fig. 20.15). Food enters the stomach from the oesophagus via the cardiac sphincter.

The stomach is anatomically divided into four regions: the cardiac, fundus, body and antrum. The lining of the stomach has folds, called rugae, and consists of columnar epithelial cells containing gastric pits which house the gastric glands. The outer layer of the stomach consist of muscular fibres arranged in circular and longitudinal fashion to facilitate the breakdown and churning of food required during mechanical digestion. Whilst the muscles provide the movement to aid mechanical digestion, the gastric enzymes (water, salts, mucus, hydrochloric acid (HCl) and pepsinogens) secreted by the gastric glands provide the chemical digestion. Once adequately churned, the food is then propelled towards the duodenum via the pyloric sphincter.

Small intestine

This is a narrow tube that is about 6 m long and divided into three parts:

Large intestine or colon

This part of the tract extends from the terminal ileum to the anus. It is made up of various segments:

Food entering the large colon has already been stripped of its useful nutrients and the remainder is ready for excretion as waste. The colon has bacteria, which solidify faeces and causes flatulence. The bacteria are important in the formation of vitamin K for blood clotting. The lining of the colon contains mucous cells, which secrete mucus to neutralise the pH of the faeces as well as helping it to move towards the anus. The faeces are then expelled out of the body by internal and external anal sphincters.

The external surface has large bulges, called haustra. These haustral patterns are important during imaging and diagnoses, as their absence denotes pathology in the region.

Liver and gall bladder

The liver is a large wedge-shaped structure, which lies in the upper right quadrant of the abdomen, just under the right hemi-diaphragm. It is roughly divided into left and right lobes, which are separated by a ligament called the falciform ligament. This ligament forms an important landmark when imaging the liver during ultrasound examinations.

The liver has various functions in metabolism, blood cell production and digestion. It produces a pigment called bile, which is housed in a specialised sac called the gall bladder. Bile is secreted via the bile duct directly into the duodenum where it breaks down or emulsifies fats, thus aiding enzyme action by digestive enzymes.

Sometimes small calcifications resembling little stones form in the gall bladder and can be seen on a radiograph; these are called gallstones. Imaging of the gall bladder can be via ultrasound or by performing a cholecystogram. The liver is easily imaged using ultrasound, CT or MRI modalities.

Pancreas

The pancreas lies posterior to the stomach in the retroperitoneal space and acts as both an endocrine (see Endocrine system, p. 283) and exocrine gland. Its exocrine function is to produce digestive juices (pancreatic juices) and enzymes that are transported via the pancreatic duct to empty into the duodenum to help in digestion.

External anatomy

Each kidney is bean shaped, with a convex lateral border and a concave medial border. The medial border is called the hilum and it is here that blood vessels, nerves and ureter enter and leave the kidney.

Three layers of tissue surround the kidney and protect it from mechanical injury and infections. The innermost layer is a fibrous layer called therenal capsule; the middle layer is a ‘fat’ layer called the adipose capsule; and the outermost layer is a dense connective tissue layer called the renal fascia. The renal fascia provides attachment for the adrenal glands (small pyramid-shaped endocrine glands situated above each kidney) as well as attaching the kidneys to the posterior abdominal wall. The connective tissue of this layer also provides the flexibility needed by the kidneys during breathing movements.

Internal anatomy

There are three distinct areas on the internal surface of the kidney: a cortex, a medulla and a renal pelvis (Fig. 20.19). The cortex forms the outermost part of the internal structure of the kidney, whilst the middle portion is called the medulla and contains cone-shaped tissue called medullary pyramids. On a longitudinal section these pyramids appear to have stripes or striations. This is because they are composed of urine-collecting tubules arranged in a parallel manner. The pyramids have ‘finger like’ projections called papillae, which empty into the minor calyces. The renal pelvis is found medially and is the dilated portion of the ureter, which has two divisions known as the major and minor calyces. The minor calyces act as cups to collect the urine from the collectingtubules of the pyramids and propels it, with the help of its smooth muscle containing layer, towards the pelvis. The pelvis acts as a reservoir to collect the urine and then transport it to the bladder for excretion.

Filtration, formation and flow of urine

Kidneys function to regulate the water, nutrient and electrolyte balance of the body. The component responsible for this activity is a microscopic, blood-filtering and functional unit called the nephron (Fig. 20.20). The nephron is associated with many renal tubules. These structures filter the blood and form urine. Collecting ducts then collect the urine, which is emptied into the minor calyx of the renal pelvis.

Each nephron is made up of a bundle of capillaries called the glomerulus, which is surrounded by a cup-shaped structure called the glomerular capsule. Together these two structures constitute the renal corpuscle and are associated with renal tubules.

Blood enters the glomerulus and is filtered. The filtrate then passes through the porous (fenestrated) epithelium of the glomerus into the glomerular capsule. From the glomerular capsule the fluid then flows into the proximal part of the renal tubule called the proximal convoluted tubule. The fluid then flows into a part of the tubule that resembles a hairpin, called the Loop of Henle; it continues to flow towards the distal part of the tubule called the distal convoluted tubule and finally into the collecting duct as urine.

When the fluid enters the glomerulus as a filtrate, it contains plasma, useful ions, blood cells and nutrients. These constituents of the filtrate are then absorbed as a result of the filtrate flowing through the renal tubule, and the fluid that remains at the distal end of the tubule empties into the collecting ducts and is now called urine.

Urine is generally a clear–pale yellow colour of watery fluid that has a slightly acidic pH due to the presence of urea, sodium, potassium, phosphate and sulphate ions, uric acid and creatinine.

Functions of the kidney

Male urethra

Female urethra

Brain

The brain is a large mass of white and grey matter that would slump and sag if it was not supported by the cranium and meninges (Fig. 20.22). The cranium is the skull vault that surrounds the brain and protects it from injury.

Meninges

Directly surrounding the brain are the meninges (Fig. 20.23). These layers of tissue protect thebrain and, together with the CSF, act as shock absorbers to prevent injury. There are three meningeal layers:

Cerebrum

This is the largest part of the brain. The obvious features of the cerebrum are two large hemispheres of convoluted, wiggly folds of tissue called gyri that are separated by deep grooves called sulci. These convolutions increase the surface area of the cortex of the cerebrum.

The two hemispheres form the left and the right lobes. The lobes are made up of grey matter on the outer cortex and the inner layers are made up ofwhite matter. The cerebrum consists of nerve tracts for communication between the lobes. The lobes are connected to each other by a dense bundle of nerve fibres called the corpus callosum. This relays impulses between the left and right lobes.

Each lobe is divided, by deep fissures, into smaller lobes which correspond with the part of cranium they lie under:

Cerebellum

The second largest part of the brain, the cerebellum lies inferior to the cerebrum and is separated from it by the central fissure. It is made up of left and right hemispheres separated by a central vermis. It is composed of an outer cortex made up of grey matter, with white matter in the inner layers. The inner white matter is made up of long and short tracts. These short tracts conduct impulses from neuron cell bodies on the outer cortex to the inner area. The long tracts conduct nerve impulses to and from the cerebellum itself.

Brain stem

The brain stem is the ‘stalk’ of the brain that acts a bridge and connects the cerebrum to the spinal cord. It is made up of three parts: the midbrain, pons and medulla oblongata.

The brain stem consists of long tracts of ascending and descending pathways that conduct impulses from the body to the cerebrum and from the cerebrum to the rest of the body. It houses animportant structure called the reticular formation that controls the vital life-sustaining role of maintaining respiration and cardiac activity. It is also responsible for regulating consciousness and levels of ‘awareness’. The reticular fibres desensitise the repetitive sounds and create levels of awareness for ‘important’ sounds.

Ventricles and cerebrospinal fluid (CSF)

The ventricles are cavities within the brain that are filled with CSF (Fig. 20.24). These cavities are lined with cuboidal epithelial cells (called ependymal cells) that make contact with the pia mater at various points along its surface to form blood vessel network structures known as choroid plexuses.

CSF is formed at the choroid plexus due to the semipermeability of the capillary network. CSF is made up of water, small amounts of minerals, ions, white blood cells and organic compounds.

Blood–brain barrier

This is a network of semipermeable membranes that acts as a barrier, selectively preventing access to certain substances (e.g. certain drugs) whilst allowing other substances such as oxygen and essential nutrients to enter freely.

Astrocytes are a special type of neuroglial cell that support the nerve cells of the CNS but do not conduct impulses. A layer of astrocytes is found just inside the pia mater covering the brain. The blood–brain barrier is made up of this layer of astrocytes in addition to the capillary wall surrounding the pia mater. The choroid plexuses are considered to be part of the blood–brain barrier.

Spinal cord

This is an elongated structure that is held together by dural ligaments and supported and surrounded by the meninges and CSF. The spinal cord is made up of tracts that form the connecting link between the brain and the rest of the body and is the centre for coordination of reflex action. It ascends superiorly from just below the foramen magnum to about the level of L1–L2 inferiorly.

From the spinal cord, 31 pairs of spinal nerves emerge (Fig. 20.25). There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and 1 pair of coccygeal nerves.

Somatic nervous system

This is composed of both sensory and motor pathways. The sensory pathway receives impulses such as touch, pain and heat as conscious perceptions. Impulses from the mouth, skeletal muscles and joints, nose, etc., are relayed via cranial and spinal nerves and are perceived at an unconscious level. The motor pathway of the somatic system is composed of motor neurons, which conduct impulses from the CNS to the skeletal muscle, enabling the muscles to voluntarily ‘act’ or contract.

Visceral nervous system

This is also composed of sensory and motor pathways. The sensory pathway receives impulses from sensory receptors of visceral systems(e.g. cardiovascular, digestive and urinary). These impulses are perceived on an unconscious level; however, some impulses are conveyed on a conscious level – e.g. pain, taste and feeling of fullness of the bladder.

The motor pathway of the visceral nervous system is known as the autonomic nervous system.

Autonomic nervous system

These are the motor pathways that respond to impulses from the visceral nervous system and secretions from endocrine and exocrine glands of the body. This system can be divided into two parts whose responses are generally antagonistic (i.e. have the opposite effect) to each other, although they function together to promote homeostasis. They are:

Nerve fibres from these systems innervate visceral organs and they tend to have the opposite effect to each other. For example the sympathetic division would constrict an artery whilst the parasympathetic division would dilate the artery. However, there are instances where both systems are stimulated to have the same effect (e.g. production of saliva).

The parasympathetic division functions to ‘conserve’ energy by slowing the heart rate and blood pressure and stimulating excretion of waste products, whereas the sympathetic division is involved in action and activity, thereby increasing heart rate and blood pressure.