4: Anatomy and Physiology

CHAPTER 4 Anatomy and Physiology

Chapter points

• This chapter covers the requirements of the NEBDN syllabus Section 4: Anatomical structures and systems relative to dental care. The chapter is divided into two subchapters:
4.1: General Anatomy and Physiology
4.2: Dental Anatomy and Physiology

4.1 General Anatomy and Physiology

• This chapter covers the requirements of the NEBDN syllabus Section 4: Anatomical structures and systems relative to dental care, part 4.1.

Definitions

• Anatomy is the study of the structure of the body.
• Physiology is the study of the working of the body – how it functions.

The structure of the body

Everything in the body has a structure and a function or purpose. Virtually all body parts are necessary for health but organs such as the brain, heart, lungs and small intestine, in particular, are essential to life. Organs are complex structures that are made up of various tissues. The tissues themselves are further built up from millions of cells, which are the smallest units of life.

The Cells

There are many different cell types, but the basic design is the same for all cells (Figure 4.1.1):

• The cell or plasma membrane (the outer membrane) – which controls the movement of water, nutrients and waste material into and out of the cell.
• The cytoplasm – which forms the main part of the cell and contains many important structures, especially mitochondria (single: mitochondrion) – the power houses of the cell. Mitochondria produce the energy needed by the cell to function. The most important molecule that is involved in energy production is called adenosine triphosphate (ATP).
• The nucleus – the control centre of the cell. The nucleus is essential to everything the cell does, and contains the key to life itself, the DNA.
image

FIGURE 4.1.1 The basic parts of a single cell.

DNA stands for ‘deoxyribonucleic acid’, and makes up the genes that form the chromosomes. The genes co-ordinate the formation (synthesis) of all proteins in the body. Proteins are essential for virtually all body structures and functions. Genes are inherited from the parents and, because they control protein synthesis, they are responsible for many of the differences between individuals (including differences in the susceptibility or resistance to disease). Gene abnormalities are the cause for many diseases.

Terms to Learn

Chromosome: the thread-like linear structure made of DNA and some other proteins. It stores the hereditary information in the genes and passes it from generation to generation by taking part in cell division.

Protein: a major class of food and the ‘building blocks’ of the body. Proteins are made of amino acids.

Humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y). The sex chromosomes determine whether a baby will be male or female; females have only X chromosomes (XX) and males have an X and a Y chromosome. Chromosomal abnormalities can cause conditions such as Down syndrome.

Cell function and growth are controlled by signals that are sent to the cell from, for example, hormones (chemical messengers). The molecules that carry the signals bind to receptors on the cell membrane, triggering molecules on the inner side of the cell membrane to carry the signal deep into the cytoplasm and to the nucleus. In this way the cell carries out the activities it is instructed to do.

Many, but not all, cells grow and divide to produce daughter cells. Careful control of growth is essential for not just the health of the cell but the health of the entire individual. If the normal pattern of growth is disturbed, it may lead to diseases such as cancer.

Term to Learn

Cancer: the condition in which the growth of some cells becomes uncontrolled.

The Tissues

Cells group together to form larger structures called tissues. The four main types of tissue in the human body, each of which performs particular functions, are:

• Epithelium – this makes up the outer layer of the skin and surface of the lining of the mouth (the mucosa), gastro-intestinal and genitourinary tract.
• Connective tissue – this makes up the supporting structures of the body, which include the tendons, ligaments, cartilage, bone and fat.
• Muscle – this tissue can contract and relax to produce movement. This includes movement of the body as a whole or just a part of the body (e.g. food is taken from one part of the digestive system to the next by the pushing movement produced by contraction and relaxation of the muscles of the digestive tract).
• Nerve tissue –this is made of special cells such as the neurones, which direct other cells in the body to perform certain functions by generating and passing on messages (signals) to them.

Find Out More

What is the difference between: (a) tendons and ligaments; (b) cartilage and bone?

The Organs

An organ is a structure that contains at least two different types of tissue that work together for a common purpose (Figure 4.1.2). Organs include the brain, heart, liver, kidneys, skin and others.

image

FIGURE 4.1.2 Development of the human organism: from cells to tissues, tissues to organs, organs to systems and from systems to a complete body.

The Systems

The main functions of the body such as breathing, circulation of blood, digestion of food require several organs to work together. Organs that function together form a system: for example the heart and blood vessels form the circulatory system, which is responsible for circulating blood throughout the body.

Terms used in anatomy

The special terms used in anatomy to describe the relationship of one part of the body to another are shown in Table 4.1.1. To help describe the position of structures in the body relative to each other and also the movement of various parts of the body in relation to each other, the body can be divided into anatomical planes (Figure 4.1.3) that correspond to the vertical and horizontal planes of space (Table 4.1.2).

TABLE 4.1.1 Terms Used in Anatomy

Term Definition
Superficial Closer to the surface
Deep Further from the surface
Anterior Closer to the front of the body
Posterior Closer to the back of the body
Superior Closer to the top of the head
Inferior Closer to the soles of the feet
Medial Closer to the midline of the body
Lateral Away from the midline of the body
Proximal Closer to the point of origin of a structure
Distal Further from the point of origin of a structure
image

FIGURE 4.1.3 The anatomical planes.

TABLE 4.1.2 Anatomical Planes

Anatomical Plane Spatial Plane Body is Divided by Plane into Portions
Coronal Vertical Anterior and posterior
Transverse Horizontal Superior and inferior
Sagittal Vertical Right and left

The circulatory system

The blood is circulated around the body to all organs, tissues and cells by the circulatory system. This system consists of:

• The blood – which carries oxygen and nutrients to all the cells of the body and removes waste products; thus its circulation is vital for survival
• The blood vessels (arteries and veins) – the network of blood vessels in which the blood circulates all around the body
• The heart – the pump that makes the blood flow in the blood vessels.

Blood

The blood is a special kind of tissue that consists of a variety of cells suspended in a solution called plasma.

Blood cells

There are three types of blood cell:

• Red blood cells (erythrocytes): these transport oxygen from the lungs to the various body tissues and organs. The oxygen binds to a pigment inside these cells called haemoglobin. When this happens the haemoglobin becomes ‘oxygenated’ and turns red in colour, so blood carrying oxygen is bright red in colour. After most of the oxygen is given off to the tissues, the haemoglobin becomes ‘de-oxygenated’ and it (and also the blood) becomes darker red in colour. If there is very little oxygen in the blood, it may even become bluish in colour (called cyanosis), and this makes certain parts of the body also appear blue, such as the lips, tongue and tips of fingers. De-oxygenated blood carries the waste carbon dioxide from the tissues back to the lungs, from where the carbon dioxide is exhaled into the air.
• White blood cells (leucocytes) – these cells are part of the body’s defence and help fight against the micro-organisms that can cause infections.
• Platelets – these play an important part in haemostasis (see Box 4.1.1).

BOX 4.1.1 Haemostasis and Wound Healing

Haemostasis

When a blood vessel is damaged, the process that normally stops the blood escaping is called haemostasis. Haemostasis occurs in several steps:

1. Almost immediately the blood vessel constricts, which slows down the bleeding (vasoconstriction).
2. The platelets start to stick to the blood vessel walls around the damaged area to form a ‘platelet plug’.
3. The blood coagulation factors are activated, leading finally to the formation of a blood clot (haematoma).
4. Later, the clot is removed as part of wound healing.

Wound Healing

Wound healing starts after the formation of the blood clot. It involves special cells called macrophages. The macrophages produce substances called ‘growth factors’ that trigger the formation of a special healing tissue called granulation tissue. Granulation tissue consists of macrophages and also another type of cell called the fibroblast. The fibroblasts produce the fibrous tissue that replaces the damaged tissue.

Within hours of an injury, the epithelium in the damaged area starts to regenerate as the surface cells of the skin or mucosa (called keratinocytes) migrate across the wound to cover it. Later, as the fibrous tissue grows stronger it produces the scarring that is seen in place of the wound.

Term to Learn

Fibrous tissue: this is a specialised tissue that contains tightly woven strands of a fibrous protein called collagen. Besides occurring normally in the body, it is found in scar tissue.

Key Point

A lack of oxygen supply to the brain (called hypoxia) can cause severe brain damage and can kill a person within three minutes.

Production of blood cells

The blood cells are produced in the bone marrow, found inside many bones. Blood cell production requires many substances called haematinics, such as iron, and vitamins – folic acid (folate) and vitamin B12. These substances are present in the food we eat and therefore a good diet is essential for blood cell production. A person who does not eat a diet that contains all the substances required for blood production may not have enough red cells and haemoglobin and the person is said to have anaemia.

Term to Learn

Anaemia: when the cells do not get enough oxygen; symptoms include feeling tired easily.

Blood cell production also requires a healthy bone marrow. People whose bone marrow is damaged (e.g. because they have had radiotherapy or chemotherapy for a cancer) may lack all types of blood cells. They can then have anaemia and they also have a tendency to catch infections due to a lack of white blood cells for defence (see Box 4.1.2 below) and they can also have a tendency to bleed (since platelets are also damaged).

BOX 4.1.2 Immunity and Inflammation

The immune system is responsible for protecting the body against potentially harmful substances that may cause damage or infection. The body’s response to such an attack is called the immune response or immunity.

The first line of defence is the intact skin and mucosa (the lining of internal body cavities). When the skin or mucosa is cut or damaged, there is haemostasis, and then inflammation is induced. Inflammation consists of increased blood flow to the area (with heat and redness), leakage of the plasma proteins from the inflamed blood vessels into the tissues (with swelling), and the release of pain-inducing chemicals from cells. Inflammation is thus recognised by the presence of:

• Heat
• Redness (erythema)
• Swelling
• Pain.

Following inflammation, special cells called macrophages and a kind of white blood cell called neutrophils are activated. These recognise, eat and kill bacteria (phagocytosis) and cleanse foreign matter from the injured site. This is the body’s second line of defence.

The other white blood cells involved in defence are the lymphocytes:

• B lymphocytes – these protect the body against bacteria by producing antibodies
• T lymphocytes – these protect mainly against viruses and fungi.

Central to the immune response are also organs such as the spleen and lymph nodes, which are together called the lympho-reticular or reticulo-endothelial system (RES)). The lymph nodes are basically collections of lymphocytes and macrophages. These cells catch and deal with pathogens or other foreign materials that have escaped from the blood into the tissues and then entered the lymph (Figure 4.1.7).

The white blood cells also release a number of proteins called cytokines, which trigger the various events involved in inflammation and healing.

A hormone produced in the kidney called erythropoietin (EPO) stimulates the bone marrow to produce red cells. People with kidney disease may lack EPO and also develop anaemia. Some athletes use commercially available EPO to increase the oxygen-carrying capacity of their blood – but this is illegal.

Terms to Learn

Chemotherapy: the use of strong chemicals (drugs) to treat cancer. Chemotherapy drugs aim to kill the cancerous cells but also have many side-effects on other normal parts of the body.

Radiotherapy: the treatment of disease (especially cancer) by exposure to an ionising radiation beam (see Chapter 14) or to a radioactive substance.

The plasma

The blood plasma is made up of many kinds of protein, for example proteins called the blood coagulation factors – these work with the platelets to help form a clot to stop bleeding. Other proteins in the blood work with the white blood cells in defending the body against micro-organisms. These are called antibodies.

Key Point

People with diseases (e.g. haemophilia) that affect the production or action of the platelets and the blood coagulation proteins can have serious bleeding after operations including tooth extraction. Therefore it is very important to find out whether a dental patient may have such a condition.

The Blood Vessels

There are three types of blood vessel:

• Arteries – these take the blood (usually oxygenated) away from the heart to the tissues.
• Veins – these return the blood (usually de-oxygenated) to the heart.
• Capillaries – these are the smallest branches of the blood vessel network and link the arteries to veins. These are also the vessels where the blood gives off the oxygen and takes up carbon dioxide from the cells (see p. 106).

The pulmonary artery and vein are the opposite of the rest of the arteries and veins, since the pulmonary artery carries de-oxygenated blood from the heart to the lungs and the pulmonary vein carries the oxygenated blood from the lungs to the heart (see Figure 4.1.5 below).

image

FIGURE 4.1.5 The circulatory system.

The Heart and the Flow of Blood around the Body

The heart is the organ that is found in the centre of the chest (the thorax). It has four chambers: two large ventricles and two smaller atria (Figure 4.1.4).

image

FIGURE 4.1.4 The chambers of the heart and main blood vessels.

The heart pumps de-oxygenated blood to the lungs via the pulmonary arteries. In the lungs the blood releases the carbon dioxide and becomes oxygenated. It then travels in the pulmonary veins back to the heart entering it at the left atrium (Figure 4.1.5). From the left atrium blood is pumped into the left ventricle. The opening between these two chambers is controlled by a valve called the mitral valve.

Find Out More

How does the valve control the flow of blood – does it allow the blood to flow in only one direction or both?

Blood is pumped out of the left ventricle into the aorta. The aortic valve controls the opening to the aorta. The aorta and its branches take the blood to all tissues and cells in the various parts of the body. Therefore the left ventricle is the most powerful heart chamber as it has to move blood all around the body. Because of this, the heart beat, which is the sound of the pumping action of this chamber, is heard and felt to the left side of the chest rather than in the centre.

Term to Learn

Aorta: the main artery of the body.

Key Point

Oxygenated blood reaches the tissues of the heart itself by way of branches of the aorta called the coronary arteries (Figure 4.1.6). If the coronary arteries get blocked, this stops the oxygen supply to the heart and causes the condition called angina or a heart attack (also called myocardial infarction or coronary thrombosis; see Chapter 2 and Chapter 17.1).

image

FIGURE 4.1.6 The blood supply of the heart.

From the arteries, the blood enters the capillaries. It is in the capillaries that the oxygen is released to the tissues and the carbon dioxide collected as a waste product. The space between the capillaries and the cells of a tissue is filled with a substance called the interstitial fluid and the gases and nutrients travel through this (see Figure 4.1.7 below).

image

FIGURE 4.1.7 The lymphatic system.

The de-oxygenated blood then returns to the heart at the right atrium via the superior and inferior venae cavae. Blood then flows from the right atrium to the right ventricle, controlled by the tricuspid valve. Blood leaves the right ventricle through the pulmonary artery (where the pulmonary valve controls flow) to the lungs.

Terms to Learn

Superior vena cava: the large vein that collects the blood from the parts of the body above the heart.

Inferior vena cava: the large vein that collects the blood from the parts of the body below the heart.

The rate at which the heart pumps blood is called the heart rate and the strength with which it pumps blood is the heart beat. The heart rate and heart beat are controlled by the brain and hormones (especially adrenaline, which increases both the rate and beat). Adrenaline release is stimulated by anxiety and exercise.

Key Point

Anxiety and exercise can increase the heart rate and the force of the heart beat.

The Lymphatic System

The lymphatic system (Figure 4.1.7) is the part of the circulatory system that cleanses it of impurities. It also forms an important part of the immune system (Box 4.1.2). The lymphatic system consists of the following parts:

• The lymph – this is the fluid that circulates within the lymphatic vessels. It is formed by interstitial fluid within the tissues (Figure 4.1.7).
• The lymphatic vessels – these are the vessels that transport the lymph.
• The lymph nodes – the knob-like structures that are usually found in clusters along the lymph vessels. They filter the passing lymph to catch its impurities (e.g. pathogens and other foreign material, stray cancer cells). The cells in the lymph nodes are called lymphocytes and form part of the body’s immune defence system (see Box 4.1.2).

The lymph is not actively pumped through the body like blood. It is moved mostly by virtue of muscle contractions. The lymph vessels carry the lymph to the neck where it then enters the veins and eventually becomes part of the blood.

The respiratory system

The respiratory system (Figure 4.1.8) has two parts:

• Upper respiratory tract – this is located in the head and neck and consists of the nose, paranasal sinuses (see p. 117), pharynx and larynx.
• Lower respiratory tract – located mainly inside the chest, this consists of the airways (trachea, bronchi and bronchioles) and the lungs.
image

FIGURE 4.1.8 The respiratory system.

Defence Mechanisms in the Respiratory Tract

The defence mechanisms in the respiratory tract include those discussed in Box 4.1.2 as well as:

• The mucociliary lining: the mucosa of the respiratory system is covered by a layer of thick slippery fluid called mucus. In addition, the cells also have tiny hair-like projections called cilia. Particles or micro-organisms that are inhaled while breathing are trapped in mucus and driven by the cilia into the pharynx to be swallowed or coughed out.
• The cough reflex – this is crucial in preventing food, drink and other material from entering the larynx (see below) and thence the lungs. If coughing is impaired (when a person is under general anaesthesia (GA) or conscious sedation for dental treatment) the person may inhale (aspirate) foreign material into the airways and the lungs. This can lead to a lack of oxygen or it can lead to a dangerous lung infection. Lack of oxygen can be because of difficulty breathing due to blocked airways or damage to the alveoli.

Key Point

If the respiratory system’s defences are impaired (as they are by general anaesthesia), foreign material can enter the lungs and block the airways or cause infections.

Find Out More

Why do people with asthma have difficulty breathing?

The Nose

Air enters the nose through the openings in the face called the anterior nares. It exits the nose into the pharynx which connects to the lungs via the larynx, trachea and bronchi (Figure 4.1.8). The mucosal lining of the nose is rich in blood vessels and glands that produce protective mucus. The plentiful blood warms the inspired air. The nose also has special cells that enable us to smell. The functions of the nose thus include:

• Olfaction (smelling)
• Warming incoming air (via the blood)
• Filtering incoming air (via the mucociliary lining)
• Humidifying incoming air
• Collecting and disposing of secretions of various cells in the upper respiratory tract.

The Pharynx

The pharynx (the throat) extends from the nose and serves as a passage for air into the larynx and trachea. It also carries the food from the mouth into the oesophagus and so is also part of the digestive system. The pharynx can thus be divided into three parts: the nasopharynx, oropharynx and laryngopharynx.

The Larynx

The larynx has three important functions:

• It is part of the respiratory passageway
• It has a flap (epiglottis) that covers the airway during swallowing to prevent food from entering the airway
• Its walls contain the vocal cords that produce the sounds during speaking.

The larynx is also close to a very important gland called the thyroid gland (see p. 116). The larynx can be visualised in the neck as the ‘Adam’s apple’. The larynx leads into the trachea (the windpipe).

Key Point

The shape and area of the opening between the two vocal cords changes continuously during speech and respiration. This helps, for example, to modulate the voice.

The Trachea and Bronchi

The trachea is the windpipe and divides into the right and left main bronchi, which carry air to the right and left lungs, respectively. The bronchi divide into smaller tubes called bronchioles.

Key Point

The right main bronchus is shorter, wider and more vertical than the left. Thus, if, for example, a small dental instrument or a part of a tooth or a filling is inhaled, it tends to enter the right bronchus because it is wider and more directly continuous with the trachea. The use of rubber dam (p. 217) should prevent such catastrophic accidents.

The Lungs

The lungs are made of the alveoli, which are clusters of cells at the tip of the terminal branches of the bronchioles. This is where the oxygen and carbon dioxide exchange in the blood occurs.

People with lung disorders may not be able to have treatment under GA and conscious sedation as their breathing may be compromised.

The digestive system

The digestive system (Figure 4.1.9) starts at the mouth and ends at the anus. It is responsible for:

• Chewing, digestion and absorption of food
• Removal of waste food and other waste matter.
image

FIGURE 4.1.9 The digestive system.

Swallowing

After being chewed (masticated) and mixed with saliva, food is swallowed. Swallowing (also called deglutition) helps carry the food into the pharynx and then into the oesophagus (the food pipe) and finally into the stomach. Swallowing is a complex process co-ordinated by the brain, which sends messages through several nerves. Swallowing is divided into three phases:

• Phase 1 is voluntary – it involves the collection of the chewed food into a bolus and the first part of the swallowing of the food bolus.
• Phase 2 – this is the passage of food through the pharynx into the oesophagus. It is involuntary.
• Phase 3 – this is also involuntary and involves the passage of food from the oesophagus into the stomach.

Key Point

Obstruction to the pharynx or oesophagus can cause dysphagia. Cancer is an important cause of such obstruction.

Terms to Learn

Dysphagia: difficulty swallowing.

Stomach

It is in the stomach that most digestion begins. The cells in the stomach walls produce a variety of substances (e.g. hydrochloric acid and pepsin) that help break down the food by chemical reactions. The secretion of these substances is regulated by complex hormonal and nerve (vagal nerve) mechanisms. The stomach cells also produce mucus and bicarbonate to neutralise the acid, which can damage the lining of the stomach itself. The stomach is not crucial to life but intrinsic factor, produced there, is essential for vitamin B12 absorption in the small intestine. The stomach is a common site for cancer and ulceration.

The Small Intestine

The small intestine is a very long tube (approx 7 m) made of three parts: duodenum, jejunum and ileum. It is the main site of digestion and absorption of food and is crucial to life. Food that has started to be digested in the stomach is fully digested in the small intestine with the help of intestinal and pancreatic enzymes. The nutrients – fats, carbohydrates and proteins – thus released are absorbed into the blood in the small intestine. The blood then takes the nutrients to other parts of the body for use or for storage.

Terms to Learn

Enzymes: the proteins that catalyse the chemical reactions in living beings.

Malabsorption: the condition in which the nutrients are not absorbed by the digestive tract.

Diarrhoea: frequent loose and watery stools.

Steatorrhoea: fatty pale stools.

Diseases of the small intestine cause malabsorption and diarrhoea or steatorrhoea. This can lead to feelings of tiredness and weakness, loss of weight and anaemia.

The Pancreas

The pancreas produces several of the digestive enzymes (e.g. amylase and lipase) and also secretes hormones (insulin and glucagon) that regulate the amount of sugar (glucose) in the blood.

The Large Intestine

The large intestine has two functions:

• Recovery of water and electrolytes (sodium and chloride) from the digestive tract
• Formation and storage of faeces.

Lying between the small intestine and the anus, the large intestine consists of:

• The caecum – this is a blind-ended pouch that carries a worm-like extension (the appendix) that can become inflamed (appendicitis)
• The colon – this forms most of the length of the large intestine
• The rectum – this is the short last segment, continuous with the anus.

The large intestine is not crucial to life, and it is a common site for cancer.

Liver

The liver is located in the right part of the upper abdomen. Its functions are:

• Turning extra glucose into glycogen, which is then stored in the liver. Glycogen is converted back into glucose when required, maintaining stable blood glucose levels. Excess carbohydrates and protein are converted to fat, which can also be converted to glucose when required.
• Making proteins, for example most of the blood clotting factors.
• Making bile, which is essential for fat digestion and absorption of some vitamins. Bile is stored in the gall bladder between meals and when a person eats, it is discharged into the duodenum.
• Producing or storing several vitamins (A, D, E, K, folate, vitamin B12), and storing minerals (e.g. copper and iron).
• Breaking down worn out haemoglobin, cholesterol, proteins and many drugs.

Terms to Learn

Carbohydrate: a major class of food, an important structural component of cells and a />

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  3. 2: Medical Emergencies and First Aid in the Dental Surgery
  4. 5: Oral Disease
  5. 10: Surgical Procedures
  6. 17: Human Diseases and Health Promotion

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Jan 8, 2015 | Posted by in Dental Nursing and Assisting | Comments Off on 4: Anatomy and Physiology

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