CHAPTER 4 Anatomy and Physiology
4.1 General Anatomy and Physiology
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.
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.
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.
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.
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.
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.
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).
|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|
|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|
BOX 4.1.1 Haemostasis and 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.
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.
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:
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.
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).
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.
Radiotherapy: the treatment of disease (especially cancer) by exposure to an ionising radiation beam (see Chapter 14) or to a radioactive substance.
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.
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 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).
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.
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.
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).
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).
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.
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.
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 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.
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:
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 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).
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.
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:
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 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.