4: Anatomy and Physiology

CHAPTER 4 Anatomy and Physiology

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):

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.

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.

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

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:

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:

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.

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.

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

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.

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).

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.

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).

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 respiratory system

The respiratory system (Figure 4.1.8) has two parts:

The digestive system

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

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