Introduction to Medical Sciences

1.1a Introduction to physiology: systems of the body

Systems of the body

Physiologists consider the human body to be a collection of functionally integrated systems. Although these systems cannot work in isolation, each can be defined in terms of its structure (anatomy) and function (physiology).

At the most basic of the levels of organization of all plants and animals lays the living building block known as the cell. An understanding of how the cell functions will form a basis for the study of those more complex organizational levels of the body, the tissues, the organs and the physiological systems.

Most cells are too small to be seen with the naked eye, but a simple microscope can reveal certain details of their structure, including the boundaries between cells.

Figures 1.1b-I to 1.1b-IV are diagrammatic represen-tations of how cells in different tissues can appear under the light microscope. These indicate the wide variety of cell shape seen in different tissues, and also that each cell contains a large central structure known as the nucleus.

Figure 1.1b-I Fibrous tissue, illustrating fiber-generating cells (fibrocytes) and collagen fibers

Figure 1.1b-II Adipose (fat) tissue, illustrating fat-filled adipose cells and minimal connective fibers

Figure 1.1b-III Tissue from a lymph node, illustrating immune cells (white blood cells and reticular cells) and supportive reticulin fibers

Figure 1.1b-IV Cardiac muscle tissue, illustrating the fiber-like cardiac muscle cells linked by connecting “intercalating discs”

The more powerful electron microscope is required to reveal more detail in cell structure. Although the precise structure of each cell is different, depending on the tissue in which it is found and the role it has to play, all cells in the human body share some basic features.

Structure of the cell

Each cell contains a plasma membrane and a number of internal structures known as organelles. This terminology reflects the fact that a single cell can be seen as a living unit in isolation, with the organelles corresponding to the organs of the body. The organelles are fluid in nature, being bound by oily fluid membranes and suspended within a fluid matrix known as the cytosol. Together, the cytosol and organelles are called cytoplasm, and this is bounded by an oily fluid plasma membrane.

The plasma membrane acts as the link between the cell and the outside world. Large molecules made of protein in this membrane make the cell unique and structurally respond to various chemicals in the cellular environment to bring about changes within the cytoplasm. In addition, the plasma membrane allows the passage of nutrients into the cell and waste out of the cell, but able to retain essential substances within the cell. This important feature of cell membranes, known as semi-permeability, is described in Section 1.1c.

The nucleus is the largest structure found within the cell. It contains genetic material in the form of chromosomes. These consist of strands of a very complex molecule called DNA (deoxyribonucleic acid). DNA is the template for the synthesis of proteins building materials in the cell. Another complex molecule with a similar structure, RNA (ribonucleic acid), takes the role of the messenger. Messenger RNA transfers the information coded on DNA out to the cytosol and to the ribosomes, where the proteins are actually made.

The other large round structures in the cytosol are the numerous cigar-shaped mitochondria, which utilize oxygen to break down the basic nutrients obtained from carbohydrates and fats (and sometimes proteins) to form an energized compound called ATP (adenosine triphosphate). ATP can be likened to a battery as it holds a readily available store of energy to “power” all processes of the cell. This process of using oxygen and nutrients to form energy is called cellular respiration. The waste product of respiration is carbon dioxide.

A large part of the remaining cytosol consists of many layers of oily membrane called the endoplasmic reticulum (ER), on which the dense ribosomes are situated. Where the ribosomes are numerous the ER is described as rough (rough ER), and where they are less numerous it is described as smooth (smooth ER). Ribosomes are the site at which the long molecule of RNA is used as a template to guide the production of proteins from simple chemicals called amino acids, out of which all proteins are made. Proteins made on the rough ER pass into the space between its membranes in preparation for transport out of the cell.

The Golgi apparatus is an extension of the ER. It takes the proteins made on the ribosomes and covers them with a membrane coating to make “vesicles” (also known as secretory granules). These can travel to the plasma membrane, so that proteins can be released into the outside environment when necessary.

Microfilaments and microtubules are fibers that can contract and cause the movement of substances from one part of the cell to another. These fibers, also known as the cytoskeleton, maintain the structure of the cell and link the various organelles.

Both meiosis and mitosis are involved in the development of the fertilized egg (zygote) into an adult human being. Meiosis in the sex organs of the parents leads to the production of the gametes. When a male gamete (spermatozoon) and female gamete (egg or ovum) meet, they fuse through the process of fertilization. The resulting zygote divides and grows by the process of mitosis.

Mutation

When the genetic material of the daughter cells from mitosis and meiosis has become damaged during the process of cell division, this is referred to as a mutation. There are three possible consequences of this. First, the mutation is so minor that the resulting cell is very similar in function to its parent. Second, the mutation disturbs the function of the daughter cells so much that they die. Third, and most significantly, the mutation may lead to disturbed function in the daughter cells, although these cells continue to live and replicate. This may or may not have a significant effect on the tissue of which the parent cell is a part.

If the parent cell is one of the cells in an embryo, a single mutation can have devastating effects, as the daughter cells may have been destined to develop into major body parts. One example of the consequences of mutation occurring during the process of mitosis is the effects of the drug thalidomide, which was developed in the early 1960s as an anti-nausea medication in early pregnancy. It was later discovered that significant limb development mutations had developed in the embryos carried by pregnant mothers using the medication.

A mutation in mitosis can also lead to cancerous change in the daughter cells. Cancer is always the consequence of a mutation that has led to impairment in growth regulation and cell survival in the mutated cells. The affected cells have abnormal and uncontrolled growth patterns, and this is how cancerous change leads to disease.

Mutation in meiosis will also lead to a defect in the genetic code in the gametes. Sometimes this means that affected gametes cannot be fertilized. However, in other cases, fertilization is possible, and the genetic code of the resulting zygote will then carry the defect. This means that every cell of the developing embryo will be affected by the mutation. This may lead to an insignificant or minor change in the function of the adult, such as color blindness, but can also result in severe inherited diseases, such as sickle cell anemia or hemophilia.

Mutation is an important process to understand in the study of pathology. Mutation in mitosis can cause certain physically based congenital diseases and cancer, and mutation of the gametes in meiosis can lead to genetic congenital diseases.

1.1c Cell transport and homeostasis

Cellular transport processes

As previously described, the cell membrane has the property of semi-permeability in which it can allow the passage of some substances but not of others. By this mechanism, the cell takes in nutrients and expels wastes. The membrane can also permit the cell to respond to changes in the outside world. An example of this is the nerve cell, which picks up information from one part of the body (e.g. an image in the eye) and relays this to another part of the body (e.g. to the brain, for recognition of the image). This property of “response to stimuli” is also dependent on the passage of substances across the cell membrane.

Movement of substances within the body, whether inside cells, outside cells or across cell membranes, always involves movement through body fluids. When substances are dissolved or suspended in water they tend to move from an area of high concentration to one of low concentration, and the movement continues until the differences in concentration are evened out as much as is physically possible. This form of movement of substances dissolved or suspended within water is called “diffusion.” For example, when orange squash concentrate is poured into a glass of water, the eventual even mix of orange and water occurs as a result of diffusion.

When a semi-permeable membrane such as the cell membrane separates two regions of fluid of different concentrations, the process of diffusion for small molecules is slowed down but not prevented. For example, if there is a strong salt solution on one side of a membrane and water on the other, the water molecules will cross the membrane via the protein channels to mix with the salt solution. The larger salt molecules cannot use the tiny pathways in the membrane proteins and so cannot make this transition. However, because the water molecules can move, over time the concentrated salt solution will become much more dilute. The process may take some time, but even despite this membrane barrier, as with the orange squash, the result is a much more even concentration of solution on either side of it. This movement of water across a semi-permeable membrane into a region of more concentrated solution is called osmosis (see below).

Diffusion of certain fat-soluble substances can actually occur through the membrane itself rather than through the protein membrane channels. This is because the membrane is made up of an oily substance and permits the free passage of molecules that can dissolve in it. Therefore, because of diffusion, a difference in concentration of oxygen on either side of a semi-permeable membrane will eventually become evened out. This will be as a result of the movement of oxygen between the oily molecules, which make up the membrane, from the more concentrated to the less concentrated side.

Facilitated diffusion

In addition to the protein channels (holes), there are also gates (also made of protein) that allow the passage of slightly larger substances such as glucose and amino acids. Movement of larger molecules via these gates is known as facilitated diffusion. The gates restrict the free passage of these substances, so that this form of diffusion is extremely slow. This means that, if one of these specialized gates is present in a membrane, only a small amount of a large molecule such as glucose dissolved within the cytoplasm of a cell will be able to pass through.

Some molecules, such as proteins, are so large that they cannot pass across the membrane by the process of diffusion. This means that any large molecules contained within a membrane, such as proteins forming the microfilaments in the cytoplasm, will be unable to leave.

In summary, the membrane is permeable to water and fat-soluble molecules, slightly permeable to glucose, and impermeable to large molecules. This illustrates the meaning of “semi-permeable.” Understanding the concept of semi-permeability is a prerequisite for making sense of the process of osmosis.

Osmosis

The process of osmosis in action is evident when limp flowers are placed in some water and the floppy stems become, after a length of time, much sturdier. What is happening in this example is that the water in the vase is moving across the plasma membranes of the plant cells into the cells themselves. This causes the cells to swell, and then the whole stem to regain its firmness. The water is moving into the cells by diffusion, because the cytoplasm contains a concentrated solution of glucose and large molecules. These larger molecules have a tendency to move in the opposite direction out of the cells, but they are unable to do this because of the barrier of the semi-permeable membrane. Therefore, most of the movement is of water passing one way across the membrane, and this, of course, benefits the plant.

In physiology, the term “osmosis” refers to the process of the transfer of water molecules from within a dilute solution with the result that a more concentrated solution on the other side of a semi-permeable membrane is made more dilute.

Osmotic pressure

In osmosis, the tendency of the water is to continue moving until the solutions on each side of a membrane are of equal strength. However, the movement of water into a concentrated solution of large molecules does not always occur indefinitely. Otherwise, in the example of limp flowers placed in water, the cells in the stems of the flowers would eventually burst. What happens is that, eventually, the process slows down to reach a stable state; the plant cells are firm, but not over-full.

The force of osmotic pressure can, in some situations, be strong enough to rupture cells. For example, red blood cells are not supported by fibrous tissue. If a drop of blood is placed in pure water (which is much more dilute than the plasma in which these cells normally circulate), the red blood cells will swell as water molecules move by osmosis into their cytoplasm. Eventually, these cells will burst under the force of the osmotic pressure.

Figure 1.1c-II illustrates what could happen to a red blood cell if it is placed into (a) a solution of the same strength (isotonic) as the fluid in its cytosol, (b) a much more dilute (hypotonic) solution, and (c) a more concentrated (hypertonic) solution. In (a) the movement of water across the cell membrane (indicated by the arrows) is balanced inwards and outwards, and the cell remains the same size. In (b) there is a tendency of a larger volume of the more dilute solution to move into the red blood cell than there is for water to move out. This will cause it to swell; if the solution is very dilute, the red cell might even burst. In (c), the tendency is for more fluid to move out of the cytosol, across the cell membrane and into the surrounding solution; the red blood cell will tend to shrivel as a result.

Figure 1.1c-II The process of osmosis

This powerful effect of osmosis explains why hospital patients are given intravenous saline solution in a drip rather than pure water. Saline is a salt solution made to be precisely the concentration of the cytoplasm of all cells, and thus the cells will neither burst nor shrivel when mixed with it.

Osmosis also explains why the bouncy and firm quality of the skin is lost in cases of dehydration. Lack of water means the blood becomes more concentrated. The cells are bathed in the fluid derived from blood. Therefore, when the body is in a state of dehydration, the fluid within the cells will be less concentrated than the surrounding fluid. Water will then move out of the cells by osmosis. This leaves the whole tissue limp, as each cell has slightly shriveled due to the cellular dehydration caused by the osmosis.

Active transport

These simple processes are at the foundation of all that enables a cell to work as part of a living organism. Transport across membranes is what allows a cell to take in nutrients and excrete waste. It also permits a response to changes in the environment, and therefore communication between cells and the ability of one cell to alter the behavior of another.

Homeostasis

Homeostasis (meaning staying the same) refers to the body’s ability to maintain a state of balance despite changing external circumstances.

Two important concepts in physiology that relate to homeostasis are those of the internal environment and the external environment. The internal environment describes all parts of the body that are contained by a tissue layer, the outside surface of which has a direct connection with the external environment. The external environment includes those parts of the body connected to the outside by orifices. Therefore, parts of the body such as the air passages leading to the lungs, the long space enclosed by the tube of the gut, and the inside of the bladder are all part of the external environment. The external environment is, of course, unpredictable and prone to change in temperature, composition of the air and the presence of other substances such as food and liquids.

The principle of homeostasis is that the internal environment is always maintained at a steady state despite changes in the external environment. For example, our body temperature remains the same whatever the outside weather through homeostasis (except in the case of illness). Homeostatic processes ensure that diverse variables, including the concentration of the blood, the levels of hormones, oxygen, carbon dioxide, blood sugar and mineral salts, the density of the bone and the muscle tone remain controlled within very narrow limits that maximize function and health.

The process of homeostasis is underpinned by a system of negative feedback loops, which involve changes to re-establish balance occurring in response to changes in the internal balance. Such a system requires a “detector” to recognize a move away from balance, a “control center” that recognizes when the move has been so great that something has to be done about it, and an “effector” that brings about a change in the body to reverse the imbalance. A thermostat is a mechanical example of a negative feedback loop in action. The detector in this case is a thermometer. The control center is the mechanism that is set to recognize when the temperature falls outside a desired range. The negative feedback loop is based on the maximum desired temperature. When the temperature becomes higher than this maximum, the effector, in this case the heating element, is switched off until the time when the temperature drops within the desired range once again.

It is clear that in chronic (long-term) illness, and also in aging, these feedback systems no longer work as effectively. In chronic illness the body never quite achieves the ideal internal environment for optimal function. For example, the blood glucose concentration of a person with untreated diabetes is at a level that is too high for the cells to remain undamaged, and in chronic rheumatoid arthritis the inflammatory processes of the body are always too overactive to be wholly beneficial, and damage to cells will also be the result. With this perspective, it becomes easy to understand how being in a state of ill health can lead to progressively worsening ill health. The way to stop this would be to reset in some way the “thermostat” of the feedback loop. However, many conventional treatments for chronic illness cannot do this. Most conventional medicines tend to act on the consequences of the imbalance in homeostasis, and not its underlying cause (which often has its root in chronic exposure of the body to unhealthy lifestyle patterns or adverse environmental influences).

In conclusion, the physiological view is that health depends on a natural tendency of the body to remain in balance. Illness is the result of a failure in this delicate system.

1.1d The tissue types

Specialization

This section explores in more detail how individual cells are specialized to perform different roles, and how specialized cells form the different tissue types.

Single-cell organisms, such as the amoeba, can survive without specialization because all parts of the cell membrane are in contact with the external environment. It is possible for a single-cell organism to obtain the food and oxygen it needs, to excrete waste and to respond to other changes in the environment, simply because all of its boundaries are in touch with the outside world. Specialization is necessary in larger organisms because most cells are not in contact with the external environment. Cells buried deep within a large organism are unable on their own to obtain nutrients and oxygen, or to excrete their waste. Likewise they will be unable to detect changes in the external environment and to respond appropriately.

•the digestive tract is specialized to obtain food for all cells in the body

•the lungs are specialized to obtain oxygen and excrete carbon dioxide for all cells in the body

•the liver and kidney are specialized to excrete other waste for all cells in the body

•the nervous system is specialized to sense changes in the environment for all cells in the body.

Tissue types

There are four recognized categories of specialized tissues – epithelial, connective, muscle and nervous tissue. All these can be subdivided into different tissue subtypes within each category, but this section focuses only on the main characteristics of the four types.

Epithelial tissue

All parts of the body that are in direct contact with the external environment are epithelial in nature, whether they are on the outside surface of the body or form the linings of the cavities and passageways of internal organs, such as the lung. Epithelial cells are specialized to protect, secrete and absorb. These are the functions that would be required of an external lining to the body, which has to perform the role of protection, taking in nutrients from the environment and the excretion of waste.

Epithelial cells are also found lining the heart, blood and lymph vessels. Here they are known as the endothelium, reflecting the fact that these epithelial cells are deep within the body rather than on the surface.

Although each cell that is produced by mitosis in the embryo contains the same genetic code, through the process of differentiation parts of the genetic code are “switched off” and other parts are activated. A cell destined to develop into nerve tissue will have the genetic code relevant to the structure of nerve tissue “switched on” and all of that part of the code relevant to other types of tissue “switched off.” Nevertheless, underlying the diversity of all the differentiated cells of the body a lifelong unity is preserved. All nucleated cells that develop throughout the lifetime of an individual will carry within them the unique DNA blueprint that defined that individual from the point of conception onwards.

1.1e Introduction to pathology and pharmacology

Pathology

The word pathology is derived from the Greek words for disease and study. The word pathogenic has the same root, meaning disease causing. The science of pathology involves understanding the various ways in which the homeostasis of the perfectly functioning body can fall out of balance.

Conventional pathologists understand that disease arises in the body either as a result of endogenous factors or because of adverse environmental conditions.

From a medical perspective, the internal causes of disease include:

•constitutional weakness (inherited disease)

•autoimmune disease

•cancerous change (although some cancers are understood to be triggered by external causes)

•degeneration of age

•negative emotions (the relation of chronic anger, depression and stress to ill health have all been studied in detail).

The external causes of disease include:

•infections

•inadequate diet

•overeating/obesity

•under-exercise

•physical trauma

•excessive heat or cold and dampness

•chemical damage/poisoning (including dietary and recreational factors such as caffeine, tobacco and alcohol) and radiation

•stressful living and working conditions.

Medical approaches to the management of disease

Confusingly, the medical profession also uses the terms primary, secondary and tertiary prevention in a slightly different way, and it is important to be clear about this, as both forms of nomenclature appear in current respected texts. Some medical authors use the term primary prevention to mean the practice of preventing any aspect of a disease from developing. An example of primary prevention used in this sense is the recent UK policy of administering the human papilloma virus (HPV) vaccine to young teenage girls. This practice has the aim of preventing wart virus infection, now strongly associated with cervical cancer, from taking hold with the onset of sexual activity in later teenage years. Secondary prevention is the practice of detecting early treatable markers of disease so that early curative treatment can be instigated. The national cervical screening program in the UK is an example of this, in which all women aged between 25 and 60 are offered regular cervical smear tests to look for early precancerous changes, which can then be treated to prevent the development of cervical cancer. In this form of naming of disease prevention, tertiary prevention refers to the approaches used to deal with established disease. Following on from the earlier examples, tertiary prevention would include the surgical and medical treatment of established cervical cancer.

According to this nomenclature, many of the approaches used by doctors to screen for disease, such as a mammography for breast cancer, blood tests for markers of prostate and ovarian cancer, and examination of the testicles for testicular cancer, are all examples of secondary prevention of disease.

It is currently generally accepted that the greatest impact that modern medicine may have on health is in the realm of the promotion of the general health of a population and the prevention of disease.

Cure of disease

Although prevention of disease may have the most significant impact on health, it is the doctor’s ability to cure diseases that can engender the greatest respect. Moreover, while medical and surgical approaches often have the most dramatic results in terms of cure, it is important to remember that healing modalities requiring less technical expertise, such as the giving of simple advice or the administration of basic nursing care, may be all that is needed to effect a full return to health.

Methods of cure include:

•medication that reverses the process of disease

•physical therapies (e.g. physiotherapy)

•lifestyle advice

•talking therapies (i.e. psychotherapy)

•surgery

•basic needs supplied through hospital care

•treatment of distressing symptoms or disability resulting from disease.

Treatment of symptoms in chronic disease

Most diseases that impact on the health of Western populations are chronic conditions such as coronary heart disease, diabetes, depression, cancer and dementia. These sorts of conditions are not necessarily amenable to cure. In a developed country, the bulk of the cost of medical care is devoted to the management of these chronic conditions and preventing their progression as far as possible. The financial burden on healthcare systems becomes ever greater as the number of people suffering from chronic disease continues to increase (matching growth in population size, adoption of unhealthy lifestyles and longevity), and as the range of evidence-based medical treatments becomes increasingly complex and “high tech.”

Methods of treatment of the symptoms of chronic disease include:

•medication that alleviates symptoms or hinders progression of disease

•surgery

•physical therapies (e.g. physiotherapy)

•advice

•talking therapies (e.g. psychotherapy)

•occupational therapy and rehabilitation

•nursing home/hospital care

•referral to other support agencies (e.g. social services).

Pharmacology

The term pharmacology is derived from the Greek meaning medicine and study. Pharmacology embraces the study of the drugs that may be prescribed for each disease. In addition, it involves understanding the complex way in which each drug interacts with the chemical and cellular processes of the body. As well as defining when a drug might be perceived to be of benefit in the treatment of disease, pharmacologists are interested in predicting possible side effects and contraindications (the situations when a drug should not be prescribed).

This text is written with the aim of making medical information more accessible to health practitioners with a non-medical background or focus. An important skill to acquire is a facility for using medical texts and online resources and for interpreting the language used within them. Very often the information presented by medical textbooks and on websites is organized in a way that follows a predictable pattern. Familiarity with this pattern can greatly assist in the process of extracting important information.

Texts on physiology and pathology

Medical textbooks are generally structured around the physiological systems, and this will also apply to texts on physiology and the pathology of disease. Physiology and pathology are descriptive subjects, and the information found in pathology texts can generally be trusted as established fact derived from scientific observation. For example, a phrase from a pathology textbook such as “…bone is a connective tissue with cells (osteocytes) surrounded by a matrix of collagen fibers…” has its origins in the scientific observation of the microscopic structure of bone. Most readers would probably not dispute such facts, even though they might hold an alternative view of the body based on an energetic foundation of bone. This technical description is simply a way to describe the material nature of healthy bone.

Similarly, most statements about pathology in medical texts are also facts based on the observation of diseased tissue. For example, the sentence, “In Paget’s disease overactive osteoblasts deposit abnormal new bone that is thickened or enlarged or structurally weak,” reflects a process that has been observed using scientific methods, including microscopy.

Texts on clinical medicine

Clinical medicine is the term used to describe the real-life practice of medicine on patients in the setting of the clinic or hospital. Texts on clinical medicine are also generally structured around the physiological systems, and may include some introductory physiology and pathology to introduce the description of clinical medical approaches system by system.

Doctors may use the term general (or internal) medicine more specifically to describe the discipline that aims to treat diseases by means of non-surgical techniques. Drug treatments are a principal therapy in the practice of general medicine defined in this way. For mainly historical reasons, the practice of surgery is considered to be a separate, albeit complementary, discipline to general medicine. Textbooks of general clinical medicine do not, therefore, describe in detail those conditions that are primarily managed by surgeons, for example, orthopedics (surgical conditions of the bones and joints), otorhinolaryngology (ear, nose and throat diseases), obstetrics and gynecology (diseases of pregnancy and women’s health), urology (diseases of the urinary tract) and plastic surgery.