Introduction to physiology

The term ‘physiology’ is derived from the Greek words meaning ‘study of nature’, and is used to describe how all living organisms work or function. The study of physiology has to underpin the study of clinical medicine, as it is only through knowledge of how biological processes function in health that the mechanisms and outcomes of disease may be understood. Throughout this text, physiological descriptions of the body systems precede the descriptions of how disease manifests in those systems. The introduction and explanation of the key concepts necessary to understand these descriptions is the purpose of this first chapter.

The study of physiology offers more to the complementary medical practitioner than simply the building blocks for the foundation of the further study of medical sciences. Central to all healing modalities is the development of an understanding and respect for the human body in health and disease. This respect enriches the way in which the practitioner communicates with patients and handles their bodies. It may well, indeed, add to the healing potential of the encounter, as through this respect the patient will feel recognised and understood. Respect comes with knowledge, and although the perspective of many complementary medical modalities is of the human body as more than a mere collection of physical parts, it is very helpful for their practitioners to acquire a sound understanding of the structure and normal functions of these physical parts. Only then can they appreciate in greater detail the importance of the functions of each part, and exactly how all the parts need to work together in a harmonious way in the healthy body viewed as a whole.

An additional benefit of a physiological understanding of the body is that it may also enhance the understanding of the energetic nature of the body parts, as understood by a number of complementary medical systems. Bodily functions are recognised and described in diverse ways, but some elements remain constant. All medical systems recognise major functional details – the lungs take in air, the heart pumps blood, the blood clots and the skin produces sweat. Although the processes of these various functions may be described very differently, an awareness that all disciplines recognise the same summary descriptions can help when translating from the language of one medical discipline to another.

As an illustration of this, in this text physiological concepts are, from time to time, translated into the language of the complementary medical discipline of Chinese medicine. In Chinese medicine the body is understood in terms of constantly interchanging and interdependent forms of Qi, the underlying energetic principle on which that system of medicine is founded. The comparisons with Chinese medicine’s concepts of physiology described in this text clearly illustrate not only the internal consistency of thought in Chinese medicine but also the possibility of translation. This process of translation has the potential to promote increased understanding and respect between conventionally trained practitioners and those complementary medical practitioners who use the language of energetics to describe the philosophical basis of their practice.

The 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).

The physiological systems of the body are listed in Table 1.1a-I. Some of the terms may appear unfamiliar (the more commonly used descriptions follow in brackets). However, as they are fundamental to the organisation of most conventional medical textbooks, it is important that their names and functions are clearly recognised and understood.

Table 1.1a-I The 12 physiological functional systems of the body

Most conventional general medical textbooks are written with chapter headings that refer to these systems. This is because conventional practitioners have learned to classify diseases as they affect each system. The in-depth study of diseases according to a physiological system is known as a ‘medical speciality’ (often spelled speciality). Usually, the term for the speciality is the name of the system together with the suffix ‘-ology’. For example, the medical speciality devoted to diseases of the endocrine system is known as endocrinology (see Q1.1a-1).

Sometimes two or more systems are considered together because their functions are related. For example, the urinary and reproductive systems may be considered together as the urogenital system, and the blood and the cardiovascular system can be considered together as the circulatory system.

Conversely, a system can be broken down into component ‘subsystems’ for study in depth. This leads to terms such as ophthalmology (the study of the eye), otorhinolaryngology (the study of the ear, nose and throat), hepatology (the study of the liver) and cardiology (the study of the heart).

The organs

Each system in the body comprises organs that work together to enable the system to perform its role in the physiology of the whole body (see Q1.1a-2).

The physiological levels

The distinct systems of the body described earlier can in turn be broken down (in some cases) into subsystems, each of which represents a collection of interrelated organs. From a physiological perspective, the body as a collection of functionally integrated physiological systems is considered the most complex level of organisation of the body. The various organs around which the systems function represent the next level of organisation of the body.

To descend further to even less complex levels, organs can be considered in terms of the different tissues of which they are made. Tissues, of which there are four main types, comprise the range of basic living materials with which organs can be built. Finally, the most simple level of organisation is the organisation of the cell. This is viewed as the single building block out of which all tissues, and therefore all organs, systems, and the body itself are made.

The physiological levels are summarised in Figure 1.1a-I.

Figure 1.1a-I • The levels of organisation of the body.

The cell

The cell is the building block that lies at the most simple of the levels of organisation of all plants and animals. It is important to understand how the cell functions in order to make sense of the physiology of those more complex organisational 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. If some tissue from the body is examined under a microscope, the boundaries between cells, the plasma membranes, and also the largest structure within the cell, the nucleus, may be distinguished.

Figures 1.1b-I to 1.1b-IV are diagrammatic representations of how cells in different tissues can appear under the light microscope. These diagrams indicate the wide variety of shapes of cells found in different tissues and how each cell within these tissues contains a large central structure, the nucleus. The nucleus also has a characteristic shape in cells of different tissues.

Figure 1.1b-I • Fibrous tissue, illustrating fibre-generating cells (fibrocytes) and collagen fibres.

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

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

Figure 1.1b-IV • Cardiac muscle tissue, illustrating the fibre-like cardiac muscle cells linked by connecting ‘intercalating discs’.

The electron microscope is a more powerful microscope that has enabled much more detail to be discovered about the structure of the cell. It has revealed that all animal cells have some basic features in common. Although the precise structure of each cell is different, depending upon the tissue in which it is found and the role it has to play, all cells in the human body have these basic features in some form. This makes sense when considering that every cell in the body has originated from the first unique cell formed at the moment of conception.

It can be helpful to use the idea of a generalised cell to study how all cells are made up and function. No single cell will be exactly like this generalised cell, but most cells will have the features portrayed in the generalised cell depicted in Figure 1.1b-V.

Figure 1.1b-V • The structure of a generalised animal cell.

The structure of the cell

Each cell contains a plasma membrane and a number of different internal structures known as the organelles (‘little organs’). 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.

Each cell is bounded by an oily fluid plasma membrane, otherwise known as the cytoplasm. Cytoplasm is a term used to describe all the fluid contents of the cell, with the exception of the nucleus.

The plasma membrane acts as the link between the cell and the outside world. Large molecules, called proteins, in the membrane make the cell unique (they give the cell its ‘immunological identity’), and respond to various chemicals in the cell’s environment to bring about changes within the cytoplasm. In addition, the membrane allows the passage of nutrients into the cell and waste out of the cell (transport across the membrane) but is able to retain essential fluids and substances within the cell. This important feature of cell membranes, known as ‘semipermeability’ is described in Chapter 1.1c.

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

The other large rounded structures in the cytosol are the numerous cigar-shaped mitochondria, which utilise oxygen to break down the basic nutrients obtained from carbohydrates and fats (and sometimes proteins) to form an energised 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, on which the dense ribosomes are situated. Where the ribosomes are numerous the endoplasmic reticulum 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 vesicles can travel to the plasma membrane, so that proteins can be released into the outside environment when necessary.

Microfilaments and microtubules are fibres that can contract and cause the movement of substances from one part of the cell to another. These fibres, also known as the cytoskeleton, maintain the structure of the cell and link the various organelles (see Q1.1b-1-Q1.1b-3).

The essential information about the ‘generalised’ cell is listed in Table 1.1b-I.

Table 1.1b-I The important characteristics of the cell

Cell replication

The replication of cells is a fundamental process within the body that begins with the first division of the fertilised egg (zygote). It is essential both for growth and for the repair of ageing and damaged tissues. There are two ways in which cells can divide, known as mitosis and meiosis. Mitosis is the process whereby a single cell divides into two identical cells following the replication and separation of the chromosomes in the nucleus. Meiosis is the process whereby a single cell divides into daughter cells, each of which carries exactly half of the genetic material of the parent cell.

Mitosis and meiosis are illustrated in Figure 1.1b-VI. In this diagram, for the sake of clarity, only a single pair of the human cell’s complement of 23 chromosomal pairs is illustrated. The diagram shows how mitosis results in two identical daughter cells, whereas meiosis results in the production of four genetically unique daughter ‘half-cells’ or gametes.

Figure 1.1b-VI • The stages of mitosis.

Mitosis reproduces the parent cell for the purposes of the growth of the tissue or the repair of damaged tissue, while meiosis produces the reproductive cells (spermatozoa and ova), also known as gametes. When the gamete is fertilised, the resulting zygote (the very first cell, which will later divide to form the embryo) will have a unique combination of genetic material, half from the mother and half from the father. It is the process of meiosis that leads to the uniqueness of every human being (see Q1.1b-4 and Q1.1b-5).

Both meiosis and mitosis are involved in the development of the fertilised 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 (spermatozoan) and female gamete (egg or ovum) meet, they fuse through the process of fertilisation. 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 disturbed function 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. The thalidomide tragedy resulted from the use of a drug developed in the early 1960s as an anti-nausea medication in early pregnancy. It was later found that this drug causes mutations in the embryo. The mutation caused by thalidomide taken in the first trimester of pregnancy affected embryonic cells that were destined to develop into limbs. A mutation in mitosis can also lead to cancerous change in the daughter cells and can occur at any stage of life. This means that the mutated cells have abnormal and uncontrolled growth patterns, and can take over and destroy neighbouring tissues.

Mutation in meiosis may result in gametes that carry a defect in the genetic code formed by the DNA. Sometimes this means that the gametes cannot be fertilised. In some cases of mutation of the gamete, fertilisation is possible and the DNA of the resulting zygote will also carry the defect. This will mean that every cell of the developing embryo will also carry the mutation. This may lead to an insignificant or minor change in the function of the adult, such as colour blindness, but can also result in very severe disease, such as sickle cell anaemia or haemophilia (see Q1.1b-6).

Mutation is an important process to understand in the study of pathology. Mutation in mitosis can cause cancer, and mutation of the gametes and the embryo is the cause of congenital disease.

Self-Test 1.1b The cell

1. What is the name of the cell part (organelle) that is responsible for producing energy?

2. Name two main internal cell parts that are composed almost entirely of membrane. How are their functions linked?

3.

(i) What ingredients does the cell require to produce energy?

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