The body has many levels of organisation that give rise to us

Multiple levels of organisation within you

Our humanity, and indeed our human body through which this and our personhood is expressed is often taken for granted. We accept ourselves as a unit, but of course there are layers of organisation that allow us to be. This article will start to look at some of the organisation and structures which combine to form our bodies.

The body has been studied in pre-scientific eras and in the current scientific paradigm for about the past 400 years. This had led to a fairly unified approach to the study, and it is useful to be familiar with the conventional angles of approach, it is therefore useful to define a few terms.

Physiology is the study of the normal function of a body or indeed any biological system, in this case the human body. Anatomy is the study of normal structure. Inevitably, in order to understand physiology, we must also learn some anatomy. Histology is the study of tissues and cytology the study of cells. Biochemistry is the study of the chemistry of living things; most of this chemistry takes place within individual cells.

Interestingly, much of our understanding of the body comes from when things go wrong. Pathology is the study of abnormal anatomy and pathophysiology means the study of abnormal body function. Histopathology studies abnormalities found in tissues. While psychology studies the normal processes of the mind, psychiatry is the study of the abnormal mind.

Cells, tissues and organisation

The body is a remarkably complex structure which performs thousands of physiological functions. This means that the structures of the body must be organised in a precise way to carry out these functions. The functional systems of the body, such as the nervous or digestive systems, are themselves made up of organs such as the brain or stomach. Organs in turn are composed of precise arrangements of groups of tissues.

A tissue is a group of similar cells, often with associated additional structural material produced and secreted by the tissue cells. Cells which compose a particular tissue are themselves composed of smaller functional units called cell organelles. These organelles can be thought of as the ‘organs’ of a cell as they perform specialised intracellular functions. Cell organelles are made up of highly structured biomolecules such as proteins, carbohydrates and fats. Large biological molecules are made up of smaller organic compounds. (Organic simply means that the molecule contains some carbon.) The organic molecules are composed of structured arrangements of atoms, which are composed of protons, neutrons and electrons. Below this level is the sub-atomic, quantum world, but that’s another topic.

Specialised cells

During growth and development cells differentiate into specific types which allows them to perform particular functions. This differentiation is initiated and regulated by genetic instructions from the genes which carry all of the required information. Different cells must have a specialised structure which results in the production of many completely different types of cell. It is this process of cell specialisation which is referred to as differentiation. Specialised cells are required to form the different types of tissue needed to construct the large structures and organs of the body. Different types of cells include nerve cells or neurones, muscle cells, liver cells, blood cells and epithelial cells. Each cell has a structure and function specific to the role that a cell or tissue is required to perform in the body. Groups of similar cells compose tissues. Groups of tissues compose organs and groups of organs compose the systems of the body.

Cells and enzymes

A cell is essentially a very complex chemical and molecular machine. A wide range of biochemistry is going on in the cell throughout its life span. In order to control the function of the cell it is therefore necessary to control the chemistry of the cell. All intracellular biochemistry is controlled by enzymes. An enzyme is a biochemical catalyst. A catalyst is something which facilitates or speeds up the rate of a chemical reaction without being used up in the reaction itself. The names of enzymes usually end in ‘ase’, for example, lipase, amylase, creatine kinase and alcohol dehydrogenase. Each chemical process is catalysed by a specific enzyme. If this enzyme is absent, the particular chemical reaction cannot proceed. Each enzyme is synthesised from the genetic information carried by a specific gene. Enzymes are complex proteins formed into a particular shape; it is because of this complexity that they require specific conditions in order to function. For example, enzymes are specific to a particular range of temperature and pH.

A rise in body temperature of one or two degrees is enough to make a person feel very unwell and a rise of six or seven degrees can be life threatening. Even relatively small changes to blood pH are life threatening. If body temperature rises beyond certain levels or there is a change in pH, the physical shape of some enzymes will be altered. As a result, they will no longer be able to efficiently catalyse some cell biochemistry; this in turn will lead to interference with the physiology of the cell, and may lead to cell injury or cell death.

Metabolism

There are basically two classifications of biochemical reactions taking place within cells. Some reactions build smaller molecules up into bigger ones; these are referred to as anabolic reactions. For example, individual amino acid units can be chemically combined to produce a large protein molecule. Other reactions break larger molecules down into smaller ones, these are catabolic reactions.

For example, fatty acids and glucose are broken down in the mitochondria into water and carbon dioxide. Metabolism is a term which describes all of the chemical reactions occurring within the cells, including anabolism and catabolism.

Cells, tissues and fluids

Water is often described as the living matrix, living processes are always ‘watery’ to some extent and take place in a hydrated environment. However, it’s not just water, but precisely the right amount of water, in just the right place at the right time. This is why water levels in various compartments of the body as precisely regulated.

In an average adult body, there is approximately 42 litres of water, comprising around 60% of body weight. Most water, normally around 28 litres, is found inside the cells which comprise the body. Although in reality this water is located in billions of individual cells it is collectively referred to as the intracellular compartment. The remaining 14 litres of the fluid in the body is located outside the cells so is termed extracellular fluid. Total blood volume in an adult is usually about 5 litres. This is located in the heart and various blood vessels, a space collectively referred to as the vascular compartment.

However, only 3 litres of the blood volume is made up of water, the other 2 litres are blood cells. This leaves approximately 11 litres of extracellular fluid which is located in the tissues. This tissue or interstitial fluid is located in the interstitial compartment which in reality is in all of the tissue spaces of the body.

Importance of fluid compartmentalisation

If you go out for a long walk on a hot day you could lose a litre or more of fluid as sweat. If you then lose your way home you could lose a further litre. This will mean you become thirsty but you will not die. Sweat is produced from water in the plasma by sweat glands. This means that during your walk, 2 litres of fluid was removed from the 5 litres of blood circulating in the body. If you lose 2 litres of blood in haemorrhage death can be the result; however, at the end of the walk your blood pressure and blood volumes will still be normal.

As a result, the fluid in the blood has been replaced despite the fact that you did not have access to a drink. Blood contains large protein molecules called plasma proteins. These molecules give the blood its osmolarity, i.e. they make it osmotic so water will diffuse into it. When water is lost from the blood to produce sweat there is less water left so blood osmolarity increases as the blood becomes more concentrated and less watery. The result of this is that water diffuses through the semipermeable membranes of the capillaries from the tissue fluid by the process of osmosis. This movement of water restores the blood volume to normal. However, if more water is sucked from the tissue fluid into the blood, the tissue fluid will, in turn, also become more osmotic. This will result in water moving from the intracellular compartment through the semi-permeable cell membranes into the interstitial compartment to maintain tissue fluid volumes. Tissue fluid does not contain proteins like plasma but osmolarity is generated by the presence of sodium (remember salt is sodium chloride).

(Perhaps you have seen the inexcusable wretched pictures of severely malnourished children with swollen tummies. This is because they are protein deficient, so their livers are unable to produce the essential proteins that are normally in the blood plasma to generate osmotic potential. Without the osmotic proteins in the blood the water is no longer osmotically sucked back into the blood so pools in the tissue spaces leading to the swelling called edema.)

So in the healthy situation, the compartmentalisation of body fluids means there is a large fluid reserve to maintain blood volumes during periods of water loss when drinks are not available. Blood volumes will only be reduced when dehydration is severe.

All of this physiology means we can keep walking until we get to the next well or oasis. This is how physiology can often be understood, as a series of mechanisms which promote our survival.

The wonder of this fluid regulating example is that the hierarchy of cells, tissues, blood vessels and tissue spaces is needed anyway. Cells are the units of life which must be supported by the circulatory system. It’s as if the body is using what is there already to provide an additional, but essential extra layer of physiological function. Having re-explained this organisation and the emergent physiology, I can see it all makes sense and I understand it well, but yet get this feeling I see so little and actually understand even less.

Dr. Lorimer Campbell

Written by

I work in the emergency department of a local hospital. For 27 year I was an academic teaching nurses and carrying out research.

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