The intracellular fluid consists of large amounts of potassium, magnesium and phosphate ions, the extracellular fluid of large amounts of sodium and chloride ions. These differences in ion concentrations is of great importance for the electronical phenomena of the cell membrane.
The cell membrane is thin and elastic. The thickness is in the order of 7.5 - 10 nm. It consists of proteins (62\%), lipids (35\%) and polysacharides (3\%). The lipids consist of phospholipids (60\%), cholesterol (25\%) and other molecules (15\%). The exact structure of the cell membrane is not known. It is assumed that the membrane has pores. Small molecules with a diameter less than 0.8 nm can diffuse freely through the membrane (diameter water molecule $\pm$ 0.3 nm, ureum $\pm$ 0.36 nm, sucrose $\pm$ 1.04 nm).
There are many different kinds of cells. Red blood cells are circulating in the blood through the vessel network and they have a biconcave (donut) shape with a diameter of 7 $\mu$m and a thickness of 2$\mu$m. They play an important part in transporting O2 and CO2 via the blood stream. Our muscles (skeleton muscles, smooth muscles, heart muscle) consist of muscle cells having a shortening mechanism. During relaxation the heart muscle cells have a cylindrical shape, with a length of 40 mm and a diameter of approximately 10 mm. Skeleton muscle cells can reach a length of millimeters. Nerve cells have totally different specific features. They have a cell body with dimensions around 10 - 50 mm. The cell bodies have an axon that can have a length of 0.5 meter in order to transmit signals over a long distance.
In spite of the large differences in structure of the various types of cells, there are also many similarities. These will not be described here, but reading relevant chapters in physiology books is recommended. The structure of muscle cells will be discussed in detail in Chapter .
The similarities between the different cell types that are of importance for this course are:
Cells use the same nutrients.
Oxygen is one of the most important substances used to obtain energy necessary for cell functions. O2 is bound to carbohydrates, fats and proteins.
CO2 is an important waste product that is released during this oxidative reaction.
products of this chemical reaction are removed to the extracellular space
Organs are conglomerates, usually of more than one cell type. To refresh our orientation of the internal human body, the position and function of the organs in the chest and abdomen will be described. Both cavities are divided by the diaphagm, a flat muscle that is also of importance for breathing. The only function of the diaphragm is that it provides the passageway for the vena cava inferior (see Chapter~\ref{ch:circulatie}), the aorta, the oesophagus and a large lymph vessel that removes lymphe from the bottom part of the body (ductus thoracicus).
The chest cavity contains the heart and the lungs. The main function of the heart is to pump blood through the circulatory system (see Chapter~\ref{ch:circulatie} and~\ref{ch:pomp}) and the lungs take care of the exchange of O2 and CO2 between open air (milieu exterieur) and blood (see Chapter~\ref{ch:ademhaling}).
The organs in the abdominal cavity are primarily of importance for:
the digestion of nutrients,
clearing the blood of waste products and
removing waste products from the body or breaking them down.
Food is stored in the stomach and that is where the digestion starts. The food is kneaded by the stomach wall and due to the high acidity of the gastric juices (pH = 4) the proteins in this food are broken down. The resulting chyme is then slowly transported via a sphincter (pylorus) to the small intestine where bile is added that is produced in the liver and transported via the bile ducts. In the small intestine the digestion of food is completed. The remaining material which has not been absorbed by the blood or lymphe, is transported to the large bowel or colon. The main function of the colon is the absorption of water (approx. 500 ml/day) and other materials from the chyme, and the production of stools or faeces (approx. 250 g/day). The blood absorbs nutrients from the intestinal tract and passes first through the liver. Substances that are toxic to the body are made harmless here and carbohydrates and sugars are stored.
The liver perfoms many other complicated functions and is of main importance to almost all the facets that have to do with the chemistry of our body. Plasma-proteins, ureum and a large number of other chemical compounds are produced by the liver. It also produces bile which is discharged into the duodenum when needed for digestion.
Controlling the volume of bodily liquids and making sure that they have the right composition is one of the major functions of the kidneys. They produce urine with which metabolic waste products (ureum, creatine), electrolytes and other waste products are removed from the body. In spite of their small size (approx 10 * 5 * 5 cm), kidneys can process approx. 1.5 ltr. blood/minute.
The spleen is part of the lymphatic defence system. Its main task is to remove old red blood cells from the blood. Furthermore, the spleen produces certain blood cells such as lymfocytes, monocytes, antibodies and in children red blood cells.
Without bones, we would be a shapeless mound of flesh, but a bag of bones can stand upright. Therefore, one of the main functions of the skeleton is to give structure and shape to the body. Without the skeleton, we would be very vulnerable. The skeleton offers protection to the vital organs, blood vessels and the nervous system. Furthermore, the skeleton constitutes a frame to which muscles and tendons can be attached. The vertebral column gives structure to the upper part of the body and supports the head. The vertebral column also protects the large bundles of nerve fibres running through it.
From in between the separate vertebrae bundles of nerve fibres go to all parts of the body. The vertebral column is firm, but yet flexible because of the intervertebral discs of fibrocartilage between the vertebrae. We distinguish 7 cervical vertebrae, 12 toracic vertebrae and 5 lumbar vertebrae. The vertebral column rests on the pelvis by means of the sacroiliac joint. Fusion of this joint is often the cause of low back pain.
The skeleton is not stiff. It can rotate and stretch and its hinged parts allow many movements. We can distinguish between the so-called monoaxial joints (elbow, fingers), biaxial joints (wrist) and triaxial joints (shoulder, hip). The capacity to move one's extremities (arms-hands, legs-feet) is increased because of the bifurcations of the bones. Each hand consists of 27 small bones and each foot of 26 small bones. The body as a whole consists of 206 different bones, of which more than 50\% is located in the hands and the feet.
Diversen:
Koolhyd. 0.3 kg
Na 0.06
K 0.15
Mg 0.02
Fe 0.003
I 0.014 e-3
Cl 0.08
P 0.6
S 0.11
Table~\ref{tab:water} shows the routes by which water leaves the body under three different conditions: normal temperatures, hot weather and prolonged heavy exercise. The insensible loss of water through the skin refers not to sweat but to diffusion. The loss of water through the lungs takes place via evaporation. If the circumstances of high water loss continue, this has of course to be compensated for by drinking. Note that during prolonged heavy exercise the water loss via the urine is much less than it is under hot weather conditions, whereas the sweat production is much higher.
In hot weather water loss via the lungs is reduced, because of the high atmospheric humidity, whereas during heavy exercise it increases, because the respiration frequency goes up. Figure~\ref{Fig:Massabalans} gives an outline of the mass balance of water and solid food. The water balance has already been described. The balance of solid food is self-evident. Note that the total weight of $O_2$ of the daily intake and the total weight of $CO_{2}$ that we exhale, are in the same order of magnitude as the amount of solid food that is consumed.
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