Physical and Chemical Defences
The skin must be the best example of a physical barrier. It consists of a thick layer of epithelial cells, with a dead and keratinised (or reinforced) surface. It is certainly a hostile environment for viral or bacterial multiplication.
Should the skin get very dirty however, or if it is broken by physical damage, then bacteria may gain entry, the infection may become established and pus or abscesses may form. Bacteria retard healing and this is why wounds and cuts should always be cleaned and washed with antiseptic, thus preventing the bacteria from multiplying. Pus is an accumulation of dead white blood cells, dead cells and fluid from animal tissue and bacteria. Skin is also covered by a film of fatty acids which help to prevent bacterial multiplication. Excessive washing, especially with detergents, removes these acids and thus renders the skin more susceptible to infection. This is of particular relevance to teats, and is one reason why chapping is so common unless emollients are added to the teat dip.The air passages (trachea, bronchi, etc.) and the intestine can be termed external surfaces because they come into contact with materials (air and food) from outside the animal’s body. In the nose there are hairs which prevent large particles from being inhaled into the lungs. Their function is supported by a microscopic layer of cilia. These small, finger-like projections, which line the surface of the trachea, move in a wave motion to propel bacteria and other smaller particles back up towards the mouth, where they can be swallowed or coughed away. In addition mucus glands produce a sticky secretion to line the airways, thus trapping any bacteria or viruses which happen to land and this prevents them from reaching the susceptible tissues of the lungs. These mechanisms are described in detail in Chapter 3.
The mouth and oesophagus have a thick horny (keratinised) lining, like skin, and this helps to prevent bacterial penetration.
The stomach, on the other hand, produces mucus and acid, partly to assist digestion, but also helping to prevent bacterial growth, whereas the upper small intestine is very alkaline, again inhibiting bacterial growth. These extremes of acid and alkaline conditions should perhaps be considered as chemical rather than physical defence mechanisms. Vaginal secretions are also acid.The eye has some interesting and rather unique defences. The eyelids close rapidly when an object is approaching, and this protects the eyeball from physical damage. If a foreign body does land on the eye however, tears are produced to wash it away and rapid blinking helps to move the object to the corner of the eye where it will cause less damage. If the surface of the eye does become damaged, blood vessels grow across the cornea to supply antibodies and rebuilding materials. This is known as pannus formation, and is described in more detail in Chapter 4.
The final type of physical defence is provided by the bacteria which normally live in and on the animal as ‘commensals’, that is they live there without causing disease. However, they compete with disease-causing (pathogenic) infections for both nutrients and space. If these normal microbe populations are disturbed, for example by a prolonged course of antibiotics by mouth, it is possible that the more serious pathogenic infections may proliferate and cause disease. This is why it is often recommended that yoghurt or other probiotics are given at the end of a course of calf scour treatment - to recolonise the gut with ‘healthy’ bacteria.
More on the topic Physical and Chemical Defences:
- Other Chemical and Physical Aspects of Culture
- In the previous section, we saw how nutrients undergo biological, chemical, and physical transformations as they are taken up by organisms and released through decomposition, ultimately returning to their original forms (or similar ones).
- CONCEPT 22.2 Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients.
- TEAT AND UDDER DEFENCES AGAINST MASTITIS
- Werner Reiss, author of the most detailed recent discussion on the subject of violence in the Greek world, defined violence as ‘a physical act', stating further that it is a ‘process in which a human being inflicts harm on another human being via physical strength’.1
- Chemical and Photochemical Reactions in Atmosphere
- The Chemical Environment
- Microorganisms modify the chemical form of nutrients
- Physical Injury
- The generation of chemical energy by autotrophs, known as primary production, is derived from the uptake of carbon during photosynthesis and chemosynthesis (see Chapter 5).
- CONCEPT 2.6 Salinity, acidity, and oxygen concentrations are major determinants of the chemical environment.
- PARTICIPATION IN PHYSICAL ACTIVITY