We are constantly surrounded by pathogens and other microbes that we need to defend ourselves against. The first line of defence against infection stops the pathogens from entering the body. These first lines of defence are called non-specific defences as they are general defences, not designed to fight a particular type of pathogen. The skin covers almost all parts of the body and acts as a physical barrier to prevent infection from pathogens.
Skin
If it is cut or grazed, it immediately begins to heal itself, often by forming a scab, which prevents infection:
A blood vessel is damaged, leaving rough edges.
Blood exits the wound. Delicate platelets in the blood are broken open by the rough edges.
Chemicals are released into the blood which causes a series of reactions.
This makes the blood clot and later form a scab.
Parts of the body that do not have skin have developed other ways to prevent infection. For example, the eyes produce tears, which contain enzymes, and these are chemical barriers.
Nose
The nose has internal hairs, which act as a physical barrier to infection. Cells in the nose produce mucus. This traps pathogens before they can enter the lungs. When the nose is blown, mucus is removed and any pathogens are trapped within it. Any remaining mucus, and the trapped pathogens, are broken down by acid in the stomach.
Trachea and bronchi
The trachea runs from the nose towards the lungs. The cells that line the trachea also have hairs called cilia, which are much smaller than those in the nose. These are called ciliated cells. The cells waft their hairs and move mucus and pathogens upwards towards the throat where it is swallowed into the stomach. Other cells called goblet cells create mucus in order to trap pathogens. The production of mucus in the airways is a physical barrier.
Stomach
Stomach acid benfits the digestive system as it helps in the breaking down of food. Additionally, It is part of the body's non-specific first line of defence. It is hydrochloric acid and, while it does us no harm, it is strong enough to kill any pathogens that have been caught in mucus in the airways or consumed in food or water. Stomach acid is a chemical barrier against infection.
Current research into neurotransmitter systems has produced the "________ " hypothesis, which states that when serotonin levels are low, other neurotransmitter systems become dysregulated and contribute to mood irregularities.
It is consistent with Punctuated
equilibrium. A theory that states that when a species show up in the fossil
record they will turn into a stable record, showing almost zero evolutionary
change through their geological history. When the species do change, this
change is rapid and split the species into tow rather than make the original
species evolve into another one.
