THE HUMAN BODY

Even in a clean and spotless place, there are probably a few germs. These microscopic living things,
such as bacteria and viruses, get on to our skin, into the food we eat, the drinks we consume and
even the air we breathe. They may also get into our body through a cut or wound.

If germs get into the body, they can start to multiply and cause problems. 

This is an infection.

But the body has several sets of defenses against germs. 

These include the skin, the moist germ-trapping linings of the breathing and digestive passageways,
the way blood clots to seal wounds and leaks, white cells and other substances in the blood,
 the thymus gland in the chest, and small lymph nodes or glands spread all over the body. 

Together, all these parts form the body's immune defense system.

The body's immune system includes several kinds of white cells in blood, body fluids, and lymph nodes.
 These white cells attack any germs that are in the body.

When the body is ill with an infection, various glands swell up. Many of these are lymph nodes.
When you are healthy they are about the size of a pea or grape, but during 
illness they can be as big as golf balls.

Lymph nodes contain billions of white cells, multiplying rapidly to fight the invading germs.

 During illness they fill with millions of extra white cells and also dead germs.

 

Tonsils and Adenoids

The tonsils are patches of lymph tissue at the upper rear part of the throat. They help to destroy foreign substances that are breathed in or swallowed. The adenoids are similar patches at the rear of the 
nasal cavity in the nose.

Thymus

The thymus gland in the front of the chest is large during childhood, but shrinks away during adulthood.
 It helps certain white cells of the immune system to develop and play their part in the body's defenses.

Spleen

The spleen is just behind the stomach on the left side.

 It makes and stores various kinds of white cells, especially the phagocytes that "eat" germs. It also makes and stores red cells for the blood, and 
generally cleans and filters blood.

Cuts and Clots

A wound or cut in the skin leaks blood from the damaged blood vessels (1).
Chemicals released from damaged cells and platelets make dissolved substances in the blood turn into a meshwork of microfibers, of the substance fibrin. This network traps blood cells (2).
Gradually the meshwork hardens into a clump or clot that seals the leak. The clot then hardens and dries further into a protective scab (3).
White cells arrive to attack any germs, and the skin begins to regrow and heal.


A typical person has around 4-5 litres of blood. The blood is the transport system by which oxygen and nutrients reach the body's cells, and waste materials are carried away. In addition, blood carries substances called hormones, which control body processes, and antibodies to fight invading germs. The heart, a muscular organ, positioned behind the ribcage and between the lungs, is the pump that keeps this transport system moving.


Your heart is about the size of your clenched fist. It has thick muscular
 walls and is  divided into two pumps. Each pump has two chambers. 

The upper, smaller, thin-walled atrium receives blood coming in from the veins. 

The blood flows through a one-way valve, which makes sure it always moves in the correct direction, into
 the larger, lower chamber called
 the ventricle. 

It has thick strong walls that contract to squeeze blood through another valve,
 out into the arteries.

Two-part Circulation

The body's circulation has two parts, with the heart acting as a double pump. 

Blood from the right side pump is dark red (bluish) and low in oxygen. 

It travels along pulmonary arteries 
to the lungs where it receives 
fresh supplies of oxygen and 
becomes bright red. 

It flows along pulmonary veins back to the heart's left side pump.


Blood leaves the left side of the heart and travels through arteries which gradually divide into capillaries. 

In the capillaries, food and oxygen are released to the body cells, and carbon dioxide and other waste products are returned to the bloodstream.

The blood then travels in veins back to the right side of the heart, and the whole process begins again.

(more is planned = as time permits)