It is easy to know the bits on the outside of your body, but most people don’t know where their thymus, or kidneys or lungs are, or what these organs do.

Heart – Your heart sits between the two lungs. The heart muscles continually circulate blood around your body. You know your heart is working because you can feel your heart beat and you can feel the blood at your pulse. The heart pumps blood containing new oxygen to every part of your body. At the same time it pumps the old blood without oxygen back through the lungs where is picks up new oxygen to repeat this cycle.

Lungs – your lungs are sponge-like organs. Every time you breathe they filter oxygen from the air through tiny vessels into the blood. It is then carried to the heart to be pumped round your body. The lungs filter carbon dioxide from your body when you breathe out.

Liver – your liver is the organ below the lungs that acts like a filter for the blood. Chemicals and impurities – such as drugs and medications – are filtered out by the liver. Other important activities occur in the liver including making and processing of many body fats. The liver is the only internal organ that can regrow.

Kidneys – the kidneys also act like filters. Some drugs are filtered more by the kidneys than the liver. Waste products filtered by the kidneys leave the body as urine. You have two kidneys and they are at the back of your body. Any blockage to your kidneys is extremely painful and can cause permanent damage. Although you are born with two kidneys, many people survive very well with just one.

Stomach and intestines – Your stomach is where food, drink and oral medications start to be broken down and processed in the body. Nutrients and drugs are absorbed through the stomach and small intestine walls. The small intestines are about five metres long. The large intestines are about 1.5 metres long.

Thymus – This is a small gland high in the chest where CD4 cells and other white blood cells develop. CD4 cells are sometimes called T cells (from ‘thymus’). The thymus is very active in children and adolescents, and becomes much less active as you grow older.

Pancreas – your pancreas is a pistol shaped gland below the liver. It releases digestive enzymes into the small intestine and hormones that control sugar levels in your blood. You can live without a pancreas but you need to take insulin to regulate blood sugar levels and take supplementary digestive enzymes.

Skin – your skin is the largest organ in the body and makes up 16% of an average body weight. It stops your body from drying out and is the main barrier against infection.

Bone – your bones are a living material and about 10% of bone cells die and are replaced each year. If bone cells are not replaced quickly enough, bones becomes brittle and break more easily. This means that over ten years you have grown and replaced your whole set of bones. A bit like a snake shedding it’s skin.

Bone marrow – Bone marrow is the soft tissue inside bones. Blood cells originally come from bone marrow.

Blood – Blood is the fluid pumped by your heart. It delivers oxygen and nutrients to every part of your body and carries waste products away. Blood contains cells (red cells, white cells, platelets etc) and plasma.

Plasma – The fluid part of blood that contains nutrients, sugars, proteins, minerals, enzymes, and other substances but with the blood cells taken out.

Lymph – Lymph is a clear fluid that contains white blood cells and antibodies. It is distributed round your body through a series of lymph vessels, nodes, and organs. The lymph system supports the blood in removing waste products from the body.

Although a lot of information about your health and HIV comes from blood tests, less than 2% of the HIV in your body is in your blood. Some researchers think this may be less than 1%. Most of the other 98% is in the lymph system, including the gut.

Lymph nodes – Lymph nodes are the little lumps that sometimes get enlarged in your neck, under your arms, and in the crease between your legs and your body. Most of the CD4 cells in your body rest and reproduce in your lymph nodes.

Further reading and web resources

There are hundreds of sites on the internet that explain basic biology, immunology and other medical terms.

The following sites may be useful on anatomy:

In the US, a 39-year-old man on death row donated his body to science. After he was executed, his body was frozen, cut into one-millimetre-thick slices, and photographed. The data were made available in 1994 on the Internet by the US National Library of Medicine.

To view two- and three-dimensional representations of the human body based on these data, visit these sites:

http://www.nlm.nih.gov/research/visible/visible_human.html

BodyQuest – a site designed to explain human anatomy to students ages 11–16. Try starting with the tour, which gives you an overview of the human body and allows you to find more detailed information:

http://library.thinkquest.org/10348/?tqskip1=1&tqtime=0326

The Atlas of the Body – An interactive exploration of muscles, internal organs, and skeleton of the human body from the American Medical Association.

http://www.ama-assn.org/ama/pub/category/7140.html