The body systems can be seen along a spectrum of scale:

body <-> systems (eg digestive) <-> organs (eg stomach) <-> tissue (eg stomach wall) <-> cell <-> molecule

Working up the scale from the molecular level, chemical behaviour and interaction are the basic foundation for increasing levels of complexity in the organism's functions.

The main constituents of the body and of food are:

Carbon, which is in all organic chemical compounds and is used as an integral part of most body functions.

Hydrogen, which is a principal constituent of water, H2O. Water makes up about 70% of the body.

Oxygen, also used in water and essential for respiration and burning of fuel i.e conversion of sugar (chemical energy) to movement (kinetic energy) and heat (energy).

These three elements are the constituents of all carbohydrates which are the source of energy for the body. Types of carbohydrates include sugars (glucose, fructose, sucrose, lactose etc), starch (found in plants and converted to simpler sugars) and glycogen (an animal starch that is an intermediary between fat and sugar).

Fats or oils (lipids) are carbohydrates with fatty acids. Fatty acids are very important because they repel water and are thus used in keeping things separate from their watery surroundings. Steroids are also a type of lipid for specialised function; they include some hormones, vitamins, cholesterol.

Proteins are highly complex organic compounds formed by amino acids. Of the 20 amino acids 10 are essential in that they can not be made by the body from simple carbohydrates and therefore must be in a diet. Proteins include enzymes (catalysts for the body's chemical reactions), DNA and certain specialised proteins like haemoglobin (for carrying oxygen in the blood and which has a very similar structure to chlorophyll the plant compound used in photosynthesis - the conversion of the sun's energy to chemical energy) and collagen (an elastic structural protein.

Body Shape

The basic shape of the body is a tube, through which food passes. Thus, food digested in the intestine is actually outside the body, which can help with keeping bad stuff, like coins and small bits of plastic, out of the body.  Unsuitable food is never actually taken into the body; if a stone is swallowed it does not get into the body but passes through the tube and out the other end!Nutition is taken into the body through the digestive tract.


The skelton is a tough flexible frame made of 206 bones. It gives the body its basic shape and protects internal organs from pressure. As well as giving shape, the joints in the skeleton allow free movement.

The skeleton is divided in to 2 parts: the axial skelton (head, spine and ribs) protecting brain, heart, lungs, digestive system etc and the appendicular (hanging) skeleton consisting of arms and legs and designed to allow free movement.

The skull is made of 22 bones of which 21 are permanently locked together to provide a rigid protection to the brain. The 22nd is the lower jaw bone which is hinged to allow eating and talking. When born the skull is not completely hardened or joined. During birth the skull is squeezed as it passes out from the mother's womb. Then as the person (mammal) grows the skull expand very quickly in the first few months to allow rapid growth of the brain.

The backbone is made of 24 very strong vertebrae to protect the spinal cord (which carries the nerves from around the body to the brain) from damage and has discs of cartilage between the vertebrae to allow flexibility.

The appendicular skeleton has 126 bones of which 106 are in the hands and feet! It is the flexibility offered by this design in hands and feet that allows homo sapiens (humans) to use tools in a sophisticated manner, eg painting, climbing, building circuit boards, preparing pastry etc. There are 14 knuckles in each hand.

Joints in the skeleton allow movement. Joints have additional features such as cartilage and synovial fluid to lubricate joints and act as shock absorbers.

Imbalanced diets can result in deterioration of both bones (beri beri) and joints (rheumatism, arthritis) resulting in reduced mobility.


The skin is the largest organ in the body. It in a protective barrier protecting internal organs from changes in moisture, temperature, light etc. It helps maintain a stable environment within the body for the complex chemical reactions taking place.

The principal activities of the skin are to feel, sweat and to produce oil. Feeling is aided by hairs (filaments of dead cells) covering the body which react to slight pressure. Sweat is used to cool the body or remove toxins. Oil is used to maintain the waterproof quality of the skin.


Muscles are the where chemical energy (in sugars) are converted to heat energy and kinetic energy (movement). Energy is released when sugar is converted (burned) with oxygen, a process called aerobic respiration. If there is insufficient oxygen, usually because the body is not fit enough to bring oxygen from lungs to muscle fast enough, anaerobic respiration (without oxygen) may take place. This releases only about 8% as much energy and has a by product of lactic acid.

Muscles are either involuntary (eg heart, digestive tract) or voluntary (eg legs and arms) and there are three main types: skeletal (voluntary, attached to the skeleton), cardiac (the heart muscle, must operate permananetly), smooth muscle (involuntary).


Nerves are the internet of the body. Nerves are made of neurons (nerve cells), which transmit electrical impulses around the body to communicate between different organs. Three main type of neuron are sensory neurons (for receiving and transmitting sensation eg touch), motor neurons (for stimulating voluntary (somatic) and involuntary (autonomic) activity in other organs), and association neurons (for communicating with one another eg brain).

The central nervous system is the main part of the nervous system and is made of the spinal cord and brain. The spinal cord is the main pipeline of nerves in the body and is an extension of the brain. It is protected by spinal vertebrae. If it is severed, the body loses the ability to communicate between brain and other organs.

The brain itself is a network of 1,000,000,000,000 (1,000 billion) neurons which continually change shape as part of the learning process. When working hard the brain can use 20% of the oxygen consumed by the body and produces enough electricity to power a small light bulb!


The drawings show the many folds of the cerebrum which allow for a large surface area to volume ratio. The grey matter, where most activity takes place is on the outside of the folds. The white matter is the network of neurons connecting the grey matter. The white matter is the brains internet connecting billions of neurons (computers) which constantly communicate with each other.

Side Section

and Front section


The diagram below shows where various brain activities are located in the cerebral cortex. The thinking part which is highly developed in homo sapiens is at the front. So, don't butt heads!

The brain processes millions of electrical impulses among its neurons every second. These impulses of electrical energy produce and electromagnetic field (as an electric current in a copper wire does). The electrical impulses appear to occur in four distinct patterns or types of brain wave:

  • alpha waves when a person is awake

  • beta waves when the brain is very active

  • theta waves when the brain is very active

  • delta waves when a person is in deep sleep (non-REM sleep).

The mind is a derivative of the brain. The mind has a number of abilities or characteristics:

  • Consciousness or awareness of physical environment.

  • Intelligence or ability to analyse and reason

  • Behaviour or a response pattern to the immediate environemnt.

  • Emotion or a state of mind.

  • Memory or information storage and recall.

Other Sense Organs

Other specialised organs for sensing the environment and converting the sensation into electrical impulses for communication to and interpretation by the brain include:

Eyes for vision or perception of light; other animals can perceive different wavelenghts of the electomagnetic spectrum such as infrared or ultraviolet wavelength perceived by some insects.

Ears for perception of sound. A healthy ear can detect sounds in the range of 20-20,000 hertz. An older person's high frequency hearing may deteriorate so that the upper end of the range is 12,000 hertz. A bat can hear frequencies of 1,000 to 120,000 hertz.

Tongue and nose for perception of chemicals in substances such as air and food (taste and smell).

Blood, heart, arteries, veins

Blood is the transport agent for the body, the heart, arteries and veins are the infrastructure where blood travels.

Blood transports food, oxygen, carbon dioxide and other chemicals, compounds and cells around the body. Plasma is the watery part of blood that allows it to flow. 55% of blood is plasma and 91% of plasma is water. Red blood cells, containing haemoglobin, transport oxygen from the lungs around the body via the arteries. Haemoglobin has a very similar structure to chloryphyll, the green in plants that allows photosynthesis. White blood cells fight disease. Platelets are bits of cell used in blood clotting.

Antigens, which are proteins attaching to cells in blood, are of two basic types A and B which lead to four groups, A only, B only, AB (A&B) and O (neither A nor B). Certain combinations of these proteins cause agglutination of antigens and antibodies, therefore one person's blood may not be compatible with another's.

The heart is a specialised muscle in the body that is constantly active. It is a pump that forces deoxygenated blood to the lungs and oxygenated blood around the body via arteries. The blood returns from lungs and body via veins. These blood vessels are muscular tubes of varying size. Arteries are stronger because they have to withstand greater pressures because blood in them is coming from the heart (pump).

Lymphatic Organs

The lymphatic system is a one way drains excess intercellular liquid, cleans it and returns the liquid to the blood. Cleaning involves fighting infection and removing worn out red blood cells. It is made up of lymph fluid, lymph vessels, lymph nodes (packed with white blood cells for fighting infection), spleen, thymus gland, tonsil, and adenoid.


Lungs are the principal organ for taking oxygen from the air into the blood and putting carbon dioxide from the blood back into the air. The lungs are made of millions of tubes getting progressively smaller until they end in a tiny sack called an alveolus. An adult has over 300 million alveoli. The surface area is about 70 m2 or about 35 times more than the surface area of an adult's skin. The wall of each alveolus, separating air from blood, is only two cells thick, as little as 0.001 mm, allowing for easy diffusion of gases.

Digestive Tract

Food is broken down in the alimentary canal which passes though the body. Nourishment is taken from the food as it is broken down and passes in to the blood stream for circulation around the body.

Food is taken in to the mouth where it is masticated by teeth. Saliva begins chemical breakdown. Food is swallowed down the oesophagus to the stomach. Gastric juice in the stomach is strongly acid (includes hydrochloric acid) and a powerful aid to digestion. The stomach has a special lining that prevents the stomach digesting itself. Food continues from the stomach to small intestine (6.5 m long but only 2.5 com in diameter) made up of the duodenum, jujenum and ileum. Then finally the large intestine (1.5 m long and 6.5 com in diameter), consisting of the caecum, colon and rectum.

Protein digestion begins in the stomach with hydrochloric acid and then continues in the small intestine. Carbohydrate digestion mainly takes place in the small intestine. Fat digestion mainly takes place in the small intestine where bile salts released by the gall bladder are specifically for fat digestion. Absorption of digested food into the body then takes place. Most absorption takes place in the ileum. The large intestine absorbs water and concentrate waste in to faeces for excretion.

Liver and Pancreas

The liver is a large internal organ weighing about 1.4 kg performing a number of critical functions.

It adjusts the composition of blood by processing red blood cells, bacteria and debris. It regulates blood sugar. It produces bile, which aids fat digestion, and stores it in the gall bladder. It is a prinicpal store for glycogen (in which excess amino acids are stored), minerals and vitamins. It removes toxic chemicals and hormones from blood.

The pancreas also produces digestive fluids and regulates blood sugar levels by regulating production of hormones insulin and glucagon.

Urinary Organs

The urinary system processes about 7 litres of blood an hour removing waste from it and returning useful materials to the blood. The kidneys are the organs that process the fluids, the bladder stores waste liquid in the form of urine. Other organs are tubes between the kidney and bladder and bladder to exit.

Reproductive Organs

The reproductive systems in male and female humans is different in form but similar in function. Both systems produce genetic material and reproductive cells each containing half of the genetic material required to make up a new individual. Both systems also control release of hormones involved in reproduction. Reproductive hormones affect an individual's behaviour as well as behaviour of organs within an individual.

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