The structure of body systems

Cardiovascular – (circulatory system)
The Circulatory System is responsible for transporting materials throughout the body. It transports nutrients, water, and oxygen to cells in the body and carries away wastes such as carbon dioxide. The heart, blood vessels and blood itself are three essential components the body needs to survive. The circulatory system consists of two circuits that blood travels through; pulmonary and systemic. The left side of the heart circulates oxygen rich blood through the body. This is known as systemic circulation. The right side of the heart circulates oxygen poor blood back to the lungs so they can be oxygenated.
(http://hes.ucfsd.org/gclaypo/circulatorysys.htm)

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Ventricles are more muscular than atria as these are the main pumps. The ‘septum’ is a muscular wall and divides the heart in half giving a left and a right side. The major vessels that act as entry and exit points are at the top of the heart Valves exist to ensure the smooth passage of blood through the heart they also ensure blood only flows in one direction.
Arteries carry blood away from the heart (always oxygenated apart from the pulmonary artery which goes to the lungs). They also have thick muscular walls and small passageways for blood. This means the blood is under high pressure in the arteries.
Veins carry blood to the heart and they always carry de-oxygenated apart from the pulmonary vein which goes from the lungs to the heart. Veins have thin walls and large internal passages for blood. They also have values to prevent blood flowing backwards.
Capillaries are found in the muscles and lungs. They are microscopic, only one cell thick
This is where gas exchange takes place as oxygen passes through the capillary wall and into the tissues and carbon dioxide passes from the tissues into the blood.
Respiratory system
This includes the nasal cavity. Pharynx, larynx. Trachea, bronchi and lungs. This system keeps blood supplied with oxygen and it removes carbon dioxide.

Air is drawn in through the nose. Mucus then moistens and cleans the air. The air is then drawn down past the pharynx and larynx (the throat). The epiglottis is used to cover the trachea also known as the wind pipe. And so the air carries on towards the two branches which go to the left and right lung. The alveoli have a single cell wall and a rich supply of blood. This is where respiration takes place. This is where Oxygen goes into the blood and carbon dioxide out.
The lungs contain more than 300 million alveoli. When you exhale or breathe out, your skeletal muscles and diaphragm return to the relax position which decrease the size of the chest cavity and therefore pushes the air out of the lungs.

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Digestive System
Digestion is the breakdown of large molecules into smaller more soluble molecules that can be absorbed into the body. Digestion happens inside the gut, and relies on enzymes.

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Organs in the digestive system include the mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus, liver and pancreas. The function of these are to break down food into absorbable units that enter the blood.
The mouth is where food enters the body, this is where digestion begins. In the mouth the food is chewed up into smaller more digestible pieces. Saliva is secreted from salivary glands in the mouth. It contains the enzyme amylase which helps break down starch into simple sugars. Saliva also moistens the food making it easier to chew and swallow.
When food is swallowed, the muscles in the oesophagus contract and relax, helping to push the food down into the stomach. These waves of muscular contractions which move food along the digestive system are called