The digestive system’s function is to break down food and to absorb its nutrients into the bloodstream, which will deliver them around the body. (Robert Wood Johnson University Hospital, 2011).. The organs are grouped into two sections- the alimentary canal and the accessory organs respectively.

Organs in the alimentary canal are the mouth, oesophagus, stomach, small intestine, large intestine and anus. Accessory organs are the liver, pancreas and gallbladder. [ (National Institute Of Diabetes and Digestive and Kidney Diseases, 2008) ] Food does not go through the accessory organs but these organs help with chemical digestion.For example, the liver produces bile, a fat-absorbing liquid stored by the gallbladder whilethe pancreas is involved in the production of enzymes (Integrated Publishing). The alimentary canal is a tube through which food passes.

There are two types of digestion- mechanical and chemical. Mechanical digestion is when food is physically broken down. Chemical digestion is when enzymes break down food. (Joe, 2009) Food starts to be broken down in the mouth. Once it is chewed up and ready to move on from the mouth, the food forms a bolus.

The bolus moves through the oesophagus- a track from the mouth to the stomach- into the stomach.The oesophagus contains muscles called peristalsis to prevent vomiting. The stomach continues breaking food down, mixing it with gastric juices and hydrochloric acid to kill harmful bacteria(GESA, 2006). The stomach is lined with mucus to stop the acid eating into the stomach wall. The bolus is now a thick liquid known as chyme.

The chyme moves through into the small intestine, where absorption occurs. The small intestine contains projections known as villi on its walls which increase the surface area of the small intestine for greater absorption of nutrients (National Institute Of Diabetes and Digestive and Kidney Diseases, 2008).Food breakdown continues and the nutrients go through the intestinal walls into the bloodstream, which transports nutrients around the body. Undigested food goes to the large intestine, a track to the anus, where it is excreted (The Nemours Foundation, 2011). The circulatory system aims to oxygenate blood, the carrier of oxygen and nutrients, and transport it around the body. The heart, blood and blood vessels play a big role in this system (Gregory, 2006).

Blood is composed of four parts- red blood cells, white blood cells, plasma and platelets (Aylward, 2006).Red blood cells carry haemoglobin which attracts oxygen. A white blood cell fights diseases and kills harmful bacteria. Platelets help form scabs/bruises to minimise bleeding.

Plasma is a clear liquid that holds blood cells and platelets. There are three types of blood vessels. Veins carry deoxygenated blue blood up to the heart. They are thin, containing valves to prevent blood going the other way. As they get further away from the heart, they get smaller and are called venules.

Arteries carry oxygenated red blood away from the heart to the rest of the body (Roldan, 2008).They are thick as they have to withstand more pressure when oxygenated blood is pumped from the heart. As arteries get further from the heart, they get smaller and are called arterioles. Capillaries provide the tissues of the body with oxygen and nutrients through blood (Ellis-Christensen,2011). The heart is a key organ in the circulatory system (The Nemours Foundation, 2011).

De-oxygenated blood enters the heart through a valve called the superior vena cava and goes down the right atrium, and through the right ventricle.Blood proceeds to the pulmonary valve and exits the heart through the pulmonary artery (the only artery carrying de-oxygenated blood) and goes to the lungs. In the lungs, a tube called the trachea branches off into the bronchi which branch into bronchioles. At the end of bronchioles are structures called alveoli (Virtual Medical Centre, 2011).

In alveoli, the exchange of CO2 for O2 occurs. Oxygenated blood comes back to the heart through the pulmonary vein. Blood goes down the left ventricle and atrium and then proceeds to the aorta, where it travels to the rest of the body. Blood is constantly circling through the circulatory system.

The respiratory system is responsible for taking in oxygen and giving off carbon dioxide (The Trustees of Princeton University, 2011). The organs that make up this system are the mouth, nose, trachea, lungs and diaphragm (The Franklin Institute, 2011). The mouth and nose take air into the body and breathes out carbon dioxide. (The Franklin Institute, 2011). The air is best taken in by the nose with tiny hairs blocking unwanted particles (Nemours Foundation, 2011).

The mouth and nose meet at the throat. The throat divides into two at the bottom- the oesophagus and the trachea.A flap of tissue called the epiglottis covers the trachea when food is swallowed to prevent food entering the trachea. (The Nemours Foundation, 2011) Air enters the trachea, which branches into bronchi. As previously discussed, bronchi eventually branch into sacs called alveoli, where the exchange of CO2 for O2 occurs. When we breathe out, the diaphragm moves upward, forcing the chest cavity to get smaller and pushing the gas in the lungs up and out.

The opposite occurs when we breathe in. (The Nemours Foundation, 2011)Cellular respiration is the consumption of O2 and creation of CO2 in cells. The Trustees Of Princeton University, 2011) It is necessary for life. The cellular respiration equation is glucose+oxygen water+energy+carbon dioxide. (RCN Telecom Services, 2011) The three systems discussed in this document interact to sustain life.

The digestive system breaks down food to provide glucose to the bloodstream. The respiratory system oxygenates blood. The circulatory system provides this blood to all living cells of the body (Palen, 2009) Therefore, it can be concluded that the digestive, respiratory and circulatory systems are necessary for cellular respiration and life.Bibliography