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Anatomy & Physiology - Body Systems

The Digestive System

The branches of science that will help you understand the body parts and functions are anatomy and physiology. Anatomy deals with the study of the human body (the components, structure and position) and physiology the study of how the body functions.

Body Systems

The body comprises several systems: Cardiovascular system, Digestive system, Endocrine system, Muscular system, Neurological system, Respiratory system and the Skeletal system.

The Digestive System

The functions of the digestive system are:

  • Ingestion - eating food
  • Digestion - the breakdown of the food
  • Absorption - extraction of nutrients from the food
  • Defecation - removal of waste products

The digestive system also builds and replaces cells and tissues that are continually dying.

Digestive Organs

The digestive system is a group of organs (Buccal cavity (mouth), pharynx, oesophagus, stomach, liver, gallbladder, jejunum, ileum and colon) that break down the chemical components of food, with digestive juices, into tiny nutrients which can be absorbed to generate energy for the body.

The Buccal Cavity

Food enters the mouth and is chewed by the teeth, turned over and mixed with saliva by the tongue. The sensations of smell and taste from the food set up reflexes which stimulate the salivary glands.

The Salivary glands

These glands increase their output of secretions through three pairs of ducts into the oral cavity and begin the process of digestion.

Saliva lubricates the food enabling it to be swallowed and contains the enzyme ptyalin which begins to break down starch.

The Pharynx

Situated at the back of the nose and oral cavity receives the softened food mass or bolus by the tongue pushing it against the palate, which initiates the swallowing action.

Simultaneously, a small flap called the epiglottis moves over the trachea to prevent any food particles from getting into the windpipe.

From the pharynx onwards, the alimentary canal is a simple tube starting with the salivary glands.

Digestive system

The Oesophagus

The oesophagus travels through the neck and thorax, behind the trachea and in front of the aorta. The food is moved by rhythmical muscular contractions known as peristalsis (wave-like motions) caused by contractions in longitudinal and circular muscle bands. Antiperistalsis, where the contractions travel upwards, is the reflex action of vomiting and is usually aided by the abdominal muscles and diaphragm's contraction.

The Stomach

The stomach lies below the diaphragm and to the left of the liver. It is the widest part of the alimentary canal and acts as a reservoir for the food where it may remain between 2 and 6 hours. Here the food is churned over and mixed with various hormones, enzymes including pepsinogen which begins the digestion of protein, hydrochloric acid, and other chemicals; all of which are also secreted further down the digestive tract.

The stomach has an average capacity of 1 litre, varies in shape, and is capable of considerable distension. When expanding, this sends stimuli to the hypothalamus, which is the part of the brain and nervous system controlling hunger and the desire to eat.

The stomach wall is impermeable to most substances, although it does absorb some water, electrolytes, certain drugs, and alcohol. At regular intervals a circular muscle at the lower end of the stomach, the pylorus opens allowing small amounts of food, now known as chyme to enter the small intestine.

Small Intestine

The small intestine measures 7m in an average adult and consists of the duodenum, jejunum, and ileum. Both the bile and pancreatic ducts open into the duodenum together. The small intestine, because of its structure, provides a vast lining through which further absorption takes place. There are a large lymph and blood supply to this area, ready to transport nutrients to the rest of the body. Digestion in the small intestine relies on its secretions plus those from the pancreas, liver, and gallbladder.

The Pancreas

The Pancreas is connected to the duodenum via two ducts and has two main functions:

  1. To produce enzymes to aid the process of digestion
  2. To release insulin directly into the bloodstream to control blood sugar levels

Enzymes suspended in the very alkaline pancreatic juices include amylase for breaking down starch into sugar and lipase which, when activated by bile salts, helps to break down fat. The hormone insulin is produced by specialised cells, the islets of Langerhans, and plays an important role in controlling the level of sugar in the blood and how much is allowed to pass to the cells.

The Liver

The liver, which acts as a large reservoir and filter for blood, occupies the upper right portion of the abdomen and has several vital functions:

  1. Secretion of bile to the gallbladder
  2. Carbohydrate, protein and fat metabolism
  3. The storage of glycogen ready for conversion into glucose when energy is required.
  4. Storage of vitamins
  5. Phagocytosis - ingestion of worn-out red and white blood cells, and some bacteria

The Gallbladder

The gallbladder stores and concentrates bile which emulsifies fats, making them easier to break down by the pancreatic juices.

The Large Intestine

The large intestine averages about 1.5m long and comprises the caecum, appendix, colon, and rectum. After food is passed into the caecum a reflex action in response to the pressure causes the ileocolic valve's contraction, preventing any food returning to the ileum. Here most of the water is absorbed, much of which was not ingested, but secreted by digestive glands further up the gastrointestinal tract. The colon is divided into the ascending, transverse and descending colons, before reaching the anal canal where the indigestible foods are expelled from the body.

Effect of exercise on the digestive system

Most exercise positively affects the digestive system helping to quell appetite and increase metabolism. Some endurance events sometimes cause competitors to have an upset stomach and diarrhoea.

Related References

The following references provide additional information on this topic:

  • LIPSKI, E. (2012) Digestive Wellness: Strengthen the Immune System and Prevent Disease Through Healthy Digestion. McGraw-Hill
  • Healthy Gut, Healthy You - The personalized plan to transform your health from the inside out.

Page Reference


  • MACKENZIE, B. (2001) Physiology - Digestive System [WWW] Available from: [Accessed

Related Pages

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