Anatomy and Physiology

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The digestive system starts with the mouth and ends at the anus. The tract would measure approximately 10 metres if it were laid out in a straight line.

The purpose of the digestive system (Figure 2.1) is to produce a chemical and mechanical breakdown of food:

• Ingestion: the act of eating

• Digestion: mechanical and chemical

• Absorption

• Defecation or elimination.

Large bowel

Liver-

Gallbladder

Large bowel

Liver-

Gallbladder

Rectum

Stomach Spleen

Small bowel

Figure 2.1 The digestive system. (Courtesy of Dansac Ltd.)

Rectum

Stomach Spleen

Small bowel

Figure 2.1 The digestive system. (Courtesy of Dansac Ltd.)

The digestive tract or alimentary canal is an epithelial lined muscular tube along which food passes by a movement called peristalsis. Peristalsis is a muscular action of dilatation followed by contractions as muscle fibres relax and contract (Jackson, 1979). This movement is not continuous throughout the tract because some organs store the food bolus whereas others churn and mix it with secreted enzymes. The food is then ingested and digested by various digestive juices and enzymes. Food is required to produce heat and energy for daily living and the food undergoes a complex process called digestion.

The enzymes that are secreted throughout the gut digest food. They convert the protein, carbohydrates and fats within food into their simplistic form, so that they can pass directly into the blood supply. The inorganic salts, water and vitamins are absorbed without chemical changes and waste products are eliminated

The digestive process changes as it passes along the alimentary tract. In the upper part of the tract it is predominantly under nervous control whereas further along hormones control the tract. The control reverts back to the nervous system in the rectum and anal canal. The food in the alimentary tract, along with a local nerve network, regulates the process of digestion.

Defecation or the elimination of waste products from the digestive process is triggered by the ingestion of food and the mass movement process of the bowel that occurs twice a day; both of these processes give us the desire to defecate. Control can be exercised to delay defecation. It is a process called toilet training, which begins at an early age and allows the control of elimination.

The Mouth

The food enters the mouth, the opening of which is protected by the lips. Food is bitten by the front teeth, incisors, passed to the back teeth, premolars and molars, to be masticated and chewed, and mixed with the saliva to form a bolus of food that will slide into the oesophagus through the action of swallowing.

The saliva is an alkaline substance secreted by the thought, sight, smell and taste of food through the salivary glands (experiment - think of citrus fruit and see if your mouth waters). With the aid of peristalsis (an involuntary muscular movement) the bolus of masticated food is propelled into the oesophagus. This peristaltic movement allows food and liquid to be swallowed together without choking.

The salivary glands are three pairs of glands comprising:

• The parotid gland: located in front and under the ears between the skin and muscle.

• The submandibular gland: located under the tongue.

• The sublingual gland: located anteriorly to the submandibular glands.

The salivary glands produce approximately 1 to 1.5 litres of saliva each day. The saliva mixes into the food with the action of chewing.

The parotid salivary gland produces salivary amylase, also known as ptyalin, which converts starch to simple sugar; the submandibular salivary gland produces salivary amylase and some mucus whereas the sublingual salivary glands produce a thicker version of mucus. The digestive process that happens when the saliva mixes with the food is instigated by the breakdown of carbohydrates. The saliva also has a cleansing function in the mouth, which allows taste buds to distinguish between sweet and savoury flavours. Saliva is rich in calcium and this is how teeth are protected from decalcification. The saliva is a lubricant that enables the bolus to be swallowed by making it moist; it also acts as an antiseptic.

Chewing continues until the food is manageable and moist; this is now called a bolus which is manipulated by the tongue and rolled into the pharynx. The soft palate closes the nasopharynx and the epiglottis moves to allow the food passage into the oesophagus (Green, 1978; Jackson, 1979; Tortora and Anagnostakos, 1981).

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