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Alimentary Canal

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Describe the structure of the alimentary canal of the human body in relation to its function Synopsis:
* nutrition
* Alimentary canal
* 4 layers
* Buccal cavity
* Oesophagus
* Stomach
* Small intestine
* Large intestine

Nutrition is the process of acquiring energy and materials for cell metabolism, including the maintenance and repair of cells and growth. In humans, digestion and absorption occur in the alimentary canal or gut. As the gut wall is continuous with the outside surface of the body, the food in the gut is considered to be outside of the body, the gut is specialised into different regions, each designed to carry out a different role in the overall processes of digestion and absorption.

The human gut is a coiled, muscular tube extending from the mouth to the anus. Accessory digestive organs are connected to the main system by a series of ducts. These produce compounds that contribute to digestion and release them into the gut. The gut consists of four distinct layers; the mucosa, submucosa, muscularis externa and serosa. Draw diagram of the digestive system

The buccal cavity is the chamber just inside the mouth where the chewing action of teeth and jaws and the tongue begin the mechanical breakdown of food into smaller pieces. The tongue has taste buds with receptors sensitive to substances. The eye and the olfactory receptors in the nose are important for stimulating the salivary glands in the mouth to secrete saliva. Salivary amylase begins the digestion of starch into maltose. Eventually the semi-solid, partially digested food particles are stuck together to form a bolus by the tongue, which then pushes it towards the pharynx from where it is swallowed into the oesophagus as a result of a reflux action. To prevent the food from entering the trachea and lungs, the larynx closes, the soft palate is pulled up and a flap of tissue called the epiglottis covers the entrance to the trachea.

The oesophagus is a narrow muscular tube lined by stratified squamous epithelium containing mucus glands which transfers food and fluids to the stomach. The smooth longitudinal and circular muscles contract in response to being stretched, creating a wave of contractions called peristalsis that moves progressively down the gut from the pharynx toward the anus. Behind the bolus the circular muscles contract, squeezing and constricting the gut. In front of the food, the longitudinal muscles contract, shortening this section of the gut and pulling it past the advancing bolus. When it reaches the end of the oesophagus it passes through the cardiac sphincter, (prevents backflow) into the stomach.

The stomach is a muscular bag which can stretch to take in food. It stores food temporarily after meals and releases food slowly into the rest of the gut by the pyloric sphincter. It continues mechanical digestion by its churning action. This is made more efficient by the fact that unlike the other regions of the gut it possesses three layers of smooth muscle instead of two. The stomach is dotted with numerous gastric pits which lead to long, tubular gastric glands formed by infolding of the epithelium. The thin mucosa of the stomach contains mucus-secreting epithelial cells. The mucus prevents the stomach from being eroded by the HCl or self-digested by pepsin.

The small intestine is divided into three; the duodenum, ileum and jejunum. The submucosa and mucosa together are folded. The mucosa possesses numerous villi whose walls are richly supplied with blood capillaries and lymph vessels and contain smooth muscle. They are in close contact with food in the small intestine. The villi possess tiny microvilli which increase the surface area for absorption. Throughout the small intestine, goblet cells secrete mucus which lubricates the food, helping its passage through the gut.

The sphincter muscle between the ileum and the caecum opens and closes from time to time to allow small amounts of material from the ileum to enter the large intestine. Most of the fluids and salts in the gut are absorbed in the small intestine. The remaining undigested food is called faeces. It is stored in the colon before egested through the anus. Two sphincters surround the anus, an internal one of smooth muscle and under the control of the autonomic nervous system, and an outer one of striated muscle controlled by the voluntary nervous system.

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