The Digestive System of Man The Digestive System It is also known as the Alimentary Canal. Food undergoes four major processes: • Digestion • Absorption • Metabolism • Assimilation The Lower Part of the Digestive System Mouth There two major processes which take place: • Mastication (Chewing): - Breaks down large food molecules. - Increases surface area of food particles. • Secretion of Saliva: - Contains salivary amylase (ptyalin) that digests starch to maltose. - Provides an alkaline medium. - Lubricants and moistens food. Esophagus • It is a mucus muscular membrane lined tube. • There occurs a process known as Peristalsis. Peristalsis: It is an involuntary process of muscular contraction forcing the bolus (food) down to the stomach. Stomach It secretes two substances: • Gastric juice: It contains 2 enzymes namely; - Pepsin for digestion of proteins to peptides. - Rennin for solidifying milk protein in young. • Hydrochloric acid: - Kills bacteria in food. - Provides an acidic media - Stimulates stomach walls to secrete more gastric juice. Duodenum It receives: • Bile: It’s a non-enzymatic green fluid - Breaks down fats into tiny droplets. - Provide alkaline media. • Pancreatic juice: - Pancreatic amylase for digestion of starch. - Lipase for digestion of fats. - Trypsin for digestion of proteins. Ileum Its walls secretes intestinal juices which contain enzymes to complete digestion: • Sucrase for digestion of sucrose to glucose and fructose. • Lactase for digestion of lactose to glucose and galactose. • Maltase for digestion of maltose to glucose molecules. • Erepsin for digestion of peptides & polypeptides to amino acids. • Lipase for digestion of lipids to fatty acids & glycerol. Liver • It is the largest organ in the mammalian body. • It secretes bile which is stored in the gall bladder. Bile breaks down fats into tiny droplets through emulsification. Roles: • • • • • Regulates sugar/glucose. Breaks down excess RBC. Storage of blood. Detoxification. Generation of heat Pancreas • It is an endocrine gland because it secretes Insulin hormone - converts excess glucose into glycogen for storage. • It is also an exocrine gland because it secretes pancreatic juice in the duodenum - pancreatic juice contains lipase, trypsin and pancreatic amylase for digestion of lipids, proteins and starch. Absorption • It occurs within the ileum in finger-like projection known as Villi. Villi • Amino acids and simple sugars like glucose, fructose diffuse through thin Epithelial cells into the blood capillaries. • Fatty acids and glycerol enter the Lacteal into the lymphatic system then finally into the blood system through the Innominate vein. Adaptations of Ileum • It is very long and wound to increase surface area. • It has millions of villi for food absorption. • Numerous blood capillaries for carrying of amino acids and simple sugars. • Lacteal for fatty acids and glycerol absorption. • Thin epithelial cells through which soluble foods easily diffuse. • Goblet cells secrete mucus which prevents enzymes from digesting ileum walls.
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