Digestion and Absorption Dr Gouree Francis MBBS

objectives Digestive system Digestive process Organs function Mechanical process Chemical process

Digestive system The Human gastrointestinal tract or digestive system is the system by which ingested food is processed by physical and chemical means to provide the body with nutrients and excrete the waste products. This system includes the alimentary canal extending from the mouth to the anus, and the hormones and enzymes assisting in digestion.

Digestive system

Functions of the digestive system Receipt, maceration and transport of ingested substances and waste products Secretion of acid, mucus, digestive enzymes, bile and other materials Digestion of ingested foodstuffs Absorption of materials Storage of waste products Excretion Ancillary functions

Organs of the digestive system Main organs Other organs Mouth – tongue, teeth Esophagus Stomach Small intestine Large intestines Rectum Anus Salivary glands Gallbladder Liver Pancreas

What is digestion? Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into the blood stream. Digestion is form of catabolism Breakdown of large insoluble molecules into smaller soluble molecules

Mechanical digestion Mechanical digestion is the physical break down of food into smaller pieces. Mastication (chewing) occurs in the mouth by the teeth, which act to cut and grind food into smaller pieces. This makes them easier to later digest as it increases the surface area of the food molecules.

Peristalsis successive contractions of muscles in the alimentary canal which form a wave of constriction that moves food along. Emulsification by Bile salts act to emulsify large fat globules into smaller fat droplets. breaks up all molecules and then puts a 'soapy' shield around them so they cant join back together. This increases the surface area of the molecule of fat so that enzymes (pancreatic lipase) can act on it and break it down to smaller, simpler forms

Chemical digestion Chemical digestion, is the chemical break down of foods into smaller pieces. Enzyme action salivary amylase is found in our saliva which acts to break down starch into simpler forms. HCl acid in the stomach helps to break down food and destroy any micro-organisms that accompanied the food.

Absorption Passage of digested end products in to blood or lymph

What happens to ingested food inside the digestive system? Digestive process

In mouth Digestion begins in the mouth Digestion inside the mouth occurs with the help of teeth, palate, lips and tongue. As food is chewed, it becomes lubricated, warmer, and easier to swallow and digest. The teeth and tongue work together to convert each bite of food into a bolus that can readily move into the esophagus

Teeth Tongue chew, break and grind ingested food into small pieces. mixes the food with saliva make the food bolus with the help of palate and teeth Propels food bolus toward pharynx when swallowing Taste sensation – taste buds

Saliva Saliva is produced in and secreted from one of the three salivary glands. Human saliva is composed of 98% water other 2% consists of other compounds such as electrolytes, mucus, antibacterial compounds (secretory IgA, lactofferin, lysozyme,peroxidase) and various enzymes

Saliva Facilitate taste sensation(A dry tongue can't taste) lubricates and protects the teeth, the tongue, and the tender tissues inside the mouth also breaks down food caught in the teeth, protecting them from bacteria that cause decay. Lubricates and moistens food and helps to break it down, which in turn makes it easier for the tongue to push the food to the back of the mouth for swallowing. Contain salivary amylase Salivary amylase breaks down starch into sugar Contain Lingual lipase – important in children as they have immature pancreas

In Oesophagus After the bolus is swallowed, it enters the esophagus where it continues to be warmed and lubricated as it moves toward the stomach by peristalsis. Act as a conduit

In Stomach In the stomach, there are three layers of muscle (which is unique as the rest of the alimentary canal only has 2). It has longitudinal, circular and oblique muscle which together contract and relax to form the churning motion which mixes food around. This aids in digestion as it slightly breaks up the food and also increases the contact the food has with enzymes and acids in the gastric juice.

The acidic environment (HCl) of the stomach and the action of gastric enzymes convert the bolus into chyme, a liquefied mass that is squirted from the stomach into the small intestine. Carbohydrates tend to leave the stomach rapidly and enter the small intestine proteins leave the stomach less rapidly and fats linger there the longest.

Role of liver and gall bladder The liver produces the bile required for the emulsification of fat, and the gallbladder stores the bile until it is needed.

Role of pancreas The pancreas supplies enzymes to digest proteins, fats, and carbohydrates. Trypsin Chymotrypsin Carboxypeptidase Lipase Amylase In addition, the pancreas produces and secretes buffers that help neutralize the stomach acids found in chyme.

In small intestine The small intestine is the principal site of digestion and absorption. There, enzymes and secretions from the pancreas, liver, gallbladder, and the small intestine itself combine to break down nutrients so that they can be absorbed. Intestinal cells also supply some enzymes – brush border enzymes Disaccharidases – maltase, lactase, sucrase Peptidases

The absorption of nutrients in the small intestine is facilitated by tiny projections called villi, which provide more surface area for absorption. The nutrients pass through the intestinal membranes into the circulatory system, which transports them to body tissues. Nutrients are then absorbed into the cells, where they are used for growth, repair, and the release or storage of energy. The overall process is called metabolism

In colon Undigested chyme proceeds from the small intestine into the large intestine (colon), where it becomes concentrated, as liquid is absorbed in preparation for excretion. Bacteria cause fermentation, which facilitates further breakdown, but absorption of nutrients from the large intestine is minimal.

In rectum Temporary storage of formed faeces. Further absorption of water Undigested portion will be eliminated via anus during defecation.