BME 5010, CHAPTER 17: THE DIGESTION AND ABSORPTION OF FOOD (PART I)

BME 5010, CHAPTER 17: THE DIGESTION AND ABSORPTION OF FOOD (PART I)

KEY WORDS	MEANING
INTRODUCTION Fig 17-1	The gastrointestinal (GI) system includes: (1) the GI tract. (2) the glandular organs that secrete substances into the tract.
Fig 17-2	The overall function of the GI tract is to process ingested foods into molecules that can be transferred along with salts and water from the GI tract into the body, where they can be distributed by the circulatory system to cells that need them. The four processes of the GI tract are digestion, absorption, secretion and motility.
	The GI tract is about 15 feet long. The lumen (or opening) inside the tract is continuous with the outside environment. That is one reason the large intestine has billions of bacteria. The GI lumen is an external environment. If the bacteria enter the internal environment (blood) they can be harmful or deadly.
Digestion	Food must be broken down into molecules to be absorbed into the body. This is called digestion.
Absorption	The food molecules move across the epithelial cell lining of the GI tract into interstitial space and then into blood or lymph. This is called absorption.
	The smooth muscle of the GI tract contracts to mix digested food and move it along down the tract.
	The GI tract will generally absorb as many nutrients as ingested. It is up to the body to store or eliminate what is not needed.
OVERVIEW: FUNCTIONS OF THE GI ORGANS Fig 17-3	Three food groups: fats, carbohydrates, proteins.
Mouth	Chewing breaks up food. Saliva is secreted by three pairs of salivary glands (exocrine). Saliva contains mucus and salivary amylase, an enzyme which breaks up polysaccharide into smaller sugars. Saliva also dissolves some food molecules so that they can react with chemoreceptors, producing the sense of taste.
Pharynx and esophagus	Contribute nothing to digestion. They are a muscular-walled pathway from the mouth to the stomach.
Stomach	A saclike organ. It stores, dissolves (into smaller molecules) and partially digests food so the small intestine can work on it. The stomach secretes HCl, pepsin, mucus and gastrin. Hydrochloric acid (HCl) dissolves particulate matter. This acid alters the ionization of food molecules and disrupts connective tissue proteins in food. Pepsin further digests proteins. Amylase from salivary glands continues to digest the carbohydrates. HCl also kills bacteria in food, though not completely. The remaining bacteria multiply in the colon. The digested food in the stomach is called chyme. Little absorption occurs in the stomach.
Small intestine	Here enzymes break down proteins and carbs further, and also break down fats. The food is digested into small molecules that can be absorbed into the intestinal wall and then into the blood stream or lymph. Vitamins, minerals and water do not need to be broken down by enzymes. Their absorption is described later. The small intestine is divided into duodenum, jejunum and ilium. Most absorption is in the first 1/4 of the small intestine, in duodenum and jejunum.
Pancreas Fig 17-4	A large elongated gland behind the stomach. It is both endocrine (insulin, glucagon) and exocrine (digestive enzymes) in function. Exocrine function: enzymes for each of the 3 food groups. Bicarbonate: to neutralize the HCl from the stomach.
Liver Table 17-1, know section A	Large gland in the upper right portion of the abdomen. Also endocrine and exocrine. Exocrine function: secretes bile. Bile breaks up fats.
Gallbladder	Stores bile. After a meal the gallbladder secretes bile into the small intestine through the bile duct.
Small intestine	Monosaccharides and amino acids are absorbed by carrier mediated transport processes. Fatty acids enter by diffusion. Mineral ions are absorbed by active transport and water follows by osmosis. The small intestine has motility through contraction of its smooth muscles. This mixes the digested food, moving molecules toward the wall to be absorbed and also moving the contents toward the large intestine. Most absorption occurs in the first 1/4 of the small intestine.
Large intestine	Only a small volume of water salts and undigested material is passed on to the colon (large intestine). The colon temporarily stores material, and absorbs salt and water. The remaining material is eliminated as feces. (about 100 ml water, 50 ml solids daily).
Volume Fig 17-5	About 800 grams of food and 1200 ml of water is consumed daily. In addition to this ingested portion, 7000 ml of water is secreted into the GI tract daily and most is reabsorbed into the body along with the ingested water.

STRUCTURE OF THE GI TRACT WALL
General structure Fig 17-6	Mucosa: (1) Surface: Highly convoluted. From the stomach on, there is a single layer of epithelial cells linked by tight junctions. The epithelium contains numerous ducts that act as exocrine glands and also has some endocrine glands. (2) Lamina propria: a supporting layer that has small blood vessels, nerves and lymph ducts. (3) Muscularis mucosa: thin smooth muscle layer. Submucosa: Contains submucous nerve plexus and blood vessels and lymph ducts. Muscularis externa: Contains (1) A circular smooth muscle layer whose contraction narrows the lumen. (2) A longitudinal smooth muscle layer whose contraction shortens the tube. (3) Myenteric plexus: a nerve network between the two muscle layers. Serosa: Thin layer of cells and connective tissue that surround the gut wall. These are anchored to the abdominal wall with connective tissue.
Intestinal surface Fig 17-7, 17-8	The surface is made larger for more absorption of nutrients by various structures: The villi are finger like projections. The surface of each villus has microvilli which increase the surface area as hairlike projections. The small intestine has a surface area of 300 m², about 600 times more than a plain tube of the same size.
Epithelial cell replacement	The epithelial surfaces are continuously being replaced. About 17 billion cells are replaced each day, and the whole epithelium in 5 days. The cells differentiate into mature cells as they migrate to the top of the villus, where they are sloughed off.
Nutrient movement from the epithelial layer	Each villus has an arteriole, a venule and a lymph duct (lacteal). After passing through the epithelium, the fat enters the lacteal and goes into the lymph system, which drains into the venous system. The other nutrients go into the venules then into the hepatic portal vein into the liver. Some of the nutrients are processed there. The nutrients then continue form the liver to the heart and then to the body.

DIGESTION AND ABSORPTION
Carbohydrates Table 17-2	Intake is 250-800 grams per days in our diet. About 2/3s is starch, a plant polysaccharide. Most of the rest is sucrose (table sugar) and lactose (milk sugar) (disaccharides). Cellulose is another plant polysaccharide. It cannot be digested and is referred to as fiber. Starch digestion: Begins in the mouth with salivary amylase which continues to work in the stomach. Starch is digested in the small intestine by pancreatic amylase. The end result is a disaccharide, maltose, and short glucose chains. Sucrose, lactose, maltose and the glucose chains are broken down into monosaccharides: glucose, fructose and galactose, by enzymes on the microvilli of the small intestine. The sugars then cross the epithelium into the blood. This is by active transport or facilitated diffusion. The sugars are too big for simple diffusion across the membrane.
Protein	40-50 g of protein is required to supply essential amino acids. An American diet contains about 125 g of protein daily. Enzymes, mucus, and broken down epithelial cells also are a protein source. Protein digestion: begins in the stomach with pepsin and in the small intestine with trypsin and chymotrypsin secreted by the pancreas. Also in the small intestine carboxypeptidase and aminopeptidase pull off amino acids that can be absorbed into the epithelium. Amino acids are transferred into the epithelium by secondary active transport coupled to sodium transport. Chains of 2-3 amino acids can be absorbed as well. Small proteins can be transported by endocytosis and exocytosis.
Fat 17-9, 17-10 17-11 17-12	25-160 grams/day in our diet. Fat digestion takes place almost entirely in the small intestine. The main digestive enzyme is pancreatic lipase. Triacylglycerol ----> monoglyceride + 2 fatty acids Large lipid droplets are converted to smaller emulsion droplets by emulsification and then to micelles (even smaller). Emulsification (fat breakdown) requires mechanical disruption and emulsifying agents which prevent smaller droplets from reaggregating. Phospholipid in food and bile salts are the emulsifying agents. They are amphipathic (have polar and non-polar ends). The non-polar end binds to the emulsion droplet. The polar end is exposed to the surface and prevents the droplets from reuniting. When a micelle breaks down it releases its lipid molecules into solution. These can diffuse across the intestinal epithelium. As more lipids diffuse into the epithelium, micelles break down to release more. In the epithelial cells fatty acids and monoglycerides are reassembled into triacylglycerol in the smooth endoplasmic reticulum (ER) and released as fat droplets called chylomicrons into the interstitial space on the other side. The chylomicrons also contain other lipids such as cholesterol and fat soluble vitamins (A,D,E,K). The chylomicrons diffuse from the interstitial space into the lacteals. The lymph eventually empties into the systemic veins.
Vitamins	Fat soluble vitamins are transported in chylomicrons as are the fat molecules. Water soluble vitamins are absorbed by diffusion or carrier mediated transport. However, B₁₂ must bind to a protein called intrinsic factor produced in the stomach. This complex is absorbed on specific cells in the small intestine.
Water and minerals Iron	Water is the most abundant substance in chyme. Most water is reabsorbed in the wall of the small intestine, which is very permeable to water. Water follows solutes via osmosis. Sodium is the most abundant solute in chyme. It is absorbed using Na,K-ATPase pumps similarly to the renal tubules. Chloride and bicarbonate are co-transported with sodium. The absorption of other minerals also occurs but their transport processes are too detailed to cover in this course. About 10% of ingested iron is absorbed into the blood every day. Most iron transported into the epithelial cells is bound to ferritin and stored as such. Iron in the GI epithelial cells is released back into the intestine when the cells slough off. Iron released into blood is bound to transferrin which circulates in the body. When more iron is needed, as in blood loss, ferritin synthesis drops, and the absorbed iron then goes into the blood to be used in hemoglobin. Too much iron in the body tends to accumulate in tissues and not be released as waste.

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