Digestion: Processes, Stages, and Tract Walls

• Organs are located within the gastrointestinal (GI) tract.
• The function of the organs is to hydrolyze, or break down, the macromolecules found in food.
• Nutrients are transported by the blood to the cells.

Processes of Digestion

• Ingestion takes food via the mouth.
• Digestion mechanically or chemically breaks down foods into their subunits.
• Mechanical digestion involves chewing in the mouth and contractions of smooth muscles in the stomach.
• Chemical digestion uses digestive enzymes to hydrolyze macromolecules into subunits.

Stages of Digestion

• Movement passes food from one organ to the next, normally by contractions of smooth muscle called peristalsis; indigestible material must be expelled.
• Absorption involves the movement of nutrients across the GI tract wall into the blood and then delivered to cells
• Elimination is the removal of indigestible wastes (defecation).

Wall of the Digestive Tract

• The lumen, an open area of a hollow organ or vessel, contains food or feces in the GI tract.
• The mucosa is the Innermost layer that produces mucus for protection and digestive enzymes.
• The submucosa is a loose connective tissue containing blood vessels, lymphatic vessels, and nerves.
• The muscularis is made of two layers of smooth muscle (inner, circular layer and an outer, longitudinal layer) that move food along the GI tract.
• serosa is the Outer lining and part of the peritoneum.

The Mouth

• Receives food and begins mechanical and chemical digestion
• The roof of the mouth separates the nasal cavity from the oral cavity, and it contains of a bony hard palate and a soft palate.
• The soft palate is made of muscle and ends in the uvula.
• Tonsils are located in the back of the mouth on either side of the tongue.
• Three pairs of salivary glands secrete saliva, which contains salivary amylase, an enzyme that begins carbohydrate digestion, and lysozyme, an antibacterial enzyme.
• The tongue is covered in taste buds and assists in mechanical breakdown and movement of food, and also forms a bolus (mass of chewed food) and moves it toward the pharynx.
• Teeth mechanically digest food.
• There are 20 smaller deciduous (baby) teeth and 32 adult teeth.
• The crown is The part of the tooth above the gum line
• The root is the portion below the gum line
• The enamel is the extremely hard outer layer of the tooth.
• The dentin is a thick layer of bonelike material in the tooth.
• Pulp contains nerves and blood vessels in a tooth
• Periodontal membranes anchor the tooth to the jawbone.
• Dental caries refers to tooth decay or cavities, and it occurs when bacteria metabolize sugar and produce acids, which erode the teeth.
• It is painful when the decay reaches the nerves of the pulp.
• Gingivitis is the inflammation of the gums that can spread to the periodontal membrane, causing loosening of the teeth
• Periodontitis results in the loss of bone and loosening of the teeth.

The Pharynx and Esophagus

• The mouth and nasal passages lead to the pharynx, which opens into both the food passage (esophagus) and air passage (trachea, or windpipe).
• The two tubes are parallel to each other; the trachea is in front of the esophagus.
• The esophagus is a muscular tube that leads to the stomach.
• Swallowing starts off voluntarily, but once food or drink is pushed back into the pharynx, it becomes an involuntary reflex.
• Food normally enters the esophagus because other possible avenues are blocked, after the soft palate moves back to close off the nasal passage and the trachea moves up under the epiglottis to cover the glottis, the opening to the larynx.
• Peristalsis is contractions that push food through the digestive tract.
• A sphincter is a ring of muscle that acts as a valve.
• It contracts to stop food from moving through and relaxes to allow food through.
• The lower esophageal sphincter is between the esophagus and the stomach
• Heartburn occurs the sphincter fails which causes stomach contents to move from the stomach into the esophagus.
• Vomiting occurs when strong contractions of the abdominal muscles and the diaphragm forces the contents of the stomach into the esophagus and oral cavity.

The Stomach

• Stores food, starts digestion of proteins, and controls movement of food into the small intestine.
• It does not absorb nutrients but it can absorb alcohol because it is fat-soluble and can pass through membranes easily.
• There are three layers of muscle in the muscularis layer (instead of two) to help in mechanical digestion, namely circular, longitudinal, and a third oblique layer.
• The mucosa has deep folds called rugae.
• The mucosa also has gastric pits, which contain gastric glands which produce gastric juice, this includes Pepsin (digests proteins), Hydrochloric acid (HCl) (pH of 2 and it kills bacteria and activates pepsin), and mucus
• The stomach would usually empties in 2-6 hours.
• Chyme is a mixture of food and gastric juice.
• The pyloric sphincter allows only a small amount of chyme to enter the small intestine at a time.

The Small Intestine

• The organ is named for its small diameter compared with the large intestine's and is very long, longer than the large intestine.
• It averages about 6 m (18 ft) in length, whereas the large intestine is about 1.5 m (4.5 ft).
• The three regions consisting of Duodenum, jejunum and ileum.
• The pancreas secretes digestive enzymes through a ductinto the duodenum, the first portion of the small intestine.
• Another duct brings bile from the liver and gallbladder into the duodenum which helps emulsifies fat as well as causing droplets to disperse in water to expose more of it to lipase.
• The pancreas produces enzymes that hydrolyzes fats into glycerol and fatty acids as well as pancreatic amylase that digests carbohydrates and trypsin which digests proteins, and also is a protease
• Pancreatic juice contains sodium bicarbonate, which neutralizes acidic chyme.
• Nutrients are absorbed in the small intestine.
• The mucosa contains fingerlike projections called villi (sing., villus) and The cells that cover the villi have microvilli which gives the villia fuzzy “brush border”
• These structures contain brush border enzymes that complete digestion which increases the surface area of the small intestine for absorption of nutrients.
• Nutrients are absorbed into the villi, which contain blood capillaries, and a small lymphatic capillary called a lacteal.
• Monosaccharides and amino acids enter the blood capillaries of a villus.
• Glycerol and fatty acids form lipoprotein droplets called chylomicrons, which then enter a lacteal.
• Nutrients are eventually carried to all the cells of the body by blood after they are absorbed.

Lactose Intolerance

• Lactose is the primary sugar in milk and Lactase—the brush border enzyme that digests lactose
• Characterized by diarrhea, gas, bloating, and abdominal cramps after ingesting milk and other dairy products because the undigested lactose causes fluid retention in the small intestine.
• Gas, bloating, and cramps occur when bacteria break down the lactose anaerobically.

The Accessory Organs

• The pancreas is located behind the stomach and most pancreatic cells produce pancreatic juice, which enters the duodenum via the pancreatic duct
• It contains sodium bicarbonate and digestive enzymes and is also an endocrine gland that secretes the hormone insulin when blood glucose levels rise. Types include Type 1 diabetes where there is not enough insulin, and It is normally diagnosed in childhood as well as Type 2 diabetes where the body's cells are insulin-resistant as well as normally occurring in adulthood; this is a result of Risk factors which are obesity, inactivity, and family history.
• The liver is the largest gland in the body and lies mainly in the upper right abdominal cavity, which is under the diaphragm and has lobules that are the structural and functional units, this brings the hepatic portal vein which brings blood to the liver from the GI tract.
• It filters the blood, removing poisonous substances and is involved in blood glucose homeostasis, storing glucose as glycogen.
• When the blood glucose is low, it releases glucose by breaking down glycogen and converts glycerol and amino acids to glucose.
• As amino acids are converted to glucose, urea is formed.
• Produces plasma proteins as well as regulating blood cholesterol levels, Bile, which contains bile salts, water, cholesterol, and bicarbonate, and bilirubin, a green pigment formed from the breakdown of hemoglobin.
• The gallbladder located just below the liver and organ are pear-shaped which stores bile.
• Gallstones are formed by a stone-like material.
• Liver disorders include hepatitis and cirrhosis, as well as liver disease, can cause bile pigments to leak into the blood, causing jaundice, which can then cause a yellowish tint to the whites of the eyes and the skin.

Hepatitis

• Hepatitis is the inflammation of the liver and has different forms.
• Hepatitis A is usually acquired from sewage-contaminated drinking water and food.
• Hepatitis B is usually transmitted from sexual contact, but also from blood transfusions or contaminated needles. It is more contagious then the AIDS virus and is spread in the same way as well as vaccines that are available for hepatitis A and B.
• Hepatitis C is usually acquired by infected blood and Can lead to chronic hepatitis, liver cancer, and death.

Cirrhosis

• Cirrhosis is a chronic disease where the liver becomes fatty and then filled with fibrous scar tissue.
• It is often seen in people with obesity and alcohol use disorder, because of malnutrition and excessive alcohol.
• The liver is forced to break down
• The liver can regenerate and recover if the rate of regeneration exceeds the rate of damage as well as during liver failure there may not be enough time to let the liver heal.
• The liver requires a transplant.
• Secretion of digestive juices is controlled by the nervous system and digestive hormones.
• The parasympathetic nervous system stimulates gastric secretion when you look at or smell food.
• The stomach produces the hormone gastrin when you ear a meal rich in protein which increases the secretory activity of gastric glands.
• Secretin is secreted by the duodenum
• Cholecystokinin (CCK) is released by the duodenum when proteins and fat are present.

The Large Intestine

• The regions of the large intestine include the cecum, the colon, the rectum, and the anal canal, and has larger diameter than the small intestine, however, it's shorter in length.
• The cecum is The first portion.
• The vermiform appendix projects off the cecum.
• Appendicitis results an inflamed appendix that can cause peritonitis, a life-threatening infection of the peritoneum.
• The colon includes the ascending colon, which goes up the right side of the body, as well as the transverse colon, which crosses the abdominal cavity, in addition to the descending colon, which passes down the left side, finally the sigmoid colon, which enters the rectum, the last portion of the large intestine and The rectum opens on the anus, where defecation, the expulsion of feces, occurs

Functions of the Large Intestine

• Does not produce digestive enzymes and does not absorb nutrients, except for certain vitamins.
• Absorbs water from feces to prevent dehydration and absorbs vitamins produced by intestinal flora, the bacteria that inhabit the intestine.
• Bacteria break down indigestible material and produce B-complex vitamins and vitamin K.
• Feces are ¾ water and ¼ solid wastes.
• The solid wastes are made up of bacteria and dietary fiber (indigestible remains).
• Bacteria digesting the indigestible materials cause the odor of feces and accounts for the gas.
• Stercobilin, a breakdown product of bilirubin, and oxidized iron cause the brown color of feces
• Defacation is ridding the body of feces and peristalsis forces feces into the rectum.
• Then, the rectal muscles contract, the anal sphincters relax, and then the feces exit the body through the anus after Stretching of the rectal wall initiates nerve impulses to the spinal cord.
• Can inhibit defecation by contracting the external anal sphincter, which is made of skeletal muscle.

Disorders of the Colon and Rectum

• Diarrhea causes the increased peristalsis and failure to absorb water from feces, due to either an infection or nervous stimulation.
• Constipation is a result of dry, hard feces and may be controlled with water and fiber.
• Hemorrhoids are enlarged, inflamed blood vessels of the anus due to chronic constipation, pregnancy, aging, and anal intercourse.
• Diverticulosis includes having pouches, weak spots in the muscularis layer.
• Irritable bowel syndrome (IBS), or spastic colon, results in the muscularis contracting powerfully but without normal coordination. (Symptoms: abdominal cramps, gas, constipation, and urgent, explosive stools)

Inflammatory Bowel Disease (IBD) Includes

• This consists of ulcerative colitis which affects the large intestine and rectum, which results in diarrhea, rectal bleeding, abdominal cramps, and urgency to defecate.
• Crohn's disease (usually in the small intestine), causes Ulcers in the intestinal wall, which are painful and bleed as they erode the submucosal layer, where there are nerves and blood vessels, resulting in The body not being able to absorb nutrients in the affected areas which results in Symptoms like diarrhea, weight loss, abdominal cramping, anemia, bleeding, and malnutrition.

Polyp and Cancer

• Polyps are small growths in the colon lining and can be benign or cancerous, if colon cancer is detected while still confined to a polyp, the expected outcome is a complete cure.
• Increased dietary fat raises the risk of colon cancer.
• Fiber in the diet inhibits colon cancer, and regular elimination reduces the time that the colon wall is exposed to cancer-promoting agents in the feces.
• Obesity-significantly overweight
• Of the greatest health problems in the United States is obesity, with almost 42% of adults and 18.5% of children being obese
• Excess body fat is associated with a higher risk for premature death, type 2 diabetes, hypertension, cardiovascular disease, stroke, osteoarthritis, and certain types of cancer.
• Obesity is defined as having a body mass index (bmi) of 30 or greater.

Classes of Nutrients

• A nutrient is a required component of food that performs a physiological function in the body.
• This provides energy, promotes growth and development, and regulates cellular metabolism as well as including carbohydrates, proteins, lipids, minerals, and vitamins.
• Carbohydrates-are either simple or complex, for example, Glucose (simple sugar), complex carbohydrates (digested to glucose).
• Although body cells can use fatty acids as an energy source, brain cells require glucose.
• Products made from refined grains, such as white bread, cake, and cookies. should be minimized in the diet as during the refinement of grains, fiber is removed, along with vitamins and minerals.
• Good sources of complex carbohydrates, such as beans, peas, nuts, fruits, and whole-grain products, are good sources of vitamins, minerals, and fiber.
• Insoluble fiber adds bulk to the feces and stimulates movements of the large intestine, preventing constipation and Soluble fiber combines with bile salts and cholesterol in the small intestine and prevents them from being absorbed.
• High intake of refined carbohydrates and fructose sweeteners can contribute too obesity, they also have a high glycemic index, and quickly increase blood glucose.
• The pancreas produces a lot of insulin to bring the level down when blood glucose levels rise rapidly.
• Chronically high insulin levels may lead to insulin resistance, type 2 diabetes, and increased fat deposition.

Proteins

• Are made of 20 different amino acids; this includes eight essential amino acids that must be attained through the diet.
• Complete proteins contain all essential amino acids and it is usually derived from animal sources.
• Non-animal sources of complete proteins are tofu, soymilk, and other processed food from soybeans.
• in the diet, incomplete proteins require a plant sources and you have to combine them to get all essential amino acids
• Also, you must Ingest essential amino acids every day because they are not stored in the body.
• An overabundance of protein intake can result in dehydration during exercise and can also leads to calcium loss in urine, that can lead to kidney stones.
• Eating red meat can lead to cardiovascular disease because it is high in saturated fats.

Lipids

• Include fats, oils, and cholesterol.
• Saturated fats are Usually of animal origin and solid at room temperature (unsaturated fats are usually liquid at room temperature).
• Butter, fat in meat, coconut oil, and palm oil include saturated fats. Unsaturated fats are dont promote cardiovascular disease.
• Corn oil and safflower oil are high in polyunsaturated fatty acids.
• Polyunsaturated oils contain the essential that include fatty acids linoleic acid and linolenic acid.
• Olive oil and canola oil have more monounsaturated fatty acids than others.
• Omega-3 fatty acids preserve brain function and protect against heart disease and include Flaxseed and cold-water fish which are excellent sources of omega-3 fatty acids.
• The risk for cardiovascular disease is increased by a diet high in saturated fats and cholesterol.
• Saturated fats contribute to the formation of atherosclerotic plaques that limit blood flow.
• Cholesterol is carried in the blood by two transport proteins: high-density lipoprotein (HDL) and low-density lipoprotein (LDL).
• HDL (the “good” lipoprotein) ends up in the liver, where the cholesterol is metabolized, while LDL (the “bad" lipoprotein) ends up being deposited in the tissues.
• Trans fatty acids (trans fats) is when unsaturated fatty acids are hydrogenated to produce a solid fat, and is found in commercially packaged goods, such as cookies and crackers.
• If something contains partially hydrogenated vegetable oil, it contains trans fats.

Minerals

• Minerals can divided into major minerals and trace minerals and major minerals are needed at quantities greater than 100 mg per day while trace minerals are less than 100 mg.
• Major minerals are structural components of tissues are in cells and body fluids and trace minerals are often part of larger molecules.
• Iron is present in hemoglobin, and iodine is a part of thyroid hormones.
• Zinc, copper, and manganese are present in enzymes.
• Occasionally individuals do not receive enough iron, calcium, magnesium, or zinc in their diets. Adult females need in their diet than males because they lose hemoglobin each month during menstruation.
• A varied and complete diet usually supplies enough of each type of mineral.
• Calcium is mainly located in Bones and teeth with helps nerve impulse conduction, muscle contraction and blood clotting.
• Also, taking calcium supplements can prevent osteoporosis (bone disease)in elderly and taking Vitamin D to prevent bone loss.
• Sodium regulates balances through an important for the regulation of water as well as the movement of materials across the plasma membrane and nerve impulses
• The average American takes in more than 3,400 mg but the recommended amount of sodium intake per day is 1,500 mg as intake can cause worsening hypertension.

Vitamins

• Vitamins are organic compounds used for metabolism and have to ingest because they are not produced in high enough quantities by the body as well as there are 13 different vitamins.
• They are often parts of coenzymes and are available in to types, fat soluble and water soluble.
• Antioxidants decrease the rate of oxidation or transfer of electrons.
• Cellular metabolism generates free radicals, which damage cells and tissues.
• The most common free radicals in cells are superoxide and hydroxide and have antioxidant characteristics such as Vitamin C, E, and A which defend the body against free radicals and are found largely in fruits and vegetables.
• Skin cells contain a precursor that is converted to vitamin D after UV exposure.
• Vitamin D is modified first in the kidneys and then in the liver until finally it becomes calcitriol and promotes the absorption of calcium by the intestines.
• Lack of vitamin D leads to rickets in children and leads to the bowing of the legs, which is caused by defective mineralization of the skeleton. Most milk is fortified with vitamin D to prevent rickets.

How to Plan Nutritious Meals

• Basal metabolism is the number of calories a body burns at rest to maintain normal body functions.
• Depending on exercise levels, some individuals may need more calories beyond the basal metabolic rate compared to a person that cannot become overweight without taking in more calories than needed.
• Limit the number of calories to an amount used daily
• 2,000 Maximum number of daily calories without exercise for a woman.
• 2,500 Maximum number of daily calories without exercise for a man.
• A diet should include food from all food groups, eating food such as fruits, vegetables, whole gains and low fat (fruit milk)
• You should choose dark green vegetables, orange vegetables and leafy green vegetables.
• Dry beans and peas are good sources of fiber and excellent protein.
• Limit potatoes and corn as well as when eating whole grains such as brown rice oat meal and wheat bread.
• You should choose fruit or snacks as topping for foods in stead of sugar.
• Choose lean meats such as poultry and fish especially if there's high an omega-3 fatty acids with limited sodium/trans fats.
• You should also include oils in moderate sizes that promote monounsaturated and polyunsaturated fats.
• You should be physically active daily.