Digestive System Vocabulary Review

Gastrointestinal Tract

• Digestive organs are located within the gastrointestinal (GI) tract.

• Function—to hydrolyze, or break down, the macromolecules found in food

• The nutrients made available are transported by the blood to our cells

• Regions of the GI tract are specialized to process food

Ingestion: intake of food via the mouth

Digestion: mechanically or chemically breaking down foods into their subunits

Mechanical digestion: physical breaking of food into smaller pieces

Chemical digestion: breaking of chemical bonds so that complex molecules are broken into smaller subunits

• The digestive organs:

  • Mouth, Pharynx (throat), Esophagus, Stomach, Small intestine, Large intestine

• Aided by several accessory glands and organs: Salivary glands, Pancreas, Liver, Gallbladder

Stages of Digestion

Digestion processes:

• Movement

  • Food is passed from one organ to the next, normally by contractions of muscle

• Absorption

  • Movement of nutrients across the GI tract wall into the blood; they are then delivered to cells

• Elimination

  • Removal of indigestible wastes through defecation

Mouth (Oral Cavity)

• Functions of the mouth

  • Begins mechanical digestion

  • Begins chemical digestion

  • Monitors food quality

  • Prepares (moistens and manipulates) food for swallowing

• Teeth, salivary glands, and tongue contribute to mouth’s functions

• Salivary glands and chemical digestion

• Release saliva into mouth

• Water moistens food

• Mucus binds food

• The enzyme, salivary amylase, begins chemical digestion of starch

Bolus: mixture of food and saliva (your food after chewing)

Peristalsis: contractions that push food through the digestive tract

• Digestion begins in the mouth and continues as food travels through the small intestine

• Most absorption occurs in the small intestine

The Stomach

Stomach: thick, walled, J-shaped organ on left side of body beneath diaphragm

• Stores food, starts digestion of proteins, and controls movement of food into the small intestine but does not absorb nutrients

  • Can it absorb alcohol? (Yes! 20% of alcohol into your bloodstream through your stomach)

• Three layers of muscle in the muscularis layer to help in mechanical digestion:

  • Circular

  • Longitudinal

  • Oblique layer

Structure of the Stomach

• The mucosa has deep folds called rugae (allows stomach expansion)

• Contains gastric pits, which contain gastric glands

• The gastric glands produce gastric juice, which contains:

  • Hydrochloric acid (HCl)—pH of 1.5-3.5

  • Kills pathogens and activates pepsinogen → pepsin (digests proteins)

  • Mucus

• Normally, the stomach empties in 2–6 hours

Chyme: a mixture of food and gastric juice

Pyloric sphincter (body part): allows only a small amount of chyme to enter the small intestine at a time

• In the epithelium, gastric pits lead to gastric glands that secrete gastric juice.

• The gastric glands (one gland is shown enlarged on the right) contain different types of cells that secrete a variety of enzymes, including HCl, which activates the protein-digesting enzyme pepsin.

Small Intestine

• Small intestine (regions: duodenum, jejunum, ileum)

  • Long, highly convoluted tube extending from the stomach to large intestine

  • Region where most digestion and absorption takes place

  • 6-8 meters in length; 2,700 square feet

• Functions:

  • Chemical digestion

    • Carbohydrates, proteins, fats, and nucleic acids are broken down into their simplest forms

    • Performed by enzymes of the pancreas and small intestine, aided by bile produced in the liver, and released into the small intestine

    • Aids in digestion and absorption of fats

  • Absorption (primary site)

    • Surface area of small intestine is increased by:

      • Pleated lining

      • Villi

      • Microvilli

• The small intestine is the main digestion organ

  • Pancreas: secretes digestive enzymes through a duct into the duodenum

  • Amylase: digestion of carbohydrates

  • Trypsin: protease enzyme that digests protein

  • Sodium bicarbonate to neutralize acidic chyme

  • Liver: produces bile that’s secreted into the duodenum; gallbladder stores bile

    • Bile emulsifies fat by mechanical digestion; causes fat droplets to disperse in water to expose more of it to lipase—enzyme that hydrolyzes fats into glycerol and fatty acids

Nutrients Are Absorbed in the Small Intestine

• The mucosa contains fingerlike projections called villi (sing., villus), and cells that cover them have microvill

  •  Give the villi a fuzzy “brush border”; contain brush border enzymes that complete digestion

  • Greatly increase the surface area of the small intestine for absorption of nutrients; contain blood capillaries and lacteal (lymphatic capillary)

    • 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

• After nutrients are absorbed, they are eventually carried to all the cells of the body by the blood.

Digestion and Absorption of Organic Nutrients

Lactose Intolerance

• Lactose is the primary sugar in milk.

• Lactase: the brush-like border  enzyme that digests lactose

Lactose intolerance: characterized by diarrhea, gas, bloating, and abdominal cramps after ingesting milk and other dairy products

• Diarrhea occurs because the undigested lactose causes fluid retention in the small intestine

Celiac Disease

• Autoimmune response against gluten protein, which is naturally found in grains such as wheat, barley, and rye

• The presence of gluten in the small intestine results in an inflammatory response, which damages the villi and microvilli of the small intestine.

• Can lead to abdominal pain, diarrhea, and malnutrition

Large Intestine (Regions: Cecum, Colon, Rectum, Anus)

• Large intestine outside ‘colon’

• 1.5 meters in length; wider and shorter than the small intestine

• Smooth inner wall, punctuated with ‘crypts’

• Absorbs leftover water and salts from the material that has not been digested

• Materials that have not been absorbed by the small intestine move to the large intestine

• Functions

  • Absorbs most of the water remaining in the indigestible food residue

  • Stores and eliminates feces

  • Absorbs water and ions

  • Contains beneficial bacteria that produce vitamins

• Material left in the large intestine after passing through the colon is called feces

Disorders of the Colon

Diarrhea: Material passes through the colon too quickly and not enough water is absorbed

Constipation: Material passes through the colon too slowly and too much water is absorbed

Accessory Organs (Liver, Gall Bladder, Pancreas)

• Food doesn’t pass through them, but they support digestion

• Pancreas

  • Pancreatic juice

    • Contains many digestive enzymes, including:

      • Amylase (breaks down carbohydrates)

      • Lipase (breaks down fats)

      • Proteases (like trypsin and chymotrypsin, which break down proteins)

  • Contains bicarbonate ions important in neutralizing the acid in chyme

  • Drains from pancreas into pancreatic duct, which fuses with the common bile duct before entering small intestine

• Liver

  • Largest internal organ

  • Nutrient-laden blood from capillaries in villi of the small intestine travels through the hepatic portal vein to the liver

  • Digestive activities of the liver:

    • Produces bile

    • Regulates blood glucose levels

    • Packages lipids with proteins to form lipoproteins, which aid transport in the blood

    • Removes poisonous substances

Disorders of the Liver

• Cirrhosis

  • Fat and then scar tissue accumulate in the liver

  • What is a common cause of cirrhosis? (Hepatitis, other viruses, and alcohol abuse)

• Hepatitis

  • Inflammation of the liver

  • Commonly caused by one of six viruses: A, B, C, D, E, and G

  • Hepatitis C has significantly worse outcomes than A and B

— — —

Colorectal Cancer

• Begins with polyp (small growths in the colon lining)

• Can be benign or cancerous

• Risk factors:

  • 50+ years old

  • Lack of physical exercise 

  • Use of alcohol and tobacco

  • Low-fibre diet

  • Family history of colon cancer

Colorectal Cancer - Screening

• Colonoscopy

  • Sigmoidoscopy (only first third of colon examined) or colonoscopy (entire colon examined)

  • A long, flexible tube (colonoscope) is inserted into the rectum. A tiny video camera at the tip of the tube allows the doctor to detect changes or abnormalities inside the entire colon

  • Colonoscopy takes about 30 to 60 minutes and screening is generally repeated every 10 years if no abnormalities are found and you don't have an increased risk of colon cancer

• Fecal Occult Blood Test

  • Lab tests used to check stool samples for hidden (occult) blood

Investigating the Risk Factors for Colorectal Cancer

• Denis Burkitt (1971) formulated the high fat/low fiber hypothesis of colon cancer.

• This hypothesis is based upon the low incidence of colon cancer and other intestinal diseases in the African countries where Burkitt conducted his surgical practice

Burkitt’s Hypothesis (Outdated)

• In populations consuming a high fiber, low fat diet, Burkitt observed virtually none of the large bowel diseases that are commonplace in westernized societies such as:

  • Hemorrhoids, diverticulitis, ulcerative colitis, Crohn’s disease, and colorectal cancer.

• His recommendations for prevention of colon cancer specify high levels of intake throughout life (40 grams of fiber daily with a high proportion of bran)

Fibre

Dietary fibre: non-digestible carbohydrate present in plants such as grains and legumes

Soluble fibre: fibre that dissolves in water or is broken down by bacteria in the large intestine

• Combines with bile salts and cholesterol in the small intestine and prevents them from being absorbed

Insoluble fibre: fibre that does not dissolve in water and is not broken down by bacteria in the large intestine

• Wheat bran or psyllium seed; adds bulk to the feces and stimulates movements of the large intestine, preventing constipation and other digestive disorders

The Parts of a Whole Grain Kernel

Whole grain: The entire edible portion of a grain, such as wheat, rice, or oats, consisting of the germ, endosperm, and bran

• During refinement of grains, fiber is removed, along with vitamins and minerals; end product is starch

• Recommendations of fibre:

  • Men: 30-38 grams per day

  • Women: 21-25 grams per day

The Gut Microbiome

• Gut Microbiome consists of microbes that are both helpful and potentially harmful

  • Symbiotic

  • Pathogenic

• Stimulate the immune system, break down potentially toxic food compounds, and synthesize certain vitamins and amino acids

  • Including the B vitamins and vitamin K

    • Eg. key enzymes needed to form vitamin B12 are only found in bacteria, not in plants and animals

• Dysbiosis: disturbance in balance

  • Infectious illnesses, certain diets, or the prolonged use of antibiotics or other bacteria-destroying medications

• Large families of bacteria found in the human gut

  • Prevotella, Ruminococcus, Bacteroides, and Firmicutes

• In the colon, a low oxygen environment → anaerobic bacteria

  • Peptostreptococcus, Bifidobacterium, Lactobacillus, and Clostridium

  • Believed to prevent the overgrowth of harmful bacteria by competing for nutrients and attachment sites to the mucus membranes of the gut, a major site of immune activity and production of antimicrobial proteins

*There’s a diagram on slide 46 but honestly I have no idea what the main point of that is

Burkitt’s Hypothesis (Revised)

Nutrition

Nutrient: a required component of food that performs a physiological function in the body

• Provides energy, promotes growth and development, and regulates cellular metabolism

• Includes carbohydrates, proteins, lipids, minerals, and vitamins

• Some nutrients are essential, meaning they must be included in the diet as the body cannot produce them

Carbohydrates

• Either simple or complex

• Provide four calories per gram

• Glucose → simple sugar

• Complex carbohydrates (e.g. legumes, fruits, starchy vegetables) are digested to glucose

• Primary function is to provide energy

• Although body cells can use fatty acids as an energy source, brain cells require glucose

• Carbohydrates can be harmful:

  • High glycemic index: Refined carbs, like those found in sugary foods and white bread, are quickly broken down into sugar, causing rapid spikes in blood glucose levels (can lead to insulin resistance over time = type 2 diabetes)

Proteins

• Proteins are chains of amino acids 

• Provide four calories per gram

• Essential aa are the 9 aa (e.g. leucine, phenylalanine) out of 20 that the body cannot synthesize and must be obtained in the diet

  • Complete protein (mostly animal sources), contains all the essential aa

  • Incomplete protein (plant sources), lack one or more of the essential aa

  • Complementary proteins are combinations of incomplete proteins that supply all of the essential aa

• The three most common complementary protein combinations are:

  • Grains (rice, corn, wheat, barley, etc.) + legumes (peas, beans, lentils)

  • Grains and milk products

  • Seeds (Sesame or sunflower) +legumes

• Proteins can be harmful

  • Idk why lol I’llask in office hours

Lipids

• Provide nine calories per gram

• Include fats, oils, and cholesterol

• Saturated fats

  • Usually of animal origin

  • Solid at room temperature

  • Butter, fat in meat, coconut oil, and palm oil

• Unsaturated fats

  • Don’t promote cardiovascular disease

  • Usually liquid at room temperature

• Lipids can be harmful

  • Saturated fats can raise LDL cholesterol

  • Trans fats can raise LDL cholesterol and lower HDL cholesterol