Liver Functions and Digestion

• The study guide discusses the functions of the liver, the layers of tissue in the gastrointestinal (GI) tract, the functions of the pancreas, the parts of the small intestine, digestion, and other relevant topics.

Liver Functions

• The liver filters and removes toxins, chemicals, and waste from the blood.
• Detoxification helps keep the body clean and healthy.
• The liver produces bile which helps digest fats, and enables the absorption of fat-soluble vitamins A, D, E, and K.
• The liver stores glucose as glycogen.
• The liver converts glycogen back into glucose to provide energy when blood sugar is low.
• The liver helps maintain stable blood sugar levels by storing and releasing glucose, and it is key for energy and overall health.
• The liver makes albumin to maintain fluid balance, and proteins for blood clotting.
• Proper cholesterol levels, managed by the liver, protect the heart and blood vessels.
• After food digestion, the liver processes nutrients like proteins, fats, and vitamins for the body's use, ensuring the body gets the nutrients needed for energy and growth.

GI Tract Layers

• The GI tract is composed of layers of tissue

Mucosa

• The innermost layer of the GI tract is directly in contact with food and liquids.
• It protects underlying tissues from digestive juices and pathogens.
• It secretes mucus, enzymes, and hormones, and absorbs nutrients.

Submucosa

• The layer beneath the mucosa contains blood vessels, lymphatic vessels, and nerves that support the mucosa.
• Blood vessels transport nutrients, and the layer's elasticity helps the GI tract expand and contract.

Muscularis

• The layer beneath the submucosa consists of smooth muscle.
• It moves food through the GI tract contracting and relaxing in a coordinated way, called peristalsis.
• It mixes food with digestive enzymes and acids in the stomach.

Serosa

• The outermost layer of the GI tract provides a smooth protective covering.
• It produces fluid to reduce friction with surrounding organs.

Pancreas Functions

• The pancreas is a gland located behind the stomach with exocrine and endocrine functions.

Exocrine

• The pancreas produces digestive enzymes and bicarbonate that assist in food breakdown in the small intestine.
• The pancreas releases digestive enzymes like amylase, lipase, and proteases, in the duodenum (the first part of the small intestine) through a duct.
• Bicarbonate neutralizes stomach acid.
• Enzymes in the pancreas help to digest carbohydrates, fats, and proteins.

Endocrine

• The pancreas has hormone-producing cells called islets of Langerhans.
• Insulin lowers blood sugar by helping cells absorb glucose.
• Glucagon raises blood sugar by signaling the liver to release stored glucose.

Small Intestine Parts

• The small intestine helps with blood sugar regulation and digestion

Duodenum

• It is located just after the stomach
• Digestive enzymes from the pancreas and bile from the liver mix with food to further break it down.
• The duodenum uses bicarbonate to neutralize the acidic food coming from the stomach.
• Enzymes break down fats, proteins, and carbohydrates into smaller molecules for absorption, otherwise called chemical digestion.

Jejunum

• Most nutrients are absorbed here

Ileum

• It is the last section of the small intestine before it connects to the large intestine.
• Vitamins, bile acids, and any remaining nutrients are absorbed in the ileum .
• Final absorption of nutrients happens here, with B12 vitamins and bile salts being recycled.

Mesenteries

• Mesenteries support and hold organs, supply blood and nerves, as well as aid in immune functions.
• Mesentery Proper supports the small intestines.
• Mesocolon supports the large intestine.
• Lesser Omentum connects the stomach and liver.
• Greater Omentum provides protection and padding to the intestines.

Peristalsis

• A wavelike muscle movement pushes food in one direction through the digestive tract.
• It occurs throughout the entire digestive system to move food.

Segmentation

• The muscle contraction mixes food and digestive juices, mainly occurs in the small intestine, and does not move food forward.

Digestion Locations

Mouth

• Digestion starts here
• Chewing (mechanical digestion) breaks food into smaller pieces.
• Saliva contains amylase (chemical digestion) that starts breaking down carbohydrates.

Esophagus

• No digestion happens here
• Peristalsis (wave-like muscle contractions) moves food from the mouth to the stomach.

Stomach

• Mechanical digestion: Stomach muscles churn and mix food
• Chemical digestion: Gastric juices containing hydrochloric acid and pepsin break down proteins.
• The stomach stores food temporarily.

Small Intestine

• Bile and digestive enzymes from the pancreas further break down food.
• Carbs, proteins, fats, vitamins, and minerals are absorbed into the bloodstream.

Large Intestine

• Water and salts are absorbed
• Undigested food is stored and waste is prepared for elimination.

Types of Digestion

Mechanical

• It is the physical breakdown of food into smaller pieces, i.e. chewing
• Occurs through chewing, churning, and peristalsis, making easier for enzymes to work during chemical digestion.

Chemical

• Enzymes and acids break down food into smaller absorbable molecules (nutrients).
• In the mouth, saliva contains amylase to break down carbohydrates.
• In the stomach, pepsin and hydrochloric acid break down proteins.
• In the small intestine, pancreatic enzymes (amylase, lipase, proteases) and bile from the liver break down carbs, fats, and proteins.

Catabolic and Anabolic Reactions

Catabolic Reactions

• They break down larger molecules into smaller ones, releasing energy.
• Digestion and cellular respiration are examples.

Anabolic Reactions

• They build smaller molecules into larger ones, requiring energy.
• Protein synthesis and photosynthesis are examples.

Large Intestine

• The large intestine is the final part of the digestive system, responsible for absorbing water, storing waste, and eliminating it.
• Cecum is the first part of large intestine and receives waste, while the appendix is a small structure with an unclear function.
• The colon absorbs water and includes ascending, transverse, descending, and sigmoid parts.
• The rectum stores solid waste, and the anus eliminates waste.
• The large intestine absorbs water and salts from undigested food, turning it into solid waste (feces).
• Bacteria in the large intestine produce Vitamin K and B vitamins.
• The large intestine forms feces by absorbing water and salts, storing waste in the rectum for elimination through the anus.
• When you have diarrhea, the large intestine fails to absorb enough water, resulting in liquid stool.

Plicae Circulares

• They are circular folds in the lining of the small intestine, increasing its surface area to aid nutrient absorption and slow down food movement for better absorption.

Stomach pH

• The stomach has a highly-acidic pH, usually around 1.5 to 3.5.
• This acidic environment is essential for food digestion and killing harmful bacteria.

Enzymes

• Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy.

Hormones

Insulin

• Released by the pancreas, lowers blood sugar by helping cells take in glucose.

Glucagon

• Released by the pancreas, raises blood sugar by signaling the liver to release stored glucose.

Somatostatin

• Inhibits/balances the release of both insulin and glucagon, helping to balance blood sugar levels.

Bile

• Bile is a digestive fluid produced by the liver that helps break down fats in the small intestine, stored in the gallbladder until it's needed for digestion.

Sphincters

• Lower Esophageal Sphincter (LES) prevents acid reflux.
• Pyloric Sphincter controls food flow into the small intestine.
• Ileocecal Sphincter regulates flow to the large intestine.
• Anal Sphincters control feces release.

Stomach Protection

• The stomach protects itself from its acidic environment with mucus secretion, bicarbonate production, tight junctions, and cell regeneration.
• The stomach's low pH (1.5 to 3.5) comes from hydrochloric acid (HCl) secretion, which aids digestion and kills bacteria.

Enzymes

Amylase

• Breaks down carbohydrates (starch) into sugars, starts in the mouth and continues in the small intestine.

Lipase

• Breaks down fats (lipids) into fatty acids and glycerol in the small intestine after release from the pancreas.

Peptidase

• Breaks down proteins into amino acids, works in the stomach and small intestine.

Lacteals

• They are tiny lymphatic vessels found in the villi of the small intestine that absorb fatty acids and glycerol, transporting fats into the system.

Nervous System in Digestion

Sympathetic

• Slows down digestion.

Parasympathetic

• Speeds up digestion.

Bacteria

• The bacteria in the large intestine help break down indigestible substances such as the fiber found in fruits, vegetables, and grains.

Bile Ducts

• Bile is a digestive fluid produced by the liver to break down fats
• Bile starts in the liver
• Bile then flows through special ducts.
• Hepatic ducts (right and left) carry bile from the liver and come together to form the common hepatic duct.
• If bile is not needed right away, it flows through the common hepatic duct to the cystic duct and is stored in the gallbladder until digestion is needed.
• When the body is ready to digest fat, bile flows from the gallbladder (through the cystic duct) and joins the common hepatic duct to form the common bile duct.
• Common bile duct carries bile into the duodenum (small intestine) where it helps digest fats.

Alcohol Absorption

• Some alcohol is absorbed directly through the stomach lining into the bloodstream, most alcohol is absorbed in the small intestine.

Digestive System Functions

Ingestion

• Taking in food and liquids through the mouth.

Digestion

• Breaking down food into smaller pieces (mechanical and chemical).

Absorption

• Nutrients from food are absorbed into the bloodstream, mostly in the small intestine.

Transport

• The fourth function is moving food and waste along.

Defecation

• Removing waste and undigested food from the body.

Bolus Formation

• Bolus is the ball of chewed food mixed with saliva that is ready to be swallowed.
• Food is chewed, mixed with saliva to form a soft, round ball.
• It's then swallowed down the esophagus.

Pancreatic Ducts

• The pancreatic ducts carry digestive enzymes from the pancreas to the small intestine to break down food which allow nutrients to be absorbed.

Pepsinogen

• Pepsinogen is the inactive form of pepsin, an enzyme that breaks down proteins in the stomach.
• It is kept inactive to prevent it from digesting the stomach lining itself, and is activated when exposed to the acidic environment in the stomach (hydrochloric acid).
• Parietal cells help create the acidic environment in the stomach
• Chief cells release pepsinogen to help digest proteins.

Potential Energy

• Is the amount of energy in an object due to its condition or state
• High potential energy is when something in a high position
• Low potential energy is when something is stable on the ground

Ulcers

• It is a sore that forms on the lining of the stomach, small intestine, or esophagus.
• The main causes are from bacteria, overuse of painkillers, excess alcohol, stress, and or spicy foods. It can result in harmful bacteria, hurt stomach lining, and irritate it.

Segmentation

• It is a type of mechanical digestion in the small intestine.
• It contracts to mix the food and move in the digestive tract.

Somatostatin

• The hormone that regulates functions like stomach acid production.
• It aids in digestion and also slows the release of hormones.