Kaplan MCAT Biology: Chapter 9 Digestive System

Intracellular digestion

Part of metabolism which involves the oxidation of glucose and fatty acids for energy

Extracellular digestion

Nutrients are obtained from food in the alimentary canal

Alimentary canal

Canal which runs from the mouth to the anus and is sectioned off by sphincters; The lumen is the location of extracellular digestion

Sphincters

Circular smooth muscles around the canal that can contract to allow compartmentalization of function

Digestion

The breakdown of food into its constituent organic molecules; Can be categorized as mechanical or chemical

Mechanical digestion

Physical breakdown of large food particles into smaller food particles but does not break down chemical bonds

Chemical digestion

Enzymatic cleavage of chemical bonds such as peptide bonds of proteins and glycosidic bonds of starches

Absorption

The transport of products of digestion from the digestive tract into the circulatory system for distribution to the body's cells and tissues

Digestive pathway

Oral cavity > pharynx > esophagus > stomach > small intestine > large intestine > rectum;

Salivary glands, pancreas, liver and gallbladder provide enzymes and lubrication to help digest food

Enteric nervous system

Collection of 100 million neurons that govern the function of the GI system; Present in the walls of the digestive tract and trigger peristalsis; Can function independent of brain and spinal cord but highly regulated by autonomic nervous system

Peristalsis

Rhythmic contractions of the gut tube in order to move materials through the system; Parasympathetic NS promotes peristalsis while sympathetic NS inhibits

Ingestion

Eating food

Oral cavity

Plays a role in mechanical and chemical digestion; Mechanical digestion in mouth involves the breaking up of large food particles by mastication; Chemical digestion also occurs here to breakdown the chemical bonds in the macromolecules that make food

Mastication

Breaking large food particles into smaller food particles via teeth, tongue and lips (chewing); Increases surface-area-to-volume ratio for better enzymatic digestion

Saliva

Consists of enzymes and is produced by three pairs of salivary glands; Helps chemical digestion and mechanical digestion

Salivary glands

Produce saliva and are innervated by the parasympathetic nervous system, which responds to not only the presence of food in the oral cavity, but also the sight or smell of food; Contains salivary amylase and lipase

Salivary amylase

Enzyme capable of hydrolyzing starch into smaller sugars (maltose and dextrins); Also known as ptyalin

Bolus

A soft mass of chewed food that has been mixed with saliva and formed by the tongue for transport into the pharynx by swallowing

Lipase

Catalyzes the hydrolysis of lipids

Pharynx

Shared pathway for both food entering the digestive system (esophagus) and air entering the respiratory system (larynx); Subdivided into three sections (ordered downwards): nasopharynx, oropharynx, laryngopharynx

Nasopharynx

Section of the pharynx behind the nasal cavity

Oropharynx

Section of the pharynx behind the oral cavity, at the back of the mouth

Laryngopharynx

Section of the pharynx above the vocal cords

Epiglottis

Cartilaginous structure which folds down to cover laryngeal inlet to prevent food from entering the larynx

Esophagus

Muscular tube which connects pharynx to the stomach; Top third is skeletal muscle (voluntary/somatic), bottom third is smooth muscle (involuntary/autonomic), and middle third is a mix

Emesis

Vomiting caused by either cognitive stimulation or exposure to chemicals, infectious agents, or physical stimulation in the posterior pharynx; Reverse of peristalsis

Upper esophageal sphincter

Consists of muscles of the oropharynx, which initiate swallowing of the bolus via peristalsis

Lower esophageal sphincter (cardiac sphincter)

Muscular ring at entrance to stomach that relaxes and opens to allow the passage of food

Stomach

Highly muscular organ that uses hydrochloric acid and enzymes to digest food creating a fairly harsh environment; Its mucosa is quite thick to prevent autodigestion; Primarily for digestion, but does absorb alcohol and aspirin directly

Fundus and body

Contain mostly gastric glands of stomach

Antrum and pylorus

Contain mostly pyloric glands of stomach

Lesser curvature

Internal curvature of the stomach

Greater curvature

External curvature of the stomach

Rugae

Folds in the lining of the stomach

Gastric glands

Respond to signals from the vagus nerve of the parasympathetic nervous system, which is activated by the brain in response to the sight, taste and smell of food; These glands contain mucous cells, chief cells, and parietal cells

Mucous cells

Produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic and proteolytic environment of the stomach

Gastric juice

Combination of secretions from chief cells and parietal cells

Chief cells

Secrete pepsinogen, which is the inactive, zymogen form of pepsin, a proteolytic enzyme

Parietal cells

Secrete hydrogen ions in the form of hydrochloric acid that cleave pepsinogen to pepsin; Secrete intrinsic factor

Pepsin

Digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments; Activated by acidic environment

Intrinsic factor

Glycoprotein involved in the proper absorption of vitamin B12; Secreted by parietal cells

Pyloric glands

Contain G-cells

G-cells

Secrete gastrin

Gastrin

A peptide hormone which induces parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents

Chyme

Acidic, semifluid mixture which is made from digested solid foods; Absorption of nutrients can be maximized after food is digested mechanically and chemically due to increased surface-area-to-volume ratio

Small intestine

Consists of duodenum, jejunum and the ileum

Duodenum

First section of small intestine that is responsible for a majority of the chemical digestion and also does some absorption; Releases enteropeptidase and secretes hormones, such as secretin and cholecystokinin

Pyloric sphincter

Muscle which controls the movement of food from the stomach into the duodenum

Brush-border enzymes

Released in presence of chyme in the duodenum; Line the duodenum and break down dimers and trimers of biomolecules into absorbable monomers; Consist of disaccharidases (maltase, isomaltase, lactase, sucrose) and peptidases

Disaccharidases

Digest disaccharides; If disaccharides are not properly broken down and digested they can be hydrolyzed by bacteria to form methane gas or can have an osmotic effect and pull water into the stool causing diarrhea

Maltase

Digests maltose

Isomaltase

Digests isomaltose

Lactase

Digests lactose

Sucrase

Digests sucrose

Peptidase

Breaks down proteins by breaking down peptides

Aminopeptidase

Peptidase which is secreted by glands in the duodenum that removes the N-terminal amino acid from a peptide

Dipeptidases

Cleave the peptide bonds of dipeptides to release free amino acids

Enteropeptidase

Enzyme critical for the activation of trypsinogen to trypsin; Master switch to turn on zymogens

Trypsin

Initiates activation cascade to activate other zymogens and activates procarboxypeptidases A and B into their active forms

Secretin

Peptide hormone that causes pancreatic enzymes to be released in the duodenum; Regulates the pH in the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas; An enterogastrone

Enterogastrone

Slows motility through the digestive tract for digestive enzymes to act on chyme more effectively

Cholecystokinin (CCK)

Secreted in response to the entry of chyme (specifically amino acids in the chyme) into the duodenum; Stimulates the release of bile and pancreatic juices; Acts in the brain to promote satiety

Bile

Complex fluid composed of bile salts, pigments and cholesterol; Major pigment of bile is bilirubin

Bile salts

Derived from cholesterol in bile; Serve a necessary role in mechanical digestion of fats and oversee chemical digestion; Have hydrophobic and hydrophilic ends (amphipathic) and serve as a bridge between aqueous and lipid environments; Emulsify fats and cholesterol into micelles for lipase to act upon them

Pancreatic juices

Complex mixture of several enzymes in a bicarbonate-rich alkaline solution which helps to neutralize the acidic chyme and provide a good environment for the digestive enzymes which work best at around a pH of 8.5

Accessory organs of digestion

Pancreas, liver, and gallbladder; Release enzymes to help digestion

Pancreas

Endocrine (regulates blood sugar) and exocrine functions (synthesizes digestive enzymes)

Acinar cells

Exocrine cells which make up a bulk of the pancreas and produce the pancreatic juices

Pancreatic amylase

Breaks down large polysaccharides into small disaccharides and is therefore responsible for carbohydrate digestion

Pancreatic lipase

Breaks down fats into free fatty acids and glycerol

Major and minor duodenal papillae

Through this, the ducts filled by acinar cells empty into the duodenum

Liver

Contains bile ducts and hepatic portal vein to communicate with the digestive system; Takes up excess sugar to make glycogen which is the storage form of glucose; Can also store fats as triacylglycerols; Provides glucose through gluconeogenesis or glycogenolysis and can mobilize fats as lipoproteins; Detoxifies exogenous and endogenous compounds

Bile ducts

Connect the liver with the gall bladder and small intestine; Bile produced by the liver travels down these ducts to be stored in gallbladder or secreted in duodenum

Hepatic portal vein

Delivers all blood draining from the abdominal portion of the digestive tract to the liver; Liver processes this nutrient-rich blood before draining into the inferior vena cava on its way to the right side of the heart

Bilirubin

Major pigment in bile and is a byproduct of the breakdown of hemoglobin; Travels to the liver to be conjugated and secreted into the bile for excretion

Conjugated

Attached to a protein

Jaundice

Yellowing of the skin caused by the liver's inability to process or excrete bilirubin

Albumin

Protein synthesized by the liver that maintains plasma oncotic pressure; Serves as a carrier for many drugs and hormones

Clotting factors

Proteins synthesized by the liver that are used during blood coagulation

Gallbladder

Located beneath the liver; Stores and concentrates bile

Biliary tree

Where gallbladder pushes bile out upon release of CCK

Jejunum and ileum

Involved in the absorption of nutrients

Villi

Small, fingerlike projections from the epithelial lining of the small intestine; Have microvilli, a lacteal, and a capillary bed for the absorption of water-soluble nutrients

Microvilli

Increase the surface area available for absorption

Lacteal

Lymphatic channel on villi that takes up fats for transport into the lymphatic system

Chylomicrons

Packaged trigycerides and esterified cholesterol molecules; Enter the blood through lacteals, small vessels which form at the beginning of the lymphatic system

Thoracic duct

Where lacteals converge and enter venous circulation; Empties into the left subclavian vein

Vitamins

Absorbed into the small intestine; Can be fat-soluble (dissolve directly into chylomicrons to enter the body) or water-soluble (absorbed along with water, amino acids, and carbohydrates across the endothelial cells of the small intestine)

Transcellularly

When water passes across the cell membrane to reach the blood

Paracellularly

When water squeezes between the cells to reach the blood

Large intestine

Primarily involved in water absorption; Divided into cecum, colon, and rectum; Home to many different species of bacteria

Cecum

An outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment to the appendix

Ileocecal valve

Where fluid exits the small intestine

Appendix

May have a role in warding off certain bacterial infections and repopulating the large intestine with normal flora after episodes of diarrhea

Colon

Divided into the ascending, transverse, descending, and sigmoid colons; Absorbs remaining water and salts from the undigested material; Turns the remaining material into feces

Feces

Consists of indigestible material, water, bacteria (eg. E. coli), and certain digestive secretions that are not reabsorbed (enzymes, bile)

Rectum

Storage site for feces

Anus

Opening through which wastes are eliminated; Consists of two sphincters (internal and external anal sphincters)

Internal anal sphincter

Under involuntary (autonomic) control

External anal sphincter

Under voluntary (somatic) control